Fixing device and image formation device

1. A fixing device including an endless belt with a resistance heating layer, a pressure roller fitting loosely in a circulation path of the belt, and a pressing roller pressing the pressure roller through the belt to form a fixing nip in conjunction with a surface of the belt, and thermally fixing an unfixed image on a recording sheet by passing the recording sheet through the fixing nip, the fixing device comprising:

a pair of annular electrodes provided circumferentially so as to sandwich a sheet-passing region on the surface of the belt therebetween;

a first power supply member pressurizing a given one of the electrodes with pressing force; and

a second power supply member positioned closer to the fixing nip than the first power supply member, pressurizing the given one of the electrodes with pressing force, and supplying power to the resistance heating layer in cooperation with the first power supply member, wherein

the pressing force applied by the first power supply member is weaker than the pressing force applied by the second power supply member.

2. The fixing device of claim 1 , wherein

when the pressure roller is viewed along a rotational axis thereof, four regions are defined by a first line passing through a pressure roller centre and a pressing roller centre, and by a second line perpendicular to the first line passing through the pressure roller centre,

a first region includes an upstream edge of the fixing nip, with respect to a sheet-passing direction,

a second region includes a downstream edge of the fixing nip, with respect to the sheet-passing direction, and

a contact surface area between the first power supply member and the belt is at least partly located within the first region or within the second region.

3. The fixing device of claim 2 , wherein

the contact surface area between the first power supply member and the belt is at least partly located within the first region.

4. The fixing device of claim 1 , further comprising:

a support member supporting the first power supply member and the second power supply member and freely swinging about a support shaft parallel to a rotational axis of the pressure roller, and

a biasing member biasing the support shaft toward the belt, wherein

the support shaft is positioned with respect to the support member such that a pressing force applied to the belt by the second power supply member under the bias is greater than a pressing force applied to the belt by the first power supply member under the bias.

5. The fixing device of claim 4 , wherein

the support member comprises a conductive member and is molded integrally with the first power supply member and the second power supply member.

6. The fixing device of claim 1 , wherein

each member of the pair of electrodes is a metal film having lower electrical resistance than the resistance heating layer.

7. An image formation device comprising a fixing device including an endless belt with a resistance heating layer, a pressure roller fitting loosely in a circulation path of the belt, and a pressing roller pressing the pressure roller through the belt to form a fixing nip in conjunction with a surface of the belt, and thermally fixing an unfixed image on a recording sheet by passing the recording sheet through the fixing nip, the fixing device comprising:

a pair of annular electrodes provided circumferentially so as to sandwich a sheet-passing region on the surface of the belt therebetween;

a first power supply member pressurizing a given one of the electrodes with pressing force; and

a second power supply member positioned closer to the fixing nip than the first power supply member, pressurizing the given one of the electrodes with pressing force, and supplying power to the resistance heating layer in cooperation with the first power supply member, wherein

the pressing force applied by the first power supply member is weaker than the pressing force applied by the second power supply member.