Induction heating device and image forming apparatus

1. An induction heating device comprising:

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 which is detected by the peak voltage detector and the effective value of the input voltage which is detected by the effective voltage detector; and

a power controller which achieves control of power to the fuser by controlling a duty ratio of ON and OFF periods of the switching element,

wherein the power controller calculates the duty ratio of ON and OFF periods of the switching element based on a value of power needed to supply the fuser, the effective value of the input voltage which is detected by the effective voltage detector, a crest factor for a sine-wave input voltage, and the actual crest factor of the input voltage which is calculated by the crest factor calculator, and

wherein the power controller calculates the duty ratio of ON and OFF periods of the switching element by using the ratio of the crest factor for a sine-wave input voltage to the actual crest factor of the input voltage which is calculated by the crest factor calculator.

2. The induction heating device as recited in claim 1 , wherein: if the actual crest factor of the input voltage which is calculated by the crest factor calculator is higher than the crest factor for a sine-wave input voltage, the power controller makes the duty ratio lower than that which is calculated if it is identical with the crest factor for a sine-wave input voltage; and if the actual crest factor of the input voltage which is detected by the crest factor calculator is lower than the crest factor for a sine-wave input voltage, the power controller makes the duty ratio higher than that which is calculated if it is identical with the crest factor for a sine-wave input voltage.

3. The induction heating device as recited in claim 1 , wherein the power controller adjusts the duty ratio to the calculated value of the duty ratio, at least during a half cycle of the input voltage obtained from an alternating current voltage by rectification.

4. The induction heating device as recited in claim 1 , wherein there are two points at which a half cycle of the input voltage obtained from an alternating current voltage by rectification and a sine-wave input voltage intersect with each other, and these two points are defined to be T 1 and T 2 (T 1 <T 2 ): the power controller calculates a duty ratio based on the ratio of the crest factor for a sine-wave input voltage to the actual crest factor of the input voltage, when a time t satisfies the following formula: T 1 ≦t≦T 2 ; and the power controller calculates a duty ratio based on the inverse ratio of the crest factor for a sine-wave input voltage to the actual crest factor of the input voltage, when the time t satisfies either of the following formulas: t<T 1 and T 2 <t.

5. The induction heating device as recited in claim 4 , wherein for a certain period including the time t satisfying either of the following formulas: t=T 1 and t=T 2 , the power controller adjusts the duty ratio to a value which is calculated if the actual crest factor of the input voltage is identical with the crest factor for a sine-wave input voltage.

6. The induction heating device as recited in claim 1 , wherein if a duty ratio of ON and OFF periods of the switching element is calculated based on the ratio of the crest value for a sine-wave input voltage to the actual crest value of the input voltage which is calculated by the crest factor calculator, a pulse width for the duty ratio is further calculated using the following formulas: PW=A×W+B; A=(α×Vrms×√{square root over (2)}/CF)+α′; and B=(β×Vrms×√{square root over (2)}/CF)+β′ (PW: pulse width; W: necessary value of power; Vrms: effective value of the input voltage; α, α′, β, and β′; constant values, and CF=Vp/Vrms wherein Vp is the peak value of the input voltage).

7. The induction heating device as recited in claim 1 , wherein if a duty ratio of ON and OFF periods of the switching element is calculated based on the ratio of the crest factor for a sine-wave input voltage to the actual crest factor of the input voltage which is calculated by the crest factor calculator, a pulse width for the duty ratio is further calculated using the following formulas: PW=A×W 2 +B×W+C; A=(α×Vrms×√{square root over (2)}/CF)+α′; B=(β×Vrms×√{square root over (2)}/CF)+β′; and C=(γ×Vrms×√{square root over (2)}/CF)+β′ (PW: pulse width; W: necessary value of power; Vrms: effective value of the input voltage; α, α′, β, and β′; constant values, and CF=Vp/Vrms wherein Vp is the peak value of the input voltage).

8. An image forming apparatus comprising:

a fuser provided with a workpiece to be heated;

the induction heating device as recited in claim 1 to heat the workpiece; and

a direct current power supplying device which performs rectification on an alternating current voltage to supply each of parts of the image forming apparatus with a direct current voltage, wherein the direct current power supplying device includes an effective voltage detector which detects an effective value of the input voltage of the induction heating device.