Circuit Design to Prevent Reverse Power Connection

Many projects of hardware engineers are completed on pegboards, but there is a phenomenon of accidentally reversing the positive and negative poles of the power supply, and it can result in many electronic components being burned out, and even the entire board being scrapped, and at the same time, another piece needs to be soldered. Is there any good solution to solve it?

First of all, carelessness is inevitable. Although it only needs to distinguish between positive and negative wires, one red and one black, and we will not make mistakes if we just connect them for one time or even ten times, but how about 1000 times? What about 10000 times? At this point, it's hard to say. Due to our carelessness, some electronic components and chips were burned out. The main reason is that excessive current can cause component breakdown, so measures must be taken to prevent reverse connections.

There are generally several methods to solve this problem.

First, diode series anti reverse connection protection circuit.

Connect a forward diode in series at the input end of the positive power supply, fully utilizing the characteristics of forward conduction and reverse cutoff of the diode. Under normal circumstances, the diode conducts and the circuit board operates.

When the power supply is reversed, the diode cuts off, the power supply cannot form a circuit, and the circuit board does not work, which can effectively prevent the problem of power supply reversal. But there will be diode voltage drop, and it is also important to choose the corresponding diode based on the power supply current.

Second, the rectifier bridge type anti reverse connection protection circuit.

Using a rectifier bridge to convert the power input into a non-polar input, regardless of whether the power supply is connected in the positive or negative direction, the circuit board still works normally.

The above uses diodes for anti reverse processing. If silicon diodes are used, there is a voltage drop of about 0.6 to 0.8V, and germanium diodes also have a voltage drop of about 0.2 to 0.4V. If the voltage drop is too large, the MOSFET can be used for anti reverse processing. The voltage drop of MOSFET is very small, it is only a few milliohms, and the voltage drop is almost negligible.

Third, the MOSFET protection circuit for preventing reverse connections.

Due to factors such as improved technology and inherent properties, MOSFET have relatively small internal resistance, many of which are only in the milliohm range or even smaller. This results in minimal losses in circuit voltage drop and power consumption, and can even be ignored. Therefore, choosing MOSFET for circuit protection is a recommended approach.

a. NMOS protection

As shown in the figure below: At the moment of power on, the parasitic diode of the MOSFET conducts, forming a circuit in the system. The potential of the source S is about 0.6V, while the potential of the gate G is Vbat. The opening voltage of the MOSFET is Ugs=Vbat - Vs, and the potential of gate is high. The ds of the NMOS conducts, and the parasitic diode is short circuited. The system is connected to the circuit through the ds of the NMOS.

If the power supply is reversed, the conduction voltage of NMOS is 0, NMOS is cut off, the parasitic diodes are reversed, and the circuit is disconnected, thus the protection formed.

b. PMOS protection

As shown in the figure below: At the moment of power on, the parasitic diode of the    MOSFET conducts, forming a circuit in the system. The potential of the source electrode S is approximately (Vbat-0.6)V, while the potential of the gate electrode G is 0. The opening voltage of the MOSFET is Ugs=0- (Vbat-0.6), and the gate electrode shows a low potential, the ds of the PMOS conducts, and the parasitic diode is short circuited. The system is connected to the circuit through the ds of the PMOS.

If the power supply is reversed, the conduction voltage of PMOS is greater than 0, PMOS is cut off, parasitic diodes are reversed, and the circuit is disconnected, thus the protection formed.

Note: The NMOS connects ds to the negative electrode, while the PMOS transistor connects ds to the positive electrode. The parasitic diode direction is oriented towards the correctly connected current direction.

The connection of the D and S poles of MOSFET: When N-channel MOSFET are usually used, the current generally enters from the D pole and flows out from the S pole, while PMOS enters from the S pole and exits from the D pole. When applied in this circuit, the opposite is true. The voltage condition for MOSFET conduction is met through the conduction of parasitic diodes. MOSFET will conduct completely as long as a suitable voltage is established between the G and S poles. After conduction, it feels like a switch is closed between D and S, and the resistance from D to S or from S to D is the same.

In practical applications, the G-pole is usually connected in series with a resistor. To prevent MOSFET breakdown, a voltage regulator diode can also be added. The capacitor connected in parallel to the voltage divider plays a soft start role. At the moment when the current begins to flow, the capacitor charges and the voltage at the G-pole gradually builds up.

For PMOS, compared to NOMS, conducting requires Vgs to be greater than the threshold voltage. As its turn-on voltage can be 0, the voltage difference between DS is not significant, making it more advantageous than NMOS.

Fourth, fuse protection.

Many common electronic products can be seen with fuses added to the power supply after disassembly. When the power supply is reversed, there is a short circuit in the circuit, and the high current will blow the fuse to protect the circuit. However, the disadvantage of this method is that it is more difficult to repair and replace, unless you choose to install a fuse with self recovery function.