Power Tran

7/29/2019 Power Tran

1/44

ECE 442 Power Electronics 1

Bipolar Junction Transistors (BJT)

NPN PNP


2/44


BJT Cross-Sections

NPN PNP

Emitter

Collector


3/44


Common-Emitter NPN Transistor

Forward bias the BEJ

Reverse bias the CBJ


4/44


Input Characteristics

Plot IB as f(VBE, VCE)

As VCE increases,

more VBE required to

turn the BE on so thatIB>0.

Looks like a pn

junction volt-ampere

characteristic.


5/44


Output Characteristics

Plot IC as f(VCE, IB)

Cutoff region (off) both BE and BC

reverse biased Active region

BE Forward biased

BC Reverse biased

Saturation region (on) both BE and BC

forward biased


6/44


Transfer Characteristics


7/44


Large-Signal Model of a BJT

KCL >> IE = IC + IB

F = hFE = IC/IB

IC = FIB + ICEO

IE = IB(1 + F) + ICEO

IE = IB(1 + F)

IE = IC(1 + 1/F)

IE = IC(F + 1)/F


8/44


(1 ) ( 1)

111

1 1

E B C

CF FE

B

C F B CEO

E B F CEO B F

FE C C

F F

C F E

F FF F

F F

I I I

Ih

II I I

I I I I

I I I

I I


9/44


Transistor Operating Point

B BEB

B

CE CC C

C C

CE CC C C

V VI

RV V

I

R RV V I R


10/44


DC Load Line

VCC

VCC/RC


11/44


BJT Transistor Switch

B BEB

B

CE CC C C

CE CB BE

CB CE BE

V VI

R

V V I R

V V VV V V


12/44


BJT Transistor Switch (continued)

CC CE CC BE CM

C C

CMBM

F

V V V V I

R R

II


13/44


BJT in Saturation

( )CC CE sat

CS

C

CSBS

F

B

BS

CSforced

B

V VI

R

I

I

IODF

II

I


14/44


Model with Current Gain


15/44


Miller Effect

vbe vceiout


16/44


Miller Effect (continued)

( ) ( )

[1 ] [1 ]

[1 ]

out cb be ce cb be be

out cb be cb be

cb cb

d di C v v C v Av

dt dt d d

i C A v C A vdt dt

C C A


17/44


Miller Effect (continued)

Miller Capacitance, CMiller= Ccb(1 A)

since A is usually negative (phase inversion),

the Miller capacitance can be much greater

than the capacitance Ccb

This capacitance must charge up to the

base-emitter forward bias voltage, causing

a delay time before any collector currentflows.


18/44


Saturating a BJT

Normally apply more base current than

needed to saturate the transistor

This results in charges being stored in the

base region

To calculate the extra charge (saturating

charge), determine the emitter current

1cse B BS BS BS I

I I ODF I I I ODF


19/44


The Saturating Charge

The saturating charge, Qs

( 1)s s e s BSQ I I ODF storage time constantof the

transistor


20/44


Transistor Switching Times


21/44


Switching Times turn on

Input voltage rises from 0 to V1

Base current rises to IB1

Collector current begins to rise after the

delay time, td

Collector current rises to steady-statevalue ICS

This rise time, trallows the Millercapacitance to charge to V1

turn on time, ton = td + tr


22/44


Switching Times turn off

Input voltage changes from V1 toV2

Base current changes toIB2

Base current remains atIB2 until theMiller capacitance discharges to zero,

storage time, ts

Base current falls to zero as Millercapacitance charges toV2, fall time, tf

turn off time, toff= ts + tf


23/44


Charge Storage in Saturated BJTs

Charge storage in the Base Charge Profile during turn-off


24/44


Example 4.2


25/44


Waveforms for the Transistor Switch

VCC = 250 VVBE(sat) = 3 V

IB = 8 A

VCS(sat) = 2 V

ICS = 100 A

td = 0.5 s

tr= 1 s

ts = 5 stf= 3 s

fs = 10 kHz

duty cycle k = 50 %

ICEO = 3 mA


26/44



27/44


Power Loss due to IC for ton = td + tr

During the delay time, 0 t td

Instantaneous Power Loss

Average Power Loss

0 0

1( )

(250 )(3 )(10 )(0.5 ) 3.75

d dt tCC CEO

d c CC CEO s d

d

V IP P t dt dt V I f t

T T

P V mA kHz s mW

( )

( ) (250 )(3 ) 0.75c CE C CC CEO

c

P t v i V I

P t V mA W


28/44


During the rise time, 0 t tr

( )

( )

( )

max

( )

( )

( ) ( )

( )( )

( ) @

2[ ]

c CE c

CSc CC ce sat CC

r r

ce sat CC c CS CSCC ce sat CC

r r r r

c m

r CCm

CC ce sat

P t v i

ItP t V V V t

t t

V VdP t I Itt V V V

dt t t t t

P t P t tt V

tV V


29/44


2

max

( )

2

max

(1 )(250 ) 0.5042[250 2 ]

4[ ]

(250 ) (100 ) 63004[250 2 ]

m

CC CS

CC CE sat

s Vt sV V

V IPV V

V AP WV V


30/44


Average Power during rise time

( )

0

1( )

2 3

(250 ) (2 250 )(10 )(100 )(1 )

2 3

42.33

rt

CE sat CC CCr c s CS r

r

r

V VVP P t dt f I t

T

V V VP kHz A s

P W


31/44


Total Power Loss during turn-on

0.00375 42.33 42.33375

42.33

on d r

on

on

P P P

P W

P W


32/44



33/44


Power Loss during

the Conduction Period

( )

( ) ( )

0 0

0

( ) 100

( ) 2( ) (100 )(2 ) 200

1

( )

(2 )(100 )(10 )(48.5 ) 97

n n

n

c CS

CE CE sat

c c CE

t t

n c CE sat CS s CE sat CS s n

n

t t

i t I A

v t V V P t i v A V W

P P t dt V I f dt V I f tT

P V A kHz s W


34/44



35/44


Power Loss during turn off

Storage time

( )

( )

( ) ( )

0 0

0

( ) 100

( ) 2

( ) (2 )(100 )

( ) 200

1 ( )

(2 )(100 )(10 )(5 ) 10

s s

s

c CS

CE CE sat

c CE c CE sat CS

c

t t

s c CE sat CS s CE sat CS s s

s

t t

i t I A

v t V V

P t v i V I V A

P t W

P P t dt V I f dt V I f tT

P V A kHz s W


36/44



37/44


Power Loss during Fall time0

( ) 1 , 0

( ) , 0

( ) 1

( ) 11 0

3( ) @ 1.5

2 2

(250 )(100 )

4 4

f

c CS CEO

f

CCCE CEO

f

c CE c CC CS

f f

c CC CS

f f f

f

c m

CC CS m

t t

ti t I I

t

Vv t t I

t

t tP t v i V I

t t

dP t V I tt

dt t t t

t sP t P t s

V I V AP

6250W


38/44


Power Loss during Fall time (continued)

0

( )

1( )

6

(250 )(100 )(3 )(10 ) 1256

6

10 125 135

ftCC CS f s

f c

f

CC f

off s f CS s s CE sat

off

V I t f P P t dt

T

V A s kHz P W

V t

P P P I f t V

P W


39/44



40/44


Power Loss during the off time

0

0( )

( )

( ) (250 )(3 ) 0.75

1

(250 )(3 )(10 )((50 5 3) )

0.315

o

o

CE CC

c CEO

c CE C CC CEO

t

o CC CEO CC CEO s o

o

o

t tv t V

i t I

P t v i V I V mA W

P V I dt V I f tT

P V mA kHz s

P W


41/44


The total average power losses

42.33 97 135 0.315

274.65

T on n off o

T

T

P P P P P

P

P W


42/44


Instantaneous Power for Example 4.2


43/44


BJT Switch with an Inductive Load


44/44


Load Lines

Power Tran

Documents

Transcript of Power Tran