For this design, the first thing to consider in both the transistor and the diode is the breakdown and blocking voltage. Since this is for an 800V BMS, the blocking voltage must be higher. Note that EV batteries are often charged to a higher voltage than the system voltage; an 800V EV can hold as much as 1000V on a full charge. Select a breakdown that gives additional margin above the full charge voltage of the EV. For this design, the breakdown for the FET and the diode must be 1200V.

The next consideration for these components, particularly the transistor, is the power, heat, and time. Similar to the resistor, a continuous characteristic such as the rated continuous drain current of a FET is not useful for two reasons: a characteristic like this is determined under an exceptional thermal scenario with no temporal consideration and the precharge cycle is a pulse, not continuous. Use the Safe Operating Area chart on the FET data sheet to determine if a FET is good. In an RC circuit like precharge, the power dissipation in the resistor is nearly one-third of the initial magnitude after one time constant. Relating that to this design, the pulse length worth considering for component sizing is 100ms. FETs sized for 1200V automotive systems often show a 100ms curve on the Safe Operating Area chart and a FET is likely to be large enough if the peak expected current through the design is beneath both the 100ms curve and the on-resistance limit curve.

The final component that requires additional scrutiny is the diode. As mentioned, a reverse blocking voltage of 1200V is needed and the diode must withstand the maximum forward current of the design. The forward current capabilities are confirmed through the continuous forward current rating. For this design, this current rating does not have to be larger than the peak current of the circuit, since the diodes non-repetitive peak forward surge current rating is likely orders of magnitude larger than this peak current. To be safe, the diode chosen for this design was chosen with a continuous forward current that is no more 60%–80% of the peak forward current. This maintains that the diode can withstand a soft short that is too low to trip the overcurrent protection. Additionally, if a design with a lower R_DS(on) and lower voltage drop is needed, a second blocking FET can be used, however this is higher cost.