The Parallel and Series Connection of Power
Semiconductor Devices
When the required current is higher than maximum available from a
single device, devices have to be connected in parallel.
When the required working voltage is higher than the breakdown voltage of
any single device, it is necessary to connect devices in series.
Devices in Parallel
When devices are connected in parallel,
the on-state voltage drop is the same on
all and current is shared according to their
static or dynamic current/voltage
characteristics.
A simple way to ensure equal current
sharing is to
b) device on-state characteristics have
a) connect a resistance in series with
each device. If the resistance is several to be closely matched. .
times higher than the device differential
resistance, equal current sharing is
achieved
Besides current equalisation under static conditions devices
have to be balanced during switching operations.
In the case of devices connected in parallel, it is important
for the turn-on delay times to be nearly the same.
As the device with the shorter turn-off
delay time starts to turn off, there is an
increase of current in the second device .
Turn-off losses increase considerably and
there is a possibility of second breakdown
Devices in Series
It order to obtain a reasonably equal sharing of the voltage among
individual devices under both static and dynamic conditions, a good
match of leakage current and switching characteristics is necessary.
We deal first with steady-state voltage
sharing by considering two devices
connected in series.
The leakage current through both
must be the same and voltage sharing
is determined by the static off-state
characteristic
To improve on voltage sharing, resistors can be put in
parallel with devices
A minimum number n in series connected devices
When VRM is the maximum working
reverse (or blocking) voltage across n in
series connected devices of maximum
repeatable voltage VRRM
VRRM = (I RRM + I ( R ) )(R + ∆R )
VRM = VRRM + ( n − 1) I ( R ) (R − ∆R )
n ≥ 1+
VRM − VRRM
VRRM
∆R
R
∆R
1−
R
1+
During the turn-on process, voltage sharing problems are associated
with differences in the turn-on delay time. The device with the longest
delay time turns on last. It is likely to be subjected to an overvoltage for
a short time
-More significant are differences in
the storage time during the turn-off
process. The device which turnsoff first is subject to a voltage
overload and can be destroyed by
the resulting electrical and thermal
breakdown
( n − 1)(Qrr max − Qrr min )
C≥
nVRWM − VWM
Usually, RC or RDC circuits are used
The use of auxiliary circuits results is a
considerable increase in the volume of
equipment and the additional number of
devices increases the probability of a
failure. It is thus important to minimise
the number of devices connected in
series or parallel.
Over-voltage and Over-current Protection of Power Semiconductor Devices
An input overvoltage arises via the input transformer during turn on and turn
off transients. When the current in the primary circuit is changed quickly,
the magnetic flux in the transformer changes and a voltage appears at the
secondary winding which can cause an overvoltage.
The use of RC-protection
If the voltage on the capacitance at the time of breaking the circuit
is V0, it follows from the conservation of energy that the maximum
voltage, Vk, is given by
1 2 1
WL = LI k = C(Vk2 − V02 )
2
2
Vk =
L 2
I k + V02
C
In circuits where internal overvoltage can
occur, RC or RDC snubber circuits are often
used to decrease the rate of rise of blocking
voltage during turn-off
The size of the snubber capacitance and
resistance depend on both the device and
the circuit.
For operating voltage
V0 ≈ 0.5VCEOsus
tp
C
5I C t f
I Ct0
C≥
=
8V0
VCEOsus
≥R≥
I CM
VCW
− I L − I rrM
The average power losses in the snubber
PZ =
1
2
CVCW
f
2
Varistors
I = AV β
Avalanche surge diode
Break Over Diodes (BOD)
Over-current Protection
Passive protective devices like fuses are not easily effective active
protection is normally required.
“I2t”-rating
The on-state current is monitored. At the moment it exceeds an
admissible level, the device is turned off by the driving circuit.
A crowbar circuit