The polarity-protection circuit in Figure 1 is a high-performance alternative to the usual series diode (often Schottky). The circuit incurs a much lower voltage drop than even the best Schottky diode. The circuit uses MOSFET devices because of their low on-resistance. For the transistors in this design, the combined on-resistance is 0.013 Ω. With a 10 A load, the voltage drop is 0.13 V at 25 °C. Compare this figure with forward-voltage drops of several hundred millivolts for Schottky diodes under the same conditions.
 |
||
| Figure 1. | This polarity-protection circuit incurs lower forward-voltage drop than the best Schottky diodes. |
|
You must use p- and n-channel transistors in series because of their intrinsic diodes. A photovoltaic isolator provides the appropriate gate drive to the MOSFETs. The performance is even better at lower currents. You can replace the two discrete transistors by a single-package, complementary-MOSFET pair, such as an IRF7389, which has a combined on-resistance of 0.108 Ω. Resistors R2 and R3 are necessary to turn off the transistors when IC1 turns off. R1 provides a nominal 12 V input.
