How Does an LLC Resonant Converter Work?

Detailed Explanation

Every hard-switched topology on this site so far, buck, boost, and flyback, switches a square wave directly across an inductor or transformer and controls output voltage by varying the fraction of each switching cycle the primary switch spends on (duty cycle). That approach works well, but it has a fundamental cost: turning a switch on while voltage is still present across it forces the switch node's parasitic capacitance to discharge through the switch, dissipating energy and stressing the switch on every single switching transition. That loss scales directly with switching frequency, which is exactly why hard-switched designs hit a practical frequency ceiling.

An LLC resonant converter avoids that cost by changing how the primary side operates entirely. A half-bridge (two switches, alternating) drives a resonant tank, a series inductor and capacitor plus the transformer's own magnetising inductance in parallel, that shapes the primary current into something close to a sine wave rather than a square wave. Because of the resulting phase relationship between the tank's current and the half-bridge's switching voltage, each switch's complementary body diode (or synchronous rectifier) conducts briefly just before that switch turns on, which pulls the switch node to that switch's own rail ahead of time. The switch then turns on across close to zero volts: zero-voltage switching (ZVS), with none of the hard-switched turn-on loss the other topologies incur.

The Resonant Tank and Gain Curve

The "LLC" name comes directly from the tank's three reactive elements: a series inductor (commonly the transformer's own leakage inductance, rather than a discrete component), a series resonant capacitor, and the transformer's magnetising inductance, which appears in parallel with the reflected secondary-side load. Together these form a resonant network whose voltage gain (output relative to the half-bridge's switching voltage) varies with switching frequency in a characteristic curve: gain peaks near the tank's series resonant frequency and falls off on either side of it.

Because gain depends on frequency rather than duty cycle, an LLC converter regulates its output by adjusting switching frequency, covered in full in the FAQ above. Design work centers on choosing the tank's component values so the required gain range (across the full input voltage and load range the converter must support) falls within a frequency band that keeps the converter in its zero-voltage-switching region, since gain, efficiency, and ZVS operation are all coupled through the same tank design rather than being independently adjustable.

Practical Examples

A PC or server power supply's main DC-DC stage is one of the most common LLC applications: converting the PFC boost stage's regulated ~380-400V DC bus down to a lower isolated output (commonly 12V) at high efficiency and high power density. See what power factor correction is for the upstream stage that typically feeds an LLC converter's input in an offline supply, and how a flyback converter works for the simpler, lower-power isolated topology LLC increasingly displaces as power level rises.

An onboard EV charger or a high-power LED driver benefits from LLC's combination of high efficiency (less heat to dissipate in a space-constrained enclosure) and higher achievable switching frequency (smaller magnetics), both of which matter more as output power climbs into the hundreds of watts and above.

Design Considerations

• Size the resonant tank around the converter's actual required gain range, not just its nominal operating point. The tank must maintain zero-voltage switching and adequate gain across the full expected input voltage and load range, not only at the single design-center condition; a tank sized only for nominal conditions can lose ZVS or fail to regulate at light load or low line voltage.
• Use a controller IC designed specifically for LLC, rather than adapting a generic PWM controller. LLC's frequency-based control and the need to manage dead time relative to the resonant tank's behaviour are different enough from duty-cycle-based control that dedicated LLC controller ICs (widely available from major analog vendors) are the standard approach rather than a general-purpose PWM controller repurposed for frequency control.
• Account for light-load ZVS loss deliberately, since the zero-voltage-switching condition depends on sufficient tank current, which drops at light load. Many LLC designs add burst-mode operation at light load specifically to maintain efficiency and avoid losing ZVS rather than running continuously at an ever-increasing frequency as load drops.
• Evaluate GaN devices for the half-bridge switches when pushing frequency further for density. See GaN vs silicon MOSFET for the switch-technology trade-off this converter's ZVS operation pairs well with, since ZVS already removes much of the switching loss a faster device would otherwise still incur at turn-on.
• Zeus Design designs resonant and hard-switched power conversion stages, including LLC tank design and controller selection, as part of complete product electronics development.

Common Mistakes

• Choosing LLC for a design where a flyback converter's simpler control loop would be sufficient. As covered in the FAQ above, LLC's efficiency and density advantage comes with real design complexity; a lower-power design that doesn't need that advantage is usually better served by the simpler, cheaper-to-develop flyback topology.
• Designing the resonant tank only for the nominal input voltage and load, rather than confirming zero-voltage switching and adequate gain hold across the converter's full specified operating range.
• Reusing a generic voltage-mode or current-mode PWM control loop design intuition on an LLC converter. Frequency-based gain control behaves fundamentally differently from duty-cycle-based control, and applying the wrong mental model when tuning the control loop produces a poorly compensated or unstable design.
• Neglecting light-load behaviour, assuming the converter's full-load ZVS performance holds unchanged as load drops, when in practice most LLC designs need deliberate light-load handling (commonly burst mode) to maintain efficiency and avoid losing zero-voltage switching.