Download - Lecture-7 : Semiconductor Power Switching Devices-4

Semiconductor Power Switching Devices-4(Lecture-7)

R S Ananda Murthy

Associate Professor and HeadDepartment of Electrical & Electronics Engineering,

Sri Jayachamarajendra College of Engineering,Mysore 570 006

R S Ananda Murthy Semiconductor Power Switching Devices-4

Structure of Power BJT

IBIC

vBE

vCE

(c)(a)

C

B E−

+−

+

C

B

(b)

C

EBE

np

nn+n−

pn+

Structure shown in Figure (a) is preferred for low voltageratings.Structure shown in Figure (b) is preferred for high voltageratings. But this tends to increase the on-state voltagedrop.


BJT as a Power Switching Device

CEV

VBE

RB

VCB

IE

VCE 1 V

CE 2

VBE

+

−V

BB

IB

IC

IB

RC

VCC

+

−

+

−

(b)(a)

0

B

E

+

−

+

−

>

C

CE configuration is always used as it offers high inputimpedance and higher current gain.When transistor is on IC = ICS = [VCC −VCE(sat)]/RC .IBS = ICS/β is the base current that just turns on the BJT.For faster turn on usually we make IB > IBS.


CE Output Characteristics of Power BJT

VCCVCE(Sat)

VCE

Quasi−

saturation

zone

IB

IC

RC

VCC

IB 4

IB 3

IB 2

IB 1

IB

increasing

Linear zone

Hard saturation line

Cutoff zone

Load Line

= 0

b

a

de

c

Off

On

Since hard saturation increases the turn-off time, powerBJTs are always operated in cut-off and quasi-saturationregions.


Darlington Connection for Higher Current Gain

So overall current gain is

and voltage drop is

T1

T2

Darlington connection gives higher β but it increasesleakage current, on-state voltage drop, and reduces theswitching frequency.


Switching Times of Power BJT

iB

ICS

ICS

ICS

tdtr

tn

tstf

tontoff

iC

IB 1

vCE

B 2−I

to

VCC

VCE(Sat)

0.9

0.1

t

t

t

(c)

(b)

(a)


Preferred Base Current Waveform for BJT

By reducinglike this, BJT is preventedfrom entering intohard saturation

0

By making IB1 > IBS for a brief duration during turn on bothtd and tr and hence ton is reduced.To turn off BJT gradually apply a negative base currentwith a peak of −IB2 which falls to zero after some time.This reduces ts and tf and hence toff .


Baker’s Clamping Circuit to Reduce Storage Time

QD1

D2 D3

D4

C

B

E0

X

D2, D3, D4 are low voltage diodes.D1 should be a high voltage diode.D4 is needed to provide path for negativebase current while turning off the BJT.

When BJT turns on, VCE tends to drop causing D1 to turnon. This decreases IB preventing BJT going into hardsaturation.When D1 is on, as all diode voltages are equal, by KVL weget VCE = VX −VD1 = VBE +VD2 +VD3 −VD1 = VBE +VD2 .


FSOA and RSOA of Power BJT

FSOA shown in Fig (a) is applicable when the BJT isturned on by applying a positive base current.RSOA shown in Fig (b) is applicable when the BJT isturned off by applying a negative base current.


Parallel Operation of BJT

T1 T2

Let permissible

Let permissible

By KVL

Let T1 and T2 be identical transistors. When they are on

Then

Suppose V

and A

then,

If IC1 increases, the voltage across emitter resistor gives anegative feedback and reduces VBE1. Then, IB1 decreasescausing a reduction in IC1. But emitter resistors cause powerloss which reduces the efficiency of the circuit.


Base Drive Isolation using Opto-coupler

Logic

drive

circuit

Amplifier

VCC

BQ

E

COpto−coupler

Since BJT needs a continuous base current, isolation ofbase drive is possible only by using an opto-coupler.But this needs additional VCC and amplifier which makesthe base drive circuit bulkier if more power BJTs are to beused.


Specifications of Power BJT BU208D



Safe Operating Area increases for pulsed operation.


Demerits of Power BJT

Though BJT is fully-controlled, it has the following demerits —Since it is continuously triggered, it needs opto-couplerisolated bulky base drive circuit.It cannot block high reverse voltage.It has low current gain (β ) which varies with collectorcurrent and temperature. Due to this, typically base currentof the order of few amperes is needed for controlling theBJT.β can be increased using Darlington connection. But thisincreases leakage current, on-state voltage drop, andreduces switching frequency.


Demerits of Power BJT

It is prone to fail due to ‘Second Breakdown’ — due to hotspots caused by concentration of current in a narrow areain the emitter junction when the device is turned ON and inthe collector junction when it is turned OFF.Its switching frequency is typically within 20 kHz.Parallel operation of BJTs is difficult due to their negativetemperature coefficient of resistance.BJT cannot be protected against overload using a fuse.It cannot withstand overload. So it should be turned offimmediately when there is overload. This requires sensingof overload by some method.

Due to the demerits mentioned above BJTs are not preferred inmodern power converters. Instead MOSFETs and IGBTs areused.


Next Lecture...

In the next lecture we will discuss some more powersemiconductor switching devices used in power electronics.

Thank You.
