Patent ID: 7365290

Claim:
An induction heating apparatus for heating a region of a thin sheet by inductive heating to a temperature at least equal to a target temperature which is higher than a magnetic transformation point of the thin sheet, said apparatus comprising: a heating inductor comprising an inductive portion for heating inductively the entire region of the thin sheet simultaneously; a power supply device for applying a high-frequency current to the heating inductor; and a current controller for reducing the application of the high-frequency current to the heating inductor to allow the temperatures throughout the entire region of the thin sheet to become more equalized and then for increasing the application of the high-frequency current to the heating inductor to increase the temperature of the entire region of the thin sheet to the temperature at least equal to the target temperature, said current controller having a frequency tracker for tracking the high-frequency current of the heating inductor corresponding to an impedance of the region to be heated, and stopping or reducing temporarily the power application of the high-frequency current to said heating inductor when a resonant frequency of the high-frequency current becomes a predetermined frequency.