P-Channel Protection MOSFET (P-Channel Protection FET)
It is the method that provides reverse polarity protection with far lower loss than a series diode. The MOSFET is connected in series with the positive line and its gate is pulled to the chassis side; it conducts with correct polarity and turns off with a reversed connection. The loss is reduced from the diode drop to the channel resistance.
The loss difference is clear: a MOSFET with 5 milliohm channel resistance produces roughly 4,5 W at 30 A, while a series diode gives off heat in the order of 18 W at the same current. The difference removes both the cooling requirement and the voltage drop.
The critical point of the design is the gate-source voltage. In most MOSFETs this value is typically limited to ±20 V. In a 12 V system, connecting the gate directly to chassis causes no problem; but in a 24 V system this means exceeding the limit and puncturing the MOSFET.
- A zener diode is placed between gate and source and the voltage is clipped at a safe level.
- The gate current is limited by adding a resistor in series with the zener.
- For the transients seen on automotive lines there must also be TVS protection on the input side.
This structure can also be used as an on-off switch; however, since the MOSFET's body diode remains conductive in one direction, two MOSFETs are connected back to back if full isolation is required.
Context where this term is used: DC motor and driver fault diagnosis matrix