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　　Circuit design of a parallel resonant inverter power supply

　　Circuit composition and design

　　The system block diagram of the power supply is shown in Figure 1. The three-phase AC voltage is converted into a DC voltage through an uncontrolled rectifier and filter circuit. The voltage is sent to the DC chopper for chopping adjustment, and becomes an approximately constant current with adjustable power. The source is then input to the inverter, which then controls the induction heating load. The DC chopper control part detects the current signal of the chopper output through the sensor, and controls the output pulse width of the PWM through the PI regulator, thereby changing the size of the chopper output current and achieving closed-loop control. The inverter control part uses a phase-locked loop frequency tracking circuit to control the operating frequency of the inverter and generate high-frequency trigger pulses to drive the power devices in the inverter circuit on and off.

　　The main circuit

　　1. The main circuit of the parallel resonant inverter power supply consists of four parts: a three-phase uncontrolled rectifier bridge, a DC chopper, a current source parallel resonant inverter and a load matching circuit (Figure 2).

　　Here, uncontrolled rectification and chopping are used to form a DC current source, mainly because of its advantages of fast protection speed and small filter size caused by high-frequency chopping. The main power devices (VT and VT1, VT2, VT3, VT4) in the chopper and inverter all use IGBT tubes. A diode is connected in series to each IGBT of the inverter bridge arm. The forward current passing through the IGBT will also all pass through the series diode. This requires the series diode to be able to pass a large forward voltage and withstand a high reverse voltage. Therefore, VD1~VD4 use fast recovery diodes. The inverter switches regularly through semiconductor switches and obtains an AC current of a certain frequency on the load side. Its frequency is determined by the operating frequency of the switch. Since it is powered by a current source, the inverter output current is approximately a square wave. The component has high resistance and a large voltage drop, while the voltage drop caused by the third and higher harmonics is small. The output voltage (i.e., the voltage across the capacitor C) can be approximately considered to be a sine wave.

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