Practical Power Application Issues for Switch Mode Power Supplies PPT

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Practical Power Application Issues for Switch-Mode Power Supplies

Fairchild Power Seminar 2006

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Agenda

• Stability of Synchronous Buck Converters: How to Design the Compensation Network

• What Non-Magnetics Designers Need to Know About Transformers

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Stability of Synchronous Buck Converters: How to Design the Compensation Network

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Part No. DescriptionMax

Io (A)Vin,op

(V)Vout(V)

Tj,op(ºC)

Fsw (kHz)

Control Mode

Compen-sation

Typ Drv Zout (Ω) Packages

Design Tools

FAN5182 + FAN5009

1 to 3-phase controller w/external drivers

90 10.8 to 13.2 0.8 to 5 0 to 125 200-1000 /phase

I-mode external 1.4 to 3.8 QSOP-20, SOIC/MLP-8

FAN6520A 8-pin PWM controller + drivers

30 4.5 to 5.5 0.8V to Vin -40 to 125 300 V-mode external 1.0 to 2.5 SOIC-8 AN-6009

FAN5069 PWM + LDO controller 30 3 to 24 0.8 to 15 -40 to 125 200-600 Summing I-mode

external 1.2 to 1.8 TSSOP-16 AN-6010

FAN5234 PWM/PFM Controller + drivers

15 2 to 24, 4.75 to 5.25

0.9 to 5.5 -10 to 150 300/600 Avg I-mode /hysteretic

internal 1.5 to 8 TSSOP-16, QSOP-16

AN-6002

FAN5236 Dual PWM/PFM Controller + drivers

15 ea. 3 to 24, 4.75 to 5.25

0.9 to 5.5 -10 to 150 300 Avg I-mode /hysteretic

internal 1.2 to 12 TSSOP-28, QSOP-28

AN-6002

FAN5026 Dual PWM Controller + drivers

15 ea. 3 to 24, 4.75 to 5.25

0.9 to 5.5 -40 to 150 300 AverageI-mode

internal 1.2 to 12 TSSOP-28 AN-6002

FAN2011/12 Integrated 1.5A synchronous buck

1.5 4.5 to 5.5 0.8V to Vin -40 to 150 1300 I-mode internal 95% max efficiency

3x3 mm MLP-6

AN-6011

FAN2001/02 Integrated 1A synchronous buck

1 2.5 to 5.5 0.8V to Vin -40 to 150 1300 / PFM

I-mode internal 96% max efficiency

3x3 mm MLP-6

FB

FAN6520A

VCC

+VOUT

Q2

COMP/OCSET RS

GND

LDRV

SW

HDRV

BOOT

5

3

4

8

2

1

67

Q1 CHF

ROFFSET

ROCSET

CFRF

CI

CBOOT

DBOOT

CBULK

CVCC

+5V

LOUT

COUT

MLP

SOIC

TSSOP

QSOP

Fairchild General Purpose Switching Regulators

3

5

Driver

Lo

Osc

Ri

Current FeedbackTransresistance

PWM Comparator

+

-

OUT

Vm

Driver

Vout

Co

ESR

RLOAD

Vin

Type 3 Error Amp +

-

OUTVref

R1

C1

VcompRbias

R2

C2

R3 C3

Synchronous Buck with Summing Current-Mode Control

6

Voltage Loop Gain Components

OutputVoltage

Gvd(s)

Gvc(s)GpwmGc(s)+

-DesiredVoltage

dVe Vc Vo

Ic

There are four elements in the output-voltage control loop:

• Subtractor element, which generates an error signal by subtracting the desired voltage from the output voltage

• Compensator element Gc(s), added by the designer to stabilize the loop and improve the loop performance

• PWM element Gpwm which defines the relationship between the compensator output signal and the duty cycle/current-mode control current (a constant)

• Control-to-output transfer function Gvd(s) or Gvc(s)

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7

G M

-180

0 dB

|T|

/ T

P M

High gain @low frequency

Wide bandwidth

-20db/dec

Gain Margin

Phase Margin

High Gain at Low Frequency

Wide Bandwidth

Desired Loop Gain Characteristics

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• Bandwidth • Related to the speed of transient response

• Phase Margin • Related to the damping of the system• Low PM causes oscillatory transient response

• Gain Margin • Gain is changed according to the variation of circuit components• Related to robustness against gain variation

Stability Terms

5

9

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

1.E+03 1.E+04 1.E+05

Frequency (Hz)

Gai

n (d

B)

0

i

Lo1p M

MRC2

1f ⋅⋅⋅π⋅

≈

i

v

Lo2p M

MR/L2

1f ⋅⋅π⋅

≈

ESRC21fo

z ⋅⋅π⋅=

-40 dB / decade

-20 dB / decade

-20 dB / decade

(dB)M0

v

0

L

Loo

2

i

0Lo

v

0

L

o

o0

MM

RESRRCLsESR

MMRC

MM

RLs1

ESRCs1M)s(M⋅

+⋅⋅⋅+⎥

⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+⋅⋅+⋅⋅+

⋅⋅+⋅=

Open Loop System Gain

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Type 1: Single pole - no phase boost

Type 2: Two poles, 1 zero - up to 90 degrees

Type 3: Three poles, 2 zeros - up to 180 degrees

The key task of the feedback amplifier’s compensation function is to boost the loop gain and phase to raise the crossover frequency (for better transient response) while maintaining adequate stability margins. The compensator type is determined by the amount of phase boost needed at the crossover frequency:

Compensator Types

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11 -60

-40

-20

0

20

40

60

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Frequency (Hz)

Gai

n (d

B)

-135

-90

-45

0

45

90

135

Phas

e (d

egre

es)

Gain Phase

-20 dB / decade

-20 dB / decade)CC(R2

1f211

gain_unity +⋅⋅π⋅=

21

212

pole

CCCCR2

1f

+⋅

⋅⋅π⋅=

12zero CR2

1f⋅⋅π⋅

=

1

2

RRlog20 ⋅

Phase boost(up to 90º in theory,75º in practice)

Type 2 Error Amp+

-

OUT

R1

Vref

C1

Vcomp

Vout

Rbias

R2

C2Type 2 Compensator

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Type 3 Compensator

-60

-40

-20

0

20

40

60

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Frequency (Hz)

Gai

n (d

B)

-135

-90

-45

0

45

90

135

Gain Phase

)CC(R21f

211gain_unity +⋅⋅π⋅

=21

212

2pole

331pole

CCCCR2

1f

RC21f

+⋅

⋅⋅π⋅==

⋅⋅π⋅=

( )3132zero

211zero RRC2

1fRC2

1f+⋅⋅π⋅

==⋅⋅π⋅

=

1

2

RRlog20 ⋅

Phase boost (up to 180º in theory, 160º in practice)

Type 3 Error Amp+

-

OUT

R1

Vref

C1

VcompRbias

R2

C2

R3

Vout

C3

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• A method to calculate the compensation circuit values from the amount of phase boost needed at the crossover frequency(1)

• Uses Venable’s K-factor:

• For a Type 3 compensator, this places both poles at the same frequency and both zeros at the same frequency, which maximizes the gain below crossover and minimizes it above crossover. (We will modify this later)

(1) Venable, H. Dean, "The K Factor: A New Mathematical Tool for Stability Analysis and Synthesis;' Proceedings of Powercon 10, March 1983. (Also “Optimum Feedback Amplifier Design …” from http://www.venable.biz/.)

⎟⎠⎞

⎜⎝⎛ °+

ϕ= 45

4tanK boost2

Venable K-Factor Calculation

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-100

-80

-60

-40

-20

0

20

40

60

80

100

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Frequency (Hz)

Gai

n (d

B) Open-Loop Error Amp

Type 3 Compensator Overall Converter Modulator + Filter

Gain-bandwidth product

Converter bandwidth (BW)

Pole due to load & output capacitance

Pole due to inductor Zero due to ESR &

output capacitance

Error Amp

DC Gain

KBW KBW ⋅

Final Loop Gain with Type 3 Compensator

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• Excel-Based Components Calculator• Output Filter L and C• Input filter capacitor ripple current• Various external R's and C's to set:

• ISENSE , ILIMIT , Soft Start, etc.• Compensation components (where applicable)• Bode plot

• PSPICE modelTests loop stability and transient response from the values chosen as a result of the calculations from the Excel spreadsheet above.• Runs in student PSPICE version (free distribution from Cadence) • Continuous time model : Excellent correlation with both switching

model and lab results• Runs fast. Allowing iterative compensation design

• MOSFET Selection / Efficiency simulation: AN-6005

Tool Suite for Switching Regulators

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Color Code:

Calculated system performance

Calculated values that can be overwritten

User Inputs

Limit condition

Spreadsheet Design Procedure Overview

Step Tab Input Output1 OutputFilter System requirements:

VIN, VOUT, IOUT(MAX)

Output filter L and C

2 Main Sheet RDS(ON) of MOSFETs, Start-up ramp, etc

External R's and C's

3 Compensation From Main Sheet and OutputFilter

Compensation components

4 InputFilter From OutputFilter. Calculates RMS ripple requirement for input filter cap.

Bode Plot Examine small signal stability on this tab

Compensation

Bode Plot

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Parameter: Value Units CommentsRL (load resistance) = VOUT / IMAX 0.075 ohm Use this value for Loop Bandwidth 38 KHz. Default is FSW/8.Compensator Type Synthesized

ValuesUser

ValuesBode plot uses synthesized

C1 = 373 390 pFC2 = 93 100 pFC3 = 1,524 1,500 pFR1 = 4.990 4.990 KΩ Do not modify heR2 = 25.47 25.47 KΩR3 = 1.243 1.243 KΩRBIAS 5.703 5.703 KΩ Do not modify hePhase Margin 76° 75°

Components calculated by this sheet (figure 24 in FAN5069 data

sheet)

These component-value calculations are based on Venable’s K-Factor method, modified to guarantee minimum phase margins of 60º at crossover & 45º at lower frequencies.

Values may be overwritten and results examined on the Bode plot.

Compensation Worksheet

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-30-20-10

0102030405060708090

100110120130140150160170

1 10 100 1,000 10,000 100,000 1,000,000f(Hz)

dB o

r deg

rees

Magnitude: Synthesized components Magnitude: User ComponentsPhase margin: Synthesized ComponentsPhase Margin: User Components

Synthesized Bandwidth is 36.3 KHz.

User Phase Margin is 75°

User Bandwidth is 33.1 KHz.

Synthesized Phase Margin is 76°

Loop Gain Bode Plot

Phase Margin

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CXCX

In

0 0

Notes:(1) Reset RLoad to desired value when switchingbetween AC sweep and transient simulation.(2) For more accurate AC sweep, descend into U1and set Esampling XFORM to 1+s/(wn*Qz)+(s/wn)**2.(3) For transient analysis, set XFORM to 1.

CZCZ

RzCZ_ESR

Out

0

Istep

TD = 3msPW = 500usPER = 10ms

I1 = 0.1AI2 = 6A

0

Time_Av g_SW

DCX_CCM

P

A C

D

L1

L

1 2

R1R1

RLoadRLoad

C3C3

Rdc

L_ESRF1F

00

SENSE

0

0

R3R3

FB

C1C1

R2R2

D

C2C2

PARAMETERS:External Components

R2 = 25.47kR1 = 4990

RBIAS = 5.703kR3 = 1243

C1 = 390p

C3 = 1500pC2 = 100p

RRAMP = 237k

Rramp

RrampU1

FAN5069

SW

PG

ND

FB

D

RAMP

COMP

R4

1u

SW

0

Vtest10mV

VinVin

RbiasRbias

PARAMETERS:Vin = 13

L = 0.8u

Rds_on_ls = 3m

RLoad = 1

Rds_on_hs = 4mf sw = 300kHz

Output Filter and System

L_ESR = 2m

CX = 10uCX_ESR = 5m

CZ = 1120uCZ_ESR = 8m

DB(V(Out)/V(SENSE))

Expressions for Probeto generate Bode plot:

Application Circuit

P(V(Out)/V(SENSE))

RxCX_ESR

PSPICE Model Schematic

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PSPICE Load-Transient Response

Time

2.4ms 2.6ms 2.8ms 3.0ms 3.2ms 3.4ms 3.6ms 3.8ms 4.0msV(Out)

1.45V

1.50V

1.55VI(L1) I(R4)

0A

5A

10A

SEL>>

Output voltage

Inductor current

Load Current

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Transformers:What Non-Magnetic Designers Need to Know

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How Do I Get a Transformer?The Options

• Go to a transformer supplier’s catalog (or website)… only if the application is simple & common

• Design & wind the prototype yourself… requires some magnetics expertise

• Request a prototype from a transformer supplier… what to tell them

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23 [Source: CoEv Magnetics]

A Catalog Transformer

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A Catalog Transformer (cont’d)

[Source: CoEv Magnetics]

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A Completed Design (from Fairchild)(1) Transformer Schematic Diagram

T1 EER28L

6

4

1

5

3

2

8

7

9

10

11

12

Np/2

N1.2V N2.5V

N6.7V

N13V

Na

Np/2

(2) Winding Specification

Pin (S F) Wire (mm) Turns Winding Method Np/2 2 1 0.40φ x 1 33 Solenoid winding N1.2V 8 7 0.32φ x 4 1 Solenoid winding N2.5V 9 8 0.32φ x 2 1 Solenoid winding N6.7V 11 10 0.40φ x 4 5 Solenoid winding

N13V 12 11 0.40φ x 2 4 Solenoid winding Np/2 3 2 0.40φ x 1 32 Solenoid winding Na 5 6 0.32φ x 1 10 Solenoid winding

Core : EER28L (Ae=81.4mm2) Bobbin : EER28L

(3) Electrical Characteristics

Pin Spec. Remark

Inductance 1 – 3 1.20 mH 70kHz, 1V

Leakage 1 - 3 8 uH (?) Short all other pins

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Design & Wind the Prototype Yourself: The Steps

• Design the power supply turn ratios, magnetizing inductance (at

the primary winding), rms winding currents

• Choose a core

• Material (Mn-Zn ferrite), shape, size

• Design the windings – the “hard” part

• Wind the prototype

• Set the gap

• Test in the circuit … adjust the design … until it works well

• Deliver the design to a magnetics supplier for high-volume production

(& check that their prototype works in the circuit!!)

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27

28

Some Terminology

BMagnetic flux density

[T,G]

HMagnetic field

intensity[A-t/m, Oe]

Slope: μ = Permeability P = Permeance L = Inductance= μr μ0 = 1/ℛ (Reluctance) = N2P = N2AL

= μAm/lm = AL (Inductance factor)

Magnetic characteristic of the:

Material Core Circuit Element

Φ = BAmMagnetic flux

[Wb, Mx]

ℱ = mmf= Hlm = ΣNi

Magnetomotive force [A-t, Gb]

λ = NΦFlux linkages

[Wb-t, Mx-t]

i = ℱ /N

Current[A]

[Units: SI & cgs]

These curves are all related by geometric constants & the number of turns!

15

29

B-H Curves for Some Magnetic Materials

• Wider at higher

frequency

• Shorter at high

temperature

(especially ferrite)


= 10 x Tesla

30

Core Utilization

“Double-ended”AC-only Operation

“Single-Ended”AC operation with DC offset


• Push-pull, Half- or Full-bridge –smallest transformer

Flyback – big transformerForward – smaller transformer

H

16

31

B

F E

D

A

C

M

L

A

BC

D

F E

K

B

F E

D

A

C

M

L

C HF

B

DG

A

E

C

F EGA

D

B

S

B

D

FJ

E

AG

C

B A

C

A

B

E

D

C

L

E

EP

C

F GA

B

D

E

ER

PLANAR E POT

PQ RM TOROID U

B

F E

D

A

C

K

M

L

EFD


Some Common Core Shapes

32

Some Common Core Shapes


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33

0.1

1

10

100

1000

10000

100KHz

250KHz

500KHz

750KHz

1MHz

100KHz

250KHz

500KHz

750KHz

1MHz

100KHz

250KHz

500KHz

750KHz

1MHz

100KHz

250KHz

500KHz

750KHz

1MHz

100KHz

250KHz

500KHz

750KHz

1MHz

Full or Half Bridge Push-Pull Forward Converter Flyback - Single Output Flyback - Multiple Output

Theo

retic

al M

ax O

utpu

t Pw

r - W

atts

ETD44

ETD39

ETD29

EE30

EFD30

EFD25

EFD20

EP17

EE13

EP13

EFD15

CEP10

EF12.6

EP10

CEP7

EP7

ER11/5

ER9.5

EE8.8

EE5

Power Output vs

Frequency & Topology


Power Output vs. Frequency & Topology

34 [Source: TDK]

Core Material Pages

18

35

Core Material Pages (cont’d)

[Source: TDK]

36

Core Shape Page

19

37

Design the Windings

• Choose primary turns for a “reasonable” gap (~0.2 to 1.5 mm)

• Choose wire:• Shape: round, ribbon, twisted, Litz• Size: 0.32φ (mm, = AWG#28), 0.25φ x 4, etc.• Material: polyurethane/nylon (Nyleze), polyimide (Kapton, TEX-E)• Thermal class: 105ºC, 130ºC, 180ºC, …• Build: single, heavy (double), triple, quad

• Determine the configuration or “stack-up”

• Determine safety agency requirements: • Use yellow polyester film tape; space terminations properly …

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Techniques to Reduce Stray Effects

• Leakage Inductance:• Lowest for adjacent, thin windings (tighter coupling)

• Eddy current losses (skin & proximity effects):

• Split the primary

• Limit conductor thickness to approx. 2 “skin depths” δ

• At 100 kHz & 80ºC, maximum wire diameter is just 0.46 mm!

• Stray capacitance noise coupling:• Use shield(s)

mm23.0S/m)10t)(4.8-A-Wb/m104)(kHz100(

1177

Cu0sw

=−××

== −ππσμπδ

f

20

39

B-H Loop before gapping B-H Loop after gapping

Gapping of the magnetic core along the flux path causes a “shearing over”of the effective B-H loop (really the Φ-ℱ loop), which lowers the effective permeability and allows unsaturated operation at higher bias operating levels.

g0

g

mm

m

g0

g

L

LgLg2

Aμl

Aμl

1Aμl

A1

1A,ANL

+=

+==

lm

lg

The Gap


40

Design for a “Reasonable” Gap

Fringing Flux

• If possible, use “standard” gap from core manufacturer

• Target between 0.2 & 1.5 mm

• Use spacer (“fish”) paper for prototyping

• Set final gap by measuring inductance

• For production, center leg is typically ground shorter


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41

Regulatory Jargon

• Safety agencies: VDE, UL, TUV, etc.

• Grade of Insulation System: • Functional, Basic, Supplementary, Reinforced

• Pollution Degree 1 (sealed), 2 (UL1950), 3 (conductive)

• Material Class I, II, IIIa, IIIb• Based on CTI (Current Tracking Index) level

42

Bobbin Winder

• May also want a solder pot, transformer electrical tester

(Adams-Maxwell 1201-3AX)

22

43

Requesting a Prototype …What a Supplier Needs to Know

• Who designed the power supply?

• What are the electrical requirements?• What is the switching/operating FREQUENCY? (Fo)

• What is the VA (Tx) or CURRENT rating (Inductor)?

• What is the TOPOLOGY? (Flyback, Push-Pull, Buck/Boost, Forward Converter, etc.)

• What are the INPUT VOLTAGES & CURRENTS?

• What are the OUTPUT VOLTAGES & CURRENTS (also known as output VA)?

• Which Pulse Width Modulator (PWM) IC?

• What Agency Approval requirements (UL1459, IEC950, etc.)?

• Any SIZE constraints/footprint already determined?

• EAU - Expected Annual Usage• Target PRICE & COMPETITION


44

Supplier’s Design Worksheet


23

45

• Turns • Give ratios instead

• Winding design• Wire sizes• Stack-up

• Core & bobbin supplier & part numbers

What a Magnetics Supplier Prefers to Keep Private

46

The Bottom Line

Without a magnetics specialist, you can do it yourself

(but it can be tricky)

Each approach has advantages:

• Do-it-yourself

• Faster prototyping & debug

• You know & control the design

• Outside expert

• Easy to find & use (ask your distributor)

• Usually can do a better job: smaller, cheaper, more

manufacturable, passes safety agencies

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47

The pdf version of the Power Seminar presentations are available on the our external website. To access or download the pdfs, please visit www.fairchildsemi.com/power/pwrsem2006.html

For product datasheets, please visit www.fairchildsemi.com

For application notes, please visit www.fairchildsemi.com/apnotes

For application block diagrams, please visit www.fairchildsemi.com/markets

For design tools, please visit the design center at www.fairchildsemi.com

For PSPICE models, please visit http://www.fairchildsemi.com/models/PSPICE/Discrete/index.html

For SMPS design tool, please visit http://www.fairchildsemi.com/whats_new/offline_smps_toolkit.html

Some of this content is presented with permission from CoEv Magnetics, a division of Tyco Electronics, www.tycopowercomponents.com/magnetics.asp

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Practical Power Application Issues for Switch Mode Power Supplies PPT

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