The power MOSFETs need to be chosen with a V_DS rating capable of withstanding the maximum HV-port voltage plus transient spikes (ringing). 100V rated MOSFETs is selected in this application.

When the voltage rating is determined, select the MOSFETs by making tradeoffs between the MOSFET R_ds(ON) and total gate charge Qg to balance the conduction and switching losses. For high power applications, parallel MOSFETs to share total power and reduce the dissipation on any individual MOSFET, hence relieving the thermal stress. The conduction losses in each MOSFET is determined by Equation 88.

where

• N is the number of MOSFETs in parallel
• 1.8 is the approximate temperature coefficient of the R_ds(ON) at 125 °C
• and the total RMS switch current I_{Q_RMS} is approximately determined by Equation 89

where

• D_max is the maximum duty cycle, either in the buck mode or boost mode.

The switching transient rise and fall times are approximately determined by:

And the switching losses of each of the paralleled MOSFETs are approximately determined by:

where

• C_oss is the output capacitance of the MOSFET.

The power MOSFET usually requires a gate-to-source resistor of 10kΩ to 100kΩ to mitigate the effects of a failed gate drive. When using parallel MOSFETs, a good practice is to use 1 to 2Ω gate resistor for each MOSFET, as shown in Figure 8-15.

If the dead time is not optimal, the body diode of the power synchronous rectifier MOSFET causes losses in reverse recovery. Assuming the reverse recovery charge of the power MOSFET is Q_rr, the reverse recovery losses are thus determined by Equation 93:

To reduce the reverse recovery losses, an optional Schottky diode is placed in parallel with the power MOSFETs. The diode needs to have the same voltage rating as the MOSFET, and it needs to be placed directly across the MOSFETs drain and source. The peak repetitive forward current rating needs to be greater than I_peak, and the continuous forward current rating needs to be greater than the following Equation 94: