Power Control for Induction Heating by Asymmetrical Pulse Density Modulation

To adjust the power of serial-resonant inverters for induction heating applications, asymmetrical pulse density modulation (APDM) power controlled strategy is described, analysed and designed. The pulse density modulation (PDM) is introduced to overcome the drawbacks of the classical control modes such as pulse width modulation, pulse frequency modulation and phase shift modulation. One characteristic of PDM is that it operates at resonant frequency with zero-current and zero-voltage switching for a wide range of output power variations. Normal PDM often considered as symmetry pattern for the power switches. While in some case of low quality of the resonant loads, the load current is in continuous in stage of low density, the phase loop locked circuit can not work efficiently, and the advantage of PDM disappears. APDM provides two times wide range of output power variations at the same operation period of PDM, and the switches also cut off at zero current switching and open at zero voltage switching