Abstract:
An image forming apparatus including an AC high voltage power supply, a DC high voltage power supply, and a high voltage power supply apparatus, which outputs an output voltage at an output end by superposing an output of the AC high voltage power supply and an output of the DC high voltage power supply. The high voltage power supply apparatus has a positive peak detector detecting a positive peak of the voltage at an output end, and a negative peak detector detecting a negative peak of the voltage at an output end. An output voltage of at least one of the AC high voltage power supply and the DC high voltage power supply is controlled on the basis of the detection result of at least one of the positive peak detector and the negative peak detector. An output voltage of the AC high voltage power supply is controlled on the basis of the detection result of the positive peak detector, and an output voltage of the DC high voltage power supply is controlled on the basis of the detection result of the negative peak detector.

Description:
This application claims priority from Japanese Patent Application No. 2004-316753, filed Oct. 29, 2004, which is hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a high voltage power supply apparatus used in an image forming apparatus and the image forming apparatus provided with the high voltage power supply apparatus, and more particularly, to a high voltage power supply apparatus used in an image forming apparatus using an electrophotographic system, and an image forming apparatus provided with the high voltage power supply apparatus and using the electrophotographic system. 
   2. Related Background Art 
   As an example of a high voltage power supply apparatus in a conventional image forming apparatus using the electrophotographic system, a configuration of the image forming apparatus, a configuration of the high voltage power supply apparatus used in the image forming apparatus and a circuit configuration of a developing high voltage power supply will be explained in order. 
     FIG. 7  shows a conventional type of an image forming apparatus, and a configuration of a color image forming apparatus of a four drum system. 
   A recording medium  110  fed by a pickup roller  111  is first detected at its tip position by a registration sensor  112 , and then conveyed by conveying roller pairs  113 ,  114  and a conveying belt  105 . Scanner units  100   a  to  100   d  successively irradiate a laser beam on photosensitive drums  101   a  to  101   d  in accordance with the detection timing of the registration sensor  112 . At this time, photosensitive drums  101   a  to  101   d , which are charged by electrostatic charge rollers  104   a  to  104   d , have an electrostatic latent image formed thereon by the laser beam irradiation, and further have a toner image formed thereon by developing devices  102   a  to  102   d  and developing sleeves  103   a  to  103   d . Then, the toner image is transferred to the recording medium  110  conveyed on the conveying belt  105  by transfer rollers  106   a  to  106   d . Then, the recording medium  110  is conveyed to a fixing device  107 , and outputted after the image is fixed. Here, the English character “a” of each of the reference characters denotes a configuration and unit for a color of cyan, “b” denotes a configuration and unit for a color of magenta, “c” denotes a configuration and unit for a color of yellow, and “d” denotes a configuration and unit for a color of black. 
   Next, a configuration of a high voltage power supply apparatus in the image forming apparatus shown in  FIG. 7  is explained using  FIG. 8 . 
   The image forming apparatus comprises four kinds of high voltage power supply apparatuses including electrostatic charge bias high voltage power supply apparatuses  30   a  to  30   d , which generate an electrostatic charge bias voltage, developing bias high voltage power supply apparatuses  31   a  to  31   d , which generate a developing bias voltage, transfer bias high voltage power supply apparatuses  32   a  to  32   d , which generate a transfer bias voltage, and transfer reverse bias high voltage power supply apparatuses  33   a  to  33   d , which generate a transfer reverse bias voltage. The electrostatic charge bias high voltage power supply apparatuses  30   a  to  30   d  form a background potential on the surface of the photosensitive drums  101   a  to  101   d , by applying the electrostatic charge bias voltage to the electrostatic charge rollers  104   a  to  104   d , thereby making the surface of the photosensitive drums set to a state that an electrostatic latent image can be formed by irradiation of a laser beam. The developing bias high voltage power supply apparatus  31   a  to  31   d  reciprocate the toner between the developing sleeves  103   a  to  103   d  and the photosensitive drums  101   a  to  101   d  which are separated from the developing sleeves  103   a  to  103   d , respectively, by applying an AC voltage to the developing sleeves  103   a  to  103   d , to thereby make a toner image formed on the electrostatic latent image. The transfer bias high voltage power supply apparatus  32   a  to  32   d  transfer the toner image to the recording medium  110 , by applying the transfer bias voltage to the transfer rollers  106   a  to  106   d . Further, the transfer reverse bias high voltage power supply apparatuses  33   a  to  33   d  return a waste toner on the conveying belt  105  to the photosensitive drums  101   a  to  101   d , by applying the transfer reverse bias voltage to the transfer rollers  106   a  to  106   d  at the time of the cleaning operation of the conveying belt  105 . Here, the waste toner returned to the photosensitive drums  101   a  to  101   d  is scraped off by cleaning blades  115   a  to  115   d , so as to be stored in the waste toner containers  116   a  to  116   d.    
   Next, an example of a circuit configuration of the developing bias high voltage power supply apparatus  31   a  in the high voltage power supply apparatus shown in  FIG. 8  is explained using  FIG. 9 . 
   As disclosed in Japanese Patent Application Laid-Open No. H10-28328, in a DC high voltage power supply  10 , an AC voltage generated by a DC driving circuit  12  is stepped up by a transformer  13  to a voltage having an amplitude of several tens of times as large as that of the AC voltage, and then smoothed by a rectifying circuit  14 , as a result of which, a DC voltage is outputted between outputs  20 ,  21 . A detection circuit  15  detects the voltage between the outputs  20 ,  21 . A DC control circuit  16  performs control so as to make the voltage between the outputs  20 ,  21  become a predetermined value determined by a DC control signal  28 , on the basis of the detection result of the detection circuit  15 . On the other hand, in an AC high voltage power supply  11 , an AC pulse signal  29  is amplified by an AC driving circuit  17  and then stepped up by a transformer  18  to a voltage having an amplitude of several tens of times as large as that of the amplified AC pulse signal  29 , as a result of which, an AC voltage is outputted between outputs  22 ,  23 . Here, the outputs  20 ,  21  and the outputs  22 ,  23  are connected in series, so that a voltage consisting of the output voltage of the DC high voltage power supply  10  superposed on the output voltage of the AC high voltage power supply  11  is outputted to an output end  25 . 
   Further, the adjustment of printing density is performed by changing the output voltage of the DC high voltage power supply  10  by means of the DC control signal  28 . At this time, the amplitude of the output voltage of the AC high voltage power supply  11  is kept constant. 
   However, the above-described conventional circuit configuration has problems that the output voltage of the AC high voltage power supply  11  easily fluctuates during load fluctuation, because the output voltage is not detected and controlled, and because the so-called open control is used, and that it is difficult to achieve the high output voltage precision at the output end because the output voltage precision at the output end is determined by a combination of the precision of the DC high voltage power supply and the precision of the AC high voltage power supply. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-described problems. 
   An object of the present invention is to provide a high voltage power supply apparatus, which is capable of performing accurate control of the output voltage, and which is hardly influenced by load fluctuation, and to provide an image forming apparatus provided with the high voltage power supply apparatus. 
   Another object of the present invention is to provide a high voltage power supply apparatus, which is capable of performing accurate control of the output voltage, and which is hardly influenced by load fluctuation, and to provide an image forming apparatus provided with the high voltage power supply apparatus. 
   Another object of the invention is to provide a high voltage power supply apparatus, which includes an AC high voltage power supply and a DC high voltage power supply, and which outputs at its output end a voltage consisting of an output voltage of the AC high voltage power supply superposed on an output voltage of the DC high voltage power supply, the high voltage power supply apparatus comprising a positive peak detector detecting a positive peak voltage of the voltage at the output end, and a negative peak detector detecting a negative peak voltage of the voltage at the output end, wherein the output voltages of the AC high voltage power supply and the DC high voltage power supply are controlled on the basis of the detection results of the positive peak detector and the negative peak detector, respectively. 
   Further objects of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a figure showing a configuration of an embodiment 1 of the high voltage power supply apparatus according to the present invention; 
       FIG. 2  is a figure showing an operating waveform of the high voltage power supply apparatus of the embodiment 1; 
       FIG. 3  is a figure showing a configuration of an embodiment 2 of the high voltage power supply apparatus according to the present invention; 
       FIG. 4  is a figure showing a configuration of an embodiment 3 of the high voltage power supply apparatus according to the present invention; 
       FIG. 5A  is a figure showing an operating waveform of the high voltage power supply apparatus of the embodiment 3 in the case of a duty of 50%; 
       FIG. 5B  is a figure showing an operating waveform of the high voltage power supply apparatus of the embodiment 3 in the case of a duty of 70%; 
       FIG. 5C  is a figure showing an operating waveform of the high voltage power supply apparatus of the embodiment 3 in the case of a duty of 30%; 
       FIG. 6A  is a figure showing an operating waveform of a conventional high voltage power supply apparatus when Vdc is controlled to be located in the center of the control range; 
       FIG. 6B  is a figure showing an operating waveform of the conventional high voltage power supply apparatus when Vdc is controlled at a maximum of the control range; 
       FIG. 6C  is a figure showing an operating waveform of a conventional high voltage power supply apparatus when Vdc is controlled at a minimum of the control range; 
       FIG. 7  is a figure showing a configuration of a conventional image forming apparatus; 
       FIG. 8  is a figure showing a configuration of a high voltage power supply apparatus in the conventional image forming apparatus; and 
       FIG. 9  is a figure showing a configuration of a developing bias circuit in the conventional high voltage power supply apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, preferred embodiments according to the present invention will be described with reference to the accompanying drawings. 
   Embodiment 1 
     FIG. 1  is a figure showing a configuration of a high voltage power supply apparatus of an embodiment 1 according to the present invention, wherein the present invention is applied to a developing bias high voltage power supply apparatus  31   a .  FIG. 2  is a figure showing an operating waveform of an output end  25  in the circuit shown in  FIG. 1 . In these figures, components common to the conventional configuration shown in  FIG. 7  are denoted by the same reference numerals, and the explanation of the components is omitted. The configuration of the high voltage power supply apparatus of the embodiment 1 according to the present invention is different from the conventional configuration shown in  FIG. 7  in that a positive peak detection circuit  60  and a negative peak detection circuit  61 , which respectively detect the positive and negative peak voltages at the output end  25 , are provided, and in that a DC high voltage power supply  10  and an AC high voltage power supply  62  are arranged to be controlled on the basis of these detection results. 
   The positive peak detection circuit  60  and the negative peak detection circuit  61  are constituted such that the positive and negative peak voltages are held by diodes  70 ,  71  and capacitors  72 ,  73 , respectively. A DC control circuit  16  provided for the DC high voltage power supply  10  performs control of a DC voltage between outputs  20 ,  21  so as to make the negative peak voltage at the output end  25  become a predetermined value determined by a negative peak control signal  26 , on the basis of the detection result of the negative peak detection circuit  61 . On the other hand, an AC control circuit  19  provided for the AC high voltage power supply  62  performs control of the amplitude of an AC voltage between outputs  22 ,  23 , so as to make the positive peak voltage at the output end  25  become a predetermined value determined by a positive peak control signal  27 , on the basis of the detection result of the positive peak detection circuit  60 . Similar to the conventional configuration, a voltage consisting of the output voltage of the DC high voltage power supply  10  superposed on the output voltage of the AC high voltage power supply  62  is arranged to be outputted at the output end  25 . 
   That is, referring to the voltage waveform shown in  FIG. 2 , the DC high voltage power supply  10  detects the negative peak voltage III at the output end  25 , and performs control of the DC voltage between the outputs  20 ,  21 , i.e., the mean voltage II of the output at the output end  25 , so as to make the negative peak voltage III become a voltage corresponding to the negative peak control signal  26 . Further, the AC high voltage power supply  62  detects the positive peak voltage I at the output end  25 , and performs control of the amplitude of the AC voltage between the outputs  22 ,  23 , i.e., the voltage amplitude between I and III at the output end  25 , so as to make the positive peak voltage I become a voltage corresponding to the positive peak control signal  27 . 
   An attenuation resistor  51  of the positive peak detection circuit  60  and an attenuation resistor  52  of the negative peak detection circuit  61 , which resistors serve as a DC load when seen from the output end  25 , allow reverse direction currents  54 ,  55  to flow, when the voltage at the output end  25  reaches the peak voltage. Here, when the current in the direction of arrow  55  is larger than the current in the direction of arrow  54 , and thus, the current flows in the direction of arrow  55 , the amount of charge of a capacitor  56  provided for a rectifier circuit  14  can be temporarily changed, but the compensation is performed in average through a diode  53 , so as to eliminate the influence of the current. 
   However, when the current in the direction of arrow  54  is larger than the current in the direction of arrow  55 , and thus, the current flows in the direction of arrow  54 , in the DC high voltage power supply  10 , the amount of charge variation of the capacitor  56  cannot be compensated for, and the voltage at the output end  25  changes to be not less than the control voltage, thereby causing an uncontrollable state. On the other hand, when a load resistor, and the like, is provided between the outputs  20 ,  21 , the amount of charge variation of the capacitor  56  can be compensated for. However, in this case, the load resistor serves as a load of the DC high voltage power supply  10 , so that there is a possibility of an increase in the cost and calorific value, and the like. Therefore, the present embodiment is constituted such that resistors having the same resistance value are used for the attenuation resistor  51  and the attenuation resistor  52 , so as to make the current in the direction of arrow  54  and the current in the direction of arrow  55  substantially equal to each other, whereby the current is prevented from flowing in either of the directions of arrow  54  and arrow  55 , in average. However, the attenuation resistor  51  and the attenuation resistor  52  are not necessarily made to be the same with each other. It is possible to stabilize the output voltage of the DC high voltage power supply  10  by making the attenuation resistor  51  have a resistance larger than the attenuation resistor  52 , so as to enable the current to flow in the direction of arrow  55 , in average. 
   Further, when the DC high voltage power supply  10  changes the output in order to control the negative peak voltage at the output end  25 , it also changes the positive peak voltage. On the contrary, when the AC high voltage power supply  62  changes the output in order to control the positive peak voltage at the output end  25 , it also changes the negative peak voltage. For this reason, in the case where the response speed of the DC high voltage power supply  10  is not much different from that of the AC high voltage power supply  62 , the control circuits of both of the power supplies perform control of the output voltage at the output end  25  at the same time, as a result of which, it takes much time to stabilize the output voltage. Therefore, in the present embodiment, the response speed of the DC high voltage power supply  10 , performing control of the output voltage on the basis of the detection result of the negative peak detection circuit  61 , is set to be about twenty times slower than the response speed of the AC high voltage power supply  62  performing control of the output voltage on the basis of the detection result of the positive peak detection circuit  60 . 
   In addition, when the circuit of the present embodiment is started, the negative peak control signal  26  is risen with a slight delay from the rise of the AC pulse signal  29  and the positive peak control signal  27 . Thereby, the behavior of the output voltage is stabilized and the overshoot of the waveform can be prevented. 
   According to the present embodiment, since the positive and negative peak voltages at the output end  25  are directly detected for performing the control, it is possible to constitute the high voltage power supply apparatus, which is capable of performing accurate control of the AD output voltage, reducing the effect of load fluctuation, stabilizing the output voltage in a short period of time, and suppressing the overshoot when the power supply apparatus is started. 
   Embodiment 2 
     FIG. 3  is a figure showing a configuration of an embodiment 2 of the high voltage power supply apparatus according to the present invention, and showing an example of a circuit, in which the present invention is applied to the developing bias high voltage power supply apparatus  31   a  to  31   d , in the output ends  25   a  to  25   d  of the color image forming apparatus of a four drum system. In  FIG. 3 , components common to the embodiment 1 shown in  FIG. 1 , are denoted by the same reference numerals, and an explanation of the components is omitted. The present embodiment is characterized in that the negative peak control signals  26   a  to  26   d  and the positive peak control signals  27   a  to  27   d  at the input end, are provided independently for each circuit, and in that the AC pulse signal  29  at the input end is made common to the four circuits. 
   In the present embodiment, the AC pulse signal  29  is always set in an operation state, and each high voltage output is controlled by setting the negative peak control signals  26   a  to  26   d  and the positive peak control signals  27   a  to  27   d . Here, by making the value of the negative peak control signals  26 A to  26 D increased and decreased together with the value of the positive peak control signals  27 A to  27 D, it is possible to accurately change the DC voltage of each high voltage output, while the peak-to-peak voltage of each high voltage output is maintained to be constant as in the conventional circuit configuration. Further, it is possible to operate only the developing bias high voltage power supply apparatus  31   d  of the black station at the time of printing a monochrome image, as in the conventional circuit configuration. 
   According to the present embodiment, in the image forming apparatus provided with a plurality of developing bias circuits, an operation, which is the same as that of the conventional circuit can be effected, even in the configuration in which the AC pulse signal  29  is made common to the respective developing bias circuits. Further, according to the present embodiment, it is possible to constitute the high voltage power supply circuit, which is capable of independently performing sufficiently accurate control of the output voltage of the respective circuits. 
   Embodiment 3 
     FIG. 4  is a figure showing a configuration of an embodiment 3 of the high voltage power supply apparatus according to the present invention, and showing an example of a circuit in which the present invention is applied to the developing bias high voltage power supply apparatus  31   a  to  31   d  in the output ends  25   a  to  25   d , as in the embodiment 2. In  FIG. 4 , components common to the embodiment 2 shown in  FIG. 3 , are denoted by the same reference numerals, and an explanation of the components is omitted. The present embodiment is different from the embodiment 2 in that the AC pulse signals  29   a  to  29   d  at the input end are independently provided for respective circuits, that the positive peak control signal  27  at the input end is made common to the four circuits, and that the negative peak control signal  26   abc  is commonly inputted into the three color circuits and the negative peak control signal  26   d  is independently inputted into only the black color circuit. 
   According to the present embodiment, the AC pulse signals  29   a  to  29   d  are independently provided, so that the duty ratio of the output of the AC high voltage power supply  62  can be independently controlled. Further, the provision of the negative peak control signal  26   d , for only the black color circuit, makes it possible at the time of printing a monochrome image to stop the generation of the color developing bias voltage by means of the AC pulse signals  29   a  to  29   c  and the negative peak control signal  26   abc , and to enable the output only of the developing bias high voltage power supply apparatus of black color  31   d  at the output end  25   d . However, the present embodiment is not limited to the configuration in which the negative peak control signal of black color is independently provided. 
   In the following, the effect of the present embodiment is explained by comparing the developing bias according to the present embodiment with the developing bias using the conventional high voltage circuit. 
   Normally, in the case where the same developing bias voltage is used, the output image density is changed in accordance with the image formation operation time and the use environment of the developing device. For this reason, in the conventional configuration, in order to obtain a stable output image density until the end of the life of the developing device, the output voltage of the DC high voltage power supply  10  of the conventional high voltage power supply apparatus shown in  FIG. 9  is changed by means of the DC control signal  28 , so that the mean value (Vdc) of the output voltage is changed, while the output voltage amplitude of the AC high voltage power supply  11  is maintained to be constant. As a result, the output image density is adjusted by the change of the mean value (Vdc). 
   Here,  FIG. 5A  to  FIG. 5C  show bias voltage waveforms at the time of development in the present embodiment.  FIG. 6A  to  FIG. 6C  show developing bias waveforms in the prior art form.  FIG. 5A  is a figure showing a bias voltage waveform of the high voltage power supply apparatus at the time of development in the case of control with a duty of 50%.  FIG. 5B  is a figure showing a bias voltage waveform of the high voltage power supply apparatus at the time of development in the case of control with a duty of 70%.  FIG. 5C  is a figure showing a bias voltage waveform of the high voltage power supply apparatus at the time of development in the case of control with a duty of 30%.  FIG. 6A  is a figure showing a bias voltage waveform of the high voltage power supply apparatus when Vdc is located approximately in the center of the control range.  FIG. 6B  is a figure showing a bias voltage waveform of the high voltage power supply apparatus in the case of control at a maximum in the control range of Vdc.  FIG. 6C  is a figure showing a bias voltage waveform of the high voltage power supply apparatus in the case of control at a minimum in the control range of Vdc. Potential differences A, B in the figures show the discharge (leak) potential between the dark potential (VD) of the photosensitive drum and the potential of the developing sleeve, and between the light potential (VL) of the photosensitive drum and the potential of the developing sleeve, respectively. In these figures, the potential is positive in the downward direction. 
   In the developing bias using the conventional circuit shown in  FIG. 9 , the maximum voltage (Vmax) and also the minimum voltage (Vmin) change with the density adjustment, which directly controls the Vdc, as shown in  FIG. 6A  to  FIG. 6C . At this time, the potential difference between the VD and the potential of the developing sleeve or the potential difference between the VL and the potential of the developing sleeve may exceed the leak potentials A, B. For this reason, even when the Vdc is changed as much as possible within the density adjustment range, it is necessary to use the AC amplitude, which prevents the Vmax and Vmin from exceeding the leak potential. 
   On the other hand, in the present embodiment, the Vdc, which is the mean value of the AC voltage amplitude, can be indirectly changed by changing the duty ratio of the developing bias, while the Vmax and Vmin are maintained to be constant, as shown in  FIG. 5A  to  FIG. 5C , whereby it is possible to use the developing bias voltage having a maximum AC voltage amplitude, which does not exceed the above-described leak potential. 
   That is, the AC voltage amplitude of the developing bias according to the present embodiment, can be made larger than the AC voltage amplitude of the developing bias using the conventional high voltage circuit. As a result, it is possible to obtain an output image, which is excellent in dot reproducibility and image uniformity of character images.