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Reverse Protection of Input Power with Low Voltage Drop 2014-04-03

Diodes are often used for protection from accidental connection of the power with reverse voltage. But they either suffer from voltage drop for a series diode, or requiring a current limit, such as a fuse, for a parallel diode connection. A parallel diode also applies a small reverse voltage equal to its forward drop under short-circuit current conditions when the reverse voltage is applied, which may or may not be a problem for the protected circuit.

For minimum forward drop and no reverse current you could use a power MOSFET. An LTspice simulation of such a circuit is shown here.

Reverse Protection.gif

Reverse Protection - MOSFET.asc

Since a MOSFET conducts equally well in both directions, the P-MOSFET is shown connected with the input going to the drain so that it can block the reverse voltage when the drain becomes negative. For normal operation, when the drain goes positive, the MOSFET substrate diode initially conducts current and the output voltage starts to rise. When the voltage is high enough that the gate-source voltage exceeds the MOSFET threshold, it starts to turn on. The value of R1 and R3 are selected to reduce the gate voltage so it won't exceed the maximum Vgs rating of the MOSFET. If the supply voltage is less then this value, then R3 can be eliminated.

When the voltage is reversed, the MOSFET substrate diode is reverse biased and the MOSFET stays biased off (Vgs = 0V) thus the MOSFET blocks the reverse voltage as desired.

The MOSFET shown has an ON resistance of 3.6mΩ, giving a voltage drop of about 55mV @ 16A as shown by Cursor 1 in the simulation. You can use a different P-MOSFET as long as the current and voltage rating are greater than the input voltage and maximum load current with adequate margin, and the ON resistance gives you the desired maximum voltage drop when conducting the load current. For voltages less than 10V the MOSFET must be a logic-level type (ON-resistance specified at a Vgs of 5V or less) so that it is fully turned on in normal operation.

Note that if you can connect the circuit in the ground (negative) lead of the circuit being protected then you can use an N-MOSFET which, due to the higher N material mobility, allows the use of a smaller chip and usually cheaper device for the same current and ON-resistance rating.
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