Online Minimum Switching Frequency Tracking Technique for Improving Reliability of IPOS Induction Heating Systems

This paper proposes an induction heating (IH) system structured by an input-parallel output-series (IPOS) for ship plate pre-heating. The proposed IH system provides a wide heating range for heating various ship plates using the IPOS structure. However, the impedance of the ship plate changes depending on the material, thickness, size, and temperature, which result in resonant frequency variations. It cannot guarantee the switching operation of the IH inverter in the inductive load region. Under the capacitive load region, the power switches fail their ZVS operation, and the system cannot linearly control power. In this paper, the minimum switching frequency tracking technique is proposed to improve the reliability of the IPOS IH heating system by guaranteeing the inductive region operation. A resonant frequency is estimated by using the impedance estimation of the resonant network. The validity of the proposed technique is experimentally verified using a 4-kW IPOS prototype IH system.