Understanding Hybrid Inverter Principles 2

LLC Resonant Converter

1. The operating frequency is less likely to experience drastic changes under load variations.
2. The increased converter gain provides more flexibility in design.

Region 1: The converter operates in an inductive region, where power transistors work in Zero Voltage Switching (ZVS) mode.

Region 2: The converter operates in a resistive region, where power transistors continue to work in ZVS mode.

Region 3: The converter operates in a capacitive region, where diodes work in Zero Current Switching (ZCS) mode.

Why the LLC Converter Operates in the Inductive Region

When operating in the capacitive region, the turn-on current, body diode freewheeling current, and the charging and discharging current of the junction capacitance all flow through the MOSFETs. This results in high stress currents on Q6 and Q5, which is why we generally prefer operating in the inductive region.

In some cases, to achieve higher output gain, we use a fixed frequency close to the resistive region of the inductive zone. However, the downside is that the frequency cannot be modulated with load changes.

Additionally，When operating in the purely resistive region, the network exhibits the highest quality factor and the best network characteristics. In the capacitive region, the network achieves zero current switching (ZCS) turn-off, making it suitable for use with IGBTs. In the inductive region, the network easily achieves zero voltage switching (ZVS) turn-on, making it more suitable for use with MOSFETs. Since medium and small power supplies commonly use MOSFETs, conventional LLC topology switched-mode power supplies are typically operated in the inductive region.

Operating the LLC converter in the inductive region is preferred for several reasons:

1. Zero Voltage Switching (ZVS): In the inductive region, the converter can achieve Zero Voltage Switching (ZVS) for the power transistors. This reduces switching losses and improves efficiency.
2. Reduced Stress on Components: Operating in the inductive region reduces the stress on the MOSFETs, particularly Q6 and Q5. High stress currents are avoided, prolonging the lifespan of these components.
3. Enhanced Reliability: The inductive region provides more stable and reliable operation. It minimizes issues such as excessive current through the MOSFETs, which can occur in the capacitive region.
4. Efficiency and Thermal Management: By reducing switching losses and stress on components, the overall efficiency of the converter is improved. This also helps in better thermal management, keeping the system cooler and more efficient.

In summary, operating in the inductive region allows the LLC converter to achieve high efficiency, reliability, and longevity of components.