Once the positive and negative terminals of a power supply are reversed, it can lead to the burning out of many electronic components, mainly due to excessive current causing breakdown of the devices. Therefore, measures must be taken in practical applications to prevent reverse polarity.

Here are four common reverse polarity protection circuit designs, all packed with valuable insights!

1. Diode Series Connection

Utilize the characteristic of forward conduction and reverse cutoff of a diode by connecting a forward diode in series at the input end of the positive power supply:

When the diode is conducting, the circuit board operates. When the power supply is reversed, the diode cuts off, preventing the formation of a circuit loop, and the circuit board no longer operates, effectively preventing reverse polarity issues.

However, it is important to note that during normal operation, the voltage drop of about 0.7V across the diode should be considered, which may cause the circuit to malfunction. Therefore, circuits with strict requirements on the supply voltage are not recommended for use.

2. Bridge Rectifier Configuration

The principle of operation of this circuit is not complex. Regardless of the polarity of the input to the full bridge rectifier, the output remains fixed. Therefore, whether it is connected correctly or in reverse, the circuit can still operate.

However, it is worth noting that the full bridge rectifier will generate a voltage drop of about 1.5V.

3. Fuse + Voltage Regulator Diode

This circuit design can prevent reverse connection as well as overvoltage. Its operating principle is as follows:

When the power supply Vin is reversed, diode D1 conducts forwardly, and the negative pressure of the load is the forward conduction voltage Vf of the diode, which is generally low and will not burn out the subsequent load circuit.

At the same time, when Vin is reversed, D1 conducts forwardly, and the voltage mainly falls on F1. At the beginning, the circuit rises rapidly until it exceeds the rated current of F1. Fuse F1 blows, and the power supply is cut off (at this time, D1 will not be burned out due to excessive current).

4. Low-Resistance MOSFETs (NMOS & PMOS)

   
   

(NMOS Reverse Polarity Protection)

(PMOS Reverse Polarity Protection)

When the power supply is normally connected, the MOSFET conducts; when the power supply is reversed, the MOSFET cuts off, achieving reverse polarity protection.

Both NMOS and PMOS can achieve reverse polarity protection functions, as discussed in previous VBsemi articles (those interested can refer to them). However, the connection methods of the two are different.

The circuit operates normally when powered on, and when the power supply is reversed, the circuit does not operate, achieving power supply reverse polarity protection.

In practical applications, it is better to add a resistor between the PMOS gate and source. Compared to NMOS, which requires Vgs greater than the threshold voltage to conduct, PMOS has an advantage because its turn-on voltage can be 0, and the voltage difference between DS is not large, making it more advantageous than NMOS.

Due to the advancement in MOSFET technology, its conduction resistance is small, often in milliohms or even smaller, resulting in very little voltage drop and power loss in the circuit, which can even be negligible.

Therefore, choosing MOSFETs for circuit protection is quite common.