2 Theory of LLC Topology Design

The LLC resonant converter is based on the series resonant converter (SRC). By using the transformer magnetizing inductor, zero-voltage switching can be achieved over a wide range of input voltage and load. Because, the LLC topology operates in resonance, it is always running at a 50% duty cycle. Therefore, the only way to control power to the load is by changing the resonant frequency. This frequency range is limited by the resonant tank defined by Lm, Lr, and Cr as shown in Figure 2-1. An LLC controller achieves the best efficiency when operating at resonant frequency at a nominal input voltage. As the switching frequency is lowered, the voltage gain is significantly increased. This behavior allows the converter to maintain regulation when the input voltage is reduced. For example, if the PFC is disabled to further minimize power at light load. Due to the nature of resonant converter, all the voltages and currents on the resonant components are approximately sinusoidal. The gain characteristic of the LLC resonant converter is analyzed based on the first harmonic approximation (FHA), which means all the voltages and currents are treated as a sinusoidal shape with the frequency the same as the switching frequency. FHA models the LLC power stage as a sinusoidal input voltage (VS) driving an effective load resistance RL as shown in figure 2-1 below.

Figure 2-1 LLC Resonant Converter