Design,real-time modelling,simulation and digital implementation of phase-locked loop-based auto-synchronising current-sourced converter for an induction heating prototype

Induction heating (IH) converters operate just above resonant frequency, at near unity power factor (UPF), to supply power to the targeted work-piece. Some power electronic converter-fed IH systems use power control strategies based on dynamic tracking of the changing resonant frequency as the work-piece gets heated up (since inductance changes). Therefore, the correct in-process determination of the resonant frequency is essential. A method of dynamically detecting the resonant frequency is by calculating the phase-shift between current and voltage continuously during the process. In case of CSI- (and VSI-) fed IH, the phase-shift between voltage and current is zero at resonant frequency. Hence one way of identifying the resonant frequency is by varying the frequency until the phase-shift is zero. For controlling this phase-shift between current and voltage waveforms, most of the controllers use a phase-locked loop (PLL) IC. In this paper, a novel method for the dynamic tracking of resonant frequency is proposed and the practical implementation of the same, using a field-programmable gate array (FPGA) based digital-PLL, is presented. The scheme is first simulated with generated off-line signal samples and then implemented on a real-time model of a CSI-fed IH application. Finally, the digital-PLL logic is implemented on controller hardware and practically tested in a laboratory-made experimental set-up of 2 kW at a nominal frequency of 10 kHz. The switching frequency is auto-synchronising. This fact is practically verified both by varying (i) the geometric dimensions as also (ii) the initial temperature of the work-piece. It is practically observed in the oscillograms that the phase gets locked in few cycles (and hence ensures quick tracking of the dynamically changing resonant frequency for this set-up).