Research Of Topologies And Control Strategies For Wide Range LLC Converter

Resonant converters are more and more attractive in industrial applications such as aeronautic power supply,DC microgrid,electric vehicle charging and so on,because of the advantages of high efficiency,low volume,high power density and good Electro-Magnetic compatibility(EMC)characteristic.Meanwhile,the wider application fields result in severe requirements to the converters including wide range of input voltage,output voltage and main loop current.The performance of the conventional resonant converters with pulse frequency modulation(PFM)will be affected,while satisfying these requirements.Therefore,for LLC the resonant converter,the research involves the optimizations of topologies and control strategies in depth based on the retaining the efficiency,volume and soft switching of the converter in the thesis,while satisfying the different wide range requirements.For wide input voltage applications,the three-level switch array is generally used and connected to the LLC resonant tank when the input voltage is high.The challenging issues are the voltage balance problem on input capacitors and power switches with the tolerances of the components.Clamping diodes and flying capacitor are commonly used as two clamp circuits for voltage balance.However,the operating principle of the clamp circuits in threelevel LLC converter is different from other three-level converters,because the mean value of the resonant tank current is always zero and pulse PFM control is generally used instead of pulse width modulation(PWM)control.And the converter with only the clamping diodes or flying capacitor is difficult to realize voltage balance.Therefore,the characteristics of three-level LLC converter with single clamp circuit are firstly analyzed.Then the operation principle of the hybrid-clamped circuit under the unbalanced voltages on input capacitors is described with the mode analysis,based on which the mechanism for voltage auto-balance is elaborated and the effects of two clamp circuits are derived.Finally,the design of the clamping diodes and the flying capacitor is also involved.For wide range output voltage applications,the output range of the LLC converter is restricted because of the limited switching frequency range under the PFM control.PWMburst hybrid control method and phase shift modulation(PSM)-burst hybrid control methods are proposed for full bridge LLC converter and half bridge LLC converter respectively.The performance of the converter such as efficiency,reliability and soft switching is optimized by using the proposed control strategies with output voltage range.The PWM or PSM control can reduce the current stress during burst-on time,and the burst control can provide enough current for all switches to realize zero voltage switching(ZVS).The selections of burst frequency and switching frequency are analyzed.And the state-plane trajectories of the LLC converter with PSM and PWM are used.The optimization design of the resonant tank parameters is analyzed based on the restriction of maximum current of the resonant tank.Finally the minimum duty ratio of PWM control and the maximum phase shift angle of PSM control are derived.For wide range output voltage applications with continuous high operating frequecny,the method of switching the operating mode of LLC converter between full-bridge mode and half-bridge mode can also expand output voltage range.The converter can operate under the high frequency conditions under two cases.However,it has some drawbacks when switching the operating mode directly.An optimized mode switching method is proposed,which mainly uses the burst mode control with only one switching cycle during burst-on time.The stability of the converter during mode morphing progress is improved based on the proposed method.For the high current applications,the efficiency will be greatly influenced because the current waveform in resonant tank is approximately sinusoidal.And it will produce high conducting loss in LLC converter.Meanwhile,a larger filter is required on the secondary side to eliminate the AC component of the current.Therefore,a multi-phase LLC converter with interleaving strategy is often used to reduce the current stress and the filter size.However,the current balance of multi-phase LLC converter is a challenging issue because of the unbalanced resonant parameters.The relationship between the current distribution characteristic and the parameter tolerance is analyzed in detail for three-phase half bridge LLC converter.Then the current distribution characteristic is also analyzed when the midpoint(common node)of the three phases is floated.Finally,a new control strategy based on phase shift control between modules is proposed to realize the complete current balance of three phases.