How does reverse polarity protection work, diode or MOSFET?
There are two common methods. A series Schottky diode is simple but leaves a drop of typically 0.3-0.4 V across it, and that drop turns into heat along with the current. MOSFET-based protection using a P-channel device makes a drop of about 50 mV at the same current; for example 50 mV with 20 mΩ RDS(on) at 2.5 A. At low current the diode is practical; at high current the MOSFET is clearly superior.
The purpose is the same: to stop current entering the device when the supply terminals are connected the wrong way round. The methods differ in cost, loss and heat.
Series Schottky diode. Current passes only in the correct direction. The price is the voltage left across it while conducting: typically 0.3-0.4 V. That drop both reduces the supply voltage and, multiplied by the current, produces heat. At 2.5 A it means a loss of around 1 W, which requires cooling. Besides, 0.4 V at 12 V is close to the whole of a 3% voltage drop budget.
P-channel MOSFET. With the correct polarity the channel conducts and only RDS(on) x I of voltage remains across it. On a 20 mΩ MOSFET at 2.5 A this is about 50 mV, that is, nearly a tenth of the diode's. The heat loss falls in the same proportion. In return, a few components are needed, such as a gate drive circuit and a zener limiting the gate-source voltage.
Selection rule:
- In control circuits of a few hundred milliamperes the diode is sufficient and the simplest solution.
- For continuous loads of the order of amperes the MOSFET is preferred; it preserves the voltage budget and the heat.
- In either case this protection does not replace the fuse and overvoltage protection; they are used together.
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