Research On Narrowed/constant-frequency Controlled Resonant Conversion With Wide Voltage-gain Range

With the rapid devoplepment of the IT,electric vehicles,and renewable energy,new challenges are constantly being raised in the implementation of power electronic conversion: wide range,high power,high efficiency,high power density,high reliability.Resonant converters have been widely employed in industrical applications due to its remarkable features,such as soft switching,high efficiency and high-power density.However,conventional pulse frequency modulation(PFM)cannot achieve wide voltage operating range and high overall efficiency at the same time.It is also difficult to realize smooth switching of transferred power for bidirectional power conversion.To solve these issues,this paper systematically studies on the narrowed-/fixed frequency resonant conversion technologies based on the high-frequency auxiliary regulation of rectifiers.With the help of the auxiliary regulation of rectifiers,a new degree of freedom is provided for the control system.To narrow the frequency range,a novel resonant conversion method has been proposed based on the reconfigurable-structure rectification.The proposed rectifiers can be switched online among basic rectifiers,such as center-tapped rectifier,full-bridge rectifier and multiplier rectifier,etc.Wide voltage operating range and high overall efficiency can be achieved at the same time.In order to narrow the frequency range,the matrix transformers are introduced.The secondary-side windings of different transformers can be connected to reconfigurable-structure rectifiers.Different recifiers can operating in different rectification mode.Accordingly,a new hybrid operating mode diffirent from the conventional recifiers,such as center-tapped rectifier,full-bridge rectifier and multiplier rectifier.Smooth transition can be achieved with the combination of PFM and pulse width modulation(PWM).Compared with conventional LLC converter,the proposed method greatly reduced the required gain range of resonant tank in each mode,increasing the overall conversion efficiency.To optimize the efficiency of normal operation and achieve wide voltage conversion for hold-up applications,a fixed-frequency resonant conversion method has been proposed based on the high-frequency active rectification.Inspired by the operation principle of the boost converter,auxiliary active switch(es)has(have)been integrated into the convensional passive rectifiers.With the help of auxiliary switches,the secondary-side winding of the transformer can be shorted and the resonant inductor or the resonant tank can be charged quickly.So that,the voltage gain range can be enlarged.The implement of active rectificaitons have been provided for basic rectifiers.In the applications with matrix tranformers,the secondary-side windings of different transformers can be connected to different active/passive rectifiers.The structures are flexible and changeable.The requirement of boost conversion can be promised without increasing excessive costs.In the applications with multiple modules,the rectifier circuits of each module are organically combined.The secondary-side windings of the transformers can be switched between series and parallel dynamically,which will increase the voltage gain range.The operating principles,characteristics and control strategies of a resonant converter based on active center-tapped rectifier,a resonant converter based on hybrid rectifier and a resonant converter with dynamic series-parallel reconfiguration-windingsare analyzed in detail as examples.The voltage gain is only dertermined by the duty cycle or the phase-shift angle,the resonant parameters and the load,but independent of magnetizing inductance.Therefore,a larger magnetizing inductance can be selected,resulting in low circulating current and improved steady-state efficiency.In order to achieve bidirectional resonant conversion,a voltage-in-phase pulse width modulation(VIP-PWM)strategy is proposed based on the characteristics of the conventional LLC converter that the voltage gain at resonance is independent of the load.The resonant converter always operating at resonance.The midpoint voltage of primary-and secondary-side strutures are in phase.By regulating the duty cycle of primary and secondary side,the step-up and step-down conversion can be achieved.The voltage gain is only determined by the duty cycle and has nothing to with the amplitude and direction of transferred power.Therefore,the direction of transferred power can be switched quickly and smoothly.Furthermore,the voltage gain of VIP-PWM control can theoretically be regulated from zero to infinite.A single-tage bidirectional resonant inverter with much more wide voltage range is proposed based on VIP-PWM.Compared with DAB-based solutions,the control of the proposed microinverter is much simpler.In addition,the VIP-PWM can be applied in multi-ports applications.Each port can operate with bidirectional power flow.Besides all the merits of VIP-PWM,power decoupling can also be achieved in circuit level without complex power decoupling algorithm.Prototypes are built with both the simulation and experimental results in detail to verify the analysis mentioned above,respectively.