LLC Resonant Converter

The LLC resonant converter is the isolated DC-DC stage found in most modern high-efficiency ASIC and server power supplies. It takes the regulated high-voltage rail produced by the PFC stage and steps it down through a transformer to the low-voltage, high-current output that feeds the hashboards. Its name comes from the resonant tank: two inductances (the resonant inductor Lr and the transformer magnetizing inductance Lm) plus a resonant capacitor Cr — L, L, C. That tank is the trick that lets multi-kilowatt supplies run efficient and cool enough to live inside a miner's chassis.

How resonance buys efficiency

Instead of hard-switching the primary MOSFETs against full bus voltage — which burns energy in every transition — the LLC tank shapes the current so that the switches turn on at zero voltage (ZVS) and the secondary rectifiers turn off at zero current (ZCS). This soft-switching behavior slashes switching loss, which is what lets the converter run at high frequency while staying cool; high frequency in turn shrinks the transformer and filter magnetics, buying the power density mining supplies need. Output regulation works differently from a PWM converter: instead of varying duty cycle, the controller shifts the drive frequency relative to the tank's resonant point, changing the tank's gain to hold the output steady across load and line variation.

Inside the APW12

In Bitmain's APW12, the LLC stage sits between the 410-420V DC bus (charged by two parallel PFC stages) and the outputs: the main switch MOSFETs — Q14, Q15, Q31, and Q32 on the schematic — are driven by a PWM controller IC (U22) and feed the transformer that produces OUT1, the adjustable 12-15V rail rated up to 233A for the hashboards, alongside a fixed 12V auxiliary output for the control board and fans. That voltage adjustability is not a side detail: firmware commands the output setpoint over I2C, and the LLC stage is what physically delivers each requested voltage at hundreds of amps. On the bench, the classic APW12 failure signature is a healthy PFC bus (410-420V present on the bulk caps) with a dead or unstable main output — pointing at the LLC primary switches, their driver, or the controller, with shorted primary MOSFETs among the most common single faults in supplies that died hard. The usual high-voltage cautions apply: bulk capacitors hold lethal charge long after power-off.

Why it dominates high-power supplies

LLC converters deliver excellent efficiency, low electromagnetic interference (soft edges radiate less), high power density, and a modest component count — exactly the combination that wins in the multi-kilowatt supplies mining and AI compute hardware demand. The trade-off is design and control complexity: frequency-based regulation, tank component tolerances, and behavior outside the designed load range are all less forgiving than a simple PWM buck, which is why LLC supplies are engineered as tightly integrated systems rather than casually modified. On the low-voltage side, the design pairs naturally with synchronous rectification — MOSFETs in place of diodes — because at 200+ amps, even a diode's forward drop would burn off hundreds of watts that the resonant tank worked so hard to save.

Efficiency where it compounds

It is worth pausing on why converter efficiency deserves this much engineering. A miner's PSU processes every watt the machine will ever turn into hashrate, around the clock, for years. The difference between a mediocre and an excellent conversion chain — a few percentage points — is pure loss, paid in heat that the cooling system must then remove at further cost. At a 3.6kW continuous draw, each percentage point is roughly 36W of waste, over 300kWh per year, per machine. Multiply across a fleet and the case for resonant conversion writes itself. The same logic guides repair decisions: a supply rebuilt with correct parts preserves its designed efficiency, while crude substitutions in the tank or rectification stages can leave a unit that "works" yet quietly taxes every kilowatt-hour it touches for the rest of its service life.