Fixing apparatuses mounted in copying machines or printers include an apparatus having an endless belt, a ceramic heater that comes in contact with the inner surface of the endless belt, and a pressure roller that forms a fixing nip portion with the ceramic heater with the endless belt therebetween. When an image forming apparatus including such a fixing apparatus performs continuous printing using small-sized sheets, a phenomenon (temperature rise in a sheet non-passing area) occurs in which the temperature of a region through which the sheets do not pass in the longitudinal direction of the fixing nip portion gently increases. If the temperature of the sheet non-passing area becomes too high, individual parts in the apparatus may be damaged, or if printing is performed using a large-sized sheet during a temperature rise in the sheet non-passing area, high-temperature offset of toner may occur in an area corresponding to the sheet non-passing area of small-sized sheets.
One of conceived techniques for suppressing a temperature rise in the sheet non-passing area is that a heat generating resistor on a ceramic substrate is formed of a material having a negative resistance temperature characteristic. The concept is that even if the temperature of the sheet non-passing area rises, the resistance value of a heat generating resistor in the sheet non-passing area decreases and therefore heat generation in the sheet non-passing area can be suppressed even if a current flows in the heat generating resistor in the sheet non-passing area. The negative resistance temperature characteristic is a characteristic in which as temperature increases, resistance decreases, and is hereinafter referred to as NTC (Negative Temperature Coefficient). Conversely, it is also conceived that the heat generating resistor is formed of a material having a positive resistance temperature characteristic. The concept is that if the temperature of the sheet non-passing area rises, the resistance value of the heat generating resistor in the sheet non-passing area rises and the current flowing in the heat generating resistor in the sheet non-passing area is suppressed so that heat generation in the sheet non-passing area can be suppressed. The positive resistance temperature characteristic is a characteristic in which as temperature increases, resistance increases, and is hereinafter referred to as PTC (Positive Temperature Coefficient).
In general, however, materials with NTC have a very high volume resistivity, and it is very difficult to set the total resistance of a heat generating resistor formed in a single heater within a range covered by a commercial power supply. Conversely, materials with PTC have a very low volume resistivity, and, as in the case of those with NTC, it is very difficult to set the total resistance of a heat generating resistor in a single heater within a range covered by a commercial power supply.
Therefore, a heat generating resistor formed on a ceramic substrate is divided into a plurality of blocks in the longitudinal direction of a heater, and in each block, two electrodes are arranged at the ends of the substrate in the lateral direction so that a current can flow in the lateral direction of the heater (the direction in which recording paper is conveyed). Further, a configuration in which a plurality of blocks are electrically connected in series is disclosed in PTL 1. With the above shape, if the heat generating resistor is made of a material with NTC, the resistance value of each block is low, and the total resistance of the overall heater can be kept lower than that if a current flows in the longitudinal direction of the heater. Further, when the heat generating resistor is made of a material with PTC, the total resistance of the overall heater can be made higher than that if a current flows in the lateral direction of the heater without dividing the heat generating resistor into a plurality of blocks.
Note that if a heat generating resistor is divided into a plurality of heat generating blocks, there is a space between adjacent heat generating blocks, leading to variations in the heat generation distribution. Thus, in PTL 1, heat generating blocks are formed into a parallelogram shape so as to prevent formation of a region where heat is not generated in the longitudinal direction of the heater.