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Polarity protector outperforms Schottky diodes.
The polarity protection circuit shown 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 MOSFGET devices because of their low on-resistance. For the transistors in this design, the combined on-resistance is 0.013 ohm. With a 10A load, the voltage drop is 0.13V at 25 deg C. Compare this figure with forward-voltage drops of several hundred millivolts for Schottky diodes under the same conditions. 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 ohm. Resistors R2 and R3 are necessary to turn off the transistors when U3 turns off. R1 provides a nominal 12V input.
EDN Design Idea, February 6, 2003.
Mike Hovenga, BAE Systems, Austin, TX