DPLL in Cooperated with PI Controller for HF-Inverter Control System

This paper presents a HF (high frequency)-inverter using a digital phase-locked loop (DPLL) technique in cooperated with PI controller to ensure its output power factor up to unity and operating frequency tracking the load-resonant frequency rapidly and precisely. 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. Using the DPLL, it can make both frequencies and phases of the output voltage U₀ and its current I0 achieve to be identical, so that the inverter always operates in the load-resonant or quasi-load resonant condition in steady-state. Furthermore the PI-DPLL control performance and the dynamic responses of the inverter system are emulated and compared by MATLAB/SIMULINK. Finally an experimental induction heating power has been implemented to evaluate the control performances. The cost-effective prototype HF-inverter power is operated successfully at outputs up to 40 Kw, at 60 KHz of switching frequency and input DC540 V