Compared to diodes, the circuit design using MOSFETs for reverse battery protection is not commonly seen. However, the commonly used diode reverse protection also has some drawbacks: high voltage drop. This can cause many issues in low-voltage current scenarios, which MOSFETs, on the contrary, solve.

Why is Reverse Protection Needed?

In DC systems, when the battery is connected in reverse, circuits powered by the battery can be damaged.

Today, we'll mainly explain in two parts: NMOS and PMOS.

Let's look at the following circuit: NMOS

When the circuit is activated, current flows from the positive terminal of the battery to the device, then through the body diode, and finally to the negative terminal of the battery.

At this point, the body diode conducts when forward biased.

When the body diode is open, existing current circulates in the circuit.

At this point, the gate-to-source voltage is:

Vgs = Vg - Vs

Where: Vg = Battery voltage Vs = Diode voltage drop

This causes a positive voltage to be applied to the gate of the MOSFET relative to the source. Therefore, the NMOS starts conducting, and the current flows through the channel rather than the diode.

Let's look at the following circuit: PMOS

When the battery voltage exists, current flows through the body diode, and the body diode conducts; this is because a positive voltage is applied to the anode side, but the battery voltage must be higher than the forward bias voltage of the diode.

When the diode is forward biased, the voltage level at the source of the MOSFET is equal to the battery voltage minus the voltage drop across the body diode.

The gate of the MOSFET is connected to the negative terminal of the battery or ground, meaning the voltage applied to the gate and source is:

Vgs = Vg - VS

Where: Vg = 0V VS = Vbattery - Vdrop (this is a positive value)

When the voltage applied to the gate relative to the source is negative, the PMOS will be activated.

However, it needs to meet the requirements of each MOSFET gate-to-source voltage, typically rated voltage limited to +/-20V.

When the MOSFET is activated, the channel will close, and the current will flow through the MOSFET rather than the body diode.

In reverse voltage circuits, in both NMOS and PMOS circuits, the circuit current is almost zero during battery reverse. This effectively prevents MOSFET current flow, thereby protecting the circuit or device connected to the battery.

Using MOSFETs as reverse battery protection, the connection method is slightly more complex compared to diodes, but it can achieve lower voltage drop and higher performance, making it very suitable for low-voltage applications.