Study Of Induction Heating Power Supply With LLC Resonant Load

Induction heating power supply is widely used in metal melting, diathermy, heating treatment, welding and so on. Its load impedance is variety, in order to make the load got the maximum heating power, the load matching is required. The method of electromagnetic coupling is usually applied to the traditional induction heating power supply, which uses the impedance transformation characteristics of high-frequency transformer to implement the load matching, but the high frequency transformer is always difficult to design, the volume of power supply is large, the cost is high and the efficiency is low. The electrostatic coupling method for implementing load matching mainly makes use of the passive components to change the circuit topology, accordingly, the load impedance is changed and the high-frequency transformer can be omitted under certain conditions. LLC resonant load is constituted by LC load of induction heating power supply and the inductance which is connected with the LC load.In the paper, the LLC resonant load is designed to implement the load matching on the basis of electrostatic coupling theory, and the characteristics and parameter design for LLC load resonant circuit are analyzed in detail. The theoretical analysis is verified by the simulation results.The phase-shifted control for induction heating power supply with LLC resonant load is discussed. The selection of control variables and the principle of phase-shifted control are analyzed, the calculation of output power and switching losses are provided. It has large turn-off losses for the induction heating power supply under phase-shifted control. Therefore, a novel inverter topology is presented, in which almost all of the inverter switches can achieve soft-switching. The theoretical analysis of phase-shifted control is proved by the simulation.Finally, the DSP-based control system of induction heating power supply is designed. According to the principle of analog phase-locked loop, the digital phase-locked loop is presented, and the principle of digital phase-locked loop to achieve frequency tracking is analyzed. The digital implementation of digital phase-locked loop combined with phase-shifted control and the hardware circuits of the control system which takes the TMS320F2812 as a core are given. Digital control system can simplify the hardware circuits of control loop, overcome the shortcomings of analog control circuit, for example, it is easy to aging, the circuit is complexity and so on. Furthermore, the software design is flexible, easy to implement various control algorithms to adapt to different induction heating process requirements.