Attributes of a good power switch are - subjects.ee.unsw ... 2 - Overview of... · ELEC4614 Power Electronics Lecture 2 - Overview 2-1 F. Rahman of power switches ... The Power Diode

ELEC4614 Power Electronics

Lecture 2 - Overview 2-1 F. Rahman of power switches

Lecture 2 - Overview of power switching devices

The Power Switch: what is a good power switch?

A

G

K

Attributes of a good power switch are:

1. No power loss when ON 2. No power loss when OFF 3. No power loss during turning ON or OFF 4. Little power required to turn it ON or OFF 5. Bi-directional? 6. Adequate voltage and current ratings 7. Low Turn-on and Turn-off times







More on classification of power semiconductors

Source: S. Bernet, “Recent developments of high power semiconductors for industry and traction applications”, IEEE Transactions on Power Electronics, Vol. 15, No. 6, Nov. 2000, pp 1102-1117.



Diodes of various sizes

(d) Thyristor (e) Thyristor (f) 3-φ IPM with IGBTs 50A, 400V 1200V, 1000A 1200V, 100A

Typical power device encapsulations

Key Attributes of power semiconductor switches 1. High breakdown voltage (BV) 2. Low On-state resistance 3. Fast switching times

There is not much freedom to enhance one without affecting the others two.

4. Low gate drive requirement



Static I-V characteristics of power semiconductor switches

For power electronic converters, only the saturation region and blocking regions of the characteristic (shaded part) are used. Majority-carrier devices such as MOSFETS, and Schottky diodes have low breakdown voltage in order to have acceptable On-resistance. These have very fast switching times. Minority-carrier devices such as BJTs, IGBTs, GTOs, SCRs, MCTs have low On-resistance, due to injection of large quantities of minority carries into the depletion layer when switched on. Such devices can have high breakdown voltage. These are however much slower than majority carrier devices.

BV

Vsat

IRON



DEVICE RATINGS SUMMARY

(Mohan, 1995)



DEVICE RATINGS SUMMARY (SEMIKRON, 2004)



Power Devices and Applications

Power rating VS frequency range of power semiconductor devices (Courtesy of Powerex. Inc)



The Power Diode

Chapter 19 of Mohan gives a good introduction to physics of pn junctions.

Junction p-n diode: static characteristic

ON

1R

Signal PN diode structure



The avalanche breakdown voltage BVBD is inversely proportional to the impurity densities.

217BD

BDd d

E1.3 10BVN 2qN

ε×≈ = V

where Nd = donor atom densities/m3 ε = dielectric constant of the depletion layer q = electronic charge in Coulomb EBD = field strength for avalanche breakdown ≈ 20

MV/m

19 310 / cm 19 310 / cm14 310 / cm

Power diode structure

The n− layer gives the high breakdown voltage capability. The lightly doped n− layer supports most of the reverse blocking voltage. The breakdown electric field in the n−

layer AK , max

d

VW

⎛ ⎞≈⎜ ⎟⎝ ⎠

giving avalanche breakdown in the n−

layer is determined by width Wd of the n− layer and the impurity density of this layer. These are selected for the required maximum VAK, or BVBD or VRRM.



The wide n− layer does not necessarily mean increased conduction voltage drop across the diode. The relative levels of impurity densities ensure that adequate conductivity is achieved when minority carriers are injected across the junctions.

Refer to diode data sheets 1-3 in the Lecture Notes webpage for diode parameters. Switching Characteristics

Rdidt

Fdidt

rrt 50 300 n sec≈ − for fast recovery diodes. ≈ several 100 μsecs for line-frequency power diodes

rrQ = Reverse recovery charge, C



S = snappiness factor = 5

4

tt

( )rr RR rr R Rrr 4 d0

2Q di / dtdi t di diI t 2 Idt S 1 dt S 1 dt

τ= = × = <+ +

( )rr d 0rr

R R

2Q 1 S ) 2 Itdi / dt di / dt

τ+= <

12 2

BD4 10 BVτ −≈ × Schottky diodes

Power Schottky diodes are formed from metal-semiconductor junctions, with an n-1 layer as shown in figure below. Note: only majority carriers.

Anode, A

Cathode, K

SiO2 SiO2

p p

n−1

n

Metal contact

Depletion layer with p guard rings

Depletion layer w/o p guard rings

Metal contact



Compared to junction (bipolar) pn diodes Schottky diodes, being a majority carrier device, have

1. much lower forward on-state voltage drop, ≈ 0.3 – 0.4V

2. much smaller turn-off times; no reverse recovery current

3. low reverse blocking voltage ratings, up to ≈ 200V 4. higher junction capacitance; the charging current of

this capacitance at turn-off is comparable to the reverse recovery current of junction (bipolar) pn diodes.

5. high efficiency and switching frequency.



The Power MOSFET • Very fast, toff ≈ 50 nsec - 500 nsec.

• Rdson increases with 2.6BDV ; typically, Rds ≈ 40 mΩ for a

500V, 15A device.

• Turned-on and -off by VGS. VGS,TH ≈ 5-20V.

• These devices are easily connected in parallel.

N-Channel MOSFET (a) symbol, (b) v-i characteristic, (c) idealized characteristic.

blocking

MOSFET with parasitic reverse diode

MOSFET with blocked parasitic and fast recovery diode



Switching of Power Devices (Resistive load)

Gate input power: ont

G G G s0

p v i dt f⎛ ⎞= ×⎜ ⎟⎝ ⎠∫ , W

Switching power loss = SW

p = shaded area × fs, W



Switching ON/OFF a resistive-inductive circuit

The turn-off problem

Suppose the switch T is ON for a while and the inductive

load current is do

LOAD

VIR

= .

Assume that the free-wheeling resistor RFW, is not present. When T is turned OFF, Io falls to zero in time tfi or trv (whichever is longer). If Vd = 5V and RLOAD = 0.1Ω, L = 30 mH, tfi = trv = 150 nsec,

(i) What is voltage Vds across the switch when it is turned off?

(ii) What role could the free-wheeling resistor play in limiting this voltage?

(iii) What role could RFW play in turning off the load current quickly?

Vd Io

FW Resistor

iT

vTT

RFW RLOAD

L



Switching a diode-clamped circuit

The load current Io is assumed to remain constant while the switch turns ON and OFF with a duty cycle at the switching frequency fs.

We assume:

• ideal devices and no reverse recovery current in the diode.

• Io = iT + iD at all time.

• Voltage vT(t) and current iT(t) transients at turn-on and turn-off occur along straight tines.

When the switch T is turned on, the diode continues to carry part of Io as iT builds up. Until iT becomes equal to Io, the diode remains in the conducting state (i.e., forward biased). The diode starts to become reverse biased after Io fully commutates to T in time tri. Thus, while iT rises from zero to Io, vT remains clamped to Vd. Once Io fully commutates to T, the voltage vT (or vAK) across switch falls to zero in time tfv.

Vd

Io Ideal Diode, D

iT

vT

iD

T



When the switch T is turned off, the diode does not conduct until the potential at the cathode of diode D rises to Vd. Thus, diode D remains reverse biased until vT rises to Vd in time trv. Thereafter, the diode becomes forward biased and starts to conduct. Thus, the current through T remains clamped to Io until vT rises to Vd in time trv. Until then iT = Io. Once vT rises to Vd, current iT falls to zero in time tfi. Note that diode clamping increases the areas of overlap during both transitions. The power loss in the switch is given by the product p = vT * iT, given by the shaded areas. Note also that the reverse recovery current of the diode at turn-on of T increases the peak current loading of switch T. The following analysis assumes ideal diode, i.e., the reverse recovery current of the diode is neglected.

The energy dissipated (power lost) in the switch during turn-on and turn-off transients can be found by multiplying the voltage vT(t) and current iT(t) of the switch and integrating the product over the duration of each transient. The calculation is simplified if the origin t = 0 for each case is shifted to time when the respective transitions begin, and if the on-state voltage of the switch is neglected in comparison with the DC supply voltage Vd.



tdon

TON

TOFF

tri tfv

iT

IO Vd

iD

IO

vT

pT

VdIO

tdoff

Vd

trv tfi

on on o onW V I t=

con d o con1W V I t2

=

coff d o coff1W V I t2

=

tcoff = trv + tfi tcon=tri + tfv

Ts = s

1f

= TON + TOFF



Thus, for turn-on transient,

dtttVItdt

tIVW ri fvt t

fvdo

ri

odons ∫ ∫ ⎟

⎟⎠

⎞⎜⎜⎝

⎛−+=

0 01 Joules

d o ri d o fv1 1V I t V I t2 2

= +

( )d o ri fv d o con1 1V I t t V I t2 2

= + = J

Similarly, it can be shown that for turn-off transient,

dtttIVtdt

tVIW rv fit t

fiod

rv

dooffs ∫ ∫ ⎟

⎟⎠

⎞⎜⎜⎝

⎛−+=

0 01 J

( )d o rv fi d o coff1 1V I t t V I t2 2

= + = J

The total switching power loss is thus given by

( ) scoffconodsw fttIVP ×+= 21 W

If the turn-on and turn-off transients are not short compared to Ts, the average power loss in the switching process, Ps, may become large compared to the loss during the ON time.

( )sw d o s con coff1P V I f t t2

= + The on-state power loss is given by



on on o on sP V I t f= W where ton is the on-time of the switch in a switching period. Note that Psw increases proportionately with fs, while PON does not, since Ton and fs are inversely proportional to each other.

Attributes of a good power switch are - subjects.ee.unsw ... 2 - Overview of... · ELEC4614 Power Electronics Lecture 2 - Overview 2-1 F. Rahman of power switches ... The Power Diode

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