Power Control of Asymmetrical Frequency Modulation in a Full-Bridge Series Resonant Inverter

The traditional power control schemes for induction heating device mainly focus on the pulse frequency modulation (PFM) and the pulse density modulation. But they cannot solve the problems of power control, efficiency, and load-adaption well. This paper presents and analyzes the asymmetrical frequency modulation (AFM) control scheme used in the full-bridge series resonant inverter. With the proposed AFM control technique, the output power is controlled by two variables: the operation frequency and the division factor. Better efficiency performance can be achieved in the medium and low output power range when compared with PFM. The principles as well as the zero-voltage switching condition of the AFM are explained and the power losses of switches are analyzed. A control algorithm that schedules the three control modes of AFM is experimentally verified with a digital signal processor based induction heating prototype. The load-adaption, noise and thermal distribution problem of switches are also analyzed.