It's vital that MOSFET power-switch drivers be protected from a reverse-battery connection. A small rectifier diode can shield against reversed batteries. But that approach is generally unacceptable for systems whose battery life is crucial. For a 6-V battery, the diode's forward voltage drop (0.6 to 0.7 V typical) produces a constant power loss of approximately 10%. This loss increases with dwindling battery voltage.
The approach shown in the figure is an improvement over single-diode protection. A p-channel MOSFET (Q1) protects the driver (U1) from reverse-battery insertion. Q1's low on-resistance of 10 mÙ or less yields a forward-voltage drop of only a few millivolts (versus hundreds of millivolts for a diode). Thus, substituting a MOSFET for the diode immediately improves efficiency.
Connecting a positive battery voltage to the drain of Q1 produces forward bias in its body diode, which clamps the source voltage one diode drop below the drain voltage. When the source voltage exceeds Q1's threshold voltage, Q1 turns on. Once the FET is on, the battery is fully connected to the system and can deliver power to U1 and the load.
An incorrectly inserted battery reverse-biases Q1's body diode. The source remains at ground potential, and the gate is connected to ground. So Q1 stays off, disconnecting the reversed battery from the system.
VDS equals the full negative battery voltage when the battery is inserted backwards. So in some applications, Q1 may have to withstand transient levels many times the battery voltage. Therefore, Q1 must be able to tolerate VGS and VDS voltages at least equal to the maximum battery voltage.
Aside from reverse-battery protection, the circuit also guards against reverse load current at startup. Q3 ensures that no load current flows until U1 enables both n-channel MOSFETs (Q2 and Q3). Otherwise, reverse load current could cause Q1 to turn on inadvertently, leaving the circuit unprotected against reversed-battery connections.