7.1.4 Turn-on and Turn-off Energy

The turn-on and turn-off energy, shown in Figure 8, represent the energy absorbed by the low-side device during the turn-on and turn-off transients of the circuit in Figure 11, respectively. As this circuit represents a synchronous buck converter, with input shorted to output, the switching energy is dissipated in the low-side device. The turn-on transition is lossy, while the turn-off transition is essentially lossless; the output capacitance of the devices is charged by the inductor current. The turn-on and turn-off losses have been calculated from experimental waveforms by integrating the product of the drain current with the drain-source voltage over the turn-on and turn-off times, respectively. The skew of probes for voltage and current are very important for accurate measurement of turn-on and turn-off energy.

The switching loss of the converter can be determined by adding the turn-on and turn-off energy in Figure 8, adjusting for the R_DRV value (shown in Figure 9). To obtain the switching loss, multiply this value by the switching frequency. The obtained loss is a sum of the V-I overlap loss (due to hard switching) and the loss caused by charging and discharging the C_OSS of both devices. Additional test-fixture capacitance, including PCB and inductor intra-winding capacitance, has not been removed from these measurements.