Field of the Invention
The present invention relates to an image forming apparatus.
Description of the Related Art
In an electrophotographic image forming apparatus, charging of a surface of a photosensitive drum is performed uniformly by a charger, and the charged surface of the photosensitive drum is exposed by a laser beam which is modulated in accordance with image data and irradiated by an exposure unit, thereby forming an electrostatic latent image. Then, by a developer (hereinafter referred to as a toner) supplied from a developing unit, the electrostatic latent image is developed and a toner image is thereby formed.
For photosensitive drum surface charge processing, an AC charging scheme that charges by causing a charging roller to abut the surface of the photosensitive drum, for example, applying a voltage that overlaps a direct current voltage and an alternating voltage to the charging roller, and discharging to the surface of the photosensitive drum is employed. In this AC charging scheme, the surface of the photosensitive drum can be charged uniformly because discharge to the positive side and to the negative side occurs alternatingly because voltage that overlaps an alternating voltage on a direct current voltage is applied.
A class-D amplifier is used for an alternating voltage generation circuit for generating an alternating voltage applied to this charging roller. This class-D amplifier converts an input signal into a pulse width modulation signal (a PWM signal), and drives a bridge circuit by the PWM signal. With this, compared to a conventional analog amplification scheme, the amplification signal can be obtained with high efficiency. With such an alternating voltage generation circuit, a high-voltage alternating voltage is caused to occur on a secondary side of a transformer by driving a primary side of the transformer by a class-D amplifier output (refer to Japanese Patent Laid-Open No. 2013-65932, Japanese Patent Laid-Open No. H7-241083, for example).
However, because the class-D amplifier performs switching driving of a bridge circuit at a high frequency, a radiation noise occurs, and because a ferrite bead or a filter is used as a noise counter-measure, this leads to a cost increase. Also, if the class-D amplifier is used by a charging high-voltage circuit of a color image forming apparatus having 4 image forming units for yellow, magenta, cyan, and black, it is necessary to charge each photosensitive drum for the four colors. For this reason, there is the problem that the frequencies of the PWM signals of the class-D amplifiers used corresponding to respective color photosensitive drums overlap, and radiation noise worsens.