Reverse Polarity Protection Circuit Design

A product that can be connected to its supply the wrong way round needs protection, and the choice of method affects the efficiency, the cost and the behaviour during the fault. The wrong method can protect the circuit and still destroy itself.

What Has to Be Protected

Applying the supply in reverse puts the full voltage across the input capacitor, the regulator and every semiconductor in the path in the direction they were not designed to block. If any of them conducts, the current is limited only by the supply.

Even when the semiconductors survive, the electrolytic capacitors do not, and a reverse charged electrolytic fails quickly and sometimes violently. Protection therefore has to act before any of these components sees the wrong voltage.

The protection also has to survive the event itself. A device that fails short during the fault protects the rest of the circuit and leaves the product unrepairable, which is acceptable in some applications and not in others.

The Series Diode

A diode in series with the supply blocks the reverse voltage completely and costs almost nothing. Its disadvantage is the forward drop, which is a few tenths of a volt for a Schottky and around seven tenths for a silicon part.

At a low current the drop is negligible, and at a high current it becomes a significant loss and a source of heat. A product drawing two amps through a Schottky diode loses more than a watt, which is often unacceptable.

The heat is the practical limit. The diode has to be rated for the forward current and provided with a thermal path, and the resulting voltage drop reduces the headroom available to the regulator.

The P channel MOSFET Method

A P channel MOSFET in the high side path conducts through its body diode when the polarity is correct, and the channel then bypasses the diode when the gate is biased. The voltage drop is the product of the on resistance and the current, which can be a few millivolts.

In the reverse connection the body diode is reverse biased and the transistor does not conduct, so the circuit is protected. The gate has to be biased correctly in both cases, which is done with a resistor and sometimes a Zener.

The P channel device has a higher on resistance than an N channel of the same size, and the choice of a low resistance part makes the voltage drop small. The cost is higher than a diode, and the drop is far lower.

The N channel Alternative

An N channel MOSFET in the low side path is cheaper for a given on resistance, and it works with the same principle. Its disadvantage is that the ground of the protected circuit is no longer the same as the supply ground.

Where the circuit is isolated from the chassis that is acceptable, and where it is not the low side switch creates a ground offset that affects every signal referenced to the supply.

A common arrangement places the N channel device in the return path with the gate referenced to the positive rail through a resistor and a Zener. The circuit is simple and the on resistance can be very low.

The Bridge Arrangement

A diode bridge at the input makes the polarity irrelevant, because whichever way the supply is connected the current flows through the correct pair of diodes. It costs two diode drops in the forward path.

The bridge is used where the polarity genuinely cannot be controlled, such as a field wiring terminal. The loss of two drops is the price of the convenience, and the heat has to be managed.

A bridge built from MOSFETs instead of diodes reduces the drop to a few millivolts, at the cost of a control circuit that has to decide which pair to turn on. The technique is used in high current applications.

Interaction with inrush and the Fuse

A fuse in series with the input protects against a fault but it does not protect against reverse polarity, because it takes far longer to operate than the time the semiconductors need.

inrush and reverse polarity are separate problems and are often confused. A reverse connection is a steady state fault, while inrush is a short event on every switch on, and the two call for different components.

Where the input is protected by a resettable device, the reverse connection holds it in the tripped state and the product simply does not work until the connection is corrected. That behaviour is acceptable and easier to diagnose than a failed diode.

Layout and Thermal Considerations

The protection device is in the main current path and it dissipates heat. Its thermal path to the board has to be as carefully designed as that of any power component, and the copper area under it matters.

Keep the input capacitor after the protection device, not before it. A capacitor placed ahead of the protection sees the reverse voltage directly and is the component that fails first.

The gate network of a MOSFET arrangement is a high impedance node with a large voltage appearing across it during a reverse connection. The resistor values have to limit the gate voltage within the rating of the device.

Choosing Between the Methods

The choice follows from the current and the supply voltage. Below a few hundred milliamps a Schottky diode is cheap and simple, and the drop it introduces is acceptable in most designs.

Above an amp the MOSFET arrangement becomes attractive, because both the drop and the heat fall sharply. The extra cost is a few cents on the bill of materials and it is usually recovered in the efficiency of the whole product.

Where the polarity can genuinely be either way, the bridge is the only arrangement that works without user intervention. Its two forward drops are the price of that convenience, and the heat has to be dealt with in the thermal design of the input stage.

Verification and Faults

Verify the protection by applying the supply in reverse with a current limited source, at the maximum voltage the product may see. The current should be small, and no component should become hot.

Repeat the test with the supply applied correctly and measure the voltage drop across the protection device at the full load current. That drop is a design parameter and belongs in the efficiency budget.

After the reverse test, apply the supply correctly and confirm that the product works. A protection circuit that saves the product but does not survive is a compromise that should be stated in the design description. The release checks that keep such a circuit consistent are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the thermal measures in our guide to PCB thermal management design.

FAQ

Is a series diode enough? It blocks the reverse voltage completely. The forward drop is the price, and at a high current the heat is the practical limit.

Why use a P channel MOSFET instead of a diode? The voltage drop falls from hundreds of millivolts to a few, which matters at a high current or a low supply.

Should the input capacitor be before or after the protection? After. A capacitor ahead of the protection sees the reverse voltage and fails.