ELEC-E8421
Components of Power Electronics
Power Transistors
Power Transistors
• Different types of power transistors
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Bipolar transistors, single or darlington connected
Metal Oxide Semiconductor Field Effect Transistors (MOSFET)
Insulated Gate Bipolar Transistors (IGBT)
Junction Field Effect Transistors (JFET), single or in cascode connection
Heterostructure Field Effect Transistors (HEMT)
Bipolar npn power transistor
• Structure of NPN power transistor. Note the wide n− region and interdigitated
structure of emitter and base.
Quasi-saturation phenomena
Hard saturation
region
Quasi-saturation
Linear (active)
region
region
• In linear region base is not widened.
• In quasi-saturation the holes (and electrons)
start to spread to n− region and thus
effectively widen the base
• In hard saturation the holes and electrons fill
the whole n− region
Quasi-saturation phenomena and transistor current
gain
Linear region
Quasi-saturation region
IC
hFE =
IB
@ U CE = constant
Current gain in saturation:
IC
βF =
IB
Quasi-saturation and Dynamic saturation time tds
• The transistor turn on slows down when it
enters in the quasi-saturation as it takes time to
fill the weakly doped collector region with holes
and electrons
Forward Bias Safe Operating Area - FBSOA
U BE > 0
• FBSOA defines how long
certain current and voltage is
tolerated
• Limited by bonding wires,
losses and second breakdown
(current concentration and hot
spots on the emitter edges)
Reverse Bias Safe Operating Area - RBSOA
• RBSOA defines how high
current can be switched off as
a function of maximum voltage
during turn-off.
• UBE is zero or negative
Current concentrates to the
middle of emitter
IC(PK)
t
UCE(PK)
t
Baker-clamp circuit
Baker-clamp
• Baker clamp prevents
the transistor from
entering into hard
saturation and thus
speeds up turn off as
there are less electrons
and holes in the weakly
doped collector region
Darlington transistor
C
• A structure consisting of two
(or more) bipolar transistors
• Gain is increased (from 10
to 100, for example) as the
driver transistor feeds base
current to the output
transistor. Thus higher
currents can be controlled
with small base current
• Output transistor cannot
saturate as the driver
transistor prevents it
B
E
Basic idea of
Darlington transistor
Darlington transistor in practice
• D1 is needed to speed up turn off
• R1 and R2 are needed to provide path for
leakage currents
• D2 is parasitic or can be separate
component (better recovery)
Equivalent circuit of Darlington transistor
Datasheet parameters of bipolar transistor
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Collector-Emitter voltage rating
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Collector current rating – NOTE: At 25 °C case temperature!
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hFE
Saturation voltage
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IB
Current gain
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IC
Base current rating
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UCEO (base open)
UCEX, UCEV (base shorted to emitter or has negative voltage)
Uce(SAT)
Delay time, rise time, storage time and fall time
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td, tr, ts, tf
Transient thermal impedance
• In addition to the transient thermal impedance (single pulse or DC), curves
are often given for different duty cycles D.
Power loss derating curves
Storage time depends much on the base voltage