FPGA-based real-time calculation of the harmonic impedance of series resonant inductive loads

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-70 kHz) induction heating cooking appliances. The load impedance of a planar inductor coupled to a pan depends on the geometry, pan material, temperature, and frequency. Real-time load measurement is desirable for a proper control of the inverter. The Goertzel algorithm is implemented to calculate the Discrete Fourier Transform (DFT) of the inverter output voltage and load current over a half mains period. From the real and imaginary amplitudes of the harmonics of voltage and current, the load impedance at those harmonics can be computed. The algorithm has been developed using a hardware description language (HDL). The digital circuit, the power converter, the signal conditioning circuits, and the sigma-delta modulators are simulated as a whole using a mixed-signal simulation tool. From these simulations the accuracy of the method can be evaluated when first order sigma-delta converters are used to sense voltage and current. Finally, the simulations are experimentally verified.