Two-stage Aviation AC/DC Converter Based On Cascaded H-Bridge With Variable Input Frequency

In the More-Electric-Aircraft(MEA)with variable frequency AC power supply system,disadvantages of AC/DC equipment commonly used in secondary power systems such as Auto Transformer Rectifier Units(ATRU)have become increasingly prominent with the rapid development of electronic equipment and the use of national defense weapons and equipment.Cascaded H-bridge rectifier(CHBR)can reduce the weight of the filter with reducing voltage stress on power device,which realize the miniaturization and weight reduction of the system,improve the power factor.When it combine with the rear stage DC/DC converter,the 28 V DC voltage can be achieved which is required for airborne equipment such as display equipment,weapon control,etc.In this paper,the multilevel topologies and modulation strategies of the two-stage rectification are firstly compared,then the cascaded H bridge rectifiers are choosen as the front stage and the parallel output half-bridge LLC resonant converters are choosen as the second stage.In the second chapter,by analyzing the working mode of cascaded H-bridge rectifier with the unipolar frequency-multiplied carrier phase shift SPWM,the expressions of the inductor current ripple,AC side bridge arm voltage and the inductive current high frequency harmonic can be derived.Combined with mathematical analysis and simulation results,the advantages of CHBR can be verified.The causes of low frequency harmonic of CHBR input current is studied,and the loss model is established to study the voltage balance control principle.The third chapter studies Principle of zero-voltage switching in half-bridge LLC resonant converter,and the two essential condition is proposed.Through the voltage gain and waveform analysis,the design method of the LLC parameter suitable for this paper is given considering reducing loss.The intermediate bus capacitor voltage balance and power balance are the key to realize the control.In the fourth chapter,several voltage balance control methods for CHBR are proposed.Then,three control schemes of two-stage rectifiers are compared and analyzed.A kind of simplified control scheme that can achieve balance automatically is used.For the aviation variable frequency condition,a controller with higher gain in the grid frequency fluctuation range is proposed.The closedloop parameters are designed,and the validity is verified with simulation.Finally,the hardware and software of a 1kW two-stage two-unit AC/DC converter experimental prototype is designed,and the experiment verifies the effectiveness of the topology and control strategy.