Small load detection by comparison between input and output parameters of an induction heat cooking apparatus

Claim 1

An induction heat cooking apparatus comprising: a semiconductor power-rated switching device, a resonant circuit formed by an induction heating coil and a capacitor means for converting a low frequency input into a high frequency output in response to the conduction of said semiconductor power-rated switching device and for heating an inductive load placed in overlying relation with said heating coil, an input detector means for sensing said low frequency input, an output detector means for sensing said high frequency output, reference means for generating a reference voltage corresponding to a user's power setting level, a feedback circuit including: a reference crosspoint detector means for sensing when said high frequency output reaches a predetermined voltage level, and a pulse-width modulated pulse generator means responsive to the output of said reference crosspoint detector means for generating a gating pulse having a duration which is a function of said reference voltage for gating said switching device into conduction thereby controlling the power level of said high frequency output to tend toward said power setting level and to attain a high value in comparison with said low frequency input when said inductive load is lower than a predetermined value, and a comparator means for comparing the outputs of said input and output detector means to generate a comparator output when the sensed high frequency output is greater in magnitude than the sensed low frequency input, and means for inhibiting said apparatus in response to said comparator output.