Patent ID: 7831163

Claim:
A heat device, which heats a developed image and fixes the developed image onto a sheet while nipping and conveying the sheet bearing the developed image by a nip portion, comprising: a fixing film having a sleeve shape; a heat source including a first heat generation member in which a heat generation amount at a center portion in a longitudinal direction of said heat source is larger than heat generation amounts at end portions in the longitudinal direction of said heat source and a second heat generation member in which heat generation amounts at end portions in the longitudinal direction of said heat source are larger than a heat generation amount at a center portion in the longitudinal direction of said heat source, said heat source contacting an inner surface of said fixing film; a pressure member for forming the nip portion with the heat source through the fixing film therebetween, a temperature detecting unit for detecting a temperature of the heat source; and a controller for controlling the heat source so that a temperature detected by the temperature detecting unit is maintained at a target temperature, wherein said controller calculates a turn-on ratio (Xn) corresponding to a required-power to maintain a temperature of said heat source at the target temperature by a proportional plus integral control according to a difference (ΔV) between the temperature detected by the temperature detecting unit and the target temperature, and controls said heat source according to the calculated required-power, wherein the turn-on ratio (Xn) is calculated based on a formula represented by Xn=In+ΔP, where ΔP represents a variable on the proportional control corresponding to the difference (ΔV), and In represents a variable on the integral control corresponding to the integration value Vn of the difference (ΔV), and wherein, during continuous conveyance of a small-size sheet, in a case where the variable (In) controlled by the proportional plus integral control in a time when the sheet is discharged from the nip portion is equal to or more than a predetermined value (Y 1 ), said controller maintains a turn-on duty ratio of a turn-on duty of the second heat generation member to a turn-on duty of the first heat generation member, and in a case where the variable (In) controlled by the proportional plus integral control in a time when the sheet is discharged from the nip portion is smaller than the predetermined value (Y 1 ), said controller changes the turn-on duty ratio of a turn-on duty of the second heat generation member to a turn-on duty of the first heat generation member to reduce the turn-on duty ratio.