Connect a battery backwards and you can fry an entire board in milliseconds. The classic fix is a series Schottky diode, but at any meaningful current it bleeds voltage and dissipates real heat. The modern fix is a P-channel MOSFET configured as an ideal diode. It conducts in the right direction with only a few milliohms of resistance and blocks reverse current cold.

The wiring is counterintuitive at first. You put the PMOS in the positive supply line with its source toward the load and drain toward the battery — backwards from how you'd instinctively draw it. The gate ties to ground (battery negative) through a resistor, typically 10 kΩ to 100 kΩ, with a Zener clamp (e.g., 12 V) from gate to source to protect Vgs.

Why this works: When the battery is connected correctly, the body diode conducts first, pulling the source up near V+. The gate sits at 0 V, so Vgs = −V+, which turns the PMOS fully on. Current then flows through the low-Rds(on) channel, bypassing the body diode. When the battery is reversed, V+ goes negative, Vgs goes positive, and the FET stays off. The body diode is also reverse-biased — nothing flows.

Concrete example: A 12 V, 3 A robotics board. A series Schottky (SS34, Vf ≈ 0.4 V) drops 0.4 × 3 = 1.2 W and loses you nearly a volt. A SUP75P03-07 PMOS with Rds(on) = 7 mΩ at Vgs = −10 V dissipates I²R = 9 × 0.007 = 63 mW. Twenty times more efficient and the load sees the full battery voltage.

Rule of thumb for FET selection:

• Vds rating ≥ 2× your supply voltage (reverse connection puts full battery across the FET)
• Vgs(th) well below your minimum operating voltage — a "logic level" PMOS turns on by ~2.5 V, but you want Vgs ≥ 4 V for low Rds(on)
• Continuous Id ≥ 1.5× your max load current, with thermal headroom
• Add the gate Zener if your supply could ever exceed ±20 V — Vgs is the most fragile parameter on a MOSFET

Gotcha: If your load has bulk capacitance, the body diode will charge it before the FET turns on. That's usually fine, but during a brief reverse-then-forward connection bounce, the cap can momentarily reverse-bias the FET while charged. For mission-critical designs, add a TVS across the input.