Application of FPGA based PLL in a CSI fed induction heating prototype

Modern induction heating (IH) power converters operate marginally above resonant frequency to supply power to the targeted work-piece at near unity power factor (upf). The resonant frequency continuously increases as the work-piece gets heated up. A possible method of dynamically detecting the resonant frequency may be to calculate the phase-shift between current and voltage continuously during the process. The phase-shift between voltage and current is ideally zero at resonance. So resonant frequency may be identified by varying the frequency until the phase-shift is zero. For this some controllers use a phase-locked loop (PLL) strategy. In this paper, the dynamic tracking of resonant frequency, using a field-programmable gate array (FPGA) based digital-PLL, is presented. The scheme is first simulated off-line. Finally, the logic is implemented on controller hardware and practically tested in a laboratory made experimental set-up of 2kW at around 10kHz.