The LLC resonant converter has become the top choice for high-efficiency, high-density power supplies—server power, telecom rectifiers, EV onboard chargers, and industrial power. Its zero-voltage switching (ZVS) capability enables very high efficiency (96–98%) at high switching frequencies.
This guide covers LLC resonant transformer design fundamentals.
Shaanxi SiRui Technology Co., Ltd. designs and manufactures LLC resonant transformers for high-efficiency power applications. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. How LLC Works
An LLC resonant converter uses three resonant elements: L_r (resonant inductor, often integrated as leakage), L_m (magnetizing inductance), and C_r (resonant capacitor). It achieves ZVS on primary switches and ZCS on secondary rectifiers.
2. Key LLC Transformer Parameters
- Magnetizing inductance (L_m): higher L_m = lower circulating current = higher efficiency, but narrower ZVS range. Typical L_m/L_r = 5–10.
- Leakage inductance (L_r): often intentionally designed into the transformer (integrated magnetics) to eliminate a separate resonant inductor.
- Turns ratio: N_pri/N_sec = (V_in/2) / (V_out + V_diode)
3. Core Material Selection
| Frequency | Core Material |
|---|---|
| 50–100 kHz | PC40, 3C95 |
| 100–300 kHz | PC95, 3C96 |
| 300–500 kHz | PC95, nanocrystalline |
| >500 kHz | Nanocrystalline, Ni-Zn |
4. Integrated Magnetics
Most LLC designs integrate the resonant inductor into the transformer by controlling leakage inductance through winding separation and interleaving. This eliminates a separate component, saving cost and size.
5. LLC vs Flyback Transformer
| Parameter | Flyback | LLC |
|---|---|---|
| Energy storage | Yes (coupled inductor) | No (true transformer) |
| Leakage inductance | Minimize (2–5%) | Design target (L_r) |
| Magnetizing inductance | Not critical | Critical (L_m) |
| Efficiency | 85–92% | 96–98% |
| Complexity | Simple | Complex |
6. Common Design Mistakes
- L_m too low: high circulating current, low efficiency
- L_m too high: ZVS lost at light load
- Leakage inductance not controlled: inconsistent resonant frequency
- Poor winding layout: high proximity loss at high frequency
- Core loss underestimated: cores run hotter than expected
7. Typical Applications
| Application | Power | Frequency |
|---|---|---|
| Server power supply | 1–3 kW | 100–200 kHz |
| Telecom rectifier | 2–5 kW | 100–300 kHz |
| EV onboard charger | 3.3–11 kW | 100–300 kHz |
| Industrial power | 500–3000 W | 100–200 kHz |
8. Sirui-Tech LLC Capability
- Integrated magnetics: L_r as leakage inductance
- High-frequency ferrite: PC95 for 100–300 kHz
- Center-tapped and full-bridge secondaries
- Custom L_m/L_r ratio per your resonant tank design
Conclusion
LLC resonant transformers must be designed together with the resonant tank. Key parameters are L_m, L_r, and turns ratio. Integrated magnetics saves cost and size. Choose PC95-grade ferrite for 100–300 kHz operation.
Need a custom LLC transformer? Contact Sirui-Tech at sales@sirui-tech.com with your voltage, power and frequency.