5 External MOSFET

Select an external MOSFET that achieves the best trade-off between power dissipation, system-level performance (such as reverse-current blocking) and solution cost. A MOSFET that typically provides 30 mV to 50 mV of forward drop at a full load current is a good starting point.

Another important parameter is the MOSFET’s maximum VDS voltage rating. During the ISO 7637-2 Pulse 1 transient event, the maximum VDS seen by the external MOSFET Q1 is the VDS_CLAMP (max) detection threshold of an ideal diode controller. Equation 1 calculates the peak current flowing through the MOSFET during the ISO 7637-2 Pulse 1 transient event:

where:

V_ISO is the negative peak of ISO 7637-2 Pulse 1 transient event,

V_OUT is the initial level of the VBATT before applying the ISO 7637-2 Pulse 1,

VDS_CLAMP is the maximum V_CLAMP threshold of the ideal diode controller and

R_S is the ISO 7637-2 pulse-generator input impedance (10 Ω).

Figure 5-1 shows the LM74701-Q1’s TVS-less performance during ISO 7637-2 Pulse 1 transient event, along with gate turn on behavior during VDS clamp operation, peak pulse current and power dissipation across the MOSFET.

The average current during an ISO 7637-2 pulse can be approximated as one-third the peak current, or (I_{ISO_PEAK}/3). So Equation 2 calculates the average power dissipated across the external MOSFET:

An ISO 7637-2 Pulse 1 transient event lasts for typically 2 ms, and the external MOSFET operates in active clamp mode for about 1 ms. Selecting a MOSFET with safe operating area (SoA) characteristics – with the load line corresponding to the VDS of VDS_CLAMP (max) and drain current (ID) greater than (I_{ISO_PEAK}/2) for 1 ms is a suitable option.

Figure 5-2 shows typical SoA characteristics and example selection criteria for the MOSFET.