How to design magnetic components for LLC resonant converters—resonant tank, transformer design, and core material selection
Introduction
The LLC resonant converter is the dominant topology for high-efficiency isolated DC/DC power conversion. Youll find it in EV on-board chargers, server power supplies, solar microinverters, and industrial DC/DC modules. Its performance depends entirely on its magnetic components—the resonant tank and the main transformer.
Shaanxi SiRui Technology Co., Ltd. supplies magnetic cores and custom magnetic components for LLC resonant converter applications worldwide. For inquiries, contact us at sales@sirui-tech.com.
1. The Resonant Tank
An LLC converter has a resonant tank consisting of Lr (resonant inductor), Lm (magnetizing inductance, integrated into the transformer), and Cr (resonant capacitor). ZVS operation is achieved when the switching frequency is above series resonance.
2. Two Integration Approaches
- Discrete Lr: Separate resonant inductor + transformer. Easy to optimize, mature design.
- Integrated Magnetics: Leakage inductance = Lr. One component, higher density. Modern trend.
3. Resonant Inductor (Lr) Design
Powder cores (Sendust, High Flux, MPP) dominate resonant inductors. Their distributed air gap handles DC bias from magnetizing current, with no fringing loss and stable inductance over temperature. Ferrite is cheaper but has gap fringing loss and noise.
4. LLC Transformer Design
- Magnetizing inductance (Lm): Part of the resonant tank, not a parasitic! Typical Lm/Lr ratio: 3–7.
- Core material: Mn-Zn ferrite (PQ/ER/ETD) most common. Nanocrystalline for premium high-density designs.
- Winding: Litz wire essential at >100 kHz. Interleaving controls leakage and capacitance.
5. Common Applications
- EV OBC: 3.6–22 kW, 100–200 kHz. Ferrite PQ mid-range, nanocrystalline premium.
- Server Bus Converters: 1–6 kW, 200–500 kHz. Low-profile ferrite shapes.
- Telecom Rectifiers: 1–3 kW, proven reliable designs.
6. Common Design Mistakes
- Getting Lm/Lr ratio wrong → poor regulation or high circulating loss
- Underestimating resonant RMS current → overheating
- Ignoring core loss at operating frequency
- Poor winding technique → uncontrolled leakage inductance
7. Custom Components from SiRui-Tech
At Shaanxi SiRui Technology Co., Ltd., we supply Mn-Zn ferrite cores (PQ, ER, ETD shapes) for LLC transformers, powder cores (Sendust, High Flux, MPP) for resonant inductors, and nanocrystalline toroids for high-density premium designs.
Visit our website: www.sirui-tech.com | Email: sales@sirui-tech.com
Conclusion
The LLC resonant converter is the gold standard for high-efficiency isolated DC/DC. As EV OBCs and server power push toward higher density, integrated magnetics and nanocrystalline cores are increasingly replacing discrete ferrite designs.