Automatic frequency control circuit null of high frequency inverter for induction heating

摘要

PURPOSE:To provide an automatic frequency control circuit of a high-frequency inverter for induction heating which provides a very precise and wide-range control. CONSTITUTION:A current signal detected by a current detector is lagged by a phase angle of about 90 deg. by means of a capacitor C and then is further lagged by thetaf (amp) by means of a first amplifier 12 and then is given to a zero-cross detector 13. A delay circuit 14 delays a switching device off-signal by Td. The output signals of the delay circuit 14 and the zero-cross detector 13 are used as set and reset signals of a signal generator 15 respectively and then the output signal thetas with no frequency component is obtained. The signal thetas is averaged by a second amplifier 16. The theta signal is input to a V/F converter circuit 21 as a control signal and then a frequency signal corresponding to the input voltage is output. After receiving the output of the V/F converter circuit, a switching device driving circuit 22 drives switching devices Q1-Q4 so as to obtain a phase difference.