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

1 . 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.

2 . A heating device according to claim 1 , wherein in the first mode the controller controls electric power to be supplied to the second heat generating member based on the detecting result of the first thermistor and a ratio of the electric power supplied to the second heat generating member to an electric power supplied to the first heat generating member.

3 . A heating device according to claim 2 , wherein the ratio of the electric power is set to be smaller as a width of the recording material, which is a length of the recording material in the longitudinal direction, is shorter.

4 . A heating device according to claim 1 ,

wherein the first thermistor is arranged in the central portion in the longitudinal direction, and

wherein the second thermistor is arranged at one end portion in the longitudinal direction.

5 . A heating device according to claim 1 , wherein in the second mode a partial heat generation amount of the heat generating member to be controlled, of the first heat generating member and the second heat generating member, is larger than a partial heat generation amount of the heat generating member not to be controlled in respective positions of the first thermistor and the second thermistor in the longitudinal direction.

6 . A heating device according to claim 5 , wherein in the first mode the partial heat generation amount of the heat generating member to be controlled is 1.1 times or more with respect to the partial heat generation amount of the heat generating member not to be controlled in respective positions of the first thermistor and the second thermistor in the longitudinal direction.

7 . An image forming apparatus comprising:

an image forming unit configured to form an unfixed toner image on a recording material; and

a heating device according to claim 1 , the heating device being configured to fix the unfixed toner image formed by the image forming unit onto the recording material.

8 . An image forming apparatus according to claim 7 , further comprising a cassette in which the recording material fed to a conveyance passage is accommodated; and

a width detecting unit provided in the cassette and configured to detect the width of the recording material.

9 . An image forming apparatus according to claim 8 , further comprising a recording material detecting unit provided upstream of the heating device in a conveyance direction of the recording material and configured to detect presence or absence of the recording material,

wherein the controller determines whether or not the recording material is passing through the nip portion based on the detecting result of the recording material detecting unit.