Inverter power source control for high frequency heater

1. An inverter power source control circuit for a high frequency heating apparatus, which is provided in an inverter power source circuit for supplying an electric power to a magnetron of the high frequency heating apparatus and operable to generate an oscillation control signal by full wave rectifying and converting an alternating current output from an alternating current power source to a direct current output and applying the oscillation control signal, based on a PWM control signal through an inverter to a switching element to thereby convert the direct current output to a desired alternating current output, which circuit comprises;

a control reference voltage setting circuit for outputting a reference voltage setting signal useable as a benchmark for generation of the oscillation control signal of a voltage set in correspondence with fluctuation of the PWM control signal, wherein by means of the oscillation control signal having the voltage so set, a current waveform of the alternating current output can be approximated to a sinusoidal wave by generating an electric current at both sides of the current waveform of the alternating current output;

an input waveform adjusting circuit for adjusting a peak portion of an input waveform, applied to the inverter to generate a peak adjustment signal required to generate the oscillation control signal, to a sinusoidal wave, wherein by means of the oscillation control signal having this peak adjustment signal, the current waveform of the alternating current output can be approximated to the sinusoidal wave through subtraction of the peak portion of the waveform using the peak adjustment signal of the sinusoidal wave; and

an oscillation control signal generating circuit for generating the oscillation control signal based on a waveform shaping signal, comprised of the reference voltage setting signal and the peak adjustment signal, and a triangular wave signal from an oscillator, wherein by means of the oscillation control signal containing the reference voltage setting signal and the peak adjustment signal, even though the PWM control signal fluctuates, the current waveform of the alternating current output from the alternating current power source can be approximated to a sinusoidal wave to thereby improve a power factor of an electric power source circuit.

2. The inverter power source control circuit for a high frequency heating apparatus as claimed in claim 1 , wherein the control reference voltage setting circuit includes a ripple generating circuit for generating a ripple in one of the PWM control signal or a detected and rectified signal, which is a detected signal outputted from a current transformer for detecting the alternating current output from the alternating current power source, a first voltage comparator for comparing the PWM control signal and the detected and rectified signal, either of which contains the ripple, with each other when the both are applied thereto, and an integrating circuit having a small time constant for smoothing an output from the first voltage comparator.

3. The inverter power source control circuit for a high frequency heating apparatus as claimed in claim 2 , further comprising an integrating circuit having a large time constant for smoothing the PWM control signal or an output of the detected and rectified signal, respectively, and

wherein the ripple generating circuit is provided in either one of those integrating circuits.

4. The inverter power source control circuit for a high frequency heating apparatus as claimed in claim 1 , wherein the input waveform adjusting circuit includes a resistor having a high resistance and a Zener diode connected in series with the resistor, wherein by utilization of a characteristic of the Zener diode when the electric current is lowered with the resistor having the high resistance, an input waveform of the full wave rectified output to be applied to the inverter is adjusted.

5. The inverter power source control circuit for a high frequency heating apparatus as claimed in claim 1 , wherein the oscillation control generating circuit includes a second voltage comparator and a third voltage comparator, each forming a differential amplifier, wherein when a signal having the adjusted input waveform from the inverter and an output from the third voltage comparator are fed to the second voltage comparator, the second voltage comparator outputs the peak adjustment signal and, also, when the reference voltage setting signal from the control reference voltage setting circuit and the peak adjustment signal from the second voltage comparator are fed to the third voltage comparator, the third voltage comparator outputs the waveform shaping signal comprised of the reference voltage setting signal and the peak adjustment signal.

6. The inverter power source control circuit for a high frequency heating apparatus as claimed in claim 1 , wherein the high frequency heating apparatus is a microwave oven.

7. The inverter power source control circuit for a high frequency heating apparatus as claimed in claim 1 , wherein the switching element comprises an insulated gate bipolar transistor element.