Assume that the power lost during a transition is, on average, half of voltage × current. For a power device switching 400 V and 10 A with turn-on and turn-off transition times of 50 ns each and a switching frequency of 100 kHz, which of the following is closest to the switching loss?

Answer: B. 20 W

The energy lost in one transition is (1/2) × V × I × t, so turn-on and turn-off together lose (1/2) × 400 × 10 × (50 + 50) ns = 200 µJ per period. Multiplying by the number of periods per second (100 kHz) gives a switching loss of 200 µJ × 100,000 = 20 W. Switching loss is proportional to frequency, so at 200 kHz it rises to 40 W. SiC and GaN devices, which switch faster, and soft switching are ways to reduce this loss.

• A. This counts only one of the two transitions. Each period has both a turn-on and a turn-off.
• B. Correct. (1/2) × 400 × 10 × 100 ns × 100 kHz = 20 W.
• C. This uses V × I instead of V × I/2 for the power during the transition.