Heating device and image forming apparatus having a heating device with first and second modes

A heating device comprising:

• a rotatable film;
• a heater provided inside the film and including
• (a) a first heat generating member having a heat generation amount that increases from both longitudinal end portions toward a central portion, and
• (b) a second heat generating member having a heat generation amount that increases from a central portion toward both longitudinal end portions, the heat generation distribution of the second heat generating member being different from that of the first heat generating member in a longitudinal direction;
• a pressing roller configured to form a nip portion, with the heater through the film, in which a toner image formed on a recording material is heated to be fixed on the recording material;
• a first thermistor provided at a central portion in the longitudinal direction of the heater and configured to detect a temperature of the heater;
• a second thermistor provided at an end portion along in the longitudinal direction of the heater and configured to detect a temperature of the heater;
• and
• a controller configured to control the temperature of the heater based on detecting results of the first thermistor and the second thermistor,
• wherein the heating device is capable of switching between:
• (1) a first mode in which the temperature of the first heat generating member is controlled based on the detecting result of the first thermistor and the temperature of the second heat generating member is controlled depending on the control of the first heat generating member, without using the detecting result of the second thermistor for temperature control, and
• (2) a second mode in which the temperature of the first heat generating member is controlled based on the detecting result of the first thermistor and the temperature of the second heat generating member is independently controlled based on the detecting result of the second thermistor, wherein in the second mode, the electric power supplied to each heat generating member is determined based on a target temperature, the target temperatures for the first and second heat generating members are set independently from each other, and the temperature control of the first heat generating member and the second heat generating member is performed with a control gain that is at least twice as large as a control gain used in the first mode,
• wherein the controller switches to the first mode when the recording material is passing through the nip portion, and the controller switches to the second mode during a period from when a trailing end of a recording material precedingly conveyed passes through the nip portion until when a leading end of a subsequent recording material passes through the nip portion, and
• wherein the controller sets a target temperature of the heater in the first mode higher than a target temperature of the heater in the second mode, and sets a gain for temperature control of the heater in the first mode smaller than a gain for the temperature control of the heater in the second mode.