A method is provided for testing switch signals of an inverter of an electric machine of a drive system of a motor vehicle. The electric machine is controlled via a pulse-width modulation generated by a control unit using a target duty cycle and a triangular-waveform voltage sequence. An actual duty cycle of a current pulse-width modulation is continuously ascertained from the switch signals and compared with the target duty cycle of the control unit.

A fault detection circuit according to the present disclosure includes a rectangular pulse comparison circuit configured to generate, for each combination of the pulse width modulated signals, detection signals each indicating a difference component between pulse widths of two pulse width modulated signals having phases adjacent to each other, and a fault diagnosis unit configured to detect a fault in an inverter based on a shift in a combination of logic levels of a plurality of the detection signals output by the rectangular pulse comparison circuit from a preset determination value. The fault diagnosis unit uses the determination value that is different for each motor rotation angle assuming in advance that two alternating-current signals having phases adjacent to each other among the three-phase alternating-current signals have the same voltage.

A short-circuit protection and localization circuit for power devices includes a first subcircuit for detecting dv/dt of a power device at turn on, a second subcircuit for short-circuit fault localization and soft turn-off of the power device, the second subcircuit including a totem-pole driver having an upper switch and a lower switch, and a third subcircuit for detecting short-circuit faults based on the output (V.sub.dip) of the first subcircuit and the output of an upper switch (V.sub.p) of the second subcircuit. The first subcircuit outputs a voltage (V.sub.dip) having a magnitude that is proportional to dv/dt of the power device. The third subcircuit outputs a signal (V.sub.sto) to the second subcircuit that causes the second subcircuit to softly turn-off the power device. The second subcircuit outputs a voltage of the upper switch (V.sub.p) and a fault-latching signal for short-circuit localization.