1.  Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively fixing a toner image on a recording sheet using induction heating in a small-size fixing unit having a simple structure produced at a low cost. 
2.  Description of the Related Art
Background image forming apparatuses, such as copiers and printers, include fixing units using an induction heating method.  The induction heating method may shorten a time period required for the fixing units to become operable after the fixing units are powered on, and may reduce energy consumption. 
One example of the fixing units includes a fixing belt, a support roller, an auxiliary fixing roller, an induction heater, and a pressure roller.  The fixing belt is laid across the support roller and the auxiliary fixing roller.  The induction heater faces the support roller via the fixing belt.  The pressure roller faces the auxiliary fixing roller via the fixing belt.  The induction heater includes an exciting coil and a core.  The exciting coil is provided along the core and extends in directions parallel to a surface of a recording sheet in conveyance and perpendicular to a conveyance direction of the recording sheet which is conveyed between the pressure roller and the auxiliary fixing roller. 
A high-frequency alternating current is applied to the exciting coil to generate a magnetic field around the exciting coil.  The magnetic field induces an eddy current near a surface of the support roller.  An electrical resistance of the support roller generates heat.  The heat is transferred to the fixing belt from the support roller.  The heated fixing belt heats and fixes a toner image on the recording sheet at a position where the pressure roller and the auxiliary fixing roller oppose each other.  In this fixing unit, it is possible to increase a surface temperature of the fixing belt to a target fixing temperature in a short time period without consuming much energy. 
Another example of the fixing units includes a fixing roller and two exciting coils.  The fixing roller includes a hollow cylinder.  One of the exciting coils is disposed in an interior of the cylinder.  The other exciting coil is disposed above and along an outer circumferential surface of the cylinder.  Each of the exciting coils includes a wire wound a plurality of times. 
In the above fixing units, however, the induction heater including the exciting coil and the core has a complicated structure.  The exciting coil including the wire wound the plurality of times requires a supporting member, and the supporting member has a complicated structure so that it may properly support the exciting coil.  Those problems make it difficult to produce a small-size fixing unit at a low cost.