8.2.2.5.2 MOSFET Selection

The LM5141-Q1 gate drivers are powered by the internal 5-V VCC bias regulator. To reduce power dissipation in the controller and improve efficiency, the VCCX pin should be connected to the 5-V output or an external 5 V bias supply. The MOSFETs used with the LM5141-Q1 require a logic-level gate threshold with R_DS(ON) specified with V_GS = 4.5 V or lower.

The MOSFETs must be chosen with a V_DS rating to withstand the maximum V_IN voltage plus supply voltage transients and spikes (ringing). For automotive applications, the maximum V_IN occurs during a load dump and the voltage can surge up to 42 V under some conditions. A MOSFET with a V_DS rating of 60 V would meet most application requirements. The N-channel MOSFETs must be capable of delivering the load current plus peak ripple current during switching.

The high-side MOSFET losses are associated with the R_DS(ON) of the MOSFET and the switching losses.

Equation 45.

Equation 46.

where

• tr = ts = 17 ns

The losses in the low side MOSFET include: R_DS(ON) losses, dead time losses, and losses in the MOSFETs internal body diode. The body diode conducts the inductor current during the dead time before the rising edge of the switch node; minority carriers are injected into and stored in the diode PN junction when forward biased. As the high side FET starts to turn-on, a negative current must first flow through the diode to remove the stored charge before the diode can block a reverse voltage. During this time, the high side drain-source voltage remains at V_IN until all the diode minority carriers are removed. Then, the diode begins to block negative voltage and the reverse current continues to flow to charge the body diode depletion capacitance. The total charge involved in this period is called reverse-recovery charge Qrr.

Equation 47.

Equation 48.

where

• t_dr and t_df are the switch node voltage rise and fall times (20 ns)
• V_D(FET) is the forward voltage drop across the low-side MOSFET internal body diode (0.8 V)
• D_Qrr is the internal body diodes reverse recovery charge (105 nC)
• R_DS(ON) is the on resistance of the MOSFETs (26 mΩ at T_J = 125ºC)

Table 4 provides parameters for several MOSFETs that have tested in the LM5141-Q1 evaluation module.