| High voltage wide input power supply is a kind of low power isolated DC-DC converter which can realize high drop ratio.As an auxiliary power supply used in the submodule of the converter valve in flexible HVDC transmission engineering,it mainly supplies power to the control and drive units inside the converter valve submodule,so the power supply plays a vital role in the safe and reliable operation of the converter valve.At present,isolated high voltage wide input power supply still has some technical difficulties,such as high voltage stress of power transistor,complex transformer design,difficult hardware circuit design and low power efficiency.Based on the current research status of high voltage wide input power supply,combined with the requirements of technical parameters and the characteristics of power input and output,a two-stage power supply structure with front and rear stages isolated from each other is adopted in this paper.The front stage of the power supply adopts an Input Series Output Series(ISOS)cascade converter composed of four double-transistor flyback circuits,so as to reduce the voltage stress of the transistor.The control mode uses peak current control to complete the conversion from high voltage input to medium voltage 400 V busbar output.The latter stage uses independent Flyback module to avoid the mutual interference of each output loop and relize multiple output.Based on the analysis of the equivalent model of high frequency transformer,the mechanism and influence of leakage inductance and distributed capacitance of transformer are obtained,and the windings mode are improved.Secondly,through the analysis of the three working modes of the high frequency transformer,flexibly combined with the input undervoltage working point of the 600 V power supply,when the ISOS cascade converter is fully loaded,the input voltage is greater than 600 V designed in the continuous current mode.Design the input voltage below 600 V in discontinuous current mode.Therefore,when the input voltage of the cascade converter is 600 V,the high frequency transformer just works in the boundary current mode,which simplifies the calculation process of transformer design.The key hardware circuit of the power supply is designed in detail.The additional fuse and anti-reverse diode in the input circuit strengthen the protection of the power supply.The improvement of the auxiliary power supply circuit and high voltage starting circuit,the power loss of this part is greatly reduced.The protection circuit only uses a few analog circuits attached to the device to realize the protection function,which further reduces the power consumption and cost of the power supply.Each PWM control signal in the drive circuit passes through the first-level triode push-pull amplifier circuit,enhancing the driving ability of the 8-channel PWM control signal.The transfer function of the power control loop was derived based on the feedback control circuit,and a single pole compensation method was selected combined with the bandwidth frequency of the power supply.It can be seen from the bode diagram of the control loop before and after compensation that the power supply stability is improved and the dynamic response speed is faster after compensation.A high voltage wide input power supply prototype for MMC-HVDC is developed.The input voltage range of the power sampling prototype is DC 350 V ～ 4000 V,and it is divided into three isolated independent outputs,namely DC 400V/20 W,DC 15V/15 W and DC15V/20 W.The test results show that the transistor of the power supply has low voltage stress,high output voltage precision,small ripple,low load adjustment rate and good dynamic performance. |
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