New control method to increase power regulation in a AC/AC quasi resonant converter for high efficiency induction cooker

Quasi Resonant (QR) converters are widely used as AC current generators for induction. These converters are particularly attractive because they use just one solid state switch and only one resonant capacitor to generate a variable frequency/variable amplitude current to feed the induction coil. When properly designed and matched with their load, these converters are known to operate in ZVS at turn on and ZCS at turn off. For that reason QR converters are considered the best compromise between cost and energy conversion efficiency. However, when the power being regulated exceeds a given limit, the resonating voltage across the solid state switch exceeds its maximum rating, leading to instantaneous and irreversible damage of the switch itself. On the other end, operation at light load implies the loss of ZVS at turn on. Those two limitation lead to a relatively narrow regulation range defined as the ratio of maximum achievable power and the minimum achievable power. In particular, when the output power being regulated falls below a given limit, the inverter fails to operate in soft switching mode, leading to a dramatic and unmanageable increase in thermal losses and Electromagnetic interference. In this paper a new control algorithm to increase the power regulation ratio in AC/AC QR converters for induction cooktop is presented. The method described in this paper substantially increases the regulation range of QR converters with no penalties in efficiency and preserving soft switching operation. Furthermore, this method does not require any additional hardware circuitry or component but relies only on special SW modulation techniques. Simulation and experimental results confirm the benefits of this method compared a standard control, improving power regulation, and achieving a power increases higher than 20% for a typical domestic induction heating load.