Research On Sliding-mode Control Strategy Of LLC Resonant Converter

LLC Resonant converter can achieve zero voltage switching (ZVS) of the primary MOSFETs and zero current switching (ZCS) of the secondary diodes within wide input voltage range. It has the advantages of strong load adaptability, high efficiency, high power density and low EMI emissions. LLC resonant converter is widely adopted in industrial power supplies and consumer electronics. Nowadays, the dynamic response requirements are consistent with the growing processor performance. However, the dynamic characteristic of LLC resonant converter varies substantially as the operation modes are altered. Meanwhile, the optocoupler parasitic capacitor and nonlinear characteristic is also variable according to the operating conditions. It is particularly challenging for the controller to maintain satisfying dynamic performance in different input voltage and load conditions.This paper proposes a novel discrete pulse frequency modulation and successfully applies sliding-mode control to LLC resonant converter. It can effectively increase the system robustness and significantly improve the dynamic performance.Firstly, the small-signal model analysis of LLC converter is given based on the extended describing function method. The detailed mathematical derivation is presented. The steady-state precision and dynamic tracking performance are verified by theoretical calculation and simulation results.In order to achieve an optimized output voltage dynamic response, the sliding surface is derived via the input-output feedback linearization. The linear subsystem is obtained, which includes the output voltage. The system stability and control parameters are designed based on the traditional linear system approaches.Finally, two prototypes are built and tested with the sliding-mode control and PI control, respectively. The experimental results verify the feasibility of sliding-mode control scheme. The steady-stage waveforms and test results are depicted. The proposed sliding-mode controller can significantly improve the dynamic performance, compared with PI controller. The alternative current-sensorless strategy is proposed to reduce the output voltage ripple for the ripple-concerned applications.The sliding-mode control scheme of LLC resonant converter can significantly improve the dynamic response. The modelling, analysis and design method in this paper have a certain importance and value on dynamic response of LLC resonant converter.