Study On Topologies And Control Of LCC Resonant Converter With Wide Soft Switching Operation Range

Recently,resonant converter is widely used due to the benefits of wide soft switching characteristic and low electro-magnetic interference(EMI).Usually,variable frequency control strategies based on pulse frequency modulation(PFM)are adopted for resonant converter,which benefits from simple control circuit,wide soft switching operation range and being deeply studied.However,researches show that variable frequency control strategy suffers from wide variation of switching frequency and low utilization of magnetic component,which limits the miniaturization of resonant converter.Although some fixed frequency control strategies based on pulse width modulation(PWM)are proposed to solve such problems,they lose wide soft switching characteristic,which undermines system efficiency.During past decades,with the development of contactless power transfer,various hybrid control strategies aimed for resonant converters are deeply studied.It is proved that hybrid control strategy can effectively reduce switching frequency variation and maintains wide soft switching operation range.However,the existing hybrid control strategies suffer from complex control algorithm,difficult parameter design and high cost.In order to sovle such issues,this dissertation focuses on LCC resonant converter,the most typical three-element resonant converter,to study the influence of circuit structure,parameter design and control strategy on the characteristics of switching frequency variation and soft switching range,aiming to build theoretical foundation of resonant converter analysis and design.The research contents of this dissertation are listed below.1).The operation modes of fixed frequency controlled LCC resonant converter are deeply studied,where the characteristics of LCC resonant converter operating in continuous capacitor voltage mode(CCVM)and discontinuous capacitor voltage mode(DCVM)are elaborated.It is shown that when converter operates in CCVM,the resonant current stress would be high;and when converter operates in DCVM,the voltage stress of rectifier diodes would be high.In order to achieve a trade-off between resonant current stress and rectifier diode voltage stress,a CCVM-DCVM boundary mode is proposed in Chapter 2.Based on fundamental harmonic approximation(FHA),a parameter design procedure is built to guarantee fixed frequency controlled LCC resonant converter operating in CCVM-DCVM boundary mode under rated operation.The proposed parameter design procedure reduces resonant current stress and rectifier diode voltage stress,which is beneficial to the element selection and cost-saving.2).In order to solve the problem of narrow soft-switching operation range of fixed frequency controlled LCC resonant converter,the fixed frequency controlled LCC resonant converters with active and passive auxiliary networks are proposed.The active auxiliary network consists of back-to-back auxiliary power switches,auxiliary capacitor leg and auxiliary inductor,and the active auxiliary network is connected to the medal point of lagging leg of LCC resonant converter.By controlling the auxiliary power switches,the active auxiliary network produces auxiliary current to assist the soft-switching operation of power switches in lagging leg.The passive auxiliary network consists of auxiliary inductor and block capacitor,and the passive auxiliary is connected between the leading leg and lagging leg.The passive auxiliary network assists soft-switching operation of power switches of both leading leg and lagging leg,without extra control circuit.Based the analysis of operation principle of active and passive auxiliary networks,the steady state characteristics of LCC resonant converter with such auxiliary networks are derived,and detailed parameter design procedures are built.It is proved that the fixed frequency controlled LCC resonant converter with active/passive auxiliary network can achieve wide soft-switching operation,while maintains high utilization of magnetic components.3).Due to the narrow soft-switching operation range in fixed frequency controlled LCC resonant converter and bulky volum of auxiliary networks,a PWM-PFM hybrid controlled LCC resonant converter is proposed.By restricting the conduction angle control signal and switching frequency control signal in control circuit,the conduction angle in the proposed resoant converter decreases with the increase of switching frequency,thus a hybrid modulation on conduction angle and swithcing frequency is achieved.The principle and implementation of PWM-PFM hybrid control are elaborated,and the relationships between parameter coefficients and control parameters are derived.Furthermore,based on FHA,the voltage regulation condition and soft switching condition of PWM-PFM hybrid controlled LCC resonant converter are derived.Theoretical analysis proves the PWM-PFM hybrid controlled LCC resonant converter benefits from smaller switching frequency variation compared with variable frequency controlled LCC resonant converter,and it has wide soft-switching operation range compared with fixed frequency controlled LCC resonant converter.A detailed parameter design procedure is built based on the theoretical conclusions,ensuring PWM-PFM hybrid controlled LCC resonant converter achieves soft-switching operation under wide input voltage range and wide output power range.4).By extending PWM-PFM hybrid control strategy to half bridge resonant converters,a dual carrier modulation technique is proposed.The dual carrier modulator alternatively chose the modulation carrier from two saw-tooth waveforms with different slop in each switching period,through which a hybrid modulation on duty ratio and switching frequency is achieved.According to the input/output polarity of dual carrier modulator,the proposed dual carrier modulation can be classified into class A dual carrier modulation and class B dual carrier modulation.It is proved that class A dual carrier modulation can be used in half bridge resonant converter operating in inductive region to achieve wide ZVS soft switching operation within narrow switching frequency variation;and class B dual carrier modulation is prefred to be used in half bridge resonant converter operating in capacitive region to achieve wide ZCS soft switching operation within narrow switching frequency variation.Based on FHA,the soft switching condition and voltage regulation condition of dual carrier modulated half bridge resonant converter are derived.Furthermore,a detailed parameter design procedure is established,through which the dual carrier modulated half bridge resonant convereter can achieve soft-switching operation under wide input voltage and wide output power range.