工作原理
An LLC resonant converter regulates its output by changing the switching frequency around the resonance of a series inductor Lr, series capacitor Cr and the transformer's magnetizing inductance Lm. Operating near resonance gives zero-voltage switching on the primary MOSFETs and low switching loss, which is why LLC is popular in server, telecom and EV charger power supplies.
This tool follows the first harmonic approximation (FHA) design flow described in Infineon's application note on LLC resonant converters. It starts from your input voltage range, output voltage and power, picks a transformer turns ratio, and then chooses the inductance ratio m = (Lr + Lm) / Lr and the maximum quality factor Qmax so the converter can still reach the required gain at minimum input voltage and full load.
The gain plot shows the normalized voltage gain versus normalized frequency for several load conditions. The peak of the Qmax curve sets the minimum switching frequency and the maximum gain available; designs should stay to the right of that peak to keep ZVS.
FHA is an approximation: it is most accurate near resonance and becomes less accurate far below it. Always confirm the final design with simulation and measurement.
公式
- Resonant frequency
- f_r = 1 / (2π · √(Lr · Cr))
- Inductance ratio
- m = (Lr + Lm) / Lr
- Quality factor
- Q = √(Lr / Cr) / R_ac
- Equivalent AC load (full-bridge rectifier)
- R_ac = (8 · n²) / π² · R_load
- Normalized frequency
- F_x = f_s / f_r
计算示例
For a 33 V nominal input (18–36 V), 400 V / 250 W output, the tool picks a turns ratio from the nominal gain point, then sizes m and Qmax so the gain at 18 V and full load stays below the peak of the Qmax curve. The resulting Lr, Cr and Lm can be read off directly and checked on the gain plot.
常见问题
What value of m should I choose?
Smaller m gives more gain range and a narrower frequency range but higher circulating current. Values between about 4 and 10 are common; the tool can optimize m automatically for your input range.
Why must I stay above the gain peak frequency?
To the right of the peak the resonant tank looks inductive, which is what enables zero-voltage switching. Left of the peak the tank becomes capacitive and ZVS is lost.
Is the FHA method accurate?
It is a good first design step and is accurate near resonance. Below resonance and at light load the real gain differs, so verify with a circuit simulator.
计算结果为工程估算值，请结合数据手册、仿真和实测验证设计。