Hence, inserting this diode will increase the size of the
circuit and also the stray elements. Therefore, it was
decided that using this diode will not improve the circuit
behaviour. As a large proportion of IGBT failures occur
after the DUT has commenced turn off, (RBSOA failure)
it was decided that this type of failure should be
undertaken in the simulation case study. The simulated
turn off failure following a standard double pulse test is
shown in Fig. 11. The figure shows the DUT output
voltage (VDUT) and its current (IDUT). After the DUT
was turned off at t=100µs, a failure is simulated at time
110µs (point ta), Fig. 12 shows the failure region in more
detail. As the DUT fails VDUT will drop and IDUT will
increase triggering the NDT action in the circuit. There is
a delay time both in the controller and the IGBT switches
so IDUT will continue to increase with a di/dt limited by
the stray inductance in the circuit.