Abstract: The present invention relates to a switching power supply a microwave oven in which DC power is changed to a pulse form by means of a switching element coupled to a primary winding of an inverter transformer to supply the power to a high frequency oscillator (hereinafter referred to as a magnetron) coupled to a secondary winding. A reference voltage is set lower than an ordinary state from the time of turning on until the oscillation of the magnetron starts. Accordingly, power supplied from the secondary winding of the inverter transformer to the magnetron is set to a low level. The reference voltage increases when oscillation of the magnetron starts, and returns to the ordinary state when the oscillation returns to the ordinary state.

Abstract: An induction heating cooking apparatus comprises a self-controlled inverter which comprises a series connection of an induction coil and a switching element, a resonance capacitor connected in parallel with the switching element, and a flywheel diode. An inverter driver circuit triggers the inverter with a starting pulse obtained from a starting signal generator. A voltage signal associated with a current flowing through the switching element is obtained by means of a current transformer. A voltage source of the inverter is implemented as a ripple current voltage source. A comparator of a gate signal generator compares a reference level changing in accordance with the ripple current voltage and the voltage detected by the current transformer, thereby rendering the switching element non-conductive when the voltage level reaches the reference level.