Study On Digital LLC Resonant Converter

LLC resonant converter has been applicate widely for its high efficiency and power density. However, there are some challenges in controlling the fast dynamic response of the resonant tank, the current limiting arid application of synchronous rectifiers. This dissertation will discuss and investigative on these problems improve the dynamic characteristics of the LLC converter and conversion efficiency.First of all, according to the operation zone and switch state, the operation of the converter in a single switch is divided into six kinds of possible linear state, and the six states to establish the corresponding equivalent circuits and differential equations. Then steady-state waveform of LLC resonant converter is analyzed in detail and current and voltage transformation in resonant tank is depicted by using the combination of the six kinds of linear state. Finally, it is deduced, when the operation frequency lower than the resonant, the secondary diode conduction time is fixed and the approximate equal to half a resonant cycle.The small signal bode of the LLC resonant converter is obtained by simulating the switch model of LLC. It can be gain the small signal transfer function fitting expressions by studying its small models of dynamic characteristics. The transfer functions show that it is quite different in the function between the operation frequency higher and lower than the resonant frequency. When the converter operating frequency is higher than the resonant frequency, it exists in a zero, at low frequency, and the high frequency zone shift with the operation frequency. While, there is a double pole at low frequency and a right half plane zero, and the high frequency zero is stable, when the converter operating frequency is lower than the resonant frequency.The small signal characteristic of the LLC converter is relatively stable, so the type compensator can be well compensated circuit. The compensator is designed and the compensation function can be obtain, by using SISOTool. The difference equation of the compensator can be gained by using analog-digital redesign, which is used to realize the closed loop control of LLC convertor with the DSP controller.Finally, a method of senseless synchronous rectification control was proposed, which avoids misjudge of the excepted duty of SR, caused by the parasitic inductance induced VdsSR phase shift in the SR MOSEFT. This method is able to track the excepted duty of SR accurately and achieve synchronous rectification well.The method proposed in this paper is verified by a LLC converter prototype, and the test results show that the output voltage can be stabilized and the synchronous recertification duty can follow the needed one well with the method.