JPH04304170A - Ac high voltage generating circuit - Google Patents

Abstract

PURPOSE:To realize downsizing and cost reduction of a high voltage transformer in an AC high voltage generating circuit by making possible to operate the high voltage transformer at a high frequency. CONSTITUTION:The AC high voltage generating circuit comprises lowpass filters 108-113 for smoothing high output voltage of a high voltage transformer 108 and means 102, 106 for subjecting a first frequency, which can pass through the lowpass filters, to pulse wide modulation(PWM) with a second frequency which can not pass through the lowpass filters. A voltage modulated through the PWM means is then applied on an input side coil of the high voltage transformer 108.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-voltage AC voltage generating circuit suitable as a power supply means for a charging means incorporated in an image forming apparatus such as an electrophotographic copying machine.

0002

2. Description of the Related Art For convenience, a copying machine or a printer using a transfer type electrophotographic process shown in FIG. 5 will be described as an example.

Reference numeral 219 denotes a rotating drum-type electrophotographic photoreceptor (hereinafter referred to as a photoreceptor drum) as an image carrier, and a photoreceptor layer (photoconductor layer) is formed on the outer peripheral surface of a conductive drum base made of aluminum or the like. is formed and is rotated clockwise as indicated by the arrow at a predetermined circumferential speed (process speed).

During the rotation process of the photosensitive drum 219, the outer circumferential surface of the photosensitive drum 219 is sequentially subjected to a uniform charging process of a predetermined polarity and potential by a charging unit 218, an exposure process L by an image information exposure unit (not shown), and a development process by a developing unit 221. Then, a transferable image (toner image) corresponding to the target image information is formed.

The transferable image formed on the photosensitive drum surface is transferred between the photosensitive drum 219 and a transfer roller 22 as a transfer means.
0, the image is sequentially transferred onto the surface of a recording material P fed at a predetermined timing from a paper feeding means (not shown), and the recording material that has undergone the image transfer is transferred via an image fixing means (not shown). It is output as an image formed product (copy, print). After the image has been transferred, the surface of the photosensitive drum 219 is cleaned by cleaning means 225 to remove contaminants such as residual developer after transfer, and is used repeatedly for image formation.

In this example, the charging means 218 is a contact charging type charging device using a charging roller. Charging roller 2
18 is basically composed of a conductive core metal and a conductive elastic layer formed concentrically with the core metal into a roller shape by molding, etc., and is pressed against one surface of the photosensitive drum with a predetermined pressing force. In this example, the photosensitive drum 1 rotates as the photosensitive drum 1 rotates. A predetermined voltage is applied to the charging roller 218 by a power supply circuit (high voltage power supply), so that the surface of the rotating photosensitive drum is charged to a predetermined polarity and potential by a contact charging method.

In order to uniformly charge the surface of a charged object such as a photosensitive drum by a contact charging member such as a charging roller, a voltage that is a combination of a predetermined DC voltage and a predetermined AC voltage is applied to the contact charging member such as the charging roller. It is effective to carry out the charging process using
No. 33, etc.).

In this example as well, a superimposed voltage of DC and AC, which is the sum of the DC high voltage power supply 217 and the AC voltage output from the high voltage transformer 211, is applied to the charging roller 218 as a contact charging member to charge the photoconductor surface. It is designed to charge uniformly. At this time, an equivalent circuit between the charging roller 218 and the photoreceptor is a circuit shown by resistors 222 and 224 and a capacitor 223, and the voltage across the capacitor 223 becomes the charging voltage of the photoreceptor.

As the frequency of the high-voltage AC voltage increases, the AC current increases due to the capacitor 223, resulting in a large load. Further, the photoreceptor cannot be uniformly charged only with a high DC voltage. If the frequency of the high-voltage AC voltage is selected under such conditions, a frequency of 1.5 KHz or less will be appropriate. However, at a frequency of 1 to 1.5 KHz, a very unpleasant sound called charging noise is generated due to vibration of the shaft of the charging roller. To solve this problem, the frequency is made very low to about 150Hz. In FIG. 5, the pulse generator 201 oscillates this 150 Hz pulse. The pulse waveform output from the pulse generator 201 is generated by a resistor 202, a resistor 203, and an operational amplifier 205.
The peak voltage is determined by the output voltage of , and is approximated to a sine wave by the resistor 227 and capacitor 225. The operational amplifier 209 is a portage follower and applies an approximated sine wave voltage to the low voltage side coil of the high voltage transformer 211 through the capacitor 210.

[0010] The resistor 216 and the capacitor 215 detect the high voltage AC component current as the voltage across the resistor 216. The voltage across the resistor 216 is the capacitor 214
It passes through diodes 212 and 213 and is rectified, and is detected as a DC voltage by resistors 208 and 206 and a capacitor 226.

This voltage is applied to the resistor 2 based on the reference voltage 207.
The output voltage of the operational amplifier 205 is amplified by the gain determined by 04 and 228. However, as described above, the peak voltage output from the pulse generator 201 is controlled by the high voltage AC component current. In other words, as the current of the high-voltage AC component increases, the peak voltage of the pulse decreases, and as a result, the peak-to-peak voltage value of the high-voltage AC voltage decreases.

In the above circuit, the pulse generator 201 is a low frequency generating means. Capacitor 215/Resistor 21
Reference numeral 6 constitutes an alternating current detecting means for detecting an alternating current flowing from the charging roller 218, which is a load, to the photoreceptor 219. Circuit elements 202-208, 212-214, 225
228 constitute a peak voltage control means for controlling the peak voltage of the low frequency generation means 201 based on the information from the alternating current detection means 215 and 216.

[0013]

[Problems to be Solved by the Invention] In the above conventional example, the high voltage transformer 211 is used at a very low frequency of 150 Hz, so the high voltage transformer 211 becomes very large, and it is difficult to miniaturize the device. was becoming a huge problem. Furthermore, the cost of the transformer was also extremely high.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to enable a high voltage transformer to be used at a high frequency in a high voltage AC generating circuit, thereby making it possible to reduce the size and cost of the high voltage transformer.

[0015]

[Means for Solving the Problems] The present invention includes a low-pass filter for smoothing a high voltage on the output side of a high-voltage transformer, and a first frequency that can pass through the low-pass filter, and a first frequency that cannot pass through the low-pass filter. The high-voltage AC voltage generating circuit is characterized in that it has means for performing pulse-wide modulation at a second frequency, and applies a modulated voltage created by the pulse-wide modulation means to a coil on the input side of the high-voltage transformer. Further, in the above, the high-voltage AC voltage generation circuit is characterized in that the pulse width in the pulse width modulation means is varied depending on the value of the alternating current generated by the high-voltage AC voltage that is the output of the low-pass filter.

[0016]

[Operation] With such a circuit configuration, the high voltage transformer can be used at a high frequency, and the high voltage transformer can be made smaller and lower in cost. That is, in general, if the cross-sectional area of the transformer core is S, and the range of change in magnetic flux density is ΔB, then
The following formula holds true.

(S・ΔB)=(Ein・Ton)/NpTon:ON
Time Np: Primary winding Ton is ON Dut
When y is Don, Ton=Don/f (f is switching frequency) (S・Np・ΔB)=(Ein・
Ton)/f, and the shape of metamorphism S·Np and the switching frequency f are inversely proportional. That is, if losses are ignored, the higher the switching frequency f is, the smaller the transformer can be.

[0018]

【Example】

<First Embodiment> (FIGS. 1, 2, and 3) FIG. 1 shows an embodiment of the present invention. Portions and members that are the same as those in the conventional example shown in FIG. 5 are given the same reference numerals (numerals in the 200s) as in FIG. 5, and repeated explanations will be omitted.

The circuit of this example includes a low-pass filter 1 for smoothing the high voltage output from the high voltage transformer 108.
09 to 113, alternating current detection means 215 and 216 for detecting alternating current flowing from the charging roller 218 to the photoreceptor, and a first frequency that can pass through the low-pass filter and a second frequency that cannot pass through the low-pass filter. Means 102 for pulse wide modulation (PWM) in frequency.
106, and is configured to vary the pulse width in the PWM means based on information detected by the alternating current detection means 215 and 216.

A waveform approximated to a sine wave similar to the conventional example is input to the non-inverting input of the operational amplifier 101. The output of the operational amplifier 101 is input to a non-inverting input of a comparator 102 and compared with a triangular wave oscillator 106. As a result, the PWMed waveform is transferred to the comparator 102 as shown in FIG.
It is output from the power MOS 107 and drives the power MOS 107. A voltage is applied to the low voltage side of the high voltage transformer 108 as shown in FIG.

A voltage boosted by the turns ratio of the high voltage transformer is also output to the high voltage output side coil of the high voltage transformer 108 as shown in FIG. This is a diode 109 and a resistor 110
・When smoothed by 111 and capacitors 112 and 113, the voltage shown in FIG. 3 is output across the capacitor 113. Below, as in the conventional example, the output peaks and
The peak voltage value is controlled. Note that the offset circuit of the transformer is omitted.

That is, the high-voltage AC voltage generating circuit of this embodiment is as follows:
low-pass filters 109 to 113 for smoothing the high voltage on the output side of the high-voltage transformer 108; and pulse wide modulation of a first frequency that can pass through the low-pass filter with a second frequency that cannot pass through the low-pass filter. It has means 102 and 106, and applies the modulated voltage created by the pulse wide modulation means to the coil on the input side of the high voltage transformer,
The high voltage transformer 108 can be used at a high frequency, and the high voltage transformer can be made smaller and lower in cost.

<Second Embodiment> (FIG. 4) FIG. 4 shows another embodiment. The same parts and members as the conventional example shown in FIG. 5 and the first embodiment shown in FIG.
The explanation will be omitted again by adding the serial number (number of serial number).

[0024] The MPU 307 converts the high voltage alternating current detected as a direct current voltage by the resistors 208 and 206 and the capacitor 226 into digital information by an A/D converter 308. Based on that information, a low frequency sine wave is created and the A/
When the D conversion value becomes larger than the target value, Vp-p of the sine wave is decreased. A sine wave created as digital information is input as a voltage by a D/A converter 306 to a non-inverting input of a comparator 102, and is compared with a triangular wave oscillator 106 and a PWMed waveform as shown in FIG. 2 is output from the comparator 102. Part 1 below
It operates in the same way as the embodiment. It goes without saying that it is also possible to output a PWM waveform as shown in FIG. 2 from the MPU 307.

In the above embodiments, the first frequency has a waveform approximated to a sine wave, but it goes without saying that a rectangular wave is also possible.

[0026]

As described above, according to the present invention, it is possible to use a high voltage transformer at a high frequency in a high voltage alternating current generating circuit, and it is possible to downsize and reduce the cost of the high voltage transformer. It is effective and suitable as a power source for applying an alternating current component to a contact charging member.

[Brief explanation of drawings]

[Figure 1] Circuit diagram of the first embodiment

[Figure 2] High voltage transformer applied voltage waveform

[Figure 3] Output high voltage AC voltage waveform

[Figure 4] Circuit diagram of the second embodiment

[Figure 5] Circuit diagram of conventional example

[Explanation of symbols]

218 Charging roller 219 as a contact charging member
Photosensitive drums 108 and 211 as charged objects High voltage transformer 217 DC high voltage power supply 201 Pulse generator (low frequency generation means) 101
Operational amplifier 102 Comparator 106 Triangular wave oscillator 107 Power MOS

Claims (2)

[Claims] 1. A low-pass filter for smoothing a high voltage on the output side of a high-voltage transformer, and pulse-wide modulation of a first frequency that can pass through the low-pass filter with a second frequency that cannot pass through the low-pass filter. A high-voltage alternating current voltage generating circuit, comprising means for applying a modulated voltage created by the pulse width modulation means to a coil on the input side of the high-voltage transformer. 2. The high-voltage alternating current voltage generating circuit according to claim 1, wherein the pulse width in the pulse width modulation means is varied depending on the value of the alternating current generated by the high-voltage alternating voltage that is the output of the low-pass filter.