A No-Voltage-Drop Bridge Rectifier

FET Bridge -300In the past, we've looked at reverse-polarity protection methods which can protect you from the diseasters of a D'oh moment.

Power Protection Circuits

Power Protection Circuits - A Closer Look at the Series Diode

Power Protection Circuits - A Closer Look at the Crowbar Circuit

Power Protection Circuits - A Closer Look at the P-Channel MOSFET Circuit


I've started incorporating the P-channel MOSFET circuit on boards I design, particularly if they will be connected by others.  A tiny SOT-23-packed MOSFET hardly takes any room on the circuit board and for systems using 5-volt regulated power, no additional components are required.

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What if you just want your circuit to work with whatever polarity power supply is connected?  The usual method is to add a bridge rectifier between the power connector and the circuit.

bridge rectifier 1

 As shown below, a power supply of either polarity may be connected on the left, with the result being the correct polarity on the right.  Almost like magic.

bridge rectifier 2

 Well, not all that magic really.  Remember in the article on using a series diode for reverse-polarity protection — the voltage drop across a diode can be as much as 0.7 volts; with the bridge rectifier circuit, we have two diode voltage drops, so our 5 volt input could be as low as 3.6 volts!  Not good if your circuit needs 5 volts.  A bridge rectifier made up of Schotky diodes can improve the situation, but there will still be a drop of around 0.6 volts.

The bridge rectifier works ok where you're supplying a higher voltage to a regulator on your board, but fails if you want to use a regulated supply to power your board.

I came across a technique in Circuit Celler to use a bridge arrangement of MOSFETs to provide the functionality of a bridge rectifier without the large voltage drop.  This works much the same way as using a P-Channel MOSFET for reverse polarity protection.  MOSFETs are available with fractional-Ohm on resistence, so the voltage drop will be very small.

FET Bridge

For a detailed explanation of how this circuit works, please see Theta Engineering's article Rectifier Bridge Has No 2Vf drop!

FET Bridge - practical-300The article does discuss an important point if this circuit is to be used at higher voltages.  The allowable gate-source voltage on most MOSFETs is about 20 volts.  At greater voltages, zener diodes must be added to the circuit to protect the MOSFETs.

The above diagram looks a bit complicated with a lot of connections to be made.  MOSFETs are available in dual packages with a P-channel and N-channel in the same package.  Using a part like Fairchild's FD8958A, in an 8-pin SOIC package, results in a clean, simple layout.

At 4.5 volts, the combined on resistance of the P-channel and N-channel FETs in the FD8958A is 0.12Ω.  If the circuit being powered draws an amp, the voltage drop will only be 0.12 volts which all but the most crictical circuits can handle.

This circuit takes up a bit more space than the P-channel MOSFET that provides reverse-polarity protection, but if you want a no-hassle power supply connection, this is a great technique to keep in mind.

Fairchild FD8958A datasheet

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