| The development of the green economy has given birth to the demand for high-voltage and high-power switching power supplies,which are widely used in military,medical,and transportation.Based on the domestic demand for the localization of nuclear magnetic resonance core technology,the paper designs an power amplifier based on a cascaded Hbridge: it outputs and amplifies the gradient sequence signal input by the current tracking spectrometer.Its operating characteristics require timely output current tracking,short lag time,and high steady-state accuracy.The thesis analyzes the working principle of the gradient power amplifier,the design process of the switching power supply system,and designs the main circuit topology of the power amplifier based on this: it is composed of power distribution unit,rectifier circuit unit,and inverter circuit unit.The power distribution unit is composed of contactors,soft-start resistors,fuses and transformers;the rectifier circuit unit is composed of a three-phase uncontrolled full-bridge converter and filter capacitors;the inverter unit is composed of four H-bridge cascades.The parameters of the main circuit are calculated and the device types are selected.The modulation strategy of the cascaded H-bridge is compared and analyzed,and the carrier phase shift method is selected.Designed the control system of power amplifier-main control board circuit,sampling circuit,protection circuit and driving circuit.Two current control techniques commonly used in cascaded H-bridges-hysteresis current control and PI loop current control are analyzed.On this basis,a current tracking control algorithm suitable for the operation requirements of power amplifiers is proposed.The algorithm consists of a fixed duty cycle(FDC)method under the dynamics of the gradient sequence signal and a feedforward same proportional integral(PI)method under steady state.The combination method and the three-point sampling method of the transition between dynamic and steady state.Finally,simulations and experiments verify that the power amplifier designed in the thesis meets the requirements of its operating characteristics. |
