One option to use IGBTs in applications with a higher switching frequency despite hard
switching is tousea switching load reduction network or snubber.
Snubbers function to reduce the load of the active switch during the switching process.
Both, networks to ease the turn-on or the turn-off process or a combination of both can
be used. To examine the behaviour of these networks, a half-bridge topology as i
The turn-on timewith hard switching andwithout load reduction network is shown i
. The stray inductance in the commutation path in this example (including the stray
inductance of D
) is 90nH. The commutation of the current from D
settle accordingly and at its peak the resulting switching losses are approximately
depicts the same systemwith a turn-on snubber. At the time of turn-on of S
i.e. at the beginning of the commutation phase, L
takes over almost the entire
voltage and the collector-emitter voltage U
rapidly falls to a low value. Also
limits the current gradient during commutation. The result is reduced switching
losses for S
. However, additional losses appear in theRLD-network of the snubber, so
overall the efficiencywill not increase.
Because the commutation time with a snubber circuit is longer than without, attention
must be paid to a longer dead time or interlock delay time between top and bottom
switch in a half-bridge topology. There will now be a minimal off time of the switch S
as, with the time constant
, the inductance L
will take a certain time to
dissipate the energy built up during commutation.
xemplifies the load-reduced turn-off using an RC-snubber. While
illustrates this reference without load reduction network,
shows the same
topologywith anRC-snubber parallel to the IGBT turning off.
During turn-off with a snubber the capacitor C
takes over the IGBT current after a
very short time. At this time the freewheelingdiodeD
Due to the snubber capacitor connected in parallel, the voltage across the IGBT rises
only slowly. As soon as the voltage across the IGBT gets to the voltage level of theDC-
bus the current of C
commutates to D
and thus the commutation process is
The energy stored in the capacitor will be dissipated within the resistor R
next turn-on of the IGBT. The result is aminimal IGBT turn-on time, which is determined
by the time constant
of the RC network. Only when the capacitor
has discharged completely will the load reduction of the following turn-off be efficient.
Note however that the energy stored in the capacitor will flow via R
current through the IGBTduring the turn-on process and increase its load accordingly.