Automatic Resonant Frequency Tracking In LLC Resonant Converters Based On THD Algorithm

In recent years, with the rapid development of communication and data industry, the requirements on the power structure and power supply products is becoming much higher than it used to be. Due to the high efficiency, high power density, high reliability, low cost, and easy in system upgrade, bus converter has become the fastest-growing part of the distributed power system. At present, the LLC resonant DC/DC converter topology, based on PWM, is mostly adopted by the industry. In order to obtain the highest efficiency, the ideal working condition to the LLC resonant DC/DC converter is to make the switch frequency the same with to resonant frequency. Due to the tolerance of the component itself and working environment, the designed switch frequency has a certain deviation from the resonant frequency, therefore caused a lot of power loss.The desired working condition for the LLC resonant converter is to have the switching frequency equal to the resonant frequency In order to achieve higher efficiency. However, since the converter works in open-loop condition, the switching frequency usually deviates from the resonant frequency in real converter prototype. In this paper, a novel switching frequency control scheme to track the resonant frequency is proposed. Theoretically, the primary side current of LLC resonant converter is a pure sinusoidal waveform when it works at resonant frequency. The proposed control algorithm based on tracking the minimum value of the total resonant current harmonic to realize the switching frequency tracking the resonant frequency. A FPGA based closed-loop digital controller is presented to calculate the total resonant current harmonic to estimate the shape of the waveform. The switching frequency can be well controlled to track the resonant frequency. As a result, the higher efficiency can be achieved.I worked out the relative simulation under MATLAB Simulink to verify the accuracy of the algorithm and control scheme. Comparing with the results of simulation in real time, accuracy and feasibility of comprehensive validation control scheme has been verified by experiments carried out with an existing LLC resonant converter and the control platform based on the FPGA development board.