Phase-locked loop of inverter based on FPGA

An induction heating system with a full bridge LLC (a kind of load form) resonant inverter is described in this paper. Subsequently, the output voltage of inverter and capacitance voltage are chosen as control variables of phase-locked loop (PLL). With regard to LLC resonant inverter, there is a phase error of 90 degrees between the two control variables. First of all, a kind of PLL with XOR Phase Detector (XORPD) is proposed. This PLL is realized by embedded function module 74HCT297 of FPGA. Then a novel PLL with Phase-Frequency Detector (PFD) is designed. Compared with the latter, the former can be more completely implemented by FPGA. However, the latter is able to adapt to more complex input signals, which is good for high frequency and high efficiency of induction heating system.