Image forming apparatus and heating method

A fixing unit, comprising:

• a first resistive heater element that increases in electrical resistance with increases in temperature;
• a second resistive heater element on a substrate with the first resistive heater;
• and
• a controller configured to:
• when a startup condition is satisfied, vary a duty ratio of electric power applied to the first resistive heater element for raising the temperature of the first resistive heater element to a target operating temperature, and vary a duty ratio of electric power applied to the second resistive heater element, and
• when the startup condition is not satisfied, set a constant duty ratio of electric power applied to the first resistive heater element for raising the temperature of the first resistive heater element to the target operating temperature.

An image forming apparatus for forming an image on a sheet, the apparatus comprising:

• an image forming unit configured to form an image on a sheet;
• and
• a fixing unit configured to fix the image on the sheet, the fixing unit including:
• a first resistive heater element that increases in electrical resistance with increases in temperature;
• a second resistive heater element on a substrate with the first resistive heater;
• and
• a controller configured to:
• when a first condition is satisfied, vary a duty ratio of electric power applied to the first resistive heater element for raising the temperature of the first resistive heater element to a target operating temperature, and vary a duty ratio of electric power applied to the second resistive heater element, and
• when a second condition different from the first condition is satisfied, set a constant duty ratio of electric power applied to the first resistive heater element for raising the temperature of the first resistive heater element to the target operating temperature.