FPGA-based real-time harmonic impedance measurement of series resonant loads by using lock-in algorithm

This paper presents a field programmable gate array (FPGA) implementation of a digital circuit that measures in real-time the first harmonic load impedance of medium-frequency (25 kHz - 75 kHz) induction-heating cooking appliances. The load impedance of these systems, which consist of a planar inductor coupled to a pan, depends on the pan material, temperature, and frequency. For this reason, real-time load measurement is desirable for a proper control of the inverter. For computing this impedance, a dual-phase lock-in algorithm is applied to the inverter output voltage and load current. The algorithm has been implemented using a hardware description language (HDL). The implemented circuit is simulated by means of mixed-signal simulation tool. From these simulations, the accuracy of the method is evaluated. Finally, the simulations are experimentally verified.