Frequency-to-output-power transfer function measurement of a resonant inverter for domestic induction heating applications

This paper presents a method to measure off-line the frequency-to-output-power transfer function of a resonant inverter in open-loop. This method is applied to a half-bridge series resonant inverter for domestic induction heating applications. A sinusoidal perturbation is injected to the nominal value of the switching frequency, leading to a perturbation in the power transferred to the load. The Discrete Fourier Transform (DFT) is used off-line to process the digitized input and outputs in order to compute the gain and phase of the transfer function at the injected perturbation frequency. The digitized input is the saw-tooth waveform used in the trailing-edge modulator. This waveform is generated with a commercial function generator that allows modulating the frequency with an internal sine waveform. Since the internal modulation signal is not accessible, the sinusoidal switching frequency perturbation is off-line obtained by processing the ramp slope of the saw-tooth waveform. The digitized outputs are the inductor current and load voltage, which are processed to obtain the moving average output power. The algorithm accuracy is firstly evaluated in simulation, leading to an appropriate method to extract the ramp slope of the saw-tooth waveform. Finally, the proposed method is experimentally verified, evaluating the validity range of the EDF model extended for several harmonics applied to resonant power converters.