Direct phase digital control method in power inverters based on dumping frequency analysis

Direct phase or indirect frequency control methods are used in series-resonant bridge power inverters for induction heating to maintain maximum power transfer as the load equivalent electrical parameters change during the heating process. The paper present mathematical analysis of the output voltage and current phase angle dependence on the resonant circuit dumping frequency when it is excited with pulse voltage with a different frequency than the resonant one. Based on this analysis and using only measurements of the deviation of the phase angle from its reference value, an improved control method is derived that calculates the new bridge switching frequency. An algorithm for digital implementation is presented. The improved method is verified by simulations and experiments on a prototype.