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Power Electronics

Power Transistors –

BJTs (GTRs)

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Power electronics

dr inż. Andrzej Smolarz

Instytut Elektroniki i Technik Informacyjnych

Politechnika Lubelska

a.smolarz@pollub.pl

smolarz.pollub.pl

E313, 081 538 4337

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Sources

M. D. Singh, Power Electronics‎, 2008 http://books.google.pl/books?id=0_D6gfUHjcEC

J.S.Chitode, Power Electronics‎, 2008 http://books.google.pl/books?id=VMC5AYf1YFwC

NPTEL Project (India) http://nptel.ac.in/downloads/108105066

Some students’‎presentations were also used

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Power BJT aka GTR

A power transistor is a vertically oriented four layer structure of alternating p-type and n-type.

Vertical structure maximizes the cross sectional area through which the current in the device is flowing.

This also minimizes on-state resistance and thus power dissipation in the transistor.

Trade-off:

thinner base – larger , lower breakdown voltage

thicker base – smaller , higher breakdown voltage

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Power transistors

Bipolar junction transistors(BJT)

Metal-oxide semiconductor field-effect

transistors (MOSFET)

Static Induction transistors (SIT)

Insulated-gate bipolar transistors (IGBT)

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BJT Construction Parameters

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Wide base width - low (<10) beta.

Lightly doped collector drift region - large breakdown voltage

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Power BJT

Practical Power transistors have their

emitters and bases interleaved as narrow

fingers.

This‎is‎necessary‎to‎prevent‎“current‎

crowding”‎and‎consequent‎“second‎break‎

down”.‎

In addition multiple emitter structure also

reduces parasitic ohmic resistance in the

base current path.

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Basic Geometry of Power BJTs

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Multiple narrow

emitters

minimize emitter

current crowding

Multiple parallel

base conductors

minimize parasitic

resistance in series

with the base.

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Characteristics of NPN Power BJT

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The power BJT is never

operated in the active region

(i.e. as an amplifier) it is always

operated between cutoff and

saturation.

Appearance of the quasi saturation

is a consequence of introducing the

drift region.

Primary breakdown parameters

BVSUS is the maximum VCE that can be

sustained when BJT is carrying substantial

collector current.

BVCEO is the maximum VCE breakdown

voltage that can be sustained when base

current is zero (opened base)

BVCBO is the collector base breakdown

voltage when the emitter is open circuited

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Breakdown – primary and second

The primary breakdown shown takes place

because of avalanche breakdown of

collector base junction.

Large power dissipation normally leads to

primary breakdown.

The second breakdown is due to localized

thermal runaway. Due to current non-

uniformity‎„current‎filaments”‎are‎formed

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N+N+ N-E

+- VCB

++

+ C

B

Reach-thru of CB depletion across base to emitter

+

+

+P

Avoidance of reach-thru

Large electric field of depletion region will accelerate electrons from emitter across base and into collector. Resulting large current flow will create excessive power dissipation.

Avoidance of reach-thru

Wide base width so depletion layer width less than base width at CB junction breakdown.

Heavier doping in base than in collector so that most of CB depletion layer is in drift region and not in the base.

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Forward Bias Safe Operating Area

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FBSOA, IB>0

Reverse Bias Safe Operating Area

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RBSOA, IB ≤‎0

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Other ratings

VEBO – maximum allowable reverse bias

voltage across the B-E junction

IB(max) – maximum allowable average base

current at a given case temperature

IBM – maximum allowable peak base current

at a given case temperature and of specified

pulse duration.

Input characteristics (iB vs. VBE) at a given

case temperature

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Switching circuit

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Switching plots (simplified)

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characteristic times

td – delay time

tr – rise time

ts – storage time

tf – fall time

tn – ON time

tO – OFF time

k – duty cycle

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Commutation circuits

Base curent peaking during turn-on

Bipolar vs. Unipolar switching

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Power Darlington

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Measured IC-VCE characteristics

low voltage BVCBO ~ 150V high voltage BVCBO >1000V

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ADVANTAGES OF BJTs

BJT’s‎have‎high‎switching‎frequencies‎since‎

their turn-on and turn-off times are low.

The turn-on loses of a BJT are small

(low UCE)

BJT has controlled turn-on and turn-of

characteristics since base drive control is

possible.

BJT does not require commutation circuits.

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DEMERITS OF BJTs

Drive circuit of BJT is complex.

It has the problem of charge storage which

sets a limit on switching frequencies.

It cannot be used in parallel operation due to

problems of negative temperature coefficient

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Applications

Switched mode power supplies

DC to DC converters

Bridge inverters (DC to AC)

Power factor correctors

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THE END … and they lived hapily ever after

Thank you for your attention

Dziękuję‎za‎uwagę

Gracias por su atención

Спасибо‎за‎ваше‎внимание

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