When you need redundant power inputs — dual supplies, battery + wall-wart, or two rails in a server backplane — you need a way to combine them so the higher-voltage source wins and neither source back-feeds the other. The old-school answer was OR-ing diodes: two Schottky diodes with anodes tied to each supply and cathodes tied together. Simple, but at 10 A a Schottky drops ~0.4 V, dissipating 4 W of heat per diode. That's a heatsink you don't want.

An ideal diode OR-ing controller replaces each diode with a low-Rds(on) MOSFET plus a controller IC that turns it on when its source rail is highest and off when reverse current threatens. The MOSFET's body diode handles the transient during turn-on/turn-off, so behavior is truly diode-like — but with millivolts of drop instead of hundreds.

How the Controller Works

• Forward conduction: Controller senses V_ds across the FET. When V_ds is positive (current flowing source→drain through the body diode), it drives the gate high, enhancing the channel and collapsing V_ds to I×Rds(on).
• Reverse detection: When V_ds goes negative by even ~10 mV (current trying to flow backward), the controller yanks the gate down in hundreds of nanoseconds to block reverse current before it becomes destructive.
• Gate drive: Uses an internal charge pump so an N-channel MOSFET can be used in the high-side position (better Rds(on)/cost than P-channel).

Real-World Example: Dual 12 V Server PSU

Two 12 V PSUs feed a common rail through LTC4359 controllers driving BSC0902NS FETs (Rds(on) ≈ 0.9 mΩ). At 20 A load:

• Schottky diode: V_drop ≈ 0.5 V → P = 20 × 0.5 = 10 W dissipated
• Ideal diode: V_drop = 20 × 0.0009 = 18 mV → P = 20 × 0.018 = 0.36 W dissipated

That's a 28× efficiency improvement in the OR-ing path alone, and no heatsink required.

Design Rule of Thumb

Size your MOSFET so I_load × Rds(on) < 30 mV. This keeps forward drop well below any Schottky and — crucially — keeps you above the controller's reverse-current detection threshold (typically 5–10 mV) so nuisance shutoffs don't happen from load transients or ripple.

Watch Out For

• Oscillation near crossover: When two supplies are nearly equal, the controller can chatter. Look for parts with built-in hysteresis (e.g., LTC4353, LM74700).
• Fault propagation: A shorted MOSFET (D-S short) defeats the entire scheme — one PSU can back-feed the other. Some controllers add a fault flag output to alert your MCU.
• Inrush at hot-plug: Pair with an inrush limiter or hot-swap controller for boards inserted live.