Design World/EE Network - Power Electronics Handbook

www.designworldonline.comA Special Supplement

Power Electronics Handbook

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2 DESIGN WORLD 2015 www.powerelectronictips.com

said “There were many more at the graduate level. Glancing at the new course catalog, it appears they have added a power electronics lab and an undergraduate packaging class. I would guess other universities have fewer power electronics courses, since that is a main research area for VT faculty.”

Of course, sometimes the breadth of subjects you’re taught isn’t as important as what you do with the material you’re exposed to. “It’s not so much about what grade level EE students are introduced to power semiconductors. It is more related to how far along a person is in his/her career when the new power semiconductor technology is introduced. We studied a spec sheet for a 2N3055 power bipolar in a 2nd-year EE class, but it wasn’t until half a decade later that power MOSFETs were just starting to be known. When a new technology (like GaN) is introduced, it is up to working professionals or students to become familiar with it, no matter their age,” said one power engineer.

And some engineers don’t see formal classwork ever having much of an impact on power.

“Personally, I feel universities will never handle power properly,” said one. “They can and do introduce product families, but devices are out of the question—there are just too many. There are enough transistors/SCRs/IGBTs/GTOs/triacs/*FETs/BJTs to keep the student busy for four years…I was lucky enough to have what I think was the best kind of instruction—a good teacher working on a project, with time to educate.”

This handbook is our small attempt at advancing power electronics education. We hope you find it worthwhile.

We asked ourselves that question recently when we ran an informal contest on our powerelectronictips.com site. We know the answer in at least one case: The first person with a perfect score was Xiucheng Huang, currently working toward a Ph.D. at Virginia Tech’s Center for Power Electronics Systems.

The rhetorical query Huang’s score brings up is whether you have to be a Ph.D. candidate these days to ace a short test on power semiconductor circuits. We asked about power circuit education on such social sites as LinkedIn and EDAboard.com. The responses we got seem to show that until recently, power electronics was a subject mainly available only in grad schools.

The most thoughtful observations came from application engineers at eGaN maker Efficient Power Conversion, who are themselves recent grads. “As an undergrad, I had no exposure to power electronics,” said one. “There was one power engineering (T&D) class (for undergrads), but that was it. At the time, anything power related was considered mature and something of a dead end. I ended up taking RF and analog classes, which turned out to be a good fit anyway. In grad school, the power electronics program was brand new and offered as a grad/senior undergrad class…Outside the context of a power electronics class, I don’t think there is much exposure to the real implementation of power transistors and diodes, or even passive components that must handle power. This is especially true for magnetic components. I don’t think you gain a real appreciation until you burn some parts and/or yourself!”

Another EPC engineer confirms the impression that power electronics has only recently become a topic for undergraduates. “At Virginia Tech, there was one course that covered power electronics at the undergraduate level,” he

of power electronics engineers

Leland TeschlerExecutive Editor@DW_LeeTeschler

NEXT GENERATION

WHO DO YOU SUPPOSE CAN ACE A TEN-QUESTION QUIZ ABOUT USING POWER TRANSISTORS?

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02 The Next Generation Who do you suppose can ace a ten-question quiz about using power transistors?

10 NEWS Tighter Energy Star regs; higher switching

frequencies; harvesting energy from lake currents; better lithium batteries; GaN FETs go into production

16 Power converters with GaN It can be tricky to analyze power supply designs incorporating GaN power devices.

24 How to evaluate cutting-edge power semiconductors

When device makers don’t give values for all the pertinent parameters, circuit designers themselves must characterize strategic operating factors.

33 Working with Wirewounds For high currents, axial-leaded wirewound resistors are often good candidates because they can handle up to 10 W and double as fuses.

37 Cool calculations It pays to do a back-of-the-envelope thermal analysis.

40 Choking off EMI/RFI in off-line switchers

Common-mode chokes help keep down RF emissions that can otherwise be problematic in modern power supply circuits.

48 Small batteries pack big wallop for wireless devices Battery makers have devised chemistries that put a lot of energy in a tiny volume.

52 Selecting power supply connector pins Designers can avoid reliability problems by acquainting themselves with the way connector pin materials stand up to repeated use and corrosion.

55 ADVERTORIAL: Thermal management and cost of ownership in power systems

58 TOP FORUM MEMBERS Bring Power Knowledge to their Peers

CONTENTS

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EDITORIAL

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Power supply makers

THE LATEST round of energy efficiency standards

for external power supplies will go into effect next year. But unlike previous standards, efficiency levels dictated by the new mandates, which power supply makers dub Level 6, could spell difficulties for power supply makers.

“We couldn’t just tweak the current design to get there,” said Andrew Johnson, product manager for external supplies, CUI Inc.“For Level 6, we had to look at the whole topology and rethink it. The redo involved everything from the transformer to the control IC.”

The new efficiency levels could come as a surprise to power supply users that aren’t closely monitoring Energy Star. “The impact on OEM customers is that they’ll soon have to swap out current models to Level 6. Foreign firms sending products into the U.S. also have to be aware of this,” said Johnson. “We’ve found that in the past, customers have been generally unaware of the changes when there have been transitions from one level to the next. Companies need to start planning now for these changes.”

CUI said it will be releasing Level 6 supplies this year, starting with desktop adapters up to 150 W. Though CUI is ready to field power supplies meeting the new mandates, company officials think the new efficiency benchmarks are tough enough to precipitate a shake out among power supply vendors. “Going from Level 4 to Level

Tougher Energy Star standards kick in next year. Some manufacturers may not be able to handle its more stringent mandates.

get ready for tighter energy efficiency regs

5, suppliers could get by with minor tweaks of their product, such as adjusting dc core length. Level 6 is not as trivial and safety issues are more important as well,” Johnson said.

Level-6 regs expand the range of products that fall under the Energy Star efficiency mandate. Regulated products will now include multiple-voltage external power supplies and products with power levels exceeding 250 W. The new rules also boost active mode efficiency by 5%.

But the biggest change, at least in the eyes of power supply makers, is no-load efficiency. Level 6 boosts it from 0.5 W to 0.1 W in supplies up to 49 W. It is the higher no-load efficiency that power supply makers find tough to meet and could force the most changes in circuit design. “We are still working through the cost impact. It could be a 5 to 10% bump. The transformer on some of the bigger

designs will require a slightly larger case,” said Johnson.

The new standard also categorizes power supplies as direct operation and indirect operation products. A direct operation product is an external power supply (EPS) that functions in its end product without using a battery. An indirect operation EPS is not a battery charger, but cannot operate the end product without the assistance of a battery. The Level 6 standard only applies to direct operation external power supplies. Indirect operation models will still be governed by the limits as defined by EISA2007.

U.S. supply makers designing products for export must wait to find out whether Level 6 will apply in other parts of the world. The EU and Canada currently are at Level 5 efficiency specs. Supply makers expect a ruling in the next year about whether they will go to the new levels.

You can tell this external power supply meets new efficiency standards by the Roman numeral VI. Manufacturers use Roman numerals I through VI as an in-ternational efficiency marking protocol.

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The Microchip name and logo and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.All other trademarks are the property of their registered owners. © 2014 Microchip Technology Inc. All rights reserved. 7/14DS00001793A

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Gallium Nitride power

Better barriers

FUJITSU Semiconductor group’s CMOS-compatible, 150-mm wafer fab in Japan has started mass production of Gallium Nitride (GaN)

power devices for switching applications. The facility is providing GaN foundry services for Transphorm, which has established the only qualified 600-V GaN device platform. The first photovoltaic power conditioner products using the GaN module from Transphorm also launched in January 2015. Other applications include ultra-small ac adapters, high-density power supplies for PCs, servers and telecom equipment, and highly efficient motion control systems.

Transphorm’s Jedec-qualified process has been combined with Fujitsu Semiconductor’s technology with improvements for high-volume, silicon-compatible device manufacturing.

Transphorm also recently secured fundamental patents in the areas of GaN power conversion and inductive load power switching circuits, and in bridge circuits. Both patents are directed toward the operation and use of GaN transistors

in applications that include half bridges. Counterparts of these patents have also been issued in China and Taiwan and are pending in several other countries. The patents belong to a bridge circuit

patent family based on Transphorm Diode-Free GaN, wherein a GaN transistor also serves the function of the conventional anti-parallel, or fly-back diode, used in traditional approaches. This not only helps eliminate diode components, but also eliminates the cost, space and energy loss associated with them—resulting in compact, higher efficiency systems.

AN advanced barrier between the electrodes in lithium-ion batteries could help head off fires and overheating that occasionally arises in

laptops or industrial applications. The barrier uses nanofibers extracted from Kevlar. The barrier helps minimize the growth of metal tendrils on battery electrodes that can otherwise lead to electrical shorts.

In normal battery operation, there are holes in the barrier that let lithium ions pass through but not electrons. If the holes in the membrane get too big, lithium atoms build up into fern-like structures called dendrites, which eventually poke through the membrane. If they reach the other electrode, the resulting structure gives a path for electrical current within the battery, shorting it out.

“The fern shape is particularly difficult to stop because of its

FETs go into mass production

make lithium batteries hard to beat

A Transphorm FET equivalent circuit and typical packaging.

A sample of the Kevlar separator material. It is described in a paper titled A dendrite-suppressing solid ion conductor from aramid nanofibers, which appeared online Jan. 27 in Nature Communications.


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www.powerelectronictips.com 2015 DESIGN WORLD 13

Move over, wave power —

PILOT projects aimed at generating electrical energy from the motion of ocean waves have gotten a lot of press. The Scottish

government, for example, has now approved the installation of a 40-MW wave energy generation facility in the Shetland Islands.

To these efforts you can now add those of a company called Vortex Hydro Energy in Michigan. Founded by two engineering professors from the University of Michigan (U of M), the firm takes a different approach to generating power than those of traditional wave generation schemes. Wave energy harvesters generally employ bobbing buoys or some similar means of extracting energy from the movement of passing waves. In contrast, the VHE device consists of cylinders that move up and down as vortexes in the current move past them, creating kinetic energy. Each bobbing cylinder moves a magnet up and down past a coil of wire, thus creating electrical current from water currents.

Unlike traditional wave generators, the VHE device needs no wave action to operate, only currents as might arise in lakes or streams. That makes it more versatile in the eyes of its creators, Michael Bernitsas of the U of M Naval Architecture and Marine Engineering department, and James MacBain, also formerly with U of M’s College of Engineering. The vortex induced vibration (VIV) that provides the kinetic energy is an extensively studied phenomenon where vortices are formed and shed on the downstream side of rounded objects in a fluid current. The vortex shedding alternates from one side of a body to the other, thus creating a pressure imbalance causing an oscillatory lift.

The cylinder oscillations happen at a relatively slow, one-cycle-per-second rate. This lets fish easily navigate around the cylinders, the creators said.

VIM is a nonlinear resonance, as opposed to linear resonance which happens only when the frequency of excitation from the flow, such as

Vortex device aims to harvest energy from water

nanoscale tip,” said University of Michigan graduate student Siu On Tung, who is also chief technology officer at Elegus Technologies, a spin-off company that will be commercializing the technology. “It was important that the fibers formed smaller pores than the tip size.”

Other membranes might have pores that are a few hundred nanometers wide. In contrast, the pores in the membrane developed at U-M are 15 to 20 nm across. That is large enough to let individual lithium ions pass, but small enough to block the 20 to 50-nm tips of the fern structures.

The researchers made the membrane by layering the fibers on top of each other in thin sheets. This method keeps the chain-like molecules in the plastic stretched out, which is important for good lithium-ion conductivity

between the electrodes, Tung said.Researchers say they can make the Kevlar material

in super-thin sheets that allow squeezing more energy into a given battery size or reducing the size of a cell with a given energy capacity. They also say Kevlar’s heat resistance could lead to safer batteries as the membrane stands a better chance of surviving a fire than most membranes currently in use.

The team is satisfied with the membrane’s ability to block the lithium dendrites, but they are currently looking for ways to improve the flow of loose lithium ions so that batteries can charge and release their energy more quickly.

Elegus Technologies expects to begin shipping the material in the fourth quarter of 2016.

BETTER BARRIERS cont.

Vortex Hydro Energy’s concept of an energy harvesting farm of VIV devices. The concept has the additional advantage of being fish friendly.

waves, equals the natural frequency of the oscillating body. In contrast, linear resonance is the principle used in energy devices with oscillating buoys, flaps, foils, or water columns. It has a limited range of large amplitude oscillations.

VHE has gotten to the prototype stage with its technology, which it calls VIVACE, or Vortex Induced Vibrations for Aquatic Clean Energy. It now has a pilot project in place in Michigan’s St. Clair River near Detroit. VHE hopes to commercialize the technology, which was invented and patented at U of M. Plans are to deploy another small prototype this summer and a bigger one next year. So far, the company has deployed cylinders about 12 ft long. Plans are to next build an installation with cylinders about 20 ft long in an apparatus that is about 27 ft high.

One irony of VHE’s technique is that the VIV effect it employs has, until now, been considered mainly a source of structural damage. The original Tacoma Narrows Bridge collapsed in 1940 when vortex shedding caused torsional vibrations that ripped the bridge apart.

REFERENCE:Vortex Hydro Energy, www.vortexhydroenergy.com


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NEW designs for switch-mode power supplies are pushing frequencies higher as a way of

cramming circuitry into smaller and smaller volumes. An indication of the trend can be seen from the recently developed Dart power supply from Finsix, a start-up with roots in the Massachusetts Institute of Technology. The Finsix device fits in the palm of your hand, but puts out 65 W, as much as a conventional brick power supply for laptops. It includes a USB connection so it can power phones as well as a computer.

The key to its small size is its switching power supply operating at VHF switching frequencies—around 100 MHz according to company literature. In comparison, ordinary switching supplies usually work in the range of tens of megahertz. The use of higher frequencies reduces the size of the circuit components involved.

VHF-range switching supplies are a hot research topic, but usually involve wide bandgap SiC or GaN power devices. However, company officials said the Dart uses only silicon power transistors.

Finsix said it uses a fully-resonant Class-Φ2 converter topology. A resonant converter is basically a switching supply that incorporates a resonant L-C circuit as a part of the power conversion process. These converters operate by putting energy into the resonant circuit and then transferring some or all of it into the load.

The Class-Φ2 converter topology, which does not originate with Finsix, was inspired by RF power amplifiers, which use resonant harmonic peaking of their input or output network to reduce the peak voltage on the semiconductor switch. But RF amp topologies can’t be borrowed unchanged to make power supply circuits; their switching devices are typically never 100% on or off, so they tend to make inefficient power supplies. Class-Φ converters address the efficiency problem by putting a high-order lumped network in the supply input to shape the waveform in a way that reduces the stress on the switching device.

A problem is that these inverters use high-order resonant structures with many energy storage components that make the circuitry complicated. The Class-Φ2 is a simplified version of a Class-Φ inverter, which, among other things, absorbs switching device capacitance into the wave-shaping network. This lets the supply put out the same amount of power regardless of changes in the device output capacitance. The components of the inverter are tuned to get a low peak voltage across the semiconductor switch and to allow near zero voltage at switch turn-on and turn-off. Judicious choice of energy storage components can yield a zero dv/dt across the switch at turn on, desirable for operating at VHF-UHF frequencies.

A point to note is that resonant converters are characterized by a storage of energy in the device parasitic capacitance. The energy moves to a complementary reactance over each cycle so that most of the energy gets recovered. This results in only a small energy loss per switch transition, so the resulting power supplies are energy efficient.

The Finsix founders came out of the Power Electronics Research Group (PERG) within MIT. Work on high-frequency supplies continues there as well. For example, a group organized by PERG head David Perreault is developing a prototype LED lamp driver that operates at frequencies high enough to use ceramic capacitors for energy storage instead of electrolytic devices. Perreault said the prototype LED driver hits a power factor better than 0.7, the level recommended for LED drivers by the EPA Energy Star program. The driver also provides a power density five to 10 times that of current commercial systems, he said.

High switchingfrequencies come to laptop chargers

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Power converters

THERE is a lot of activity in wide-bandgap semiconductors these days. They have the ability to operate

at much higher voltages, frequencies and temperatures than conventional semiconductor materials. Power circuits built around wide-bandgap devices can be cheaper and more energy efficient than their silicon counterparts.

One type of wide-bandgap technology for making power transistors uses gallium nitride grown on top of silicon. Enhancement-mode transistors, such as eGaN FETs from EPC, have qualities resembling those of power MOSFETs but with better high-speed switching, lower on-resistance, and a smaller size than their silicon predecessors.

But eGaN transistors aren’t one-for-one replacements for silicon MOSFETs. Nor can eGaN power supply circuits be analyzed in exactly the same way as those created with MOSFETs.

In that regard, we recently ran an informal online contest that asked contestants to solve ten story problems centered on power supply design with eGaN devices. Winners received a copy of the hardcover textbook, GaN Transistors for Efficient Power Conversion, 2nd Edition.

The problems in our challenge proved to be difficult for most people. Only a handful of entrants got a perfect score and earned a copy of the book.

In the interest of promoting power supply education, here are the questions and the solutions that eluded a majority of contest entrants. Engineers engaged in cutting-edge power design would be well served by spending time understanding the concepts behind the answers.

It can be tricky to analyze power supply designs incorporating GaN power devices. Here are the basics of calculating losses and device currents when using these exotic semiconductors.

Alex Lidow | CEOEfficient Power Conversion Corp.

with GaN

eGaN_EE_Feb_Vs11.indd 16 3/6/15 2:54 PM

www.powerelectronictips.com


Synchronous Buck Converter

Switching node waveform

MOSFET Body Diode

Td (on) Td (off) Td (on) Td (off)

1000

100

10

10.2

VGS = 0V

TJ = 150oCTJ = 25oCTJ = -40oC

VSD, Source-to-Drain Voltage (V)

0.30 .4 0.50 .6 0.70 .8 0.91 .0

In the same buck converter,

VIN = 12 V, VOUT = 1.2 V, IOUT = 20 A, fsw = 1 MHz.. If the Td(on) = Td(off) = 20 ns,

Calculate both the eGaN FET and MOSFET body diode power loss. Again, both operate at room temperature (25 C).

ANSWER:The power loss in the body diode is the product of the frequency, output current, and the voltage drop across the body diode (Vsd) times the amount of time the diode is conducting. Recall fsw = 1MHz, Td(on) + Td(off) = 40 ns.

For the MOSFET, Vsd = 0.7 V at 20 A and 25ºC from the supplied curve.

Then Pdiode(MOSFET) = 40 ns x 1 MHz x 20 A x 0.7 V = 0.56 W.

From the supplied curve for the eGaN FET, Vsd = 2 V at 20 A and 25ºC.

Pdiode(eGaN) = 40 ns x 1 MHz x 20 A x 2 V = 1.6 W.

Consider the buck converter circuit in the accompanying diagram, the switch-mode waveform, and eGaN and conventional MOSFET body diode operating curves provided here. In the buck converter, assume

VIN = 12 V, VOUT = 1.2 V, IOUT= 20 A, and fSW = 200 kHz. Td(on) = Td(off) = 20 ns.

Calculate both the MOSFET and eGaN FET body diode power loss. They are both operating at room temperature (25 C).

Q1:

ANSWER:The power loss in the body diode is the product of the frequency, output current, and the voltage drop across the body diode (Vsd) multiplied by the amount of time the diode is conducting. Given fsw = 200 kHz., Td(on)+Td(off) = 40 ns; Pdiode = (Td(on) + Td(off))

x fsw x Iout x Vsd

For the MOSFET, Vsd = 0.7 V at 20 A and 25ºC;

Pdiode(MOSFET) = 40 ns x 200 kHz x 20 A x 0.7 V = 0.112 W.

For the eGaN FET, Vsd =2 V at 20 A and 25ºC;

Pdiode(eGaN) = 40 ns x 200 kHz x 20 A x 2 V = 0.32 W.

Q2:


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500

400

300

200

100

000 .5 11 .5 22 .5 33 .5 44 .5 5

VSD - Source to Drain Voltage (V)

25oC125oC

eGaN FET Body Diode

Same buck converter.

VIN = 12 V, VOUT = 1.2 V, IOUT = 20 A, fsw = 1 MHz. If the Td(on) = Td(off) = 2.5 ns,

Calculate both the eGaN FET and MOSFET body diode power loss. We’re still at 25ºC.

ANSWER:The power loss in the body diode is the product of the frequency, output current, and the voltage drop across the body diode (Vsd) times the amount of time the diode is conducting. Thus Td(on)+Td(off) = 5 ns. For the MOSFET, Vsd = 0.7 V at 20 A and 25ºC.

Pdiode(MOSFET) = 5 ns x 1MHz x 20 A x 0.7 V = 0.07 W.

For the eGaN FET, Vsd = 2 V at 20 A and 25ºC.

Then Pdiode(eGaN) = 40 ns x 1MHz x 20 A x 2 V = 0.2 W.

Q3:

Q4:Same buck converter.

VIN = 12 V, VOUT = 1.2 V, IOUT = 20 A, fsw = 200 kHz.

Calculate the inductor and capacitor values needed to achieve a ripple current (dIL) of 5 A and an output ripple voltage (dVpp) of 50 mV. (Hint: Remember from the inductor law, VL = L di/dt. The duty cycle of the buck converter is given as D = VOUT/VIN.)

ANSWER:For questions four and five, refer to the diagram of the buck converter output waveform. Solving the inductor law relationship for L, L = VL / (di/dt).

For the capacitor, use dIL(RMS)/dt = Cmin x dVpp ; dIL/(8 x fsw) = Cmin

x dVpp . For 200 kHz, the ramp time for the inductor is: dt = 0.1/200 kHz = 0.5 цs; di = 5 A; VL = (12 V – 1.2 V); L= (10.8 V) /(5 A / 0.5 цs) = 1.08 uH;

Cmin = 5 A /(8 x 200 kHz x 50 mV) = 62.5 uF





Q5:Same buck converter.

VIN = 12 V, VOUT = 1.2 V, IOUT = 20 A, fsw = 1MHz.

Calculate the inductor and capacitor values needed to achieve a ripple current (dIL) of 5 A and an output ripple voltage (dVpp) of 50 mV. (As in question four, remember from the inductor law, VL = L di/dt. The duty cycle of the buck converter is given as D = VOUT/VIN.)

ANSWER:The calculation process proceeds similarly to that in question four, but here L= (10.8 V) /(5 A / 0.1 us) = 0.22 uH;

Cmin = 5 A /(8 x 1 MHz x 50 mV) = 12.5 uF.

2.5 kW 5 kW 10 kW

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Q6:In the accompanying diagram of the buck converter,

VIN = 12 V, VOUT = 1.2 V, IOUT = 25 A.

How much is the power loss reduced if the loop inductance is reduced from 1.5 nH to 0.5 nH? fsw =1 MHz, L = 0.33 μH (Hint: To calculate the peak current, it might be helpful to remember that V = Ldi/dt.)

ANSWER:Calculate the peak current from the relationship V = Ldi/dt; dt(on)=1.2 V/12 V x 1 MHz =0.1μs;

(12-1.2) V = 0.33 uH x (di/0.1 μs); di = 3.27 A; Peak current (Ipeak) = (di/2) + 25 A = 26.6 A; The energy saved in the loop inductance, E_LLoop = (1.5-0.5)/2

x Ipeak2 = 0.3538 J; The power saved in the loop inductance,

P_LLoop = 0.3538 x 1 MHz = 0.3538 W.

ANSWER:Calculate the peak current by V = Ldi/dt; dt(on) = 1.2 V/12 V x 5 MHz = 0.02 μs; (12-1.2) V = 0.1 μH x (di/0.02 μs); di = 2.16 A; Peak current (Ipeak) = (di/2) + 25 A = 26.08 A; Energy saved in the loop inductance, E_LLoop = (1.5 – 0.5)/2

x Ipeak2 = 0.34 J; Power saved in the loop inductance,

P_LLoop = 0.34 x 5 MHz = 1.7 W.

In the buck converter,

VIN = 12 V, VOUT = 1.2 V, IOUT = 25 A.

How much is the power loss reduced if the loop inductance is reduced from 1.5 nH to 0.5 nH? Fsw = 5 MHz, L = 0.1 μH (As in question six, to calculate the peak current, it might be helpful to remember that V = Ldi/dt.)

Q7:


Buck converter output waveform

Schematic for loop inductance




ANSWER:The duty cycle D = 1/12 = 8.33 %; TON = 21 ns; From the BSZ0901NS data sheet, VDS = 30 V, RDS(on)max = 2.1 mΩ, ID = 25 A. Qrr = 2 nC, VSD (30A)= 0.75 V, Freq = 4MHz. Cycle time = (1/4 MHz) T = 250 ns; TDeadtime = 2 x 25 ns = 50 ns; Then sync FET conduction time Tsync = (250 ns – 21 ns – 50ns) = 179 ns. Diode conduction losses during dead time = Vsd

x Isd x tdeadtime

x fSW; Sync FET conduction losses = (IOUT2 + dIL2/12) x RDS(on)

x Tsync/T. The diode conduction losses arer: 0.7 V x 25 A x 50 ns x 4 MHz = 3.5 W; Sync FET Conduction Losses = 0.94 W;

Total = 4.44 W

For a 12-V input, 1.0-V output, 4-MHz 25-A buck converter, the designer wants to get the best performance and minimum loss on the low-side (Sync) FET. The controller has a minimum dead time of 25 ns. dIL = 4 A. Calculate the Sync FET losses assuming a BSZ0901NS. (Hint: From the BSZ0901NS data sheet, VDS = 30 V, RDS(on)max = 2.1 mΩ, ID=25 A. Qrr = 2 nC, VSD (30A) = 0.75 V, Freq = 4 MHz)

Q8:

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ANSWER:

ANSWER:

From the EPC2023 data sheet, VDS = 30 V, RDS(on)max = 1.3 m Ω, ID = 25 A, Qrr = 0.0, VSD(30A) = 2.2 V. As in question eight, diode conduction losses are 2.2 V x 25 A x 50 ns x 4 MHz = 11 W; Sync FET conduction losses = 0.58 W;

Total = 11.58 W.

Thus the sync FET losses are substantially higher compared to a regular MOSFET with a fixed minimum dead time of 25 ns for the controller.

The control FET duty cycle D = 1/12 = 8.33%; TON = 21 nS. Sync FET conduction time = (250 ns – 21 ns – 50 ns) = Tsync = 179 ns. Diode conduction losses during dead time = Vsd x Isd x tdeadtime x fSW = 2 W,

Sync FET conduction losses = (IOUT2 + dIL2/12) x RDS(on) x Tsync/T = 0.58 W. 2 + 0.58 = 2.58 W.

The sync FET losses drop substantially when a Schottky diode is used in parallel if the dead time is long (2 x 25 ns).

For a 12-V input, 1.0-V output, 4-MHz 25-A buck converter, the designer wants to get the best performance and minimum loss on the low side (Sync) FET. The controller has a minimum dead time of 25 ns. dIL = 4 A. Calculate the Sync FET losses assuming a EPC2023.

(Hint: From the EPC2023 data sheet,

VDS = 30 V, RDS(on)max = 1.3 mΩ, ID =25 A. Qrr = 0.0 , VSD (30A)= 2.2 V, Freq = 4 MHz)

For a 12-V input, 1.0 V output, 4 MHz, 25-A buck converter, the designer wants to get the best performance and minimum loss on the low side (Sync) FET. The controller has a minimum dead time of 25 ns. dIL = 4 A. Calculate the Sync FET losses for a EPC2023 with a Schottky diode in parallel with a Vf = 0.4 V.

(Hint: From the EPC2023 data sheet, VDS = 30 V, RDS(on)max = 1.3 m Ω,

ID = 25 A, Qrr = 0.0, VSD(30A) = 2.2 V, cycle time = (1/4 MHz) T = 250 ns;

TDeadtime = 2 x 25 ns = 50 ns.)

RESOURCESGaN quiz on powerelectronictips.com, www.powerelectronictips.com/pop-quiz-ace-

egan-fet-quiz-win-textbook-egan-uber-guru/

GaN Transistors for Efficient Power Conversion, 2nd Edition, www.wiley.com/WileyCDA/WileyTitle/

productCd-1118844769.html

Efficient Power Conversion Corp., http://epc-co.com/epc

Q9:

Q10:


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How to evaluate

INVESTMENTS in power semiconductor

devices continue to grow. Performance and reliability also improve each year through the employment of new structures and new wide bandwidth materials such as SiC and GaN.

Particularly with respect to these new power devices, it is important to evaluate operating properties. Sometimes device makers don’t provide figures for all the parameters necessary for designing power circuits. It’s the responsibility of the device user to characterize power devices for use in designs that employ them. Similarly, product assurance engineers and incoming inspection technicians must fully characterize the power devices used in their products.

Energy consumption is a factor in the use of any power device. Thus, it is important to understand the sources of energy loss in power devices. These losses can be divided into three main components: conduction loss, switching loss and driving loss. The interaction between input stimuli and basic device properties (on-resistance, gate resistance and junction capacitances)

Ryo Takeda Alan WadsworthKeysight Technologies

cutting-edge power semiconductors

Power_Semiconductor_EE_Feb_Vs11.indd 24 3/6/15 2:57 PM



determines the level of loss. Thus, the evaluation of device characteristics generally involves examining these parameters. In addition, as power device performance improves, on-resistances drop and operational currents rise.

The typical way of evaluating semiconductor operational qualities has been with a curve tracer. As a brief review, a curve tracer basically applies a swept (automatically continuously varying with time) voltage to two terminals of the device under test and measures the amount of current the device passes at each voltage. This voltage-versus-current graph displays on an oscilloscope screen. The main terminal voltage can often be swept up to several thousand volts, with load currents of tens of amps available at lower voltages.

When testing three-terminal devices, such as transistors and FETs, the curve tracer connects to the control terminal of the device, such as the base or gate terminal. For transistors and other current based devices, the curve tracer steps the current to the base or other control terminal. By sweeping the input voltage or current through a range of main terminal voltages, the curve tracer generates a

group of V-I curves for each step of the control

signal. This group

of curves makes it very easy to determine, for example, the gain of a transistor.

One difficulty with curve tracers arises because semiconductor devices have internal sources of energy storage. These internal sources can degrade the accuracy of the V-I curves the curve tracer generates. In addition, there can be thermal effects within the semiconductor device that arise because of internal heating when the device handles current. It can be difficult to compensate for both the heating effects and internal storage when measuring device parameters. These effects can be particularly substantial at the VHF switching frequencies and high power levels at which wide bandgap semiconductors sometimes operate.

However, the use of VHF and higher switching frequencies brings many benefits in modern electronics. The circuits can be physically smaller than those operating at conventional power supply frequencies on the order of 1 MHz. For example, smaller power electronics make it possible for mobile devices to squeeze into more compact packages and help lighten up hybrid/electric vehicles for better fuel economy.

As frequencies rise, switching loss eventually exceeds conduction loss. Switching loss strongly depends on the power device’s junction capacitances (parasitic capacitive elements between gate, drain

Power device manufacturers are likely to use the B1505A Power Device Analyzer/Curve Tracer because it scales to high power levels and is the only instrument available able to characterize high power devices from the sub-picoamp level up to 10 kV and 1,500 A. These capabilities allow evaluation of novel new devices such as IGBTs and materials such as GaN and SiC.


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and source), which in-turn depend strongly on the drain-to-collector voltage.Conventional test equipment cannot measure capacitance at dc biases greater

than 100 V. For example, power loss from the charging and discharging of parasitic capacitance from the drain-to-source and from the source-to-the-gate comprise the primary components of switching loss in the case of a resonant converter employing FETs. The parasitic capacitances have voltage dependency in the nano-farad range because the power device’s depletion region modulates the applied varying operational voltages. These capacitances are conventionally measured by an LCR meter. But the maximum voltage of an integrated LCR voltage source is limited to around ±40 V. So, most device data sheets do not include capacitance measurement data with more than ±40 V bias.

Circuit designers traditionally use curve fitting to estimate capacitance in their design work for voltages above 40 V. However, curve fitting is impractical when the semiconductor device is built with trench or super-junction structures. In addition, complicated manufacturing processes induce additional variations in device performance, for example, a capacitance gap between the high side and low side of a FET. Designers have to identify performance differences when selecting a power device and when analyzing failures. Accordingly, it has become essential to characterize device capacitance from actual chip and module-level measurements.

The difficulty of getting accurate high-voltage measurements has prompted some device manufacturers to create their own in-house test systems. Unfortunately, the process of creating the connections needed to make correct input, output or reverse-transfer capacitance at high drain/collector voltage biases is complicated. This often causes connection mistakes and reduces productivity.

Gate charge (Qg) is another important parameter relevant to power loss at high switching frequencies. Gate charge is related to the parasitic junction capacitances. Its measurement can be complicated because, like junction capacitances, it is dynamic. It is also inversely proportional to drain-source resistance. Again, there are no commercially available bench-top solutions to measure Qg for high-power

Typical B1506A measurement screen displays. The B1506A measures and evaluates numerous parameters that include those for IV (breakdown voltage and on-resistance); three-terminal capacitances (Ciss, Coss, and Crss) with high-voltage bias; gate charge; switching time; and power losses.

BV & Leakage up to 3 kV

Rg & Ciss vs. Vgs

Saturation voltage characteristics

Gate charge (Qg)

IV up to 1.5 kA

Up to 3 kV Ciss, Coss, Crss


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aerospace and defense, rail or medical equipment. Power devices used in these applications must be reliable because their failure could cause the loss of human life.

Device breakdown voltages and leakage currents are obvious reliability concerns. In addition, designers must understand how devices behave over temperature for two reasons: Power devices innately operate at high temperatures because they self-heat, and power electronic products often operate in harsh environments of both extreme heat and cold. Thermostatic chambers are often used for characterizing operation over temperature, but they have drawbacks. Most problematic is that they usually require long cable extensions run from the measurement resources to the chamber. This arrangement introduces both residual resistance and inductances that can cause oscillations during the voltage stepping involved in making measurements. It also takes a lot of time and effort to monitor the test system for long time periods. In the real world, many people just abandon device characterization across temperature, though it is important for product reliability.

Although not strictly related to reliability, counterfeiting is another important concern for power devices. The accidental use of counterfeit semiconductors can reduce credibility and cause financial loss. The possibility of counterfeiting forces power-device users to evaluate inversely related parameters (such as on-resistance and capacitance) to ensure they meet all specifications.

Of course, most power circuit designers are not device physics experts and do not regularly evaluate power devices. The learning curve on power device test equipment typically entails some wheel spinning on the part of the circuit designer.

Circuit designers typically follow a three-step process when it comes to working with power devices: 1) Measure device characteristics to select an appropriate device, 2) Use simulators to design and verify circuit behavior, and 3) Build the circuit and evaluate it. There are iterations involved in the circuit design, assembly and evaluation process until the circuit meets its specifications. Unfortunately, it can take many cycles to complete the hardware verification loop if the simulation is off because the device data is inaccurate or the device models lack capacitance parameters that are critical at high switching frequencies.

Consequently, instrumentation suppliers have developed specialized equipment optimized for characterizing modern power semiconductors. An example is the Keysight Technologies B1506A.

The B1506A can evaluate all key power device parameters. It displays device tests in data-sheet format and lets users measure key device parameters without specialized training. Pass/Fail capabilities, as well as

Power device data sheets typically show behavior across only a limited range of operating conditions and the process of obtaining key datasheet parameters can be complicated. By providing an automated, easy-to-use way to extract power device parameters, the Keysight B1506A Power Device Analyzer for Circuit Design is equipped to overcome these difficulties.

devices. Some device manufacturers have devised in-house Qg test systems. However, these systems require ultra-high-power sources that can simultaneously supply high voltage and high current.

Besides being dangerous, Qg test systems constructed using ultra-high-power sources aren’t particularly accurate. The poor accuracy arises because it is difficult to supply large, constant currents to the gate terminal without reducing both the device turn-on time and the measurement accuracy. In addition, there can be measurement disruptions because of unpredictable influences from stray inductance in the test circuit or voltages. These voltages can get superimposed on the gate due to the gate resistance Rg. Furthermore, if the constant gate current is set too low, then the turn-on time rises. This results in more power applied to the DUT and creates the possibility of device damage.

Finally, it can be difficult to design a circuit that can supply the appropriate constant current for the measurement. For all of these reasons, Qg test systems designed in house have trouble making reproducible measurements. If deployed to multiple sites across, the world they can often have correlation issues.

Reliability is important for all types of power electronics applications, but especially so for applications in automotive,




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data-sheet-format report generation, are also available. Device evaluation work that previously took several days to a week can now take place in less than an hour. The B1506A has a wide current/voltage range (up to 1,500 A/3 kV) to handle a broad range of power devices. Similarly, the B1505A Power Device Analyzer/Curve Tracer is available when even higher voltages (up to 10 kV) are involved.

The B1506A is more accurate than conventional test equipment. It has traceability to international standards for both low current and voltage measurements and also for high current (>1,000 A) and high voltage (up to 3 kV) measurements. These capabilities are especially valuable for SiC or GaN devices, which have small leakage currents in the picoamp range. The B1506A can provide data that will be consistent, not only within a given company, but also across multiple companies in different locations. Moreover, the B1506A has a simple DUT connection scheme designed to help eliminate connection errors. It employs TO-type device connections that don’t require cables. IGBT modules connect easily with furnished cables and connectors. For other types of packages, a universal-type fixture is available.

Plug-in adapter (above) for testing a power semiconductor in a TO package. These types of devices require no cabling for tests in a B1506A tester. IGBT modules can be connected easily with furnished cables and connectors. A universal-type fixture (left) handles other types of packages.


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The B1506A also supports the characterization of device parameters across temperature. If devices don’t require characterization at cold temperatures, a thermal plate can sit in the test fixture.

An optional adapter that interfaces with an inTEST Thermostream system allows a wider temperature range (from -50° C to more than 200° C). It minimizes the test cable length which maximizes current carrying capacity and reduces the risk of oscillation.

Designers must measure device junction capacitances at thousands of volts of dc bias to fully characterize modern power electronics. In addition, it is essential to evaluate both IV and CV characteristics to to detect counterfeit or substandard devices. However, the connection scheme for making these measurements can be complicated because it involves complex connections utilizing dc-blocking capacitors and ac-blocking resistors. The B1506A handles this job using test fixture internal circuitry that configures itself correctly for each measurement. This eliminates the need for the external RC network. Typical parameters measured this way include input and output capacitance, gate-drain and gate-source capacitance, and reverse-transfer capacitance.

Gate charge (Qg ) is notorious for being hard to measure. Often measurement equipment situated at different locations will record different values for Qg from the same device. The B1506A can

DEVICE PARAMETERS SUPPORTED BY B1506A

CHARACTERISTICS CATEGORY PARAMETERS

STATIC Threshold voltage V(th),Vge(th)

Transfer Characteristics Id-Vgs, Ic-Vge

On resistance Rds-on, Vce(sat)

Gate leakage current Igss, Iges

Output leakage current Idss, Ices

Output Characteristics Id-Vds, Ic-Vce

Breakdown voltage BVds, BVces

GATE CHARGE Gate Charge Qg, Qg(th), Qgs, Qgd, Qsw, Qsync, Qoss

CAPACITANCE Gate Resistance Rg

Device Capacitance Ciss, Coss, Crss, Cgs, Cgd, Cies, Coes, Cres

POWER LOSS Conduction loss, driving loss/switching loss calculated at specified duty cycle

The first power device analyzer specifically made for circuit design is the Agilent B1506A. It automatically characterizes all power device parameters across a wide range of operating conditions and temperatures ranging from -50 to 250° C, at up to 1,500 A and 3 kV.


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A thermal plate can go in the test fixture to help maintain thermal qualities during high-temperature tests. For wide temperature ranges (from -50° C to more than 200° C) an optional adapter (above) can interface with an inTEST Thermostream system to minimize the test cable length. The Thermostream system (left) basically generates air at the right temperature and flow rate to make tests of interest.

perform gate charge measurements at up to 3 kV of drain-to-source bias with extremely high repeatability. It uses an innovative technique that measures gate charge in two passes. The first measurement takes place at high current with low voltage; the second at high voltage with low current. This technique reduces the power needed to measure Qg and boosts measurement safety.

Because the B1506A uses SMU (Source Monitor Unit) technology with internal feedback, it can easily control the gate current. There is no need to select an optimal gate resistor or to create a special driving circuit for each device. The B1506A can make accurate and reproducible Qg measurements merely by adjusting gate turn-on time.

The B1506A can measure all basic device parameters (IV, CV, Rg and Qg), so it can also easily calculate all contributors to power loss (such as conduction and switching loss).

Quantifying power loss is extremely important for selecting the optimal device in a power electronics circuit. Moreover, understanding junction capacitance as a function of dc bias, in addition to IV device behavior, greatly improves the accuracy of circuit simulation results. The net result is a drastically improved power electronics circuit design process.

Power device manufacturers often have broad measurement requirements (such as on-wafer test, ultra-high-voltage test up to 10 kV, GaN current collapse characterization and so on) beyond those of circuit designers. The B1506A isn’t the best choice for such demands. In this case, the B1505A is better because it has a modular architecture scalable up to 10 kV/1,500 A with leakage current measurement capabilities down to the sub-picoamp level. Thus, the choice between the B1505A and the B1506A depends on a user’s particular measurement application needs.

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Working with

WHEN a task calls for dissipating kilowatts of power over an extended period, wirewound resistors are often a logical choice. These

resistors are known for their ability to take high amounts of current. Wirewound resistors feature several parameters that are adjustable. This gives them advantages over film resistors.

Resistor manufacturers can tailor wirewounds to specific design requirements and to optimize their performance. Design engineers should understand what resistor attributes are adjustable and how adjustments can affect the performance of the device and the system.

Wirewound resistors are inherently inductive because they are manufactured in the exact same way as wirewound inductors. The amount of inductance can vary widely from size to size and from value to value, but typically ranges from around 10 nH to 10 μH. Designers frequently request wirewound resistors made with lower inductance or that are wound non-inductively. To get a wirewound with less inductance, resistor manufacturers typically use a smaller wire size, which has a higher resistivity. Smaller wire delivers the desired resistance value with fewer turns. One side effect of this practice is a reduced ability to handle a power pulse because the wire mass has a lower resistance.

An Ayrton Perry winding method can create a non-inductively wound resistance. Ayrton Perry winding uses two elements wound in opposite directions on a single part, with both elements being essentially double the requested resistance value. The winding technique helps cancel the magnetic fields and lowers the inductance to between 0.1 and 1 nH typically. However, though they are dubbed non-inductive, non-inductively wound resistors still have some inductance. In addition, the Ayrton Perry winding process requires

For high currents, axial-leaded wirewound resistors are often good candidates because they can handle up to 10 W and double as fuses.

Kory SchroederDirector of Product Marketing and Engineering Stackpole Electronics Inc.

Wirewounds

The SM Series (above) is an example of a wirewound product in a surface mountable package. Capable of replacing multiple ceramic flat chips, one SM resistor can handle high continuous power while emitting relatively little heat.

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EWT Series vitreous enamel-coated tubular wirewounds are frequently used for handling high-power, load-dump applications in vehicular applications. The robust wire element withstands thousands of high-power load dumps without failure or significant resistance shift.

two precision wind operations. So it has a dramatically higher manufacturing cost, around twice that of ordinary wire-wound parts.

The need to handle higher power, or pulse power, can force use of a wire with a larger diameter, which lowers the wire resistivity. This practice can dramatically boost the wire mass available to dissipate electrical energy. There are two potential side effects. First, resistor inductance rises because it takes more turns to get a given resistance value. Second, there may also be some concerns about handling high voltages. Sufficiently high voltages can cause arcing between windings. Thus, windings spaced closely together are more prone to arcing, especially when there is a void present in the coating or molding.

There are two ways of designing wirewounds to withstand high voltage. The easiest and least expensive is to reduce the wire size, raising the resistivity of the wire element. This allows winding fewer turns, creating wider spacing between windings. The wide spacing minimizes the risk of internal arcing whether or not there is a

void in the coating or molding around the part. This process also lowers the inductance of the part. As a result, the resistor would be able to handle less overall pulse power or pulse energy. These factors may or may not be relevant to a specific design. But it is important that designers understand the trade-offs associated with the ability to handle high voltages.

Simply reducing the wire diameter will not work for applications characterized by both high voltage and high pulse energy. These applications require element wire that is coated. The coating protects the resistor from arcing even if the windings touch each other. However, coated wire is much more difficult to weld and has higher manufacturing costs, so it is only used when absolutely necessary.

Of course, one potential drawback of any power circuit device is the possibility of overheating and potentially catching fire under the influence of a sufficiently high overload. Resistor manufacturers, such as Stackpole Electronics, can build wirewounds with a reliable and repeatable fusing characteristic.

One way of obtaining a fusing


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action is to use coating materials that concentrate the thermal energy so it doesn’t dissipate into the ambient air. Then with an overload, the part fuses quickly and typically with low thermal stress to the PCB and surrounding components. This method does not reduce the part’s ability to withstand short-term pulses. Another method of creating a fuse action is to reduce the wire size. The advantage to this method is that the resulting resistor is still straightforward to manufacture and less expensive than the first option. One limitation is that the component will be less able to handle short-term pulses.

Because of the continued demand for smaller and lighter electronics, it’s now possible to get single-layer varistors and molded wirewounds in surface-mount packages. Surface-mount varistors use what is essentially a 5- or 7-mm single layer of semiconductor material pressed into a disk. The current and energy handling range for this type of device is somewhat limited.

However molded wirewounds in surface-mount packages can now be found with power ratings of up to 5 W. Though there is a price premium for these devices, they are comparatively small, light and reliable. Single-layer varistors in surface-mount packages have become more widely used, particularly with the development of surface-mounted wirewounds rated higher than 3 W.

All in all, wirewound resistors used in series with varistors are far more reliable than any film resistor because they are more robust and require significantly more energy to fuse. Moreover, the resistance of metal-electrode, leadless-face resistors and surface-mounted, pulse-withstanding chips will both begin to shift with prolonged exposure to surges. Eventually the devices will fuse. When this happens, the protection scheme is effectively gone and the protected circuitry is exposed.

The KAL Series wirewound resistor technology uses a large ceramic core molded into an aluminum housing to optimize heat transfer. The KAL Series can be tailored to handle many different types of requirements such as low inductance, non-inductively wound, high surge, high voltage and fusibility. These resistors can also have different types of terminations such as tabs, flying leads and crimped connectors.



Cool calculations

POWER electronics can generate a lot of heat, so much so that many modern power supplies

incorporate fans or blowers to keep temperatures down. The task of cooling is often complicated by packaging constraints. The real estate available for power circuits can be pretty small. So the thermal analysis of these circuits generally calls for use of a computerized fluid dynamics package or other software.

Nevertheless, it can be convenient for circuit designers to know in which ballpark they are playing before cranking up a thermal analysis program. When working with cooling fans, a few calculations can show how much cooling to expect from a fan given the temperatures involved.

To begin, the use of a fan basically improves the heat transfer

It pays to do a back-of-the-envelope thermal analysis before cranking up a CFD program for power electronics.

Steve GriffithsManager of Technical Sales and MarketingAmetek, Dynamic Fluid Solutions Business Unit

Photo courtesy of iStock

coefficient of the components that are transferring heat to the surrounding air. To figure out the requirements for the fan, the designer must determine the amount of air necessary. A good approximation for how much air is required comes from the mass flow relationship:

Eq.1: q = w Cp ∆t

where q = the amount of heat the air absorbs, btu/hr; w = mass flow of air, lb/hr.; Cp = specific heat of air, btu/

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Cooling_EE_Feb_Vs6.indd 37 3/6/15 3:09 PM

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Fan Performance

CUBIC FEET PER MIN. CFM (L/SEC)

50(23.6)

100(47.2)

150(70.8)

200(94.40)

250(118.0)

SYSTEM RESISTANCE CURVE∆P = K(Q)N

OPERATINGPOINT

2.0(50.8)

1.5(33.1)

1.0(25.4)

.5(12.7)

PERFORMANCE CURVE

lb°F; and ∆t = temperature rise of the air in °F. This equation yields the following relationship which applies more directly to the forced air cooling of electronics:

Eq.2: Q = (178.4 x ti x W )/ (∆t x P)

where Q = airflow needed, ft³/min; ti = inlet temperature, °R (°F + 460°); ∆t = temperature rise across the equipment,

°F; W = power the equipment dissipates, kW; P = barometric pressure at the air inlet, in. of Hg. Here it is assumed that the air picks up all the heat to be dissipated. In other words, the relationship ignores conduction and radiation as well as natural convection effects on the external surfaces of the equipment.

For standard conditions of 70° F and 29.92 in. Hg, Equation 2 reduces to the familiar

Eq.3: Q = (3,160 x W)/∆t

A good rule of thumb for cooling is that a 15° F temperature rise is probably enough. Planning for a higher temperature rise tends to oversize the air-moving device.

The only way to move air around electronics is to create a pressure drop. The situation is analogous to causing the flow of electrical current by applying a voltage across a resistance. The resistance to flow is defined by an equation having the form:

Eq.4: ∆P = K ρ Qn

where ∆P = pressure drop, in. H2O; Q = airflow, ft3/min; K = a constant determined by the qualities of the system; n = a constant that depends on the type of flow; ρ = density of air, lb/ft3. The constant n will assume a value from 1 to 2 depending on whether the flow in the system is completely laminar (n = 1) or completely turbulent (n = 2). For most electronic equipment, n will be nearly 2. You can assume this value for calculation purposes unless there is data showing it to be otherwise.

The air flow path through electronics is typically complex. So pressure drop

calculations do not yield easily to the simple fluid flow equations. If there is a physical prototype available, it is generally best to test the equipment for its flow resistance qualities.

Suppliers of cooling fans now typically provide fan performance curves (actually a family of curves) covering the full range of performance with flow rate on the X axis and static pressure on the Y axis. This format provides a quick snapshot of how a given fan model and size behaves.

Static pressure is the resistance to airflow (friction) caused by the air moving through the air handling system—pipes, ducts, hoses, filters, hood slots, air control dampers or louvers. In the case of electronics, there are usually few air handling components to worry about. But the point is that static pressure should include the pressure drop through all of the ductwork on the inlet and outlet of the fan, plus the pressure drop through any filters, control dampers and so forth, or through any other system components that restrict airflow.

Thus, to determine what kind of fan can cool the electronics, the designer must calculate a system air resistance curve (equation four) for the application at hand and plot this equation on the family of fan performance curves. Then the air flow required is that at the intersection of the performance curve and the resistance curve. At this operation point, the pressure available from the fan to force air through the assembly is equal to the pressure necessary for that flow.

However, designers are concerned with the amount of power needed to accomplish cooling. The factors influencing air-mover operating efficiency

A typical fan performance curve and a system resistance curve. The intersection of the two curves denotes the best operating point for the fan whose behavior is depicted by the curve.




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Fan Performance

CUBIC FEET PER MIN. CFM (L/SEC)

50(23.6)

100(47.2)

150(70.8)

200(94.40)

250(118.0)

SYSTEM RESISTANCE CURVE∆P = K(Q)N

OPERATINGPOINT

2.0(50.8)

1.5(33.1)

1.0(25.4)

.5(12.7)

PERFORMANCE CURVE

the motor consumes, or A/I. No can also be expressed as the product of blower efficiency, motor efficiency and the efficiency of the cooling device, usually denoted Ne:

Eq.8: No = Nf x Nm x Ne

To maximize efficiency, and minimize noise, the air mover generally is operated at 50% or more of its peak efficiency. The best efficiency point, BEP, is usually in the middle two-thirds of the flow rate range.

There are a few other points that are important when fans or blowers cool electronics. The vast majority of cooling equipment used for electronics are axial fans. They use fan blades to draw air in and discharge the air in the same axial direction. The other main type of cooling equipment is a centrifugal blower. A centrifugal blower has a scroll rather than a fan blade or propeller. It draws air into its inlet, through the scroll wheel, and discharges it at 90° out through a housing outlet.

Finally, it should be pointed out that these calculations assume the use of standard air, which is 70° F, 29.92 in. barometric pressure (at sea level) and 0.075 lb/ft3. Of course, air density changes with temperature and/or barometric pressure variations (at higher altitudes). Usually, applications in cooling electronics can ignore these effects, but manufacturers typically supply temperature and/or altitude conversion factors that can be used in making the corrections to standard conditions if the planned environment warrants it.

are the flow and pressure range, the operating speed range, the motor powering the air mover, and any electrical device that modifies the power supplied to the motor. These variables influence the efficiency at specific operating points as spelled out by

Eq.5: A = (Q x ∆P)/6,350

Eq.6a: S = (T x N)/5,252

Eq.6b: S = A/Nf

Eq.7: I = S/Nm

where A = air horsepower, hp; Q = air flow, ft3/min; ∆P = pressure drop, in. H2O; S = motor shaft horsepower, hp; T = motor torque, lb-ft; N = motor speed, rpm; Nf = fan or blower efficiency; I = motor input horsepower, hp; Nm = motor efficiency.

The overall efficiency of the cooling system No is just air horsepower divided by the power


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Choking off EMI/RFI

ELECTROMAGNETIC interference (EMI) and radio frequency interference (RFI) are natural byproducts of switching power supplies. Much of this noise arises as a direct result of the large voltage swings caused by short-duration charging and discharging in the power supply circuitry. Common mode noise, which is generated through the power line and returns to

the source through some type of ground path, is a key problem as well.

Power supply designers can use several filtering techniques to control such EMI/RFI. One of the more effective techniques is to place a common mode choke (CMC) after the full-wave diode rectifier. This helps meet numerous rules and regulations that define electromagnetically compatible (EMC) power supplies. In the U.S., the

Jim EarleyPresidentPremier Magnetics

in off-line switchers

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A typical toroidal CMC from Premier Magnetics.

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Federal Communications Commission (FCC) is responsible for regulating EMI emissions. Part 15 of FCC rules specifies that any spurious signal above 10 kHz be subject to regulation. It also specifies that radiated emissions must be controlled between 30 MHz and 1 GHz, while conducted emissions must be controlled in the frequency band between 450 kHz and 30 MHz.

The regulatory agency overseeing EMC in Europe is the International Special Committee on Radio Interference, or CISPR. The committee’s third edition of CISPR, Pub 22, (known as CISPR 22 or EN55022) defines those regulations and has been adopted by much of the European community. Other countries have developed their own regulations as well. The Federal Republic of Germany, for example, requires compliance with its own VDE 0871 or VDE 0875 regulations. Canada sets limits similar to the FCC Part 15 in its C108.8-M198 rules. Generally speaking, all current regulations cover frequencies in the 10 kHz to 30 MHz range.

Both the FCC and CISPR standards divide electronic equipment into two classes. Class A equipment encompasses devices designated for commercial, business and industrial environments. Class B devices include personal computers, calculators and similar devices typically used in residential settings. In general, the standards define more severe constraints for Class B devices because they are more likely to sit close to radio and TV receivers in the home. Typically, Class B limitations are about three times more restrictive than Class A limits (~10 dB). But there are a few differences. CISPR 22 requires certification over a 150 kHz to 30 MHz frequency range for conducted emissions rather than the 450 kHz to 30 MHz range specified by the FCC. In addition, the FCC specifies limits for frequencies above 1 GHz; the CISPR does not.

Beginning in the 1990s, regulatory agencies began harmonizing the different RF emissions standards for digital devices. The revised rules let electronics developers certify equipment

by simply demonstrating that their device complies with either the FCC Part 15 or CISPR 22 regulations. One caveat: The same standard must be used for both conducted and radiated emissions. Today, conducted and radiated emission limits specified in FCC Part 15 and in CISPR 22 are within a few dB of each other over prescribed frequencies. So designers can use either regulation without compromising the accuracy of the measurement or certification process. The FCC retained its standard for emissions above 1 GHz, but adopted the test distances specified in CISPR 22. It is also important to note that FCC limits are given in microvolts while CISPR limits are measured in dB-microvolts, so the units must be converted for a direct comparison.

COMMON-MODE CHOKESMany electronic circuits need CMCs to filter out noise that is common to, or coupled to, the power and network lines. A CMC features two identical windings with the current in each winding flowing in opposing directions. The live and return currents are of the same magnitude because they are from the same power source. But the direction of the magnetic flux lines created by the current flowing into the first winding opposes the flux lines created by the return current in the second winding. These two magnetic pluses cancel each other out to create a theoretical net flux of zero.

As a result, the choke presents little inductance or impedance to the differential-mode currents. This means the CMC core will not saturate because of the amplitude of the main currents. High-frequency noise currents, however, are of much lower amplitude. They will see a high impedance because of the common inductance of the windings and will be severely attenuated or filtered out.

It is possible to use discrete inductors and capacitors in front-end filtering instead of CMCs. Discrete components do offer a small cost advantage. In many cases, however, that cost advantage may be more than offset by a need for flux bands or shields on the main transformer. A flux


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band, basically a circumferential copper shield around the entire transformer, can complicate the design. It is often left floating, but may be connected to the secondary ground. If connected this way, the transformer may need reinforced insulation between its primary and secondary for safety reasons. If the transformer uses an air gap on its outer limbs, a fringing flux from the gap causes severe eddy current losses in the flux band. Additionally, the ends of this band should be soldered together.

All in all, avoiding use of a CMC may incur not only a more complex transformer, but also a need for additional inductors and/or capacitors to filter the power lines. This design strategy drives up system cost and complexity. For example, adding a flux band or shield to a typical 30- to 50-W SMPS transformer will boost component cost by 20 to 30%. Moreover, this approach may make FCC compliance more difficult. It often takes relatively powerful power-line filters to meet conducted noise specifications. Yet safety regulations limit the size of the capacitors fitted between the supply line and ground plane. This limitation can often result in filtering that doesn’t cure the common-mode interference problem it tries to address.

Designers who opt to add capacitors for filtering can face additional issues. Electrolytic capacitors added after the rectifier diodes are more likely to fail from overvoltage, heat or other factors. Film capacitors are also undesirable because of UL/CE requirements and their loss of capacitance over time.

To reduce common mode noise,

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Shown here is a ferrite core CMC from Premier Magnetics. CMCs using E- or U-shaped cores come in similar packages.


The Seventh Annual IEEE Energy Conversion Congress and Exposition (ECCE 2015) will be held in Montreal, Canada, on September 20‐24, 2015. ECCE is the pivotal international conference and exposition event on electrical and electromechanical energy conversion field. To be held for the first time outside USA, ECCE 2015, in Montreal, Canada, will feature both industry‐driven and application‐oriented technical sessions, as well as industry expositions and seminars. ECCE 2015 will bring together practicing engineers, researchers and other professionals for interactive and multidisciplinary discussions on the latest advances in various areas related to energy conversion. ECCE has grown to become the foremost technical conference and exposition around electrical and electromechanical energy conversion. It focuses on solutions that are industrially oriented. People from a broad spectrum of the energy conversion industry and academia gather to interact in a convivial and innovative atmosphere, a perfect blend of state of the art, technical prowess and commercial opportunities. LATEST NEWS: ECCE has received record number of digests for 2015!

The ECCE organizing committee invites organizers interested in organizing Special Sessions and attending as an Industry Exhibitor! The Special Sessions are aimed at industry and the showcasing of new industrial work. Such sessions consist of oral presentations only, without written papers and are strongly oriented towards the interests of industry, as well as towards the interaction of it with academia. Presentations might be of a somewhat more overview and commercial nature than those related to the papers in the standard technical sessions.

Presentations are solicited on any subject pertaining to the scope of the conference described in its Call for Papers (obtainable from http://2015.ecceconferences.org). Those that will address the following aspects of growing interest and innovation are encouraged:

Standard development for power electronics systems / products; Power Supply on Chip (PwrSoC) and related technology; High Efficiency, flicker free LED light fixtures; DC Microgrid: trend, requirement, and technologies; Innovative materials for improved components and/or systems in electrical and electromechanical energy conversion; Components and systems for electrical applications in the oil & gas and mining sectors; Technologies and systems for large, cycle‐efficient and cycle‐intensive energy storage; Modelling of materials oriented to improve the estimation of the energy efficiency in components and systems; Reliability, diagnostics and prognostics of components and modular systems.

For exhibitors: ECCE has a large exhibition hall that is traditionally the centre for social and other activities and represents an outstanding location for trade exhibiting. Please contact Steve Sprague about this – see contact below.

Proposal Submission Guidelines Special Session organizers are requested to submit a maximum five page proposal summarizing the proposed Special Session with 4 or 8 presentations. The proposal should contain the session title, session organizer, title of each presentation, presenter for each presentation (with a short biography) and a summary of each presentation. Please submit the proposal directly to ECCE 2015 Technical Program Committee Chairs via email at [email protected].

For more conference information, please visit http://2015.ecceconferences.org.

For exhibiting at ECCE 2015, please contact conference Exhibition Chair at [email protected].

For more about Montreal and its surrounding areas, please visit http://www.tourisme‐montreal.org/.

For submission and information regarding the ECCE 2015 Special Sessions, please contact the ECCE Technical Program Committee Chairs ([email protected]).

ECCE 2015 Technical Program Chairs: Dan Ionel, Regal Beloit Corp., USA; Xinbo Ruan, Nanjing University of Aero. & Astro., China; Nasir Uddin, Lakehead University, Canada; andBin Wu, Ryerson University, Canada;

Call for Special Session Organizers and Industry Exhibitors

Important Dates March 31st, 2015: Special Session proposal submissions deadline (maximum five pages)

May 1st, 2015: Notification of session acceptance

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CMCs whose ferrite cores are toroid-shaped are the most effective. Their continuous, unbroken magnetic flux path maximizes magnetic coupling between windings and minimizes leakage inductance. However, the core material, the high number of windings and the use of a plastic mounting base make toroids more expensive than other CMCs. Furthermore, isolation between windings is typically limited to withstand less than 1.5 kV, which may limit their use in some applications.

A combination of efficient filtering and low cost can come from CMCs with ferrite cores shaped like an E or a U. These devices offer isolation between the two windings up to 3 kV, which helps meet UL and other safety agency requirements while ensuring FCC compliance. In general, E cores can handle higher power levels and can provide a higher level of inductance. The U core is slightly less expensive than an E core because of its simpler geometry but has less inductance capability.

Designers can also use this style of CMC as an output filter. Long cable runs

that are connected to a safety ground act like antennas when common mode currents flow through them. A CMC filter can limit radiated EMI on long output cables where the output return is connected to safety earth ground.

When selecting CMCs, there are a few points to keep in mind. Most CMC line filters use standard ferrite material, but the grade of ferrite in the choke may depend on the inductance required. Higher inductance requires a more permeable core. However, designers needn’t specify the number of wire turns on the chokes; the number of turns is standard for a given part number. The user need only select a choke that meets inductance, current and rated voltage requirements. Circuit designers must only concern themselves with the choke’s rated current, VA rating, inductance and package options.

Finally, the physical size of the CMC package is generally determined by the amount of electrical current it carries, (which primarily determines the wire size), plus the inductance needed.

A typical low-profile, common mode filter for low-to-medium power applications.

Typical CMC application circuitAC input

Line

Neutral

GND

Cx = 0.1uF Differential Mode Noise Currents

Common Mode Noise CurrentsCy = 2,200 pF

Load

Line

Neutral

GND

CX CX

LCM1

LCM2

CY

CY

RESOURCESPremier Magnetics www.premiermag.com


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The battery of the future is here today. Tadiran bobbin-type lithium thionyl chloride (LiSOCL2) batteries feature an annual self-discharge rate of just 0.7% per year: so energy efficient that they allow low power consuming wireless devices to operate for up to 40 years on a single battery. No one else even comes close. Tadiran lithium batteries also feature the highest capacity, highest energy density, and widest temperature range of any lithium cell, plus a glass-to-metal hermetic seal for added ruggedness and reliability in extreme environments.

For a battery that lasts as long as your device, run with Tadiran.

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* Tadiran LiSOCL2 batteries feature the lowest annual self-discharge rate of any competitive battery, less than 1% per year, enabling these batteries to operate over 40 years depending on device operating usage. However, this is not an expressed or implied warranty, as each application differs in terms of annual energy consumption and/or operating environment.

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Small batteries

GENERALLY speaking, every penny counts when it comes to designing devices for a mass

consumer market. That’s one reason smartphones use relatively inexpensive, rechargeable lithium polymer batteries for power. Constructed from a thin, flexible material, lithium polymer batteries can reduce product profile while satisfying the power requirements of a device that typically gets replaced every few years.

There’s another kind of lithium battery that can sometimes be an excellent power source for consumer devices: a consumer-grade rechargeable lithium-ion (Li-ion) battery, which is relatively inexpensive and readily available. But these cells have a limited service life, about five years and 500 recharge cycles.

Short-lived rechargeable Li-ion batteries don’t work well for industrial applications, which are often characterized by the need for long battery life. Consequently, battery makers have developed more robust Li-ion cells. One in this category is the TLI Series Li-ion rechargeable battery. The AA-sized cell can last for up to 20 years and 5,000 full recharge cycles while delivering 15-A pulses. These small but powerful batteries can also work and recharge at extreme temperatures (-40 to 85° C), have a low self-discharge rate of 5% per year, and are constructed with a glass-to-metal hermetic seal to resist leakage and corrosion.

Remote wireless devices are predominantly powered by bobbin-type lithium thionyl chloride (LiSOCL2) batteries. These cells are characterized by the highest capacity and energy density of any lithium chemistry. Certain brands of bobbin-type LiSOCL2 batteries can operate for up to 40 years and have an extremely wide temperature range. These qualities make them candidates for long-term deployment in applications drawing a low rate of current where the device spends a majority of its time in a “dormant” state. The low-rate design of bobbin-type LiSOCL2 batteries makes them poorly suited for applications that draw high continuous current. But they can be optimized to handle these applications

Battery makers have devised chemistries that put a lot of energy in a tiny volume.

Sol JacobsTadiran Batteries

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Surgical device maker BioAccess made TLM Series lithium batteries an option for its surgical drills, which previously used Alkaline battery packs. Six AA-size TLM-1550HP batteries did the job of 15 AA-size Alkaline cells and weighed a third less while taking up only 40% of the original volume.

by including a patented hybrid layer capacitor (HLC). The HLC helps deliver high-current pulse loads that often arise during the initiation of data acquisition and transmission.

The choice of a power supply can become more complicated for applications that require both high pulses and high continuous current. Typical applications in this category include single-use devices for military and aerospace, surgical power tools and safety devices, such as automatic external defibrillators (AEDs), that can remain dormant for years but must operate reliably in a life-and-death emergency.

Battery makers have developed technologies to meet these challenging power requirements. One development in this area is the TLM Series lithium metal-oxide battery. This battery is constructed with a carbon-based anode, a multi-metal-oxide cathode and an organic electrolyte.

pack big wallop for wireless devices

Tadiran_EE_Feb_Vs5.indd 48 3/6/15 3:11 PM

200 Broad Hollow Rd., Farmingdale NY • 631-249-0001 • fax 631-249-0002 • www.batteryholders.com

What are the chances of shock or vibration? Who is the target user and how much dexterity can they be assumed to have? Whether you design defibrillators or glucose meters or some other product, we offer progress reports, prototypes, and design testing to fulfill our pledge of excellence to you.

IS YOUR DEVICE STATIONARY OR PORTABLE?

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batteries an optional power supply upgrade for its surgical drills. The drills previously used Alkaline battery packs exclusively. The Alkaline batteries performed well and were reliable, but heavy. Substituting six AA-size TLM-1550HP batteries reduced battery weight by more than a third (36%) and took up only 40% of the original volume. An equivalent Alkaline battery pack would need 15 AA-size Alkaline cells to do the same job as six AA-size TLM cells.

Use of a TLM-1550-HP battery pack also permitted the surgical drill to deliver faster drilling speeds, more active drill time (30 to 40 sec at a time for up to 20 to 30 cycles), more instantaneous power and more stall torque. The overall result was more efficient drilling cycles with less operator fatigue.

TLM Series batteries can operate for up to 20 years with an annual self-discharge rate of less than 1% per year. These small but powerful cells feature a nominal voltage of 4 V and up to 2 Wh of energy, with a discharge capacity of 135 to 500 mAh. They are capable of handling 5-A continuous loads and up to 15-A pulses. These batteries also offer a wide operating temperature range (-40 to 85° C), and comply with MIL-STD 810G specs for vibration, shock, temperature shock, salt fog, altitude, acceleration (50,000 g) and spinning (30,000 rpm). TLM batteries also conform to UN 1642 and IEC 60086 standards for crush, impact, nail penetration, heat, over-charge and short circuit, and can ship as non-hazardous goods.

Examples help illustrate where TLM Series batteries can provide benefits. In one case, an air-to-ground missile originally used a battery pack consisting of 19 silver-zinc cells. Substitution of high-power lithium metal-oxide cells reduced battery pack size by 30% and weight by 75%, 2.2 Kg versus 0.5 Kg. The new battery pack also had 3.5 times greater energy density. Use of a TLM Series battery pack saved space by eliminating the need for a squib, gas generator and heater that a silver-zinc battery pack required.

In another case, BAE Systems deployed TLM Series batteries as part of a 60-mm mortar guidance system for a Darpa initiative called ODAM (Optically Directed Attack Munitions). TLM-1530-HP lithium batteries powered the system’s laser-guided optical seekers. TLM Series cells can operate at much colder temperature (-40 ° C) and have a much longer shelf life (20 years versus 5 years) than CR-2 consumer batteries that were also candidates for this application.

Another common application for TLM Series batteries is in powering surgical screwdrivers and power tools. These applications require a small, ergonomic power supply with the capacity to deliver a high rate of current. Many such handheld medical tools formerly used Alkaline batteries, which are extremely bulky and self-discharge at a high rate.

BioAccess, a surgical device manufacturer, made TLM Series lithium

An air-to-ground missile originally used a battery pack consisting of the19 silver-zinc cells visible here. Substitution of high-power, lithium metal-oxide cells reduced battery pack size by 30% and weight by 75%. The new battery pack also had 3.5 times greater energy density.

RESOURCESTadiran Batteries, www.tadiran.com

One application for LiSOCL2 batteries is in FastTrak electronic toll tags, which must sit in the high summer temperatures that arise under automotive windshields.

Tadiran_EE_Feb_Vs5.indd 50 3/6/15 3:11 PM

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Selecting power supply

EXAMINE the input and output

connectors on most open-frame power supplies and you’ll likely find parts from a supplier like Molex or JST. These connectors are inexpensive, readily available, reliable and easy to use. In addition, use of these readily available parts makes it easier for designers to second-source a power supply, if necessary, when some standardization exists.

Many power supply manufacturers will specify the name of the mating connector series in their product documentation. But they will often leave it up to the user to determine the actual part numbers. This usually provokes a call to power supply tech support for a recommendation.

An example illustrates why power supply makers adopt this practice. Consider the industry standard low-power 2 X 4-in. single output power supply. The Molex KK 09-50-3041 housing is widely specified as an output mating connector. Made of nylon, it has a friction lock and four circuits: two for the plus output and two for the minus output.

When looking for the mating pin, one has to be a little more careful. The suggested pin for the connector

is available in two materials: brass and phosphor bronze.

Brass is a common material for contacts and pins. It is inexpensive, has good conductivity and is generally dependable in a benign, low-temperature environment like an office. Brass loses flexibility as it ages and, under repeated stress, is subject to crystallization, which significantly lowers it conductivity. However, most power supplies don’t undergo repeated connect/disconnect cycles, so loss of conductivity usually isn’t a problem.

Phosphor-bronze mating pins should be considered for more challenging environments. Phosphor bronze is an alloy of copper with 3.5 to 10% of tin and a phosphorus content of up to 1%. The phosphorus serves as a deoxidizing agent during melting. These alloys are tough, strong and have a low coefficient of friction. They are particularly helpful for applications at higher temperatures because heat can make brass contacts lose their spring properties. If there is some vibration, the lack of spring in a bronze pin can cause reliability problems. Phosphor-bronze contacts do not lose their spring. However, brass is more

Designers can avoid reliability problems by acquainting themselves with the way connector pin materials stand up to repeated use and corrosion.

David NortonTDK-Lambda Americas

connector pins

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conductive than phosphor bronze, so check current rating capability.

Phosphor bronze is more expensive than brass, currently about $0.13 compared to $0.05 for brass (1,000-piece pricing from a distributor). For a 2 X 4-in. power supply, the additional few cents could add $0.56 to the bill-of-material cost. Designers should consider the environment and desired field life. As a note, on higher power 2 X 4-in., open-frame power supplies, there are alternatives to the single-point-of-contact KK style pins like those used with the Molex 09-50-1041 housing (SPOX series). These have multiple points of contact for lower resistance.

As Molex advised

Different terminals have different performance and different characteristics.

There are several other metals that can be found in power connector pins and plugs. Here are a few details about how other metals compare:

• Beryllium copper is often used for its excellent conductivity and thermal properties. It has the best electrical conductivity of any spring alloy of comparable hardness. It beats other copper-based spring alloys in terms of resistance to fatigue and strength. So it is generally a candidate for applications experiencing numerous insertion and withdrawal cycles. But it costs more than any other basic contact material.

• Nickel-silver alloys are actually copper alloys with nickel and often zinc. The typical makeup is 60% copper, 20% nickel and 20% zinc. These alloys resist oxidation but may be susceptible to stress corrosion, though not to the extent of brass.

• Several materials can be used for plating connector pins. Gold is an excellent conductor and has a low contact resistance. Hard gold platings are applied in applications characterized by numerous insertion/withdrawal cycles. Gold can be impregnated with graphite to handle super-high insertion/withdrawal needs.

• Of course, gold is expensive, so alternative plating materials have been developed. Silver is a general-purpose plating for power contacts. But it tarnishes when exposed to air. The resulting oxide layer can be problematic for low-level circuits, less so for power contacts. Like gold, silver has become expensive, so suppliers have developed alternatives.

• Nickel is slow to react with air, so it resists corrosion and is relatively conductive. It often serves as an undercoat for contacts destined for high-temperature-use to prevent migration of plating materials. It also has good wear resistance.

• Tin has good conductivity and is relatively inexpensive. But it has poor wipe resistance and best suits connectors that experience few mating cycles.

• Finally, rhodium is sometimes found in connectors that need exceptional wear qualities. It is not as conductive as gold or silver, but its conductivity is generally acceptable when the material is deployed as a thin plating.

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THERMAL MANAGEMENT

TO AN engineer, power management is also thermal management.

Excess heat generated by power systems places additional burdens on their cooling technology. Even the most efficient system will have performance and reliability issues if served by an inefficient cooling method. Power and heat are two sides of the same coin, and both must be addressed in order to achieve higher power densities.

High temperatures can damage electronic systems or, at best, the protective circuitry will shut them down. Therefore, it is essential that engineers pay careful attention to the actual thermal performance of the system and be aware of every aspect of the thermal cooling path. Chassis design and tolerances, as well as all conductive elements, are critical to system performance.

HEAT VS POWER DENSITYCreating more efficient electronics to lower waste heat, and heat-extraction systems to manage it, are at the core of increased power density. In the case of rugged high-performance power systems, the physical constraints of the form factor and packaging required place an even greater burden on thermal management. As a result, power systems have serious power-density issues that severely challenge system designers.

Specified by many major defense contractors, Behlman’s VPXtra power supplies are used in their latest advanced VPX-based systems, due to Behlman’s ability to deliver superior power, efficiency and thermal management. Behlman’s careful attention to detail in electrical and mechanical design, along with judicious use of materials to

Overcoming power density and the related thermal issues in rugged, hi-rel power supplies.

Jerry HovdestadDirector COTS Engineering Behlman Electronics

and cost of ownership in power systems

Figure 1: Behlman VPXtra1000CD; VPXtra1000CM and VPXtra1500CS 6U power supplies.

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VPXtra1000CM dc-to-dc power supplies,

VPXtra1500CS can also be used to power devices from other manufacturers having similar wide-range dc inputs.

Behlman VPXtra 6U power supplies are also designed to meet the applicable sections of MIL-STD-461 for conducted emissions, conducted susceptibility and radiated susceptibility. In accordance with MIL-HDBK-217F Notice 2, MTBFs are 336,000 hours for VPXtra1000CD; 277,000 hours for VPXtra1000CM; and 209,000 hours for VPXtra1500CS (see Figure 1).

FITTING SMALLER FORM FACTORSBehlman’s latest VPXtra power solution addresses the growing need for a smaller form factor. The Behlman VPXtra500M is a 3U COTS dc-to-dc power supply. It is a rugged, highly reliable, conduction-cooled, switch mode unit that is VITA 62 and Open VPX compliant, delivering up to 560 W of dc power through six outputs (see Figure 2). The 12, 3.3 and 5 V main outputs can also be paralleled for higher power.

The VPXtra500M accepts 18 to 36 Vdc input, is compliant with MIL-STD-704, and can supply

address the issues of heat transfer versus weight, are key. For example, copper can transfer roughly twice the heat of aluminum, but weighs almost twice as much. Behlman uses the most advanced techniques available to maximize thermal performance, including state-of-the art heat pipes that help optimize control.

Addressing the issues of circuit efficiency and thermal management, Behlman’s Xtra-Cooling technology breakthrough uses a unique card-edge conduction-cooling methodology. This enables 6U power systems, such as Behlman’s dual-output VPXtra1000CD, to deliver an unprecedented 1,000 W of clean, regulated 12 Vdc power and 3.3 Vdc auxiliary power, with a typical efficiency of 90% from a wide-range 28-Vdc input.

Behlman’s VPXtra1000CD is a rugged, highly reliable switch mode unit, well-suited for high-end industrial applications, as well as for mission-critical military airborne, shipboard, vehicle and mobile systems. The 1000CD is OpenVPX Vita 62 compliant, and standard features include overvoltage, short circuit and thermal protection, with no minimum load requirement.

The VPXtra1000CM COTS dc-to-dc power supply is a rugged, highly reliable, conduction-cooled, switch mode unit. It is a VITA 62, Open VPX compliant, 6U power supply that delivers 700 W of dc power through five outputs, and its 12-V output can be paralleled for higher power and redundancy.

The Behlman VPXtra1500CS gave VPX design engineers the ability to obtain a dc output from a three-phase ac source to support design initiatives requiring power factor correction and input current harmonic reduction. Although initially conceived to power Behlman’s VPXtra1000CD and

Figure 2: Behlman Electronics’ VPXtra500M 3U COTS dc-to-dc power supply is a rugged, highly reliable, conduction-cooled, 560-W switch mode unit that is VITA 62 and Open VPX compliant.


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high-power dc output, requiring no minimum load and providing overvoltage, short circuit, overcurrent and thermal protection. Behlman’s Xtra-Cooling technology, with its unique card-edge conduction-cooling, enables these power supplies to operate at full power up to a maximum rail temperature of 85° C.

All VPXtra power supplies incorporate Behlman’s Xtra-Reliable design and Xtra-Rugged construction. They are designed and manufactured to meet VITA 47 for shock, vibration, and humidity, and MIL-STD-461 for EMI.

COST OF OWNERSHIP The U.S. Navy has long led the way in quantifying what’s important to a reliable power supply design, driving the use of COTS, in order to take advantage of established reliability and cost savings. This experience led the Navy to revise its guidelines and publish NAVSO P-3641A, which replaces NAVMAT P-4855-1A. This paper outlines the total cost of ownership and highlights the benefits of modified COTS power supplies, such as those built by Behlman.

NAVSO-P-3641A goes on to say:

Developing Power Systems Utilizing High Density DC/DC Modules—Reliable power supply designs with high output power density are generally achievable using standard switching power supply components and topologies widely available in industry today. To minimize cost and the time to develop a power system, existing high-density dc/dc modules are often incorporated into the design. High-density modules are available that provide electromagnetic compatibility, thermal management, prime power rectification, input voltage multiplication, power factor correction and non-isolated and isolated user output voltage conversion.

The Navy recognized that the design approach to using modules is not simple, and a number of other components will be required to meet key criteria, such as EMI filtering, input ac-to-dc rectification, hold-up capacitance, output filtering, auxiliary circuitry (sequencing, BIT, On/Off), cooling and packaging. Behlman’s years of designing advanced hi-rel power systems, including extensive module selection experience, EMI input and output filtering, hold-up, transient protection, efficient cooling practices for both conduction and convection, parallel and N+1 Designs and control, and BIT circuits, has led to the creation of more than 75 COTS power products that are ruggedly built and designed to meet the rigors of high-end industrial and military applications on aircraft, shipboard, ground or mobile platforms.

ABOUT BEHLMAN ELECTRONICSBehlman Electronics is a subsidiary of Orbit International and a key member of the Orbit Power Group. Behlman designs and manufactures a wide array of power products including ac power supplies, frequency converters, inverters, uninterruptible power supplies (UPS), COTS (dc-dc, ac-dc and dc-ac), power sources and VPX/VME power supplies, as well as modified standard power supplies and railroad signal sources for commercial, industrial and mission-critical applications. All product information and data sheets are immediately available at www.behlman.com.


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“Well-Known Member” and retired university professor MikeMI joined Electro-Tech-Online.com in 2009. He worked in the industry for 30 years at companies including General Instruments, Fairchild Semiconductor, Signal Processing Circuits, Analog Devices, Performance Semiconductor and Hewlett-Packard. Some of the projects he contributed to included work in bioengineering, prosthetics, IC chip design, sensors, robotics and avionics.

Now in his 70s, Mike likes to spend his time with ham radio, avionics and flying his own plane in and around Big Springs Ranch, Texas.

Another “Well-Known Member,” Ron’s education is in electronic design. He took a path into engineering management early on in his career, and didn’t practice as much as perhaps he would have liked to. But like many of his engineering peers, he still likes the problem-solving tasks.

WHY ON FORUMS:

EDAboard member and “Super Moderator” Andre lives in Rio de Janeiro where he works as a senior software development EE and specializes in power electronics, microcontrollers and DSP. Andre has worked on R&D projects for various applications including imaging, medical, chemical, automotive and mobile phones. His experience with design tools includes products from Mentor Graphics, Xilinx and Texas Instruments.

WHY ON FORUMS: “Of all the forums I have attended, this one appeared to be the most comprehensive, having the widest variety on the subjects covered.What motivates me to help others is the challenge to test my own knowledge of a subject. Sometimes, someone responds with a better solution. I not only give help, but also get help and improve my own knowledge.”

Top Forum Members

MOST engineers can point to a time or two in their career when they’ve been helped along by one of their

peers. Sometimes the answers to a few key questions spell the difference between days or weeks of wheel spinning and getting on with the job at hand.

That’s why we’d like to recognize some of the frequent contributors on our two online engineering communities, EDABoard.com and Electro-Tech-Online.com (ETO). These are communities of forums, blogs and articles containing a lot of useful advice for

After I retired, I discovered ETO and circuit simulators. Now I can problem solve and design circuits. Almost as good as chocolate.

I could sit and do crosswords and watch TV, but I prefer to keep my mind active by doing what I know how to do by helping newbies with their electronics projects. The trick is to guide them to knowledge rather than just show them how to do it.

What motivates me to help others is the challenge to test my own knowledge of a subject.

Aimee KalnoskasContibuting Editor

Bring Power Knowledge to their Peers

MEMBER PROFILES

“MIKMI”

“RONV” AKA RON VITULLO “ANDRE_TEPROM” AKA ANDRE LOUIS

CASTRO DE RAMOS

individuals trying to solve engineering problems. We did some digging into the numerous

conversations on these sites around power-electronics challenges. We uncovered a few of the top forum contributors who have a particular penchant in this field. We were able to persuade some of them to share a few details about their backgrounds and, perhaps more importantly, about what motivates them to spend their valuable time on the forums.

We hope our little shout out gives others more appreciation for sharing their knowledge and participating in forum conversations. Everyone benefits from shared insights.

As one top contributor said, “It’s almost as good as chocolate.” Can’t top that.

Aimee_EE_Feb_Vs7.indd 58 3/6/15 3:13 PM



EDABoard “Super Moderator” SunnySkyguy started life as an EE in 1975 designing a VLF Doppler navigation receiver for the world’s first unmanned floating Automated Weather Station. Tony worked on nuclear instrumentation built to survive rough handling and designed Power over Ethernet boxes for Avaya. He held managerial roles in contract manufacturing and design services for EMS companies and created a virtual design engineering organization.

“Experience is worth every penny,” Tony said, and after more than 30 years as an EE, he decided to retire and do some part-time consulting. He maintained that his first year in R&D after graduating was akin to 10 years of experience. When he switched jobs, the next 20 years of long hours and hard work gave him what he felt was another 40 years of experience.

WHY ON FORUMS: “With the same appreciation for my mentor’s help, I give back to the Engineering world, regardless of where they live. It’s also a good way to keep the mind sharp with good questions.”

A native of Dhaka, Bangladesh, Tahmid joined Electro-Tech-Online in 2008 and is currently a teaching assistant at Cornell University in Ithaca, N.Y. He loves electronics and learns and practices in his home laboratory. Tahmid uses microcontrollers and embedded computers extensively in power electronics, robotics applications and in bioinstrumentation applications to solve real-world problems.

Tahmid’s worked on several power electronics projects, specifically with respect to the development of low-cost backup power systems for rural and urban regions in times of electricity shortage. Tahmid has also worked with solar energy based products, using solar panels and batteries to provide off-grid power to remote rural regions for powering lights, fans, televisions and more.

I give my personal experience and knowledge and, in exchange, I get to learn more through interactions with other engineers.

There is a certain bliss that you can only get when helping others.

MEMBER PROFILES

“SUNNYSKYGUY” AKA TONY STEWART “TAHMID” AKA

SYED TAHMID MAHBUB

Experience is worth every penny

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Orion Fans ............................................................................ 39orionfans.com

Panasonic Corporation ........................................................ 27na.industrial.panasonic.com

Pico Electronics Inc. ............................................................ 51picoelectronics.com

Pioneer Magnetics ......................................................... 19, 21pioneer-mag.com

Premier Magnetics, Inc. ...................................................... 32premiermag.com

Renco Electronics, Inc. ........................................................ 35rencousa.com

Tadiran Batteries .................................................................. 47tadiran.com

Teledyne LeCroy .................................................................. 29teledynelecroy.com

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ID(cont)

TC=25°C(A)

RDS(on)

maxTJ=25°C

(Ω)

Ciss

typ(pF)

Qg

typ(nC)

trr

(ns)PD

(W)

RthJC

max(°C/W)

PackageStyle

VDSS

max(V)

PartNumber

POWER

D

SG

IXYS’ Surface Mount Power Device (SMPD) TechnologyLose The Weight! Put Your Design On A DIET!

Lighter weight, more power (ultralow profile, energy efficient, and rugged)

www.ixys.com

CONFIGURATIONSBuckBoostFull-bridgeHalf-bridgePhase legSingle

APPLICATIONSDC-DC convertersBattery chargersSwitching and resonant power suppliesDC choppersTemperature and lighting controlsMotor drivesE-bikes and electric and hybrid vehiclesSolar invertersInduction heaters

SMPD ADVANTAGESUltra-low and compact package profile(SMPD: 5.3mm height x 24.8mm length x 32.3mm width)(Mini SMPD: 5.3mm height x 16.8 length x 29.5mm width)Surface mountable via standard reflow process(Available in Tape & Reel packaging) Low package weight (SMPD: 8g, Mini SMPD: 5g)Up to 4500V ceramic isolation(DCB)Low package inductanceExcellent thermal performanceHigh power cycling capability

EUROPEIXYS [email protected]+46 (0) 6206-503-249

USAIXYS [email protected]+1 408-457-9042

ASIAIXYS Taiwan/IXYS [email protected]@ixyskorea.com


SMPD IGBTs

IC25

(A)

VCE(sat)

maxTJ=25°C

(V)

Eoff

typTJ=125°C

(mJ)

RthJC

maxIGBT

(°C/W)

Configuration Package styleVCES

(V)Part Number

32925023

1.83.52

3.1

1.73.550.86

-

10.311.131.25

BoostCopacked (FRED)

Half-bridgeFull-bridge

SMPD-BSMPD-XSMPD-ASMPD-A

12001200600

2500

IXA20RG1200DHGLBMMIX1Y100N120C3H1MMIX2S50N60B4D1MMIX4G20N250


SMPD Power MOSFETs

6001026324

0.00130.02

0.0430.29

40000280001860019000

590335250310

100200250300

830570520500

0.180.220.240.25

SMPD-XSMPD-XSMPD-XSMPD-X

40300500

1000

MMIX1T600N04T2MMIX1F160N30TMMIX1F132N50P3MMIX1F44N100Q3

SMPD-X

Mini SMPDSMPD-B


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© Allied Electronics, Inc 2015. ‘Allied Electronics’ and the Allied Electronics logo are trademarks of Allied Electronics, Inc. An Electrocomponents Company.

1.800.433.5700

The most trusted brands in Power, all under one roof.

THE ULTIMATE HOUSE

alliedelec.com/PowerHouse

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Features

• Ultralow EMI/EMC Emissions• Spread Spectrum Frequency

Modulation• Input Voltage: 3.4V to 42V• Ultralow Quiescent Current in

Standby: 2.5µA• Low Dropout: 100mV at 1A

All Conditions• Small 3mm x 4mm QFN Package

Low EMI/EMC Passes CISPR25 Class 5, 95% Efficient at 2MHzThe LT®8640 joins our growing family of ultralow quiescent current high voltage monolithic synchronous buck regulators. Its inherent ultralow EMI/EMC emissions, coupled with spread spectrum capabilities, allows it to comfortably meet CISPR25 class 5 radiated emissions requirements for automotive environments. The LT8640 consumes only 2.5µA of quiescent current while regulating an output of 3.3V from a 12V input source. It also delivers up to 5A of output current with efficiencies as high as 96%.

42V, 5A Silent Switcher® More Power & Spread Spectrum

VIN VOUT

LT8640

, LT, LTC, LTM, Linear Technology, the Linear logo and Silent Switcher are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners

Load Current (A)0.5

60

Effic

ienc

y (%

)

Power Loss (W

)

65

75

80

85

100

95

1.5 2.5 3

70

90

0

0.38

1.13

1.50

1.88

3.00

2.63

0.75

2.25

1 2 3.5 4 4.5 5

1MHz2MHz

12VIN to 5VOUT

Efficiency

Actual SizeDemo Circuit

www.linear.com/product/LT8640

1-800-4-LINEAR

Simplified LT8640 EMI/EMC Curve, IOUT = 4A

-505

101520253035404550

0 100 200 300 400 500

Ampl

itude

(dBu

V/m

)

Frequency (MHz)

CISPR25 Class 5 Pk Limit LT8640 Fixed Frequency Mode

LT8640 Spread Spectrum Mode

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