In a power supply system, a high-side (HS) insulated-gate bipolar transistor (IGBT) has a first collector, a first gate, and a first emitter. A low-side (LS) IGBT has a second collector coupled to the first emitter, a second gate, and a second emitter. A gate drive circuit is coupled to the first gate of the HS IGBT and the second gate of the LS IGBT. A control circuit is coupled to the gate drive circuit. The control circuit is configured to control the gate drive circuit for biasing the HS IGBT to a HS saturation, and determine a HS degradation of the HS IGBT based on a HS digitized gate voltage of the HS IGBT in the HS saturation.