Image forming apparatus having a developing bias control unit

An image forming apparatus comprising:

• an image support for supporting an electrostatic latent image on a surface of the image support;
• a developing unit having a developing agent support, the developing agent support retaining a developing agent, including a toner and carriers, contained in the developing unit, and the developing unit converting the latent image on the image support into a toner image by causing the toner to adhere to the surface of the image support;
• a developing bias supplying unit for supplying a developing bias voltage to the developing agent support of the developing unit, the developing bias voltage being one of a DC bias voltage and an AC bias voltage;
• a timer for measuring a non-driven period of the developing agent in the developing unit;
• and
• a control unit for selecting one of the DC bias voltage and the AC bias voltage from the developing bias supplying unit based on the non-driven period measured by the timer, and for controlling the developing bias voltage at an output of the developing bias supplying unit such that the selected one of the DC bias voltage and the AC bias voltage is supplied to the developing agent support.