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Transistors in Parallel
Why connect transistors in parallel?
• Connect in parallel to
handle high currents
• Need to be closely
matched for equal
current sharing.
• Connect resistors in
series with the
emitters to get 45 to
55% current sharing.
Dynamic Current Sharing
Consider an increase in IE1
• If IE1 increases,
– L1(dIE1/dt) increases
• Voltage L2(dIE1/dt)
increases, increasing
current IE2 (shifting, or
sharing the increase)
Turn-on and Turn-off times
During turn-on,
di I I
= =
dt t
t
L
CS
r
r
Turn-on and Turn-off times
During turn-off,
dv V V
=
=
dt
t
t
S
f
CS
f
Transistor Chopper
di
limits
dt
dv
limits dt
Approximate Switching Waveforms
Equivalent Circuit at turn-on
di V
=
dt L
di I
=
dt t
Vt
L =
I
s
s
L
r
s
s
L
r
Equivalent Circuit at turn-off
At turn-off
• Q1 becomes open
• IL flows through CS, charging it towards VS
• The capacitor voltage appears across Q
dv I
V
=
=
dt C
t
It
C =
V
L
S
S
L
f
s
S
f
When C charges to VS, Dm turns on
Looks like an RLC circuit
L
R =2
=R
C
S
S
S
T
 =RC =
3
S
S
S
S
1
3R C = T =
f
1
R =
3f C
S
S
S
s
S
s
S
critical
Discharge time = TS/3
Insulated-Gate Bipolar Transistor IGBT
Collector -- C
Gate -- G
Q1
IRG4BC10U
Emitter -- E
Output Characteristics
Transfer Characteristic
Structure
When VGE>VT – Channel Induced
Induced channel
Equivalent Circuit
Driver Circuit