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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
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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