Batteries are the most convenient power source, providing voltage to electronic circuits. There are many other ways to power electronic devices such as adapters, solar cells, etc., but the most common DC power source is batteries. Usually, all devices come with anti-reverse protection circuits, but if you have any battery-powered device that does not have reverse polarity protection, you must always be careful when replacing the battery, otherwise it may blow up the device.
Therefore, in this case, the anti-reverse protection circuit will be a useful complement to the circuit. There are some simple ways to protect a circuit from reverse polarity connections, such as using a diode or diode bridge, or using a P-channel MOSFET as a switch on the HIGH side.
Reverse polarity protection using diodes
Using a diode is the easiest and cheapest way to reverse polarity protection, but it has a leakage problem. When the input supply voltage is high, a small voltage drop may not matter, especially when the current is low. But in the case of a low-voltage operating system, even a small amount of pressure drop is unacceptable.
It is well known that the voltage drop on a general-purpose diode is 0.7V, so we can limit this voltage drop by using a Schottky diode because it has a voltage drop of about 0.3V to 0.4V and can also withstand high current loads. Be careful when choosing a Schottky diode, because many Schottky diodes have high reverse current leakage, so make sure to choose a diode with a low reverse current (less than 100uA).
Thunderbolt Electronics has specially developed ultra-low Vf Schottky diodes and ultra-low leakage Schottky diodes, suitable for anti-reverse use.
At 4 amps, the power loss of the Schottky diode in the circuit is:
4 x 0.4V = 1.6W
In ordinary diodes:
4 x 0.7 V = 2.8W
Therefore, The energy-saving effect of Schottky in the circuit is obvious, if the circuit current is large, you can also choose a SCHOTTK diode in a DO-277 package, such as the Thunderbolt Electronics SS10U60.
Rectifier bridge stack anti-reverse connection protection
We can also use a full-bridge rectifier for reverse polarity protection as it has nothing to do with polarity. But the bridge rectifier consists of four diodes, so in the above circuit of a single diode, the amount of power waste will be twice as much as the power waste.
Reverse-polarity protection using P-channel MOSFETs
Reverse polarity protection using P-channel MOSFETs is more reliable than other methods because of its low voltage drop and high current capability. The circuit consists of a P-channel MOSFET, a Zener diode, and a pull-down resistor. If the supply voltage is lower than the gate-to-source voltage (Vgs) of the P-channel MOSFET, only a MOSFET without diodes or resistors is required. You only need to connect the gate terminal of the MOSFET to the ground.
Now, if the supply voltage is greater than Vgs, the voltage between the gate terminal and the source must be reduced. The components required to manufacture the circuit hardware are mentioned below.
§ P Channel FET models are selected according to current and voltage
Resistors (100k)
§ 9.1V Zener diode
circuit diagram
The working principle of the reverse polarity protection circuit using the P-channel MOSFET
Now, when you connect the battery according to the circuit diagram, with the correct polarity, it causes the transistor to turn on and allow current to flow through it. If the battery is connected backwards or with reverse polarity, the transistor is turned off and our circuitry is protected.
This protection circuit is more effective than other protection circuits. Let's analyze that when the battery is connected in the correct way, the P-channel MOSFET will turn on because the voltage between the gate and the source is negative. The formula for finding the voltage between the gate and the source is:
Vgs = (Vg - Vs)
When the battery is not connected correctly, the voltage of the gate terminal will be positive, and we know that the P-channel MOSFET is only turned on when the voltage of the gate terminal is negative (the minimum -2.0V or less of this MOSFET). Therefore, whenever the battery is connected in the opposite direction, the circuit will be protected by the MOSFET.
Now, let's talk about the power loss in the circuit, the resistance between the drain and the source is almost negligible when the transistor is on, but for more accuracy, you can browse the datasheet of the P-channel MOSFET. For the LMAK30P06 P-channel MOSFET, the static drain source on-resistance (RDS(ON)) is 0.020Ω (typ). Thus, we can calculate the power loss in the circuit as follows:
Power Loss = I2R
Suppose the current flowing through the transistor is 1A. So the power loss will be
Power Loss = I2R = (1A)2*0.02Ω = 0.02W
As a result, the power loss is about 27 times smaller than in circuits using single diodes. This is why using P-channel MOSFETs for reverse polarization protection is much better than other methods. It's a little more expensive than diodes, but it makes the protection circuit safer and more efficient.
We also used Zener diodes and resistors in the circuit to prevent exceeding the gate-to-source voltage. By adding resistors and a 9.1V Zener diode, we can clamp the gate source voltage to a maximum of minus 9.1V, so the transistor remains safe.
Of course, the anti-reverse circuit of MOS can also use Nmos to truncate the circuit, the truncation is the negative circuit, our general concept is to switch the positive electrode, just like the home light switch, is installed on the fire line, not the zero line.