Abstract:
An apparatus for applying a voltage across an image carrying member carrying a toner image and a print medium, the toner image being transferred from the image carrying member to the print medium during an image transfer process. The apparatus includes a first voltage receiver disposed on a first side in reference to the print medium, a second voltage receiver disposed on a second side opposite the first side, a voltage supply unit adapted to apply the voltage selectively to the first voltage receiver and to the second voltage receiver, and a voltage adjustment unit adapted to compensate for variations of the voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 10/679,317 filed Oct. 7, 2003 now U.S. Pat. No. 7,110,685 that claims the benefit of Korean Application No. 2002-80913, filed Dec. 17, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power supply apparatus for transferring an image in an image forming machine, and more particularly, to a power supply apparatus for transferring an image in an image forming machine for supplying power to a transfer roller and a transfer backup roller for transferring an image which is created on an intermediate transfer medium by a plurality of photoreceptors to a print medium. 
   2. Description of the Related Art 
   Generally, one of the typical apparatuses for forming an image on a print medium using an intermediate transfer medium is a single pass color image forming machine. The single pass color image forming machine has a plurality of color image forming units each forming an individual color image on the intermediate transfer medium in a single pass. A color image forming machine creates a predetermined image by precisely overlapping color images transferred from each color image forming unit on the intermediate transfer medium and transfers the created image onto a print medium. Conventionally, the plurality of color image forming units are consecutively arranged in the order of the intermediate transfer medium moving direction respectively developing images in cyan C, magenta M, yellow Y, and black B. The color image forming machine in the present description refers to an apparatus forming a color image on a print medium such as a color printer or a color copier. 
   An example of an image forming machine for printing an image using such an intermediate transfer medium is shown in  FIG. 1 . 
   Referring to  FIG. 1 , the image forming machine comprises a plurality of color image forming units  10 C,  10 M,  10 Y,  10 K, an intermediate transfer medium  30 , a transfer device  40 , and a transfer power supply apparatus  50 . 
   The plurality of color image forming units  10 C,  10 M,  10 Y,  10 K are arranged in the order of the moving direction (shown by the arrow) of the intermediate transfer medium  30  forming images respectively in cyan C, magenta M, yellow Y, and black B. Each color image forming unit  10 C,  10 M,  10 Y,  10 K is provided with a photosensitive medium transferring an image onto the intermediate transfer medium  30 , a charging roller (not shown) charging the photosensitive medium with a very high voltage, and a development roller (not shown) developing an electrostatic image created on the photosensitive medium in each color. An intermediate transfer roller  20 C,  20 M,  20 Y,  20 K is provided for each color image forming unit  10 C,  10 M,  10 Y,  10 K having the intermediate transfer medium  30  interposed therebetween for transferring the image created by each color image forming unit  10 C,  10 M,  10 Y,  10 K to the intermediate transfer medium  30 . 
   Each color image created by the plurality of color image forming units  10 C,  10 M,  10 Y,  10 K is consecutively overlapped on the intermediate transfer medium  30 , thereby creating a predetermined color image, and the intermediate transfer medium  30  conveys the color image to the transfer device  40 . The intermediate transfer medium  30  is comprised of polyamide, polycarbonate, and urethane which are resilient. 
   The transfer device  40  comprises a transfer roller  41  and a transfer backup roller  42 , for transferring the color image created on the intermediate transfer medium  30  onto a print medium  60  supplied from a feed device (not shown). 
   The transfer roller  41  allows the image formed on the intermediate transfer medium  30  to be transferred onto the print medium  60  by supplying a constant voltage while pressing the print medium  60  against the intermediate transfer medium  30  by a predetermined pressure. The transfer roller  41 , which is a resilient body with a low degree of rigidity, presses the intermediate transfer medium  30  with a low pressure and is electrically formed to be supplied with a high voltage. 
   The transfer backup roller  42  is comprised of a resilient body with a low degree of rigidity or an electroconductive metal, and supports the intermediate transfer medium  30  pressed by the transfer roller  41 . The transfer backup roller  42  is grounded. 
   The transfer power supply apparatus  50  supplies a predetermined transfer voltage to the transfer roller  41  in order to form an electrostatic field for transferring the image created on the intermediate transfer medium  30  onto the print medium  60 . The transfer power supply apparatus  50  comprises a positive power unit supplying positive polarity voltage to the transfer roller  41 , a negative power unit supplying negative polarity voltage to the transfer roller  41 , and a power shifting member selectively connecting the transfer roller  41  with the positive power unit or the negative power unit. The transfer power supply apparatus  50  controls the power shifting member in order to reverse the polarity of the power supplied to the transfer roller  41  relative to the image during the transfer process when the image on the intermediate transfer medium  30  is being transferred onto the print medium  60  and to once again reverse the polarity during the cleaning process when no image is transferred onto the print medium  60 . That is, the transfer power supply apparatus  50  supplies a voltage having the opposite polarity to the polarity of the image created on the intermediate transfer medium  30  for the image to be transferred onto the print medium  60  during the transfer process and supplies the same polarity to that of the polarity of the image created on the intermediate transfer medium  30  so that the image is not transferred to the transfer roller  41  during the cleaning process. 
   Therefore, visible images created on the photoreceptors of the plurality of color image forming units  10 C,  10 M,  10 Y,  10 K through charge, exposure, and development processes are transferred onto the intermediate transfer medium  30  and are precisely overlapped with each other by the electrostatic force formed between the intermediate transfer rollers  20 C,  20 M,  20 Y,  20 K and the photoreceptors. The image created on the intermediate transfer medium  30  is transferred again onto the print medium  60  by the transfer roller  41  and the transfer backup roller  42 . Since the transfer roller  41  presses against the intermediate transfer medium  30  by a predetermined pressure, and the transfer power supply apparatus  50  supplies a predetermined transfer voltage to the transfer roller  41  transferring development agent to the print medium  60 , the development agent such as ink or toner forming an image on the intermediate transfer medium  30  is effectively transferred to the print medium  60 . 
   During the cleaning process when no image is transferred from the intermediate transfer medium  30  to the print medium  60 , the transfer power supply apparatus  50  supplies a transfer voltage to the transfer backup roller  42 , which is the same transfer voltage transferring the image created on the intermediate transfer medium  30  to the print medium  60 . Accordingly, the development agent on the intermediate transfer medium  30  does not contaminate the transfer roller  41  when no image transfer is made to the print medium  60 . 
   However, when the temperature around the image forming machine varies, resistance of the intermediate transfer medium  30  or the transfer roller  41  varies, and when the resistance of the intermediate transfer medium  30  or the transfer roller  41  varies, the output of the transfer power supply apparatus  50  varies. When the output of the transfer power supply apparatus  50  varies, the quantity of the development agent being transferred from the intermediate transfer medium  30  to the print medium  60  varies, thereby causing a problem that the quality of the image transferred onto the print medium  60  varies depending on the surrounding environment. In other words, the conventional transfer power supply apparatus of an image forming machine cannot maintain a predetermined degree of quality of the image transferred onto the print medium regardless of the change in the external environment. 
   In addition, since the conventional transfer power supply apparatus  50  supplies a necessary voltage by connecting the transfer roller  41  either to the negative power unit or the positive power unit using the power shifting member when switching the mode from the transfer process to the cleaning process or vice versa, the output voltage loaded in the transfer roller  41  largely varies, thereby causing poor quality of the image transferred on the print medium  60  or ineffective cleaning of the intermediate transfer medium  30 . 
   Therefore, there is a need for a transfer power supply apparatus of an image forming apparatus which can maintain the stable quality of the print image by supplying an appropriate voltage to a transfer roller according to the change in the temperature of the surrounding environment during the transfer process, and supplying an appropriate voltage to a transfer backup roller according to the change in the temperature of the surrounding environment during the cleaning process. 
   SUMMARY OF THE INVENTION 
   An aspect of the invention is to solve at least the above problems and/or disadvantages and to provide the advantages described hereinafter and/or other aspects and advantages. 
   Accordingly, one aspect of the present invention is to solve the foregoing problems by providing a transfer power supply apparatus of an image forming machine which can maintain a predetermined degree of the quality of an image on a print medium by minimizing the change in the output of a transfer voltage by supplying an appropriate voltage according to the change in the temperature of the surrounding environment to the transfer roller during the transfer process and to the transfer backup roller during the cleaning process. 
   The foregoing and/or other aspects and advantages are realized by providing a transfer power supply apparatus of an image forming machine applying a voltage in order to transfer an image created on an intermediate transfer medium onto a print medium conveyed between a transfer roller and a transfer backup roller, the transfer power supply apparatus comprising a voltage computation unit computing a transfer voltage corresponding to resistance of the intermediate transfer medium, an output voltage generator generating the transfer voltage according to the output from the voltage computation unit, and a switching unit selectively applying the transfer voltage output from the output voltage generator, wherein the switching unit connects the transfer voltage output from the output voltage generator to the transfer roller when the image created on the intermediate transfer medium is transferred to the print medium, and connects the transfer voltage output from the output voltage generator to the transfer backup roller when the image is not transferred to the print medium. 
   The voltage computation unit comprises a transfer electric current recognizer measuring an electric current flowing in the intermediate transfer medium, a resistance computation unit computing resistance of the intermediate transfer medium from the electric current measured in the transfer electric current recognizer, and a voltage determination unit determining the transfer voltage to be output by the resistance calculated from the resistance computation unit. 
   The voltage determination unit may compute an output voltage according to the resistance using a resistance-voltage table. 
   The switching unit comprises a grounding unit, a first switching unit disposed to have one point of contact connected to the transfer roller and the other point of contact selectively connected to either the output voltage generator or the grounding unit, and a second switching unit disposed to have one point of contact connected to the transfer backup roller and the other point of contact selectively connected to either the output voltage generator or the grounding unit, wherein the second switching unit is connected to the grounding unit when the first switching device is connected to the output voltage generator. The first switching unit and the second switching unit may be relays. 
   Another aspect of the present invention is to provide a transfer power supply apparatus of an image forming machine applying a voltage in order to transfer an image created on an intermediate transfer medium onto a print medium conveyed between a transfer roller and a transfer backup roller. The transfer power supply apparatus comprises a first transfer power supply apparatus comprising a first voltage computation unit computing a transfer voltage corresponding to resistance of the intermediate transfer medium, a first output voltage generator outputting the transfer voltage according to the signal output from the first voltage computation unit, and a first switch disposed between the first output voltage generator and the transfer roller, and a second transfer power supply apparatus comprising a second voltage computation unit computing a transfer voltage corresponding to resistance of the intermediate transfer medium, a second output voltage generator outputting the transfer voltage according to the output from the second voltage computation unit, and a second switch disposed between the second output voltage generator and the transfer backup roller. The second switch is turned off, preventing the second transfer power supply apparatus from applying a transfer voltage to the transfer backup roller when the first switch is turned on and the first transfer power supply apparatus supplies a transfer voltage to the transfer roller. 
   The first and second output voltage generators comprise a transfer electric current recognizer measuring an electric current flowing in the intermediate transfer medium, a resistance computation unit computing resistance of the intermediate transfer medium from an electric current measured from the transfer electric current recognizer, and a voltage determination unit determining the transfer voltage to be output from the resistance computed from the resistance computation unit. 
   The foregoing and/or other aspects of the present invention are also achieved by an image forming machine comprising an intermediate transfer medium on which images developed in a plurality of developing units are overlapped thereby forming a predetermined color image, a transfer roller transferring a color image created on the intermediate transfer medium onto a print medium, a transfer backup roller supporting the transfer roller, and a transfer power supply apparatus comprising a voltage computation unit computing a transfer voltage corresponding to resistance of the intermediate transfer medium, an output voltage generator outputting the transfer voltage according to the output from the voltage computation unit, and a switching unit applying the transfer voltage output from the output voltage generator selectively to the transfer roller and the transfer backup roller. The transfer power supply apparatus controls the switching unit for a voltage output from the output voltage generator to be connected to the transfer roller when the color image created on the intermediate transfer medium is transferred to the print medium, and to be connected to the transfer backup roller when the color image is not transferred to the print medium. 
   In addition, the foregoing and/or other aspects of the present invention are achieved by providing an image forming machine comprising an intermediate transfer medium on which images developed in a plurality of developing units are overlapped thereby forming a predetermined color image, a transfer roller transferring a color image created on the intermediate transfer medium onto a print medium, a transfer backup roller supporting the transfer roller, a first transfer power supply apparatus comprising a first voltage computation unit computing a transfer voltage corresponding to resistance of the intermediate transfer medium, a first output voltage generator outputting the transfer voltage according to the output from the first voltage computation unit, and a first switch disposed between the first output voltage generator and the transfer roller, a second transfer power supply apparatus comprising a second voltage computation unit computing transfer voltage corresponding to resistance of the intermediate transfer medium, a second output voltage generator outputting the transfer voltage according to the output from the second voltage computation unit, and a second switch disposed between the second output voltage generator and the transfer backup roller. The second switch is turned off, preventing the second transfer power supply apparatus from applying a transfer voltage to the transfer backup roller when the first switch is turned on and the first transfer power supply apparatus supplies a transfer voltage to the transfer roller. 
   As described above, according to the transfer power supply apparatus of an image forming machine in accordance to the present invention, a predetermined degree of the quality of an image on a print medium can be maintained by minimizing the change in the output of a transfer voltage by supplying an appropriate voltage according to the change in the temperature of the surrounding to the transfer roller during the transfer process and to the transfer backup roller during the cleaning process. 
   Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and/or other aspects of the present invention will be more apparent by describing embodiments of the present invention with reference to the accompanying drawings, in which: 
       FIG. 1  schematically shows an image forming unit of an image forming machine comprising a conventional transfer power supply apparatus; 
       FIG. 2  schematically shows an image forming unit of an image forming machine comprising a transfer power supply apparatus according to an embodiment of the present invention; 
       FIG. 3  is a block diagram showing a transfer power supply apparatus of an image forming machine according to the embodiment of the present invention of  FIG. 2 ; 
       FIG. 4  is a diagram illustrating the operation of the transfer power supply apparatus of  FIG. 3 ; 
       FIG. 5  is a graph showing the change in resistance and a transfer voltage of an intermediate transfer medium according to the change in the temperature of the surrounding environment; and 
       FIG. 6  shows a transfer power supply apparatus of an image forming machine according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Hereinafter, a transfer power supply apparatus of an image forming unit according to an embodiment of the present invention will be described in greater detail with reference to the accompanying drawings. Like reference numerals refer to like elements throughout the description and the drawings. 
     FIG. 2  schematically shows an image forming unit forming a predetermined image on a print medium in an image forming machine comprising a transfer power supply. Referring to  FIG. 2 , the image forming machine comprises a plurality of color image forming units  10 C,  10 M,  10 Y,  10 K, an intermediate transfer medium  30 , a transfer device  40 , and a transfer power supply apparatus  100 . 
   The plurality of color image forming units  10 C,  10 M,  10 Y,  10 K are arranged in the order of the moving direction (shown by the arrow) of the intermediate transfer medium  30  forming images respectively in cyan C, magenta M, yellow Y, and black B. Each color image forming unit  10 C,  10 M,  10 Y,  10 K is provided with a photosensitive medium (photoreceptor) transferring an image onto the intermediate transfer medium  30 , a charging roller (not shown) charging the photosensitive medium with a very high voltage, and a development roller (not shown) developing an electrostatic image created on the photosensitive medium in each color. An intermediate transfer roller  20 C,  20 M,  20 Y,  20 K is disposed to each color image forming unit  10 C,  10 M,  10 Y,  10 K having the intermediate transfer medium  30  interposed therebetween, transferring the image created by each color image forming unit  10 C,  10 M,  10 Y,  10 K to the intermediate transfer medium  30 . 
   Each color image created by the plurality of color image forming units  10 C,  10 M,  10 Y  10 K is consecutively transferred onto the intermediate transfer medium  30  to overlap with each other, thereby creating a predetermined color image, and the intermediate transfer medium  30  conveys the color image to the transfer device  40 . The intermediate transfer medium  30  is comprised of polyamide, polycarbonate, and urethane which are resilient. A drive roller  32  moves the intermediate transfer medium  30  in the direction shown by the arrow. 
   The transfer device  40  comprises a transfer roller  41  and a transfer backup roller  42 , transferring the color image created on the intermediate transfer medium  30  onto a print medium  60  supplied from a feed device (not shown). 
   The transfer roller  41  allows the image formed on the intermediate transfer medium  30  to be transferred to the print medium  60  by supplying a constant voltage while pressing the print medium  60  against the intermediate transfer medium  30  with a predetermined pressure. The transfer roller  41 , which is a resilient body with a low degree of rigidity, presses the intermediate transfer medium  30  with a low pressure and is electrically formed to be supplied with a high voltage. 
   The transfer backup roller  42  is comprised of a resilient body with a low degree of rigidity or an electroconductive metal, and supports the intermediate transfer medium  30  pressed by the transfer roller  41 . The transfer backup roller  42  is grounded. 
   The transfer power supply apparatus  100  supplies a predetermined voltage to the transfer roller  41  and its structure is shown in  FIG. 3 . Referring to  FIG. 3 , the transfer power supply apparatus  100  comprises a voltage computation unit  120 , an output voltage generator  110 , and a switching unit  130 . 
   The voltage computation unit  120  computes an appropriate voltage for transferring an image from the intermediate transfer medium  30 , which has a resistance varying according to the change in the surrounding temperature, to the print medium  60 . The voltage computation unit  120  comprises a transfer electric current recognizer  121  measuring an electric current flowing in the intermediate transfer medium  30 , a resistance computation unit  123  computing resistance of the intermediate transfer medium  30  from the electric current measured from the transfer electric current recognizer  121 , and a voltage determination unit  125  determining a voltage to be outputted from the resistance computed from the resistance computation unit  123 . 
   A precise voltage needs to be supplied to the transfer device  40  for maintaining a predetermined degree of image quality transferred onto the print medium  60 . Visible images created on photoreceptors of the plurality of color image forming units  10 C,  10 M,  10 Y,  10 K through charge, exposure and development processes are transferred on the intermediate transfer medium  30  to overlap with each other by an electrostatic force supplied between the intermediate transfer rollers  20 C,  20 M,  20 Y,  20 K and the photoreceptors. The image created on the intermediate transfer medium  30  is conveyed between the transfer roller  41  and the transfer backup roller  42  and transferred onto the print medium  60  by an electrostatic force. The electrostatic force applied to the image on the intermediate transfer medium  30  is determined by the transfer voltage supplied to the transfer roller  41 . How effectively the development agent can travel from the intermediate transfer medium  30  to the print medium  60  can be expressed by the following transfer efficiency (η): 
                 η   =         Q   T         Q   T     +     Q   R         ×   100   ⁢   %             Equation   ⁢           ⁢   1               
where Q T  is the amount of development agent transferred to a print medium, Q R  is the amount of development agent remaining on an intermediate transfer medium, transfer efficiency (η) is the rate of the development agent transferred from the intermediate transfer medium  30  to the print medium  60 .
 
   Thus, in order to achieve transfer efficiency, it is necessary to supply an appropriate voltage to the transfer roller  41 . That is, if the transfer efficiency is maintained at a stable level, the quality of an image transferred to the print medium  60  can also be maintained at a stable level. 
   The transfer efficiency should be set so as to transfer the image from the intermediate transfer medium  30  to the print medium  60  at a maximum level and such transfer efficiency is called optimum efficiency. An appropriate voltage to be supplied to the transfer roller  41  to achieve optimum efficiency varies according to the value of resistance of the intermediate transfer medium  30 . However, since the resistance value of the intermediate transfer medium  30  varies according to the temperature of the surrounding environment, if the surrounding temperature varies, it becomes difficult to achieve optimum efficiency although a transfer voltage obtaining optimum efficiency at a particular temperature is supplied to the transfer roller  41 . In order to maintain optimum efficiency without the influence of the change in the surrounding temperature, it is necessary to change the transfer voltage supplied to the transfer roller  41  according to the resistance value of the intermediate transfer medium  30 . Accordingly, the variance in resistance of the intermediate transfer medium  30  according to the temperature should be known and an example of the relationship of temperature and resistance is shown in  FIG. 5 . Referring to  FIG. 5 , as the temperature rises, the resistance of the intermediate transfer medium  30  decreases (curve  1 ), and the electric current increases (curve  2 ). Therefore, the resistance of the intermediate transfer medium  30  according to the temperature can be known by measuring the electric current flowing in the intermediate transfer medium  30 . 
   The voltage determination unit  125  computes voltage for gaining optimum transfer efficiency according to the resistance of the intermediate transfer medium  30  calculated from the resistance computation unit  123 . In one implementation, a predetermined table (resistance-voltage table) showing values of voltage for gaining the optimum transfer efficiency with respect to the resistance of the intermediate transfer medium  30  is established and stored in a storage device (not shown) and a value of voltage corresponding to the resistance calculated in the resistance computation unit  123  in the table is transmitted to the output voltage generator  110 . 
   The output voltage generator  110  outputs transfer voltage according to a signal output from the voltage computation unit  120  and supplies it to the transfer device  40 . The output voltage generator  110  comprises a transfer voltage controller  111  outputting a voltage corresponding to the signal input from the voltage computation unit  120 , a pulse width controller  113  adjusting and controlling the pulse width for establishing an output voltage at an appropriate output value, a high voltage converting unit  115  converting the output voltage into a high voltage, and a high voltage generator  117  outputting the high output voltage to the transfer device  40 . 
   The switching unit  130  connects the high output voltage output from the output voltage generator  110  either to the transfer roller  41  or the transfer backup roller  42 , and comprises a first switching unit  131 , a second switching unit  132 , and a grounding device  135 . ( FIG. 4 ) 
   The first switching unit  131  is disposed to have one point of contact connected to the transfer roller  41  and the other point of contact selectively connected to the high voltage generator  117  of the output voltage generator  110  or the grounding device  135 . The first switching unit  131  may use a relay, a solenoid, or a mechanical contact point shifting device using a motor, which can switch an electrical contact point according to a signal of a control unit (not shown) of an image forming machine. 
   The second switching unit  132  is disposed to have one point of contact connected to the transfer backup roller  42  and the other point of contact selectively connected to the high voltage generator  117  of the output voltage generator  110  or the grounding device  135 . The second switching unit  132  also uses the same contact point shifting device as the first switching unit  131 . 
   If either of the first switching unit  131  or the second switching unit  132  is connected to the output voltage generator  110 , the other of the first and second switching units  131 ,  132  is connected to the grounding device  135 . That is, if the first switching unit  131  is connected to the output voltage generator  110  and a high voltage from the output voltage generator  110  is supplied to the transfer roller  41 , the second switching unit  132  is connected to the grounding device  135  whereby the high voltage is not supplied to the transfer backup roller  42 , and vice versa. 
   Hereinafter, the operation of transferring an image to a print medium  60  by a transfer power supply apparatus  100  having the above described structure will be described. 
   When a print command is received, the control unit (not shown) operates the plurality of color image forming units  10 C,  10 M,  10 Y,  10 K for developing each color image on each photoreceptor and transferring the developed images onto the intermediate transfer medium  30  to precisely overlap with each other. The image created on the intermediate transfer medium  30  by the plurality of color image forming units  10 C,  10 M,  10 Y,  10 K is conveyed to the transfer device  40  together with the intermediate transfer medium  30  moved by the drive roller  32 . 
   When the part of the intermediate transfer medium  30  having the image created enters between the transfer roller  41  and the transfer backup roller  42 , a high voltage is supplied to the transfer roller  41 , and the transfer backup roller  42  is connected to the grounding device  135 . At this time, a voltage having a polarity forming an electrostatic field conveying development agent of a created image from the intermediate transfer medium  30  to the print medium  60  is applied to the transfer roller  41 . For example, a positive voltage is applied to the transfer roller  41  when the development agents are negatively charged. Describing the operation of the transfer power supply apparatus  100 , one contact point of the first switching unit  131  is connected to the high voltage generator  117  of the output voltage generator  110  and the contact point of the second switching unit  132  is connected to the grounding device  135 . Accordingly, the high voltage generated from the output voltage generator  110  is applied to the transfer roller  41 . The electric current flowing between the high voltage converting unit  115  and the high voltage generator  117  is measured by the transfer electric current recognizer  121 . 
   During the printing process, if the temperature around the image forming machine varies, the resistance of the intermediate transfer medium  30  varies. Then, the electric current flowing in the high voltage generator  117  varies and therefore the transfer electric current recognizer  121  recognizes the changed value of the electric current. Accordingly, the resistance computation unit  123  computes a value of resistance corresponding to the electric current recognized by the transfer electric current recognizer  121 . The voltage determination unit  125  then extracts a value of a voltage corresponding to the value of resistance computed by the resistance computation unit  123  from the resistance-voltage table and transmits the extracted value as an appropriate voltage to the transfer voltage controller  111 . The output voltage generator  110  then generates a high voltage corresponding to the appropriate voltage input into the transfer voltage controller  111 . That is, the output voltage generator  110  detects the electric current flowing between the high voltage converting unit  115  and the high voltage generator  117  and sends the detected electric current to the pulse width controller  113  for the pulse width controller  113  to generate a voltage corresponding to the appropriate voltage input into the transfer voltage controller  111 . 
   When an image is completely transferred onto a sheet of print medium  60 , a high transfer voltage is applied to the transfer backup roller  42  and the transfer roller  41  is connected to the grounding device  135 . When a high voltage is applied to the transfer backup roller  42 , an electrostatic field drawing development agents is formed between the transfer backup roller  42  and the intermediate transfer medium  30  and therefore the development agents on the intermediate transfer medium  30  are not transferred to the transfer roller  41 . Accordingly, the transfer roller  41  can be prevented from being contaminated by the development agents. Describing the operation of the transfer power supply apparatus  100 , one contact point of the second switching unit  132  is connected to the high voltage generator  117  of the output voltage generator  110 , and the contact point of the first switching unit  131  is connected with the grounding device  135 . Thus, the high voltage generated from the output voltage generator  110  is applied to the transfer backup roller  42 . The electric current flowing between the high voltage converting unit  115  and the high voltage generator  117  is measured by the transfer electric current recognizer  121 . Even when the surrounding temperature changes between the printing processes, i.e., during the cleaning process, the development agents on the intermediate transfer medium  30  are not transferred to the transfer roller  41  since the transfer power supply apparatus  100  detects resistance of the intermediate transfer medium  30  by the transfer electric current recognizer  121  and applies a transfer voltage appropriate for the corresponding temperature to the transfer backup roller  42 . 
   When an image created on the intermediate transfer medium  30  is transferred to a new print medium  60 , the switching unit  130  is operated for applying a high transfer voltage to the transfer roller  41  and therefore the transfer backup roller  42  is connected to the grounding device  135 . 
   As described above, a transfer voltage is applied to the transfer roller  41  and transfer backup roller  42  using a single transfer power supply apparatus  100  having a switching unit  130 . An embodiment of the present invention in which a separate transfer power supply apparatus is connected to the transfer roller  41  and the transfer backup roller  42  respectively as shown in  FIG. 6  is illustrated below. 
   Referring to  FIG. 6 , a first transfer power supply apparatus  200  is connected to the transfer roller  41  and a second transfer power supply apparatus  300  is connected to the transfer backup roller  42 . 
   The first transfer power supply apparatus  200  comprises a first voltage computation unit  220 , a first output voltage generator  210 , and a first switch  230 , and the second transfer power supply apparatus  300  comprises a second voltage computation unit  320 , a second output voltage generator  310 , and a second switch  330 . The first and second voltage computation units  220 ,  320 , and the first and second output voltage generators  210 ,  310  are identical to the voltage computation unit  120  and the output voltage generator  110  in the above described embodiment and therefore will not be described again. 
   The first switch  230  is disposed between the transfer roller  41  and the first output voltage generator  210 , and turned on for a high voltage to be applied to the transfer roller  41  when the image on the intermediate transfer medium  30  is transferred to the print medium  60 . When the image is completely transferred to the print medium  60 , the first switch  230  is turned off. 
   The second switch  330  is disposed between the transfer backup roller  42  and the second output voltage generator  310 . The second switch  330  stays off when an image on the intermediate transfer medium  30  is transferred to the print medium  60 , and is turned on for a high voltage to be applied to the transfer backup roller  42  during the cleaning process. 
   The control unit (not shown) of an image forming machine has the first switch  230  turned on, allowing a high voltage to be applied to the transfer roller  41 , and the second switch  330  turned off, preventing a voltage from being applied to the transfer backup roller  42  during the transferring process. During the cleaning process, the second switch  330  is turned on, applying a voltage to the transfer backup roller  42 , and the first switch  230  is turned off, preventing a voltage from being applied to the transfer roller  41 . 
   The image forming machine comprising a transfer power supply apparatus according to the present invention described above may obtain an optimum transfer efficiency at all times since an appropriate transfer voltage is applied to the transfer roller based on the resistance of the intermediate transfer medium. In addition, because the polarity of the transfer voltage applied to the transfer roller or the transfer backup roller during the transfer process and the cleaning process is the same, there is no occurrence of change in an output voltage according to the change in polarity. 
   In the above, a case in which an image created on an intermediate transfer medium is transferred onto a print medium when a transfer voltage is applied to a transfer roller has been described. However, when the voltage of the opposite polarity is used, the same transfer power supply apparatus according to the present invention may be applied as the image created on the intermediate transfer medium is conveyed to the print medium when a transfer voltage is supplied to the transfer backup roller. 
   The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.