Three-Phase to Single-Phase Multiresonant Direct AC-AC Converter for Metal Hardening High-Frequency Induction Heating Applications

A new multiresonant three-phase utility frequency ac (UFAC) to high-frequency ac (HFAC) direct power converter for the industrial metal hardening induction heating (IH) applications is presented in this article. The proposed ac-ac converter features direct frequency conversion with the wide range of soft switching by means of the minimized numbers of bidirectional switches. The conducting current of bidirectional switches can be reduced effectively owing to the multiresonant tank while keeping a high power in the IH load. Accordingly, the practical power converter with simplicity, cost-effectiveness, and high efficiency can be realized on the basis of a simple pulse-frequency modulation (PFM). The circuit topology and operating principle of the proposed converter are described, after which the design procedure of the multiresonant tank and switching frequency is presented. The performances on the soft switching and the steady-state PFM characteristics of the ac-ac converter are evaluated in experiment with the 1.7 kW/85-90-kHz prototype. Finally, the feasibility of the proposed ac-ac converter is evaluated from a practical view point.