Efficient digital adaptive control for full-bridge series resonant inverter for surface metal heating

Induction heating is a well-known technique to produce very high temperature at reduced time for very specific applications such as in melting steel, brazing and hardening. The series resonant converter has proved to be better suited to surface metal treatment and hardening. Efficiency reaches its highest level in the resonant mode, where switching losses are minimized by operating the IGBT at a resonant frequency for switching at the zero-crossing point of current or voltage. However, as heat rises during the metal hardening process, the metal exhibits parameter variations, which ultimately affect the overall system performance. Therefore, an inverter system with an effective load-adaptive control circuit with a short response time is highly sought in order to allow the system to operate efficiently regardless of any variation of the load parameters. This will return the system under resonant mode to allow the system to operate at its optimal performance. In this presentation, an analysis of a series resonant inverter performance is conducted, in which the control frequency is monitored and continuously adapted using a phase-locked loop aimed at metal hardening. The control technique is characterized by its flexibility, low-cost hardware and short response time. The system comprises microcontroller-based analogue and digital circuits. The experimental bench is built for the purpose in the range of 1 to 5 kW, with 1 to 50 kHz operating frequency. First, the process's principle and the design procedure of the inverter system with the proposed control scheme are described through modelling and simulation. Then, experimental results are presented to show the validity of this technique and to evaluate the system performance.