Improved control strategy of 1 MHz LLC converter for high frequency resolution

High switching frequency is one of the effective methods to improve power density for a LLC resonant converter. However, conventional controllers such as a digital signal processor (DSP) and an analog controller have a performance of the limited frequency resolution which introduces high primary and secondary side current variations and poor output voltage regulation at high switching frequency. In this paper, a hybrid control method combining the pulse frequency modulation (PFM) and the pulse width modulation (PWM) is proposed to overcome the limited frequency resolution performance. The proposed hybrid control method is analyzed with its control flow chart and operational principles. The improved voltage regulation performance is compared with the conventional PFM control and verified by simulation and experimental results using a 240 W prototype converter operating at 1 MHz switching frequency.