Research On Wide Gain LLC Converter Based On Switch-Controlled-Capacitor

DC/DC converters are widely used in DC microgrids,among which LLC resonant converters have the advantages of small size,light weight,high power density,soft switching and high efficiency.LLC resonant converter adopts frequency modulation and voltage regulation.The operating frequency range of the converter needs to be adjusted according to the requirements of the voltage regulation range in the parameter design,resulting in the utilization of magnetic components,the reduction of power density,and the increase of cost;when the operating frequency is far away from resonance at frequency,its efficiency is significantly reduced.In order to solve the above-mentioned problems,this paper proposes an LLC resonant converter based on controllable switched capacitors.The research is carried out from the aspects of commutation process,soft switching characteristics,voltage gain model,efficiency optimization method and soft start method.Aiming at the problem that the parameters of LLC resonant converter are highly coupled and cannot be adjusted independently under the frequency modulation and voltage regulation mode,an LLSC resonant converter with equivalent resonant capacitance and switching frequency as control variables is proposed.The controllable switching capacitor can change the equivalent capacitance value by adjusting the conduction delay angle.Under ideal conditions,the equivalent resonant capacitance value can continuously change between(C_r/2,+?).The proposed converter can adjust the equivalent resonant capacitance and switching frequency at the same time,and can adjust the output voltage in a wide range in a narrow operating frequency range.The working principle,commutation process and soft switching characteristics of the proposed converter are analyzed.The results show that the proposed converter can adjust the output voltage without changing the operating frequency;The switch tube constituting the inverter bridge can realize zero voltage on,the rectifier diode can realize zero current off,and the switch tube in the controllable switching capacitor can realize zero voltage on and zero voltage off,the converter has good soft switching characteristics.The experimental results verify the feasibility of the proposed voltage regulation principle and structure.In order to study the output characteristics of the proposed converter,the voltage gain model of LLSC resonant converter with equivalent resonant capacitance and switching frequency as control variables is established,and then the parameter design method is given.The output voltage gain model shows that the output voltage gain increases first and then decreases with the increase of equivalent resonant capacitance or switching frequency.There are many magnetic components in the converter,and there is a risk of soft switching failure by adjusting the output voltage in a wide range through the switching frequency.Therefore,the proposed converter selects the equivalent resonant capacitance as the main control quantity.The quantitative relationships among load,ratio of excitation inductance to resonant inductance,transformer transformation ratio and minimum and maximum voltage gain are deduced;The time-domain expressions of resonant current,excitation current and resonant capacitor voltage are established,and the voltage and current stress of the device are analyzed.According to the above model,the parameter design method of the proposed converter is given,and the experimental prototype of LLSC converter is developed.The experimental prototype of the improved converter is developed,and the experimental results prove the correctness of the voltage regulation principle,voltage gain model and parameter design method of the proposed converter.The switching frequency has a great influence on the loss of resonant converter.Therefore,a cooperative control method is proposed to reduce the loss of converter by adjusting the equivalent resonant capacitance and switching frequency at the same time.The relationship between switching loss,conduction loss of diode and conduction loss of diode and switching frequency is analyzed.The results show that the switching frequency can directly affect the excitation current,the amount of charge transferred in a single cycle and the time proportion of output current,and then affect the loss of switching tube and diode.The proposed cooperative control method first selects the appropriate operating frequency point according to the target output voltage gain and the maximum operating frequency curve,and then realizes the closed-loop regulation of the output voltage through the equivalent resonant capacitance value.The experimental results show that the proposed method can reduce the turn-off loss of the switch and the conduction loss of the diode.Aiming at the problem that the filter capacitor needs to be charged when starting the resonant converter and large impulse current may damage the converter,a soft start method using parallel resonance is proposed.Under ideal conditions,the parallel resonant unit can generate infinite impedance at the parallel resonant point.Therefore,this paper proposes a soft-start method for the resonant converter using the parallel resonant unit,which significantly expands the buck region while retaining the original characteristics of the proposed converter.The range of voltage regulation is low,and the frequency reduction soft start is realized;a new series resonance point is generated,which can provide a low-impedance energy path for high-order harmonics,further reduce the effective value of the resonant current,and reduce the on-state loss of the switch tube and the rectifier diode.The prototype experimental results show that the proposed method can effectively suppress the starting current.In summary,the improved converter has two control variables,the conduction delay angle and the switching frequency,which can improve the performance of the converter through cooperative control.The improved converter has the advantages of high utilization of magnetic elements,narrow operating frequency range,wide gain range,and high overall efficiency.