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FPGA-Based Gain-Scheduled Controller for Control Optimization of Resonant Converters Applied to Domestic Induction Heating

Fig. 1. Induction heating appliances: main parts of an induction heating cooktop.
Induction heating has become the technology of choice for domestic applications (Fig. 1) due to advantages in efficiency, quickness, safety, and accurate power control. One of the main elements to achieve this performance is the resonant inverter used to supply the inductor that generates the electromagnetic field to heat the induction target, i.e. the pot.

The resonant tank of the inverter is composed of the inductor-pot system. One of the most challenging points when designing an IH appliance is to accurately adjust the control system, since the resonant tank changes with the frequency, temperature, pot materials and geometry. In this paper, a gain-scheduled controller is proposed to improve the dynamic performance under different operating conditions. The proposed approach takes advantage of digital control techniques to obtain improved performance and safety. The proposed controller (Fig. 2) combines information about the operating conditions, i.e. inverter switching frequency and duty cycle, with a real-time impedance identification system to adapt the controller gain. As a result, transient performance is improved over the wide range of operating and load conditions of an induction heating appliance.

The proposed approach has been tested with a 3-kW resonant inverter. Fig. 3 shows several transients comparing a classical PI controller with the gain-scheduled one. It can be seen that the proposed controller improves the performance in complete operation range, whereas the PI controller is just optimized for a single operation point.


Fig. 2. Proposed gain-scheduled controller block diagram.
Fig. 3. Experimental results for several output power steps at different operating conditions for a standard PI controller and the proposed gain-scheduled controller.
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