Research On Self-oscillating LLC Resonant Converter

LLC resonant converter has the advantage of increasing the switching frequency while maintain the high efficiency, leading to small size, light weight and high power density. Self-oscillating converters can reduce the cost, compared to the converters with external controllers and drive circuits, and they have found wide applications where the performance is not strictly required. This paper is dedicated to the exploration of self-oscillating drive techniques for LLC resonant converters.This thesis firstly analyzes the operation principle and characterisctics of LLC resonant converter, and it is found that the voltage conversion ratio is independent of the load when LLC resonant converter operates at the resonant frequency. Base on this characteristic, this thesis proposes a self-oscillating drive circuit for LLC resonant converter by employing a current transformer which senses the resonant inductor current, and the self-oscillating frequency is kept to be equal to the resonant frequency, thus the output voltage is nearly constant at any load in the case of constant input voltage. By adopting the linear control theory, the self-oscillating frequency is derived, and choice of the value of the resonant inductor is given to ensure the self-oscillating frequency be equal to the resonant frequency.The self-oscillating drive circuit is further improved to increase driving speed for the power switches. A simple circuit for starting up the LLC converter is also proposed as well. Furthermore, the LLC resonant converter is improved by splitting the resonant capacitor over two capacitors, which are in parallel with the input voltage source, and the middle point of the two capacitors is connected to the resonant inductor. Meanwhile, two diodes are added in parallel with each of the two capacitors to clamp the capacitor voltages. Such arrangement can reduce the inrush current in the resonant inductor during the starting up process.A prototype of the self-oscillating LLC resonant converter has been built and tested in the lab, and the experimental results are given to verify that effectiveness of the proposed self-oscillating drive circuit and the improve LLC resonant converter.