The LLC resonant converter is the gold standard for medium-to-high-power isolated supplies where efficiency above 95% matters — think server PSUs, OLED TV power stages, EV onboard chargers, and high-end audio amps. It earns that crown by exploiting zero-voltage switching (ZVS) on the primary MOSFETs and zero-current switching (ZCS) on the secondary rectifiers, making the switching losses that plague hard-switched flybacks and forwards nearly vanish.

The topology looks like a half-bridge driving a resonant tank made of three elements: a series inductor Lr, a series capacitor Cr, and the transformer's magnetizing inductance Lm in parallel with the reflected load. That's where the "LLC" name comes from — two L's and a C. The half-bridge generates a square wave at variable frequency; the tank filters it into a quasi-sinusoid that excites the transformer.

The magic is in the gain curve. The tank has two resonant frequencies:

• fr1 = 1 / (2π·√(Lr·Cr)) — the series resonance, with the load attached
• fr2 = 1 / (2π·√((Lr+Lm)·Cr)) — the lower resonance when the load is light

Operating right at fr1 gives unity voltage gain, ZVS, and ZCS simultaneously — peak efficiency. Sweeping the switching frequency above fr1 drops the output voltage (buck-like region); sweeping below fr1 toward fr2 boosts it. So regulation is done by frequency modulation, not duty cycle. Duty stays at a fixed 50% with a small deadtime for ZVS handoff.

Design rule of thumb: Pick the inductor ratio Ln = Lm / Lr between 4 and 10. Lower Ln widens the gain range (good for wide input swings) but increases circulating current and hurts efficiency at nominal line. A typical 400 V to 12 V server PSU uses Ln ≈ 6 with fr1 around 100–150 kHz.

Concrete example: A 600 W telecom rectifier converts 390 V DC bus to 48 V at 12.5 A. Designer picks Lr = 60 µH, Cr = 28 nF, giving fr1 ≈ 123 kHz. With Lm = 360 µH (Ln = 6) and an 8:1 transformer, the converter runs at 120 kHz under nominal load with measured efficiency of 96.5% — a hard-switched forward in the same slot would top out near 91%.

The catches: light-load regulation is tricky (frequency runs away upward), and the gain curve becomes capacitive above a critical frequency, killing ZVS and frying MOSFETs. Dedicated controllers like the NCP1399 or UCC25640x handle the burst-mode tricks needed for low-load efficiency and add overcurrent protection that respects the resonant nature of the tank.