1. Intermediate frequency heating power inverter output power
In order to obtain different output powers to control the temperature of the heating load, the controlled parameter is actually the rms value of the intermediate frequency voltage across the load. The rms value of this voltage is similar to the DC voltage U output by the chopper. Therefore, by controlling the voltage value of the chopper, the output power of the inverter can be controlled, that is, the temperature of the heated load can be controlled.
The chopper composed of V constitutes an adjustable DC power supply, which is controlled by a given unit, a single-chip microcomputer and a driving device. Change the magnitude of the DC voltage value by changing the duty cycle of V turn-on and turn-off. Compared with the controllable rectifier circuit, the chopper is used to adjust the U value. Due to the high frequency chopping, the L volume and inductance value of the filter can be greatly reduced (H order of magnitude), and it also has the advantages of fast protection action.
2. Working frequency of intermediate frequency heating power inverter
In order to obtain the maximum output power, in normal operation, the operating frequency of the inverter should be close to the natural oscillation frequency of the parallel resonant circuit. For different heating loads, the equivalent inductance L and resistance R may change with the heating process, so even if the tuning circuit C is unchanged, the natural resonant frequency of the resonant circuit changes. In order to ensure that the inverter always keeps the operating frequency close to the natural frequency of the resonant tank during operation, the circuit adopts a quasi-resonant working state, that is, when the load voltage crosses zero, V1.4 in the arm starts to communicate with V2.3. commutation. Different from the ideal resonance mode, the fundamental phase difference of the output voltage and current is zero due to the overlapping on-time and switching time of the indispensable upper and lower arm components. This control method is also called zero-phase parallel resonance. The control to maintain the zero-phase resonance working state is completed by the detection unit and the single-chip microcomputer.
3. Protection measures for intermediate frequency heating power inverter
In the inverter, when the IGBT is commutated, protective measures should be taken to prevent the device from being damaged. Common protection measures are:
(1) Cut off the gate control signal through the detected overcurrent signal to realize overcurrent protection;
(2) Use a snubber circuit to suppress overvoltage and limit excess du/dt.
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