Fixing device, image forming apparatus having the same and image forming method

Abstract

A fixing device, an image forming apparatus having the same include a fixing member having an outer peripheral face with which a sheet to be transported is brought into pressure contact, an exciting coil for induction heating of a heat generating layer of the fixing member, and an RF power supply circuit for applying a voltage of a certain drive frequency to the exciting coil. A degaussing coil is placed in small size sheet non-passing regions corresponding to end sections of the outer peripheral face of the fixing member with respect to the width direction of the sheet. A changeover switch opens or closes the degaussing coil in accordance with the size of the sheet to be transported. A control section is provided for changing and setting a drive frequency of the RF power supply circuit along with opening or closing of the degaussing coil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a view showing an outlined structure of a fixing device in one embodiment of the present invention;

FIG. 2 is a view showing a cross sectional structure of a fixing roller in the fixing device;

FIG. 3 is a view showing a cross sectional structure of a pressure roller in the fixing device;

FIG. 4 is a view showing the fixing device in FIG. 1 as viewed from the top;

FIG. 5 is a schematic view showing the layout of an exciting coil and degaussing coils in the longitudinal direction of the fixing roller, and more particularly their corresponding relationship with a region through which a paper sheet passes;

FIG. 6 is a view showing a structure of a changeover switch in the fixing device;

FIG. 7 is a view explaining a method for observing an inductance L_S and an effective resistance R_S of an IH unit in the fixing device;

FIG. 8 is a view specifically showing an example in which an RF inverter for applying a current to the IH unit in the fixing device is constituted of a parallel resonant circuit;

FIG. 9 is a view showing drive waveforms of the parallel resonant circuit constituting the RF inverter of the fixing device;

FIG. 10 is a view specifically showing an example in which the RF inverter for applying a current to the IH unit in the fixing device is constituted of a series resonant circuit;

FIG. 11 is a view showing drive waveforms of the series resonant circuit constituting the RF inverter of the fixing device;

FIG. 12 is a view showing an inductance L_S and an effective resistance R_S of the IH unit in the fixing device in states of the degaussing coil being opened and closed, respectively;

FIG. 13 is a graph showing a relationship (resonance waveforms) between electric power inputted into the IH unit and drive frequencies in states of the degaussing coil 34 being opened and closed, respectively;

FIG. 14 is a flowchart showing a control sequence executed by a control circuit in the fixing device;

FIG. 15 is a view showing a part excerpted from a drive frequency table for the fixing device; and

FIG. 16 is a view showing a structure of an image forming apparatus having the fixing device in one embodiment of the present invention.

Claims

1. A fixing device, comprising: a fixing member having an outer peripheral face with which a sheet to be transported is brought into pressure contact;an exciting coil placed along the outer peripheral face of the fixing member to form an elongated shape with respect to a width direction of the sheet to be transported for induction heating of a heat generating layer of the fixing member;an RF power supply circuit for applying a voltage of a certain drive frequency to the exciting coil so as to make the heat generating layer of the fixing member generate heat through the exciting coil;a degaussing coil placed in a small size sheet non-passing region corresponding to a part of the outer peripheral face of the fixing member with respect to the width direction of the sheet along the exciting coil;a changeover switch for opening or closing the degaussing coil in accordance with a size of the sheet to be transported; anda control section for changing and setting a drive frequency of the RF power supply circuit along with opening or closing of the degaussing coil.

2. The fixing device according to claim 1, wherein the RF power supply circuit includes a capacitor connected in series to the exciting coil to equivalently constitute a series resonant circuit, andwhen the degaussing coil is changed from close to open, the control section moves the drive frequency to a side higher than a resonance frequency of the series resonant circuit in a state of the degaussing coil being opened, whereas when the degaussing coil is changed from open to close, the control section moves the drive frequency to a side higher than the resonance frequency of the series resonant circuit in a state of the degaussing coil being closed.

3. The fixing device according to claim 1, wherein the RF power supply circuit includes a capacitor connected in series to the exciting coil to equivalently constitute a series resonant circuit, andwhen the degaussing coil is changed from close to open, the control section moves the drive frequency in accordance with a resonance frequency of the series resonant circuit shifting to a lower side, whereas when the degaussing coil is changed from open to close, the control section moves the drive frequency in accordance with the resonance frequency of the series resonant circuit shifting to a higher side.

4. The fixing device according to claim 1, comprising: a drive frequency table containing predetermined values of the drive frequencies which should be set in accordance with set power amounts for making the fixing member generate heat and with opening or closing of the degaussing coil.

5. The fixing device according to claim 1, wherein the control section moves the drive frequency of the RF power supply circuit by a specified amount along with opening or closing of the degaussing coil.

6. The fixing device according to claim 5, wherein the amount of the drive frequency of the RF power supply circuit that the control section moves along with opening or closing of the degaussing coil is constant over an entire region of set power amounts for making the fixing member generate heat.

7. The fixing device according to claim 5, wherein the amount of the drive frequency of the RF power supply circuit that the control section moves along with opening or closing of the degaussing coil is variably set in accordance with a certain set power amount for making the fixing member generate heat.

8. The fixing device according to claim 1, wherein the control section controls the RF power supply circuit in such a way that after changing and setting the drive frequency of the RF power supply circuit along with opening or closing of the degaussing coil, an actual amount of electric power for making the fixing member generate heat continues to be a certain set power amount.

9. The fixing device according to claim 8, wherein after changing and setting the drive frequency of the RF power supply circuit, the control section maintains the drive frequency of the RF power supply circuit on a side slightly higher than the resonance frequency of the series resonant circuit.

10. The fixing device according to claim 1, wherein the fixing member is a cylindrical roller.

11. The fixing device according to claim 1, wherein the fixing member is an endless belt.

12. The fixing device according to claim 1, wherein the small size sheet non-passing region corresponds to end sections on both sides of the outer peripheral face of the fixing member with respect to the width direction of the sheet.

13. An image forming apparatus, comprising: an image forming section for attaching toner onto a sheet; andthe fixing device according to claim 1.

14. An image forming method with use of an image forming section for attaching toner onto a sheet and the fixing device according to claim 1, comprising the steps of: attaching toner onto a sheet and feeding the sheet to the fixing device by the image forming section;opening or closing the degaussing coil in accordance with a size of the sheet, and changing and setting a drive frequency of the RF power supply circuit along with opening or closing of the degaussing coil by the control section; andfixing the toner on the sheet by applying a voltage of the changed drive frequency to the exciting coil by the RF power supply circuit so as to make the heat generating layer of the fixing member generate heat through the exciting coil.