Llc resonant converter, control unit, and method of controlling the same

An LLC resonant converter, comprising:

• a square wave generator, having a first switch and a second switch connected to the first switch in series,
• a resonant tank coupled to the square wave generator,
• a transformer having a primary side and a secondary side, and the primary side coupled to the resonant tank,
• a synchronous rectifying unit coupled to the secondary side, and having a first synchronous rectifying switch and a second synchronous rectifying switch, and
• a control unit configured to receive an output voltage of the resonant converter, acquire a frequency control command according to the output voltage, and provide a first control signal to control the first switch, a second control signal to control the second switch, a first rectifying control signal to control the first synchronous rectifying switch, and a second rectifying control signal to control the second synchronous rectifying switch,
• wherein when the frequency control command is higher than a phase-shift frequency, the first control signal and the first rectifying control signal are frequency-variable and phase-fixed, and the second control signal and the second rectifying control signal are frequency-variable and phase-fixed;
• when the frequency control command is lower than the phase-shift frequency, the first control signal and the first rectifying control signal are frequency-variable and phase-shifted, and the second control signal and the second rectifying control signal are frequency-variable and phase-shifted.