Abstract: An induction heating device including a coil which inductively heats a workpiece of a fuser with an input voltage obtained from an alternating current voltage by rectification; a switching element coupled in series with the coil; a peak voltage detector which detects a peak value of the input voltage; an effective voltage detector which detects an effective value of the input voltage; a crest factor calculator which calculates an actual crest factor of the input voltage based on the peak value of the input voltage and the effective value of the input voltage; and a power controller which achieves control of power to the fuser by controlling the duty ratio of ON and OFF periods of the switching element.

Abstract: An induction heating device comprises: a resonant circuit provided with a coil heating a workpiece inductively and a capacitor, the coil and the capacitor being coupled in parallel with each other; a switching element coupled in series with the resonant circuit; a power supply which applies to the resonant circuit, a direct-current input voltage whose instantaneous value can be changed with the lapse of time; and a power controller which controls power to the workpiece by turning the switching element ON at a certain time and changing an ON period of the switching element, and the power controller is characterized by turning the switching element ON with a delay of an integral multiple of a resonant period of the resonant circuit if a value of power to the workpiece is equal to or lower than a first threshold which is predetermined.

Abstract: While a recording sheet is passing through a fixing nip formed between a heating roller 41 having a heating layer and a pressing roller 42 pressed against the roller 41, a coil 43 is exited by high-frequency power from a power source and causes the layer to generate heat. An unfixed image on the recording sheet is fixed by the heat. The power source detects a zero cross timing of a rectified alternating current, detects an instantaneous value at a point a predetermined period after the zero cross timing, calculates an effective power of the alternating current based on the instantaneous value, and controls the output power to be a desired value determined based on the effective power and the surface temperature of the roller 41. Thus power input to the power source can be quickly detected and power output to the coil can be quickly and stably controlled.