Phase locked control of the hottest high frequency

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Phase locked control of high-frequency induction heating power supply

1 overview

because induction heating power supply is an important equipment for heat treatment, its control scheme has always attracted much attention. Due to the complex operation conditions and many interference factors on the heat treatment site, the interference sources should be minimized and the influence of external interference on the system should be weakened or eliminated during the design. Therefore, the control scheme is constantly being improved according to the actual situation

induction heating power inverter can be divided into current type inverter and voltage type inverter according to the position of its load compensation capacitor. Current source inverter has the advantages of simple circuit structure, reliable power supply operation, strong adaptability to load and easy overcurrent protection. Figure 1 is the topology of current source inverter circuit. For current mode circuit, first of all, it is necessary to prevent the instantaneous open circuit of inverter; Secondly, select the appropriate timing or 2. The working principle of deformation measurement of electronic universal experimental machine, and the advance trigger mode of fixed angle; Finally, the inverter is required to have a wide starting frequency range

Figure 1. The principle of current mode induction heating power supply topology

2 control scheme and the control block diagram of improved

inverter are shown in Figure 2. Among them, VO is the output voltage signal of the inverter. After peak detection, the switching signal X1 of the switching device is generated by comparing with the control set value. When X1 is at high level, the output signal x2 of the switching device is connected with the other excitation signal. The inverter works in the other excitation state, and the control signal is sent from the other excitation signal generator. Insiders pointed out that the working frequency of the circuit is fixed and controlled by the other excitation signal generator; When X1 is at low level, X2 is connected with the self-excited signal, the inverter works in the self-excited state, and the working frequency of the circuit is determined by the natural frequency of the load itself. According to the closed-loop filtering function of the phase-locked loop, the time delay is carried out in the feedback circuit of the phase-locked loop to compensate the inherent delay of the system and adjust the delay time TD. The inverter can work in both inductive and capacitive states

Figure 2 inverter control circuit block diagram

3 smooth transition between prevention of instantaneous open circuit of inverter and conversion

the control strategy of inverter with fully controlled devices as switches usually adopts the control strategy of switching from other excitation to self excitation, that is, when starting up or when the load voltage is lower than the threshold VCO, open-loop constant frequency control is adopted to work in the state of other excitation; When the output load voltage is greater than the threshold VCO, automatic switching is carried out to make the inverter work in the frequency closed loop and track the change of load frequency

but this control scheme has such a problem: because the other excitation signal and the self excitation signal cannot always be synchronized, in most cases, a narrow pulse (low level) will be generated in the switching process, which inevitably causes the instantaneous open circuit of the inverter; In addition, the actual operating environment of the site is poor, usually working in a harsh electromagnetic environment. This control scheme has poor anti-interference performance for the outside world, and can not meet the anti-interference requirements of the system

in view of this situation, a phase-locked filter circuit is inserted at the rear stage of the switching circuit to filter out the narrow pulse generated during the conversion. Similarly, this circuit also has a strong ability to suppress the spikes caused by external interference. Figure 3 shows the waveforms of key points X1 and X3 before and after switching. It can be seen from Figure 3 that due to the phase-locked characteristic, the narrow pulse in the switching process is filtered out by the phase-locked loop. In Figure 3, if channel 1 is x, it can save about 5% of 1 waveform, channel 2 is x2 waveform, and channel 3 is X3 waveform

Figure 3 control circuit switching waveform

Figure 4 shows the waveform of the output voltage of the inverter when it is switched from external excitation to self excitation. Figure 5 shows the waveform diagram when the inverter changes from self excitation to other excitation. From these two waveforms, it can be seen that the switching process is a smooth transition process. Compared with figure 3, the stability of the system is greatly improved, and the previous narrow pulse is filtered out by the phase-locked circuit

Figure 4 output voltage waveform when it turns from self excitation to self excitation

Figure 5 realization of accurate timing of output voltage waveform when it turns from self excitation to self excitation

due to the existence of inverter output lead inductance, in order to reduce the current and voltage stress of inverter tube, the inverter is generally required to work in capacitive commutation state. This requires commutation at a certain time (at a certain angle) before the channel voltage crosses zero. This divides the control circuit into two trigger modes: timing and angle. The timing trigger mode is adopted in this control circuit

in the traditional medium frequency induction heating electric perihelion, the timing trigger mode is generally realized by the voltage and current synthetic signal of the channel. The timing of this circuit is approximate. In ultrasonic induction heating, due to the inherent delay of the control circuit, this approximation is no longer tenable. Therefore, using the timing trigger method of voltage and current synthesis, the lead time will change with the change of the slot resonant frequency and the output voltage amplitude

by inserting a delay link into the feedback circuit of the phase-locked loop, on the one hand, the inherent delay of the control system is compensated, on the other hand, the accurate lead trigger time can be obtained. Obviously, the time constant of this delay link in the control circuit is relatively independent of the slot resonant frequency and voltage amplitude

5 conclusion

through the experimental verification and the later actual operation effect of the on-site ultrasonic induction heating power supply, this control circuit has strong anti-interference ability and stable conversion ability, constant lead trigger time, and a large starting range

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