Synchronous Rectification

Synchronous rectification (SR) is the technique of replacing a power converter's rectifying diode with an actively controlled MOSFET. Rectification — forcing current to flow one way — traditionally used diodes, which cost a fixed voltage drop while conducting: roughly 0.35–0.5V even for a good Schottky. A MOSFET switched on at exactly the right moments behaves like a resistor of a few milliohms instead, dropping perhaps 0.05–0.1V at the same current. At the low voltages and brutal currents that feed mining hardware, that difference is not a refinement — it is the dominant efficiency lever on the secondary side.

How it works

In a synchronous buck converter, the freewheeling diode is replaced by a low-side MOSFET driven in complement to the high-side switch: when one conducts, the other must not. The controller inserts a brief dead time between switching one device off and the other on, because any overlap creates shoot-through — both switches momentarily shorting the input rail, a destructive fault. In isolated topologies, SR MOSFETs on the transformer secondary are timed by a dedicated controller that senses when current wants to flow and gates the FETs accordingly, emulating an ideal diode. The engineering cost of SR is exactly this timing complexity: a diode needs no gate driver, no controller, and can never shoot through. The reward is that conduction loss scales with resistance rather than a fixed junction drop, and modern power MOSFETs make that resistance extraordinarily low.

Why low-voltage, high-current rails demand it

The arithmetic is stark. A diode's ~0.4V drop is negligible against a 400V rail but catastrophic against a 12V one: at 233A — the rated output of an APW12 — a diode rectifier would burn roughly 90W per conduction path as pure heat, an instant several-percent efficiency loss plus a serious cooling problem. SR MOSFETs cut that loss several-fold, which is a large part of how a modern mining PSU reaches its 93–95% efficiency rating while packing kilowatts into a shoebox. The lower the output voltage and the higher the current, the more SR pays — which is precisely the regime of ASIC power delivery, from the PSU's 12–15V output down to the sub-volt core rails.

Where you meet it in mining hardware

An APW12 is a textbook tour of the technique's habitat: dual PFC front ends boost the AC input to a high-voltage DC bus, an LLC resonant converter handles isolated step-down with soft switching, and synchronously rectified secondaries deliver the low-voltage, high-current output to the hashboards. On the hashboard itself, the multiphase buck converter stages that regulate the ASIC domain voltage are synchronous bucks by definition — at hundreds of amps and fractions of a volt, diode rectification is simply not a viable design.

On the repair bench

For a technician, SR changes the failure picture. Rectification faults are no longer a single dead diode but can involve the SR MOSFETs, their gate driver, or the timing controller — and a shorted SR FET typically fails dramatically, often taking fuses and neighbors with it. When diagnosing a dead PSU secondary, test the SR MOSFETs for gate-to-source and drain-to-source shorts the same way you would check any switching FET, and remember that a converter can also fail quietly into diode-mode conduction through the MOSFET body diodes: the rail still comes up, but efficiency drops and the secondary runs mysteriously hot. Hot but working is a symptom, not a pass.

The technique also explains a spec-sheet subtlety worth internalizing: PSU efficiency ratings are strongly load-dependent, and synchronous designs shape that curve. SR losses scale with the square of current through the FETs' on-resistance, so efficiency peaks in the middle of the load range and sags toward full load — one more reason running a power supply at its absolute limit costs more than the nameplate arithmetic suggests. When you undervolt a fleet for efficiency, part of the win comes from the ASICs and part from pulling every stage of the power chain, synchronous rectifiers included, back toward its sweet spot.