This enhancement has been incorporated in Release 14 Service Pack 3 (R14SP3). For previous product releases, read below for any possible workarounds:
It is not necessary to use RC snubbers with two-level and three-level bridges using forced commutation devices MOSFET/diodes or IGBTS/diode pairs. A purely resistive snubber with very large resistance is sufficient. You can specify, for example, Rs=1e6 ohms Cs=inf.
The two-level bridge will function correctly if you use dead time (pulses sent to upper and lower switches not complementary). See the following pwm2levelDT model where 4 us dead time is implemented on phase A:
As soon as Q1 switch turns off, the complementary diode D2 turns on. Notice that a purely resistive snubber (Rs=1e6 ohms) is used. This is equivalent to using no snubber.
However, the use of dead-time was not foreseen for SimPowerSystems 3.0 (R13SP1) with the 3-level bridge. If the 4 switches from top to bottom in one arm are numbered Q1 Q2 Q3 Q4, it means that pulses sent Q1 and Q3 must be complementary, and that pulses sent to Q2 and Q4 must be also complementary. If these pulses are not complementary (use of dead-time), you will observe voltage glitches resulting from inductive current chopping. This is because diodes fail to turn on when the current is chopped.
See the following pwm3levelDT model where a dead time can be specified on phase A pulse pattern. If DT is nonzero, voltage glitches are observed:
