Abstract:
A combined charge/recharge xerographic power supply is provided that utilizes one power supply to drive the charge pin scorotron and recharge discorotron grids of a electrophotographic or xerographic system. The power supply uses recycled power from the pin scorotron grid to drive the discorotron grid. In particular, the power supply uses power that is dissipated in the traditional shunt regulator attached to the pin scorotron grid terminal to drive and provide active current to the discorotron grid through a series-pass regulation circuit. Thereby providing reduced electromagnetic emissions and reduced unit manufacturing costs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to systems and apparatus for recycling scavenged power from a pin scorotron grid to drive a discorotron grid in an electrophotographic or xerographic system. 
     2. Description of Related Art 
     The xerographic imaging process is initiated by charging a charge retentive surface, such as that of a photoconductive member, to a uniform potential. The charge retentive surface is then exposed to a light image of an original document, either directly or via a digital image driven laser. Exposing the charged photoconductor to light selectively discharges areas of the charge retentive surface while allowing other areas to remain unchanged. This creates an electrostatic latent image of the document on the surface of the photoconductive member. 
     Developer material is then brought into contact with the surface of the photoconductor material to develop the latent image into a visible reproduction. The developer typically includes toner particles with an electrical polarity that is the same as, or that is opposite to, the polarity of the charges remaining on the photoconductive member. The polarity depends on the image profile. 
     A blank image receiving medium is then brought into contact with the photoreceptor and the toner particles are transferred to the image receiving medium. The toner particles forming the image on the image receiving medium are subsequently heated, thereby permanently fixing the reproduced image to the image receiving medium. 
     Electrophotographic or xerographic laser printers, scanners, facsimile machines and similar document reproduction devices must be able to maintain proper control over the systems of the image forming apparatus to assure high quality output images. For example, the level of electrostatic charge on the photographic member must be maintained at a certain level to be able to attract the charged toner particles. 
     FIG. 1 shows an exemplary embodiment of an image forming apparatus  100  having a photoreceptor  120 . The image forming apparatus  100  can be a xerographic printer or other known or later developed xerographic device. It should be appreciated that the specific structures of the image forming apparatus are not relevant to this invention and thus are not intended to limit the scope of this invention. 
     As shown in FIG. 1, one or more latent images can be generated on the photoreceptor  120  in any well known manner, by controlling one or more of a number of different developer units  150 A,  150 B,  150 C and  150 D using controller  110 . 
     In many xerographic machines, where high image quality targets are desired, the photoreceptor is first charged using a pin scorotron device, and then recharged, or charge leveled, by a discorotron device. For example, as shown in FIG. 1, in the direction of movement of the photoreceptor  120 , as indicated by the arrows, to lay a first level of toner onto the photoreceptor, the photoreceptor  120  is charged by charge/recharge device  130 E having a pin scorotron and a discorotron device. Next, the charge laid by the charging device is exposed by exposing unit  140 E and finally, the toner is developed by developing unit  150 E. The process continues in the direction of movement of the photoreceptor until all layers of toner are laid to complete an image-on-image full-color image forming process. Once the full-color image is finished, the completed image is transferred to a sheet of image recording media  160 . 
     The charging procedure of the charge/recharge device is performed to produce a very uniform charge on the photoreceptor. This uniform charge is especially important in the image-on-image type xerographic color machines, as shown in FIG. 1, where the photoreceptor may be buried under multiple layers of toner. Typically, the pin scorotron device is set to charge the photoreceptor to a voltage slightly higher than the final voltage, and the discorotron is then used to discharge the photoreceptor uniformly to the desired voltage. 
     FIG. 2 represents a typical configuration of a charge/recharge system  200  that is usable in a xerographic system. The left side of the configuration represents the pin scorotron device  270 , while the right side of the configuration represents the discorotron device  210 . In the pin scorotron device  270 , a high-voltage DC signal is applied to the pins  240  by a pin current supply  250 . The applied voltage is sufficiently high to cause corona discharge at the pins  240 . This discharge provides a path for a pin current to be applied to a pin scorotron grid  245 . The pin scorotron grid  245  is located between the photoreceptor  120  and the pins  240  so that the majority of the pin current is absorbed by the pin scorotron grid  245 . The grid is held at a constant voltage by the pin scorotron grid voltage control circuit  260 , which is a simple shunt regulator type circuit. The pin scorotron grid voltage control circuit  260  operates in a linear manner to achieve a variable resistance network to ground. The resistance of the pin scorotron grid voltage control circuit  260  can be controlled to either increase or decrease its voltage drop to achieve the desired grid voltage. 
     The discorotron device comprises a shield  225  formed of aluminum or the like and having an open lower end, a corona discharge electrode  230 , such as a glass coated tungsten wire or the like, extending within the shield  225 , and a discorotron grid  235  disposed opposite the opening of the shield  235  and between the shield  225  and the photoreceptor  120 . The discorotron device  210  operates in much the same manner as the pin scorotron device  270 . The discorotron grid  235  is typically driven by an active power source, such as the grid voltage active control circuit  215 . The discorotron high-voltage AC source  220  is connected to the corona discharge electrode  230  to produce a corona discharge. 
     SUMMARY OF THE INVENTION 
     As shown in FIG. 2, the pin scorotron device  270  and the discorotron device  210  are driven by separate power supplies. However, there is available power in the pin scorotron grid voltage control circuit  260  that can be recycled and used to drive and control the discorotron grid  235 . 
     The inventors have discerned that the power that is dissipated in the pin scorotron grid voltage control circuit  260  can be used to drive the discorotron grid  235 . 
     This invention provides systems and apparatus that provide reduced power dissipation in the high voltage power supply. 
     This invention separately provides possible direct programming of the voltage applied to the photoreceptor and the voltage between the pin scorotron grid and the discorotron grid rather than by indirect programming of the voltage applied directly to the pin scorotron grid and the discorotron grid. 
     This invention separately provides reduced electromagnetic emissions and increased arc immunity of the discorotron due to a better controlled xerographic current path. The reduced emissions is achieved because the discorotron grid is not driven by an active power supply. 
     In various exemplary embodiments of the systems and apparatus of this invention, the active power source that is typically used to drive the discorotron grid is removed. According to the systems and apparatus of this invention, the discorotron grid instead utilizes a combined circuit which uses the power dissipated in the traditional shunt regulation circuit that drives the pin scorotron grid to drive the discorotron grid through a series pass regulation circuit. The current flow of the combined circuit naturally flows in a direction to allow shunt regulation of the pin scorotron grid while also providing an active drive voltage for the discorotron grid. 
     These and other features and advantages of this invention are described in or are apparent from the following detailed description of the apparatus and systems according to this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of this invention will be described in detail with respect to the following drawings, in which like reference numerals indicate like elements, and wherein: 
     FIG. 1 depicts an exemplary embodiment of a xerographic image forming apparatus in which an exemplary combined charge/recharge xerographic power supply according to this invention may be used to charge a photoreceptor; 
     FIG. 2 depicts an exemplary representation of a typical configuration of a charge/recharge device; 
     FIG. 3 depicts an exemplary representation of a typical pin scorotron grid voltage control circuit; 
     FIG. 4 depicts an exemplary embodiment of a charge/recharge circuit using a combined charge/recharge xerographic power supply according to this invention; and 
     FIG. 5 is a schematic diagram of one exemplary embodiment of the circuit elements of the combined charge/recharge xerographic power supply of FIG. 4 according to this invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 3 depicts in greater detail an exemplary representation of a typical grid voltage control circuit  260 . The grid voltage control circuit  260 , which is a simple shunt regulation circuit, contains seven cascaded pnp bipolar transistors that would be connected directly to the pin scorotron grid  245 . This circuit, while effective in providing adequate power to drive the pin scorotron grid  245 , is ineffective in providing reduced power dissipation in the high voltage power supply, which will improve electromagnetic emission profiles. 
     FIG. 4 depicts an exemplary embodiment of the charge/recharge xerographic power supply  400  according to this invention. As shown in FIG. 4, the charge/recharge xerographic power supply  400  comprises the pin scorotron device  270  and the discorotron device  210 . In the pin scorotron device  270 , as in conventional systems, a high-voltage DC signal is applied to the pins  240  by the pin current supply  250 . The pin scorotron grid  245  is located between the photoreceptor  120  and the pins  240 . 
     The discorotron device  210 , as in conventional systems, comprises the shield  225  formed of aluminum or the like and having the open lower end, the corona discharge electrode  230 , such as a glass coated tungsten wire or the like, extending within the shield  225 , and the discorotron grid  235  disposed opposite the opening of the shield  225  and between the shield and the photoreceptor  120 . The discorotron high-voltage AC source  220  is connected to the corona discharge electrode  230  to produce the corona discharge. 
     However, as shown in FIG. 4, the separate pin scorotron grid voltage control circuit  260  and the separate grid voltage active control circuit  215  of the conventional system are replaced by a single combined charge/recharge power supply  500 . That is, the pin scorotron grid  245  is held at a constant voltage and the discorotron grid  235  is driven by the combined charge/recharge power supply  500 . This configuration recycles the power provided from the pin scorotron grid  245  to drive the discorotron grid  235  through a series pass regulation circuit. FIG. 5 shows the current flow direction and demonstrates that the current from a shunt regulation circuit naturally flows in a proper direction to allow shunt regulation of the pin scorotron grid  245  while also providing an active drive voltage for the discorotron grid  235 . 
     FIG. 5 shows in greater detail a schematic diagram of one exemplary embodiment of the circuit elements of the combined charge/recharge xerographic power supply  500 . The combined charge/recharge power supply  500  has two main sections  501  and  502 . The first main section  502  is a pin scorotron grid voltage control circuit  502 . The second main section  501  is a high side gate drive circuit  501 . 
     In FIG. 5, the pin current supply  250 , pins  240  and the pin scorotron grid  245  are represented by current source  554  and resistors  551  and  553 , respectively. Also in FIG. 5, the discorotron grid is represented by resistor  555 . The discorotron high voltage AC source  220  and corona discharge electrode  230  are not shown in FIG. 5 because they have no particular bearing on the invention. 
     As shown in FIG. 5, the pin scorotron grid voltage control circuit  502  includes a positive terminal of a voltage source  503  connected to a first node  505  through a first resistor  504 . The negative terminal of the voltage source  503  is connected to ground  556 . Also connected at the first node  505  are a gate of a first p-channel MOSFET  507  and a second resistor  506 . A drain of the first p-channel MOSFET  507  is connected to the common ground  556 . A source of the first p-channel MOSFET  507  is connected to the drain of a second p-channel MOSFET  509 . 
     The second resistor  506  is connected at a second node  508  to a gate of the second p-channel MOSFET  509  and a third resistor  510 . Similarly, a source of the second p-channel MOSFET  509  is connected to a drain of a third p-channel MOSFET  511 . 
     A third resistor  510  is connected at a third node  512  to the gate of the third p-channel MOSFET  511  and a fourth resistor  513 . Similarly, the source of the third p-channel MOSFET  511  is connected to the drain of a fourth p-channel MOSFET  514 . 
     The fourth resistor  513  is connected at node  515  to the gate of the fourth p-channel MOSFET  514  and a fifth resistor  516 . Similarly, the source of the fourth p-channel MOSFET  514  and the other end of the fifth resistor  516  are connected to a fifth node  517 . Also connected at the fifth node  517  are a sixth resistor  519 , the source of a first n-channel MOSFET  520  and a first pull-up resistor  518 . 
     The sixth resistor  519  is connected at a sixth node  521  to the gate of the first n-channel MOSFET  520  and a seventh resistor  522 . Similarly, the drain of the first n-channel MOSFET  520  is connected to the source of a second n-channel MOSFET  523 . 
     An eighth resistor  527  is connected at a seventh node  524  to the seventh resistor  522 , a ninth resistor  525  and the gate of the second n-channel MOSFET  523 . Similarly, the drain of the second n-channel MOSFET  523  is connected to the ninth resistor  525  at an eighth node  526 . Also connected at the eighth node  526  is a second pull-up resistor  550  and a tenth resistor  529 , which is a part of the high side gate drive  501 . This configuration makes up the pin scorotron grid voltage control circuit  502 . 
     The high side gate drive circuit  501  includes the positive terminal of a variable voltage source  549 , which is connected to a ninth node  547  through an eleventh resistor  548 . The negative terminal of the variable voltage source  549  is connected to ground  556 . Also connected at the ninth node  547  is the gate of a fifth p-channel MOSFET  546  and a twelfth resistor  543 . The drain of the fifth p-channel MOSFET  546  is connected to ground  556 . Similarly, the source of the fifth p-channel MOSFET  546  is connected to a tenth node  544 . Also connected at the tenth node  544  is a first tap terminal  545  and the drain of a sixth p-channel MOSFET  542 . 
     A thirteenth resistor  538  is connected at an eleventh node  541  to the gate of the sixth p-channel MOSFET  542  and the twelfth resistor  543 . Similarly, the source of the sixth p-channel MOSFET  542  is connected to a twelfth node  539 . Also connected at the twelfth node  539  is a second tap terminal  540  and the drain of a seventh p-channel MOSFET  536 . 
     A fourteenth resistor  535  is connected at a thirteenth node  537  to the gate of a seventh p-channel MOSFET  536  and the thirteenth resistor  538 . Similarly, the source of the seventh p-channel MOSFET  536  is connected to a fourteenth node  532 . Also connected at the fourteenth node  532  is a third tap terminal  533  and the drain of the eighth p-channel MOSFET  531 . 
     The fourteenth resistor  535  is connected at a fourteenth node  530  to the gate of the eighth p-channel MOSFET  531  and the other end of the tenth resistor  529 . Similarly, the source of the eighth p-channel MOSFET  531  is connected to a fifteenth node  528 . Also connected at the fifteenth node  528  is a fourth tap terminal  534  and the other end of the eighth resistor  527 . 
     As shown in FIG. 5, the high side gate drive circuit  501  is connected to the pin scorotron grid voltage control  502  at the eighth and fifteenth nodes  526  and  528 , respectively. 
     Active current is supplied to the discorotron grid through the first pull-up resistor  518 . The first pull-up resistor  518  is connected to ground  556  through the discorotron grid terminal load resistance. In this instance, the discorotron grid terminal load of the discorotron grid  235  is shown as a fifteenth resistor  555 . 
     In operation of the combined charge/recharge power supply  500 , as the voltage of the variable voltage source  549  is varied, the gate-to-source voltage of the first and second n-channel MOSFETs  520  and  523  is varied through the cascaded configuration of the high side gate drive circuit  501 . Additionally, the voltage of voltage source  503  serves as the discorotron analog error voltage. The voltage supplied by the voltage source  503  serves to bias and stabilize the current supplied to the fifteenth resistor  555 . 
     The second pull-up resistor  550  is connected between the eighth node  526  and a sixteenth node  552  to provide a path for current flow and shunt regulation of the pin scorotron grid  245 . A sixteenth resistor  551  and the pin scorotron grid terminal load of the pin scorotron grid  245 , which is shown in FIG. 5 as a seventeenth resistor  553 , are connected at the sixteenth node  552 . The seventeenth resistor  553  is also connected to ground  556 . A current source  554  is connected to the sixteenth resistor  551 . The current source  554  serves to drive the pin scorotron grid  245 . 
     There are two constraints in the circuit shown in FIG.  5 . The first constraint is that the voltage at the discorotron grid terminal load, i.e., at the fifteenth resistor  555 , cannot exceed the voltage at the pin scorotron grid terminal load, i.e., the voltage at the seventeenth resistor  553 . In this instance this means that the voltage at node  517  cannot be made more negative than the voltage at node  526 . This constraint arises because the voltage supply for the discorotron grid  235  is derived from the pin scorotron grid  245 . The second constraint stems from the same instance, in that the current flow into the terminal of the discorotron grid  235  cannot exceed the current flow from the terminal of the pin scorotron grid  245 . 
     The first constraint can be overcome by adding a small transformer coupled DC to DC converter in series with resistor  550 , with the positive terminal connected nearest to node  552 . This source would allow the pin scorotron grid voltage to be maintained at a less negative voltage than required at the discorotron grid terminal. Using this method, several tens of volts are capable of being added to the output of the discorotron grid  235 . 
     The second constraint does not particularly affect the operation of a system using this invention. This is true because, as previously discussed, the majority of the pin current is collected by the grid in the pin scorotron device  270 . Thus, only a small portion is actually used to charge the photoreceptor  120 . Similarly, only a small amount of DC current is required at the discorotron grid terminal to recharge the photoreceptor  120 . 
     While this invention has been described in conjunction with the exemplary embodiment outlined above, it is evident that many alternative modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiment of the inventions as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and the scope of the invention.