mosfet reverse battery protection

How to Use MOSFET as Reverse Battery Protection

You may not know how to use MOSFET as reverse battery protection. The most common method is using a diode. However, diode voltage drop is high and this will create issue in low voltage circuits. That is the reason many use MOSFET as reverse battery protection due to its very low on state voltage drop.

Why need reverse battery protection? In DC systems, when the battery is reveresed the circuit that uses the battery as a source will get damaged. That is why the need to install reverse battery protection arise.

P-Channel MOSFET as Reverse Battery Protection

P-Channel MOSFET Reverse Battery Protection Basic Connection

There are two variants of enhancement type MOSFET. It could be N-channel or P-channel. Both of them can be used as reverse battery protection. Let us start with the P-channel. Below illustration tells how to use a P-channel MOSFET as a reverse battery protection. The MOSFET must be installed in the positive rail of the battery. The drain must be connected to the positive polarity of the battery. The source must be connected to the positive of the device that being powered. The gate must be connected to the battery negative terminal or to the system ground.

How P-Channel MOSFET Works as Reverse Battery Protection

When the battery voltage is present, the current will flow to the body diode. The body diode will conduct because the anode side is applied by a positive voltage. However, the battery voltage must be higher than the forward bias voltage of the diode. When the diode is forward biased, the level of the voltage in the source of the MOSFET will be battery voltage minus the body diode voltage drop. In short, it is a positive level. The gate of the MOSFET is tied to the battery negative terminal or to the ground, this means the voltage applied to gate to source is

VGS = VG – VS

VG = 0V (because it is tied to the ground)

VS = Vbattery – Vdrop (this is a positive value)

So,

VGS = VG – VS = 0V – Positive Voltage = a negative voltage

A P-Channel MOSFET or simply PMOS will activate once the voltage applied to gate to source is negative. However, it needs to satisfy the gate to source voltage requirement per MOSFET datasheet. When the MOSFET activates, the channel will close and current will flow to it rather than to the body diode.

P-Channel MOSFET as Reverse Battery Protection Basic Requirements

1. Basic Circuit Connection

Follow the suggested connection above.

2. Gate to Source Threshold Voltage

Not enough to have a negative voltage applied on gate to source as explained above, the level requirement must be met. Below is an example gate to source threshold voltage from a MOSFET datasheet. In order to turn on the MOSFET, the difference between the battery voltage and the body diode must the higher than -1V.

For low voltage system, it is better to select a MOSFET with a very low gate to source threshold voltage like above table shows.

3. Maximum Gate to Source Voltage

The maximum gate to source voltage of the MOSFET must not be exceeded. Otherwise, it will be damaged. Below is an example maximum gate to source voltage rating of a MOSFET.

4. Current Rating

The drain current rating of the PMOS must be higher than the actual current that will flow into it. Otherwise, it will be cooked. Below is an example current rating specified in the datasheet.

5. Power Rating

Power rating is essential as this is the capability of the MOSFET to handle heat. The computed power dissipation must be lower than the rating of the device. Below specification table specifies a power dissipation rating of 8.3W at 25’C. Therefore, the actual computed power dissipation must be lower than this with ample of margin.

6. Operating Temperature Range

You need also to mind the ambient temperature where the MOSFET will be installed to avoid failure.

N-Channel MOSFET as Reverse Battery Protection

Below is an N-Channel MOSFET installed in a circuit to function as reverse battery protection. The NMOS is installed in the negative rail of the battery. The drain must be connected to the negative terminal of the battery. The source must be connected to the device negative rail or ground. The gate must be connected to the positive terminal of the battery.

How N-Channel MOSFET Works as Reverse Battery Protection

During start-up of the circuit, the current will start to flow from the Battery positive terminal, going to the device, going to the body diode and finally to the negative terminal of the Battery. During this time, it is the body diode that is conducting as it is being forward biased.

When the body diode has turned on, there will be now current circulating the circuit. Then, the gate to source voltage will be:
VGS = VG – VS

VG = Vbattery

VS = diode drop

So,

VGS = VG – VS = Vbattery – diode drop

This will result to a positive level applied to the gate to source of the MOSFET. Thus, the NMOS will start to conduct and the current will flow to the channel instead to the body diode.

N-Channel MOSFET as Reverse Battery Protection Basic Requirements

1. Basic Circuit Connection

Follow the suggested connection above.

2. Gate to Source Threshold Voltage

To set MOSFET as reverse battery protection, it is not enough to bias the gate to source with a positive voltage. The required level must be satisfied as well.

For low voltage system, it is better to select a MOSFET with a very low gate to source threshold voltage like above table shows.

3. Maximum Gate to Source Voltage

Example datasheet of an NMOS stating the maximum gate to source voltage rating.

4. Current Rating

Below is an example current rating specified in the datasheet of an NMOS.

5. Power Rating

Below table specifies a power dissipation rating of 8.3W at 25’C. Therefore, the actual computed power dissipation must be lower than this with ample of margin. Otherwise, the device will burn.

6. Operating Temperature Range

Example operating temperature rating.

Circuit Simulation on How MOSFET Works as a Reverse Battery Protection

During Normal Voltage Application

Below are simple simulations for both PMOS and NMOS MOSFET as reverse battery protection.

When the circuit just started, it is evident that the body diode is conducting first before the channel did.

During Reverse Voltage Application

In both circuits, during reverse battery, the circuit current is zero. This means that the NMOS and PMOS is not allowing current to flow thus protecting the circuit or the device that connects to the battery.

MOSFET as Reverse Battery Protection Versus Diode

1. Connection

MOSFET – bit complex

Diode – easy

2. Voltage Rating

MOSFET – gate to source voltage is limited to mostly +/-20V

Diode – high voltage rating

3. Voltage Drop

MOSFET – very low, suitable for very low voltage application

Diode – high, may not suitable to very low voltage systems

4. Start-up Voltage Drop

MOSFET – diode drop

Diode – same voltage drop during running ideally

5. Price

Both are comparable

6. Current Rating

Both are comparable

7. Power Dissipation

MOSFET – higher capability due to very small voltage drop

Diode – compromised by higher voltage drop

8. Space

MOSFET – small package can handle larger power dissipation due to a lower voltage drop

Diode – can be bulky if need to handle big power dissipation as the voltage drop is higher

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