Research On The Frequency Tracking System Of HF-Inverter Power Controlled By PI-DPLL

In the case of the inverter control system with a load series resonant circuit, the equivalent load parameters, e.g., resonant capacitor, resistance and inductance are variable along with their operating states and temperatures so as to vary the inherent resonant frequency. Hence the di/dt stresses on the insulated gate bipolar transistors (IGBTs) used in the inverter as switching devices and their switching losses are enlarged. In order to get the maximum output power and its conversion efficiency while the resonant frequency is changed, a PLL control scheme is often applied to keep the inverter-operating frequency tracking the series load-resonant frequency to maintain the output current and voltage in identical phase and the inverter system operating in zero-current-switching (ZCS) mode. However the pull-in process for PLL tends to be slow in generally, it will cause the settling time of the control system too long and the PLL synchronization will lose maybe when there is a sudden load disturbance. As a result, the DPLL in cooperated with PI controller for HF-Inverter control system is introduced in this paper.In the dynamic condition, when the error between the frequencies of the operation and load resonance of the proposed inverter and its frequency is greater than or equal to a specified error range, only the PI controller is utilized to pull rapidly the operating frequency into the phase-locked loop locking range. Otherwise the operating frequency is only controlled by the DPLL when the error is smaller than the presetting range.The model of PLL is constructed in PSPICE environment by analyzing the principle and characteristic of PLL. Also the models of the inverter controlled by the PI-DPLL are constructed in MATLAB/SIMULINK environment. It provides a useful and realizing method to analyze as well as design a PI-DPLL controller in inverters.At last, an induction heating power supply by PI-DPLL control is designed and turned out, combined with DSP technology. It is operated successfully at outputs up to 10kw, at 50kHz of switching frequency and input DC540V.The results of simulation and experiment show that the HF-Inverter using the DPLL technique in cooperated with PI controller has simple main circuit, good characteristic of frequency tracking, high output frequency, high output stabilization, high input power factor, also it indicates that the DPLL technique in cooperated with PI controller has a brilliant future in application in the HF-Inverter power.