Heater control device, fixing device, and image forming apparatus

1. A heater control device that performs phase control of AC power and supplies phase-controlled power to a heater, the heater control device comprising:

a control unit configured to, in the phase control, gradually increase an on-duty ratio until a target power amount is supplied to the heater; and

a judgment unit configured to judge whether or not the on-duty ratio is within a predetermined range of on-duty ratios including a 50% on-duty ratio, wherein

the control unit (i) increases the on-duty ratio by a first amount while the on-duty ratio is within the predetermined range, and (ii) increases the on-duty ratio by a second amount while the on-duty ratio is not within the predetermined range, the first amount being greater than the second amount.

2. The heater control device of claim 1 , wherein

the first amount is such that, when the on-duty ratio is increased by the first amount, the on-duty ratio after increase is no longer within the predetermined range.

3. The heater control device of claim 2 , wherein

the first amount equals a difference between an upper limit and a lower limit of the predetermined range.

4. The heater control device of claim 1 , wherein

in addition to judging whether or not the on-duty ratio is within the predetermined range, the judgment unit judges whether or not the on-duty ratio is within another range of on-duty ratios that is separate from the predetermined range, the predetermined range and the other range being set due to a current value of a harmonic current of a specific order that is to be restricted having a plurality of peaks, the current value changing as the on-duty ratio changes and equaling or exceeding a predetermined threshold value at the peaks, and

the control unit increases the on-duty ratio by a third amount while the on-duty ratio is within the other range, the third amount being greater than the second amount.

5. A fixing device that fixes an unfixed toner image onto a recording sheet by causing the recording sheet to come into contact with a fixing rotational body, the fixing device comprising, as a power supply control unit for controlling power supply to a heater that heats the fixing rotational body, the heater control device of claim 1 .

6. The fixing device of claim 5 , wherein

the heater is a resistance heating element.

7. The fixing device of claim 6 , wherein

the fixing rotational body is a fixing belt that includes a heating layer composed of the resistance heating element.

8. The fixing device of claim 6 , wherein

the fixing rotational body is a fixing belt, and an elongated heating member including a heating layer composed of the resistance heating element comes into sliding contact with an inner circumferential surface of the fixing belt and heats the fixing belt.

9. The fixing device of claim 5 , wherein

the fixing rotational body is a hollow fixing roller, and the heater is a halogen heater that is disposed inside the hollow fixing roller.

10. An image forming apparatus comprising the fixing device of claim 5 .

11. A heater control device that performs phase control of AC power and supplies phase-controlled power to a heater, the heater control device comprising:

a control unit configured to, in the phase control, gradually increase an on-duty ratio until a target power amount is supplied to the heater by executing control of increasing the on-duty ratio by a fixed amount at a predetermined cycle; and

a judgment unit configured to judge whether or not the on-duty ratio is within a predetermined range of on-duty ratios including a 50% on-duty ratio, wherein

the control unit (i) executes the control at a first cycle while the on-duty ratio is within the predetermined range, and (ii) executes the control at a second cycle while the on-duty ratio is not within the predetermined range, the first cycle being shorter than the second cycle.

12. The heater control device of claim 11 , wherein

a cycle length of the first cycle is equal to or longer than a half-cycle of the AC power.

13. The heater control device of claim 11 , wherein

in addition to judging whether or not the on-duty ratio is within the predetermined range, the judgment unit judges whether or not the on-duty ratio is within another range of on-duty ratios that is separate from the predetermined range, the predetermined range and the other range being set due to a current value of a harmonic current of a specific order that is to be restricted having a plurality of peaks, the current value changing as the on-duty ratio changes and equaling or exceeding a predetermined threshold value at the peaks, and

the control unit executes the control at a third cycle while the on-duty ratio is within the other range, the third cycle being shorter than the second cycle.

14. A fixing device that fixes an unfixed toner image onto a recording sheet by causing the recording sheet to come into contact with a fixing rotational body, the fixing device comprising, as a power supply control unit for controlling power supply to a heater that heats the fixing rotational body, the heater control device of claim 11 .

15. The fixing device of claim 14 , wherein

the heater is a resistance heating element.

16. The fixing device of claim 15 , wherein

the fixing rotational body is a fixing belt that includes a heating layer composed of the resistance heating element.

17. The fixing device of claim 15 , wherein

the fixing rotational body is a fixing belt, and an elongated heating member including a heating layer composed of the resistance heating element comes into sliding contact with an inner circumferential surface of the fixing belt and heats the fixing belt.

18. The fixing device of claim 14 , wherein

the fixing rotational body is a hollow fixing roller, and the heater is a halogen heater that is disposed inside the hollow fixing roller.

19. An image forming apparatus comprising the fixing device of claim 14 .