In LLC circuit, why say: Is the resonance current greater than the excitation current when fs & gtfR 1?

To understand LLC, we must first understand softswitch. For the common topology, when the switch tube is switched, the voltage and current between the D-S of MOSFET overlap, thus generating switching loss. As shown in the figure.

In order to reduce the switching overlap, two soft-switching methods, zero-current switching (ZCS) and zero-voltage switching (ZVS), are proposed. For ZCS: keep the switch tube current at zero when it is turned on and reduce it to zero before it is turned off. For ZVS: reduce the voltage of the switch tube to zero before turning on and keep it at zero when turning off. The earliest soft-switching technology was realized by lossy snubber circuit. From the point of view of energy, it is to transfer the switching loss to the buffer circuit and consume it, thus improving the working conditions of the switch tube. This method does not improve the efficiency of the converter, and even reduces the efficiency. At present, the soft switching technology studied no longer uses loss buffer circuit, and it is not loss transfer but resonance technology that really reduces switching loss. Resonant converters are divided into full resonant converters, quasi-resonant converters, zero-switching PWM converters and zero-switching PWM converters. The resonant element of the full resonant converter always works in the resonant state, while the resonant element of the quasi-resonant converter only participates in a certain stage of energy conversion, not the whole process. Based on quasi-resonance, the zero-switching PWM converter controls the resonance process of the resonant element by adding an auxiliary switching tube. The auxiliary resonant circuit of zero-switching PWM converter only works for a period of time when the switch tube is turned on and off, and stops working at other times.

Fully resonant converter is mainly composed of switching network and resonant circuit, which makes the current flowing through the switching tube become sine wave instead of square wave, and then tries to make the switching tube conduct at a certain moment to realize zero-voltage or zero-current switching.