HEATING FIXING DEVICE

摘要

A heating fixing device has a simple structure and a function of preventing a heating member from being overheated excessively by detecting a temperature rise of the heating member with good follow-up ability, even immediately after temperature increase and independent of the operation mode such as a continuous operation. A threshold value set section (44) sets a threshold value differently with the mode such as a warm-up mode or a fixing operation mode. A threshold value judging section (43) judges the threshold value of a switching frequency controlled by a frequency control section (40) using the threshold values different with the mode. The frequency control section (40) varies the switching frequencies of switching elements (35, 36) to supply the power needed in each mode to an excitation coil (24). The frequency control section (40) prevents an excessive temperature rise in each mode by stopping the drive of the switching elements depending on the judgment result by the threshold judging section (43).