Research And Design Of Bridge-type LLC Resonant Converter With Wide Output Range

The high-frequency and modular technology of the power converter makes the converter small,lightweight and compact,and is a powerful means of achieving high-performance mobile power.In the DC-DC converter,resonant converter with high frequency,small size and high efficiency conforms to the development trend of small and light.Electric car power is a typical representative of power.The Electric car power needed to achieve small size,light weight,and the output voltage also need a wide range of continuous adjustment.LLC resonant converter has a wide range of frequency adjustment and voltage regulation range,which can achieve primary zero voltage switching and secondary diode zero current switching.LLC resonant converters have high efficiency and high power density characteristics,which is suitable for electric vehicle charging applications.In this paper,a LLC resonant converter with digital frequency conversion control and wide output voltage range is designed.Firstly,the equivalent circuit model of LLC resonant converter is established by using the fundamental approximation method.According to the equivalent model,the relationship between voltage gain and the load,resonant network parameters and frequency is calculated.By analyzing the voltage gain curves in different load and the inductance ratios,the effects of load and inductance on the operating frequency range are obtained.Then,according to the input impedance characteristic of the LLC resonant converter,the ZVS and ZCS are discussed in the different frequency range,and the optimal working range of the LLC resonant converter is determined.At last,the design of resonant parameters of LLC resonant converter is given by analyzing the boundary condition of ZVS in the whole load range.For the wide voltage range of electric vehicle charging,the switch frequency is high in the full bridge LLC operating mode.The parasitic capacitance of the rectifier diode and the leakage inductance of the transformer have a great effect on the voltage gain of the LLC resonant converter when the switch frequency is high.In order to reduce the switching frequency to achieve the wide output range of full bridge LLC,the variable mode controlled LLC resonant converter(full-bridge LLC mode becomes half-bridge LLC mode)is proposed to reduce the switching frequency range of the converter,reduce the switching frequency and reduce the circuit characteristics by the parasitic parameters.In the process of closed-loop design,the extended describing function method is adopted for the small-signal model.The stability equation of the LLC resonant converter is obtained by analyzing the state equation of the resonant converter in the steady state by small signal disturbance,establishing the system transfer function,and designing the control system compensator to realize the wide output range.Through the simulation of LLC resonant converter,the zero voltage switching in different working areas is verified.The closed-loop simulation is carried out for the two modes of the battery charging,and the feasibility of the proposed method has been proved.Finally,a test converter controlled by digital controller with input voltage of 100 V,maximum output current of 4A and power of 500 W has been designed.The experimental results verified the design scheme.