LLC Resonant Converter Design: A Step-by-Step Engineering Guide

LLC Topology — What It Is and Why It Works

LLC stands for the three reactive elements in the resonant tank: Two Inductors (L) and one Capacitor (C).

The Two Resonant Frequencies

fr1 (series resonance): $f_{r1} = \frac{1}{2\pi\sqrt{L_r C_r}}$

fr2 (parallel resonance): $f_{r2} = f_{r1} \sqrt{\frac{L_r}{L_r + L_m}}$

The LLC converter achieves Zero Voltage Switching (ZVS) on primary switches across the entire load range, which is its most important advantage over phase-shifted full bridge topologies.

Step 1 — Define Your Design Specifications (3.3 kW Example)

Step 2 — Calculate the Transformer Turns Ratio

$n = \frac{V_{in(nom)}}{2 \times (V_{out} + V_{diode})} = \frac{400}{2 \times (58.8 + 0.6)} \approx 3.37$ Practical ratio selected: 3.4 : 1

Step 3 — Design the Resonant Tank

Using an inductance ratio $m = 4$ and quality factor $Q = 0.35$:

Characteristic Impedance (Zo): 4.24 Ω

Series Inductance (Lr): 4.5 μH

Resonant Capacitance (Cr): 235 nF

Magnetising Inductance (Lm): 18 μH

Step 4 — Transformer Design

At 154.5 kHz, an EE55/28/21 core (N87 ferrite) is selected.

Primary Turns: 22

Secondary Turns: 7

Wire: Litz wire (250-strand 0.1mm for primary, 500-strand for secondary)

Summary

The LLC design process involves iteration. Your first pass parameters rarely survive contact with real component values and gain verification. Use our Design Calculator to automate these calculations.