Abstract:
A power supply device including: a transformer generating, from an input voltage, low output voltage and high output voltage; an upper limiter circuit receiving the high output voltage and controlling the high output voltage not to exceed a maximum; a power controller performing feedback control on the input voltage so that the low output voltage matches a target voltage; an operation mode acquirer configured to acquire an operation mode of an image forming device; and a target value controller configured to change the target voltage depending upon the acquired operation mode. The target voltage when consumption amount of current with the high output voltage is relatively great ensures that the high output voltage does not fall below a minimum of a rated voltage range of the high output voltage, and is higher than the target voltage when consumption amount of current with the high output voltage is relatively small.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on application No. 2015-214443 filed in Japan, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (1) Field of the Invention 
         [0003]    The present disclosure relates to a power supply device and an image forming device. In particular, the present disclosure relates to a technology for guaranteeing low electric power loss and accurate supply of voltages when load size is increased. 
         [0004]    (2) Related Art 
         [0005]    A typical power supply device used in an image forming device is capable of outputting multiple voltages by including multiple transformers and multiple control units each provided to control a different one of the transformers. In Japan, commercial power sources output 100 V AC power. For example, a power supply device receiving input of such power and outputting two different DC voltages, namely a low voltage (e.g., 5 V) and a high voltage (e.g., 24 V), is provided with a combination of a control circuit and a transformer for each voltage. In such structure, each combination of a control circuit and a transformer performs feedback control to output a voltage within a rated voltage range. 
         [0006]    In view of such conventional technology, a proposal is being made of a power supply device that includes only one combination of a control circuit and a transformer that outputs both a low voltage and a high voltage, in order to reduce cost and device size. Refer to Japanese Patent Application Publication No.: H08-211790 for one example of such a power supply device. According to this technology, due to the necessity of controlling output low voltage with higher accuracy compared to output high voltage, the output low voltage is monitored for feedback control. 
         [0007]    With such feedback control, the output high voltage may fluctuate considerably depending upon the amount of load current. In view of this, Japanese Patent Application Publication No.: H08-211790 also proposes changing the target voltage of the low voltage in the feedback control depending upon the output high voltage. 
         [0008]    However, when the necessary amount of high voltage power increases to 300 W or greater due to certain circumstances (e.g., due to the inclusion, in an image forming device, of a sheet post-processing device that performs stapling of a stack of sheets with images formed thereon), guaranteeing accuracy of both the output low voltage and the output high voltage becomes difficult. Specifically, with the conventional technology disclosed in Japanese Patent Application Publication No.: H08-211790, accuracy of both the output low voltage and the output high voltage can no longer be guaranteed should the necessary amount of high voltage power become higher than around 200 W. 
         [0009]    In view of this, a proposal is being made of a power supply device that includes an upper limiter circuit that limits output high voltage to be within ±10% of a rated voltage, while controlling output low voltage to be at a target voltage. Refer to Japanese Patent Application Publication No.: H09-093924 for one example of such a power supply device. However, this technology also has a drawback in that electric power loss occurs at the upper limiter circuit, and a consequent increase in power consumption occurs. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of such problems, the technology pertaining to the present disclosure aims to provide a power supply device and an image forming device that reduce electric power loss occurring when load size of a load to which high voltage is output is increased. 
         [0011]    One aspect of the present disclosure is a power supply device for an image forming device that is provided with a first load and a second load that operates at a higher voltage than the first load, and that operates by switching between operation modes, the power supply device including: a transformer configured to generate, from an input voltage, a low output voltage for the first load and a high output voltage for the second load; an upper limiter circuit configured to receive the high output voltage from the transformer, and control the high output voltage not to exceed a maximum; a power controller configured to perform feedback control on the input voltage so that the low output voltage matches a target voltage; an operation mode acquirer configured to acquire an operation mode of the image forming device; and a target value controller configured to change the target voltage depending upon the acquired operation mode, the target voltage, when the acquired operation mode is a high current consumption mode, ensuring that the high output voltage does not fall below a minimum of a rated voltage range of the high output voltage, the target voltage, when the acquired operation mode is a low current consumption mode, being lower than the target voltage when the acquired operation mode is the high current consumption mode, the image forming device consuming a greater amount of current with the high output voltage in the high current consumption mode than in the low current consumption mode. 
         [0012]    In the power supply device pertaining to one aspect of the present disclosure, preferably, when the acquired operation mode is the low current consumption mode, the greater the amount of the current with the low output voltage to be output to the first load, the lower the target voltage set by the target value controller. 
         [0013]    In the power supply device pertaining to one aspect of the present disclosure, preferably, when the acquired operation mode is the high current consumption mode, the greater the amount of the current with the low output voltage to be output to the first load, the higher the target voltage set by the target value controller. 
         [0014]    In the power supply device pertaining to one aspect of the present disclosure, preferably, the target voltage controller judges the amount of the current with the low output voltage to be output to the first load based on an operation state of the first load or a device structure of the first load. 
         [0015]    In the power supply device pertaining to one aspect of the present disclosure, preferably, the first load includes at least one of a hard disk drive attached to the image forming device, a facsimile communication unit of the image forming device, and a print controller unit of the image forming device. 
         [0016]    In the power supply device pertaining to one aspect of the present disclosure, preferably, when in the high current consumption mode, the image forming device is capable of performing image forming immediately, and when the acquired operation mode is the high current consumption mode and the current with the low output voltage is to be output to the first load at a predetermined amount or more, the target voltage set by the target voltage controller equals a minimum of a rated voltage range of the low output voltage. 
         [0017]    In the power supply device pertaining to one aspect of the present disclosure, preferably, the target voltage controller judges the amount of the current with the high output voltage to be output to the second load based on an operation state of the first load or a device structure of the second load. 
         [0018]    In the power supply device pertaining to one aspect of the present disclosure, preferably, the second load includes: an image reader unit of the image forming device, the image reader unit including an automatic document feeder; and a post-processor unit of the image forming device, the power-processor unit performing post-processing on a sheet stack, the sheet stack composed of one or more recording sheets already having images formed thereon, and when the acquired operation mode is the high current consumption mode and both the image reader unit and the post-processor unit are to be operated, the target voltage set by the target voltage controller equals a maximum of a rated voltage range of the low output voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    These and other objects, advantages and features of the technology pertaining to the present disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings, which illustrate specific embodiment(s) of the technology pertaining to the present disclosure. 
           [0020]    In the drawings: 
           [0021]      FIG. 1  is a perspective view illustrating the appearance of an image forming device  1  pertaining to an embodiment of the technology pertaining to the present disclosure; 
           [0022]      FIG. 2  illustrates main components of the image forming device  1 ; 
           [0023]      FIG. 3  is a block diagram illustrating the structure of 5 V loads; 
           [0024]      FIG. 4  illustrates main components of a power supply device  100 ; 
           [0025]      FIG. 5  is a flowchart illustrating main operations of the power supply device  100 ; 
           [0026]      FIG. 6  illustrates transition between different operation modes of the image forming device  1 ; 
           [0027]      FIG. 7  shows graphs illustrating examples of current-voltage characteristics of 24 V loads; and 
           [0028]      FIG. 8  shows graphs illustrating relationships between electric power loss and current amount for the 24 V loads. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    The following describes an embodiment of a power supply device and an image forming device pertaining to the present disclosure, with reference to the accompanying drawings. 
       [1] Structure of Image Forming Device  1   
       [0030]    The following describes the structure of an image forming device pertaining to the embodiment. 
         [0031]    The image forming device pertaining to the embodiment is a multi-function peripheral (MHP) having a so-called tandem system. The image forming device pertaining to the embodiment has, for example, a scanner function, a color printer function, a copier function, and a facsimile function.  FIG. 1  illustrates the image forming device pertaining to the embodiment as image forming device  1 . The image forming device  1  includes: an image reader  110 ; a sheet post-processor  120 ; and an image former  130 . The image reader  110  includes an automatic document feeder (ADF) and a scanner. The ADF feeds documents one by one to the scanner, and the scanner generates image data for each document. 
         [0032]    The sheet post-processor  120  is arranged inside an in-body space of the image forming device  1  that is located between the image reader  110  and the image former  130 . The sheet post-processor  120  performs post-processing including alignment and stapling with respect to a sheet stack ejected from the image former  130  after image forming. Here, the sheet stack may be composed of only one recording sheet or two or more recording sheets. A sheet stack having received post-processing at the sheet post-processor is ejected onto a paper eject tray  160  attached to a front side of the image former  130 . 
         [0033]    The image former  130  forms an image on each recording sheet P supplied from a paper feeder  150 . The image former  130  performs the image forming based on image data generated by the image reader  110  or image data received from other devices. Each recording sheet P carrying an image formed thereon by the image former  130  is ejected to the sheet post-processor  120 . Further, the image former  130  has an operation panel  140 . For example, the operation panel  140  provides visual presentation of information to users of the image forming device  1  and receives instructions from users of the image forming device  1 . In addition, the image former  130  includes a power supply device  100 . The power supply device  100  receives 100 V AC power from a commercial AC power source, and supplies DC power to components of the image forming device  1 . 
         [0034]      FIG. 2  illustrates main components of the image forming device  1 . As illustrated in  FIG. 2 , the image former  130  includes a control unit  200  and an imaging unit  210 . The control unit  200  receives image data from the image reader  110  or from other devices. When the control unit  200  receives image data for one recording sheet P, the imaging unit  210  forms a toner image corresponding to the image data. Here, the toner image that is formed may be a color toner image or a monochrome toner image. When the imaging unit  210  forms a color toner image, the color toner image is formed by superimposing toner images of the colors yellow, magenta, cyan, and black one over another. The paper feeder  150  picks up recording sheets P housed in a paper feeder cassette  151  one by one by using a pick-up roller  152 . The paper feeder  150  thus feeds a recording sheet P to the image former  130 . 
         [0035]    The recording sheet P having been fed to the image former  130  is transported further by transport rollers  153  to timing rollers  154 . The timing rollers  154  transport the recording sheet P to image transfer rollers  211  at an appropriate timing, where the recording sheet P receives transfer of a toner image. Subsequently, the toner image having been transferred onto the recording sheet P is heat-fixed onto the recording sheet P at a fixing device  220 , before being transported to a paper eject unit  230 . 
         [0036]    When an image is to be formed only on a front side of the recording sheet P, the recording sheet P is guided to paper eject rollers  232  by a guiding claw  231  in the paper eject unit  230 . After being guided to the paper eject rollers  232 , the recording sheet P is ejected onto the sheet post-processor  120 . When one or more recording sheets P composing a sheet stack have been ejected onto the sheet post-processor  120 , the sheet post-processor  120  ejects the sheet stack to the paper eject tray  160  after performing post-processing on the sheet stack or without performing any post-processing on the sheet stack. Here, whether the sheet post-processor  120  performs or does not perform post-processing depends upon user instruction. 
         [0037]    Meanwhile, when images are to be formed on both sides of the recording sheet P, the recording sheet P is guided to reverse rollers  233  by the guiding claw  231 . The reverse rollers  233  first cause the recording sheet P to advance on and along a reverse guide plate  123 , and then stop the recording sheet P from advancing any further with the trailing end of the recording sheet P between the reverse rollers  233 . Subsequently, the reverse rollers  233  rotate in the opposite direction to send the recording sheet P onto a reverse path  234 . 
         [0038]    After the recording sheet P travels through the reverse path  234 , an image is formed on the back side of the recording sheet P. Then, the recording sheet P is ejected onto the sheet post-processor  120 . Processing following this point is similar to that described above when an image is formed only on the front side of the recording sheet P. That is, when one or more recording sheets P composing a sheet stack have been ejected onto the sheet post-processor  120 , the sheet post-processor  120  ejects the sheet stack to the paper eject tray  160 . 
         [0039]    Note that the image forming device  1  is provided with an undepicted drive motor that achieves the rotation of rollers (e.g., the pickup roller  152 , the transport rollers  153 , the timing rollers  154 , the image transfer rollers  211 , the paper eject rollers  232 , and the reverse rollers  233 ). Further, the image forming device  1  is provided with a clutch that is operable to transfer the drive force of the drive motor to the rollers, and that is also operable not to transfer the drive force to the rollers. In addition, the image forming device  1  may be provided with a solenoid for purposes such as the transport of recording sheets P. 
         [0040]    Further, the image forming device  1  is provided with a fan for discharging high temperature air inside the device to the outside of the device and thus discharge heat generated by components such as the fixing device  220 . 
         [0041]    Further, the control unit  200  includes a control circuit that, via the operation panel  140 , provides information to users of the image forming device  1  and receives instructions from users of the image forming device  1 . The control circuit is connected to a hard disk drive (HDD) that stores image data. In addition, the control unit  200  is connected to a LAN and is capable of, for example, receiving print jobs from other devices and transmitting/receiving facsimile data. 
         [0042]    Further, the operation panel  140  includes a liquid crystal display (LCD), and the LCD includes light-emitting diodes (LED) providing LCD backlight. 
         [0043]    The power supply device  100  receives power from the AC power source  240 , and outputs DC power to the components of the image forming device  1 . Specifically, the power supply device  100  is capable of outputting DC power with two different voltages, 24 V and 5 V. For example, the 24 V DC power is supplied to the motor, the clutch, the solenoid, and the fan, among the components described above. Thus, the image reader  110  receives the 24 V DC power, in order to cause the ADF and the like to operate. Further, the sheet post-processor  120  also receives the 24 V DC power. In the following, loads receiving the 24 V DC power are referred to as 24 V loads. Meanwhile, the 5 V DC power is supplied to the loads described in the following (referred to as 5 V loads). For example, the 5 V loads include the control unit  200  and the operation panel  140 . 
       [2] 5 V Loads 
       [0044]    The following describes the 5 V loads. The 5 V loads include the control unit  200  and several other components that are controlled by the control unit  200 . 
         [0045]    As illustrated in  FIG. 3 , the 5 V loads are classified into (i) main 5 V load components, (ii) first additional 5 V load components  310 , and (iii) second additional 5 V load components  320 . In the embodiment, the main 5 V load components include the control unit  200  and the operation panel  140 . The first additional 5 V load components  310  include a hard disk drive (HDD)  311 , a facsimile communication unit  312 , and a print controller  313 . The second additional 5 V load components  320  include a Universal Serial Bus (USB) interface (I/F)  321 , a USB memory  322 , and an authentication device  323 . 
         [0046]    The control unit  200  includes: a central processing unit (CPU)  301 ; a read-only memory (ROM)  302 ; and a random access memory (RAM)  303 . When the image forming device  1  is turned on, the CPU  301  reads out a boot program from the ROM  302  and launches the boot program, and operates while using the RAM  303  as a working storage. The CPU  301  also performs processing by reading programs, image data, etc., from the HDD  311 . 
         [0047]    The operation panel  140  includes a touch panel and one or more hardware keys, and receives control from the control unit  200  via a bus  330 . The touch panel is composed of a touch pad and an LCD. The LCD is composed of a liquid crystal panel and LED backlight, both of which operate by receiving the 5 V DC power. 
         [0048]    The facsimile communication unit  312  is controlled by the control unit  200 , and performs facsimile communication with other devices. The print controller  313  is also controlled by the control unit  200 , and controls the operations of the paper feeder  150 , the imaging unit  210 , the fixing device  220 , and the paper eject unit  230  to perform image forming based on image data stored in the HDD  311 . 
         [0049]    The USB I/F  321  is used by the control unit  200  for accessing USB devices. In the embodiment, the USB memory  322  and the authentication device  323  are connected to the control unit  200  via the USB I/F  321 . 
         [0050]    The control unit  200  is capable of storing image data to the USB memory  322  and reading image data from the USB memory  322 . Further, the authentication device  323  performs authentication of users attempting to use the image forming device  1 . For example, the authentication device  323  performs authentication by using biometrics such as user fingerprints, and notifies the result of the authentication to the control unit  200 . 
       [3] Operation Modes of Image Forming Device  1   
       [0051]    The following describes operation modes of the image forming device  1   
         [0052]    The operation modes of the image forming device  1  include a sleep mode, a standby mode, a scan mode, and a print mode. 
         [0053]    The image forming device  1 , in the print mode, may simply perform only image forming, may perform image forming while performing document reading with the image reader  110 , or may perform image forming while performing sheet post-processing with the sheet post-processor  120 . In addition, the image forming device  1 , in the print mode, may perform image forming while operating both the image reader  110  and the sheet post-processor  120 . In this case in particular, a great amount of 24 V current needs to be output. Note that in any case, the image forming device  1  performs image forming by operating the image former  130  and the paper feeder  150 . 
         [0054]    The image forming device  1 , in the sleep mode, does not cause any of the 5 V loads or any of the 24 V loads to operate. Thus, the amount of current to be output from the power supply device  100  is smallest when the image forming device  1  is in the sleep mode. 
         [0055]    The image forming device  1 , in the standby mode, causes only the 5 V loads to operate. Further, the image forming device, in the scan mode, causes the 5 V loads to operate, and in addition, causes only the image reader  110  among the 24 V loads to operate. Due to this, the amount of current to be output from the power supply device  100  in the standby mode and the scan mode is in between the amount of current to be output from the power supply device  100  in the print mode and the amount of current to be output from the power supply device  100  in the sleep mode. 
         [0056]    Thus, the print mode can be referred to as a high current consumption mode. Meanwhile, each of the sleep mode, the standby mode, and the scan mode can be referred to as a low current consumption mode. 
       [4] Structure of Power Supply Device  100   
       [0057]    The following describes the structure of the power supply device  100 . 
         [0058]      FIG. 4  illustrates the main components of the power supply device  100 . As illustrated in  FIG. 4 , the power supply device  100  is a so-called AC/DC switching converter. The power supply device  100  receives 100 V AC power from the commercial AC power source  240  and supplies the 24 V loads (indicated by reference symbol  491  in  FIG. 4 ) and the 5 V loads (indicated by reference symbol  492  in  FIG. 4 ) with power. 
         [0059]    In the power supply device  100 , point a of a bridge rectifier D 411  is connected to line L of the AC power source  240 , and point b of the bridge rectifier D 411  is connected to line N of the AC power source  240 . The bridge rectifier performs full-wave rectification of the AC power. Further, in the power supply device  100 , a positive terminal of a primary smoothing capacitor C 421  is connected to point d of the bridge rectifier D 411 , and a negative terminal of the primary smoothing capacitor C 421  is connected to point c of the bridge rectifier D 411 . The primary smoothing capacitor C 421  smoothens the full-wave rectified power. 
         [0060]    Further, in the power supply device  100 , a starting power terminal of a power controller  451  is connected to line N of the AC power supply  240  via a rectifier diode D 415  and a starting resistor R 461 . Thus, the power controller  451  receives supply of starting power from the AC power supply  240 . Further a primary coil  432  of a transformer T 431  and the primary smoothing capacitor C 421  are connected in parallel via a switch SW 441 , with the switch SW 441  connected to the negative terminal of the primary smoothing capacitor C 421 . The switch SW 441  switches on and off, to control voltages output to the secondary coil  433  and a secondary coil  434  of the transformer T 431 . 
         [0061]    The secondary coil  433  and a rectifier diode D 412  compose a series circuit. This series circuit is connected in parallel with a smoothing capacitor C 422 , with a cathode terminal of the rectifier diode D 412  connected to a positive terminal of the smoothing capacitor C 422 . This parallel circuit is further connected in parallel with an upper limiter circuit  481 , which limits 24 V output to be no greater than a predetermined maximum voltage. 
         [0062]    The secondary coil  434  and a rectifier diode D 413  compose a series circuit. This series circuit is connected in parallel with a smoothing capacitor C 423 , with a cathode terminal of the rectifier diode D 413  connected to a positive terminal of the smoothing capacitor C 423 . This parallel circuit is further connected in parallel with an output voltage monitor  471 . The output voltage monitor  471  compares 5 V output with a target voltage value, and outputs a feedback signal indicating the difference between the output voltage and the target voltage value to a feedback terminal of the power controller  451 . 
         [0063]    The output voltage monitor  471  receives, from the control unit  200 , a combination of an operation mode signal indicating an operation mode of the image forming device  1 , and a load signal indicating job load size. According to these signals, the output voltage monitor  471  changes the target voltage value that it uses depending upon the operation mode of the image forming device  1 . 
         [0064]    The power controller  451  inputs, to the switching element SW 441 , a power control signal that is in accordance with the feedback signal. This achieves pulse width modulation (PWM) control of output voltage. 
       [5] Operations of Power Supply Device  100   
       [0065]    The following describes operations of the power supply device  100 , with reference to  FIG. 5 , which is a flowchart illustrating main operations of the power supply device  100 . 
         [0066]    The power supply device  100  refers to the operation mode signal from the control unit  200  (S 501 ). When the operation mode signal indicates that the operation mode of the image forming device  1  is the print mode (YES in S 502 ), the power supply device  100  checks the operation state of the 24 V loads (S 503 ). 
         [0067]    In the embodiment, the power supply device  100  changes the target voltage value that it uses depending upon the operation state of the 24 V loads. Specifically, the power supply device  100  judges that total load size of the 24 V loads is no smaller than a predetermined first threshold value (YES in S 504 ) when judging that both the image reader  110  and the sheet post-processor are to be operated, according to the load signal. When YES in S 504 , the power supply device  100  then checks the operation state of the 5 V loads (S 505 ). 
         [0068]    The power supply device  100  judges that total load size of the 5 V loads is no greater than a predetermined second threshold value (YES in S 506 ) when judging that only the main 5 V load components among the 5 V loads are to be operated according to the load signal. When YES in S 506 , the power supply device  100  uses a maximum target voltage value (S 507 ). In the embodiment, the maximum target voltage value is, for example, 5.1 V. 
         [0069]    Meanwhile, when the operation mode signal indicates that the operation mode is not the print mode (NO in S 502 ) and indicates that the operation mode is either the standby mode or the scan mode (YES in S 521 ), the power supply device  100  checks the operation state of the 5 V loads without checking the operation state of the 24 V loads (S 522 ). In subsequent S 523 , the power supply device  100  judges that the total load size of the 5 V loads is no smaller than a predetermined third threshold value (YES in S 523 ) when the main 5 V load components, the first additional 5 V load components  310 , and the second additional 5 V load components  320  are all to be operated, according to the load signal. When YES in S 523 , the power supply device  100  uses a minimum target voltage value (S 524 ). In the embodiment, the minimum target voltage value is, for example, 4.9 V. Further, the predetermined third threshold value is greater than the predetermined second threshold value. 
         [0070]    Meanwhile, in the following cases, the power supply device  100  uses an intermediate target voltage value (S 510 ): (i) when the operation mode is not the print mode, the standby mode, or the scan mode (NO in S 521 ); (ii) when the operation mode is the print mode and at least one of the image reader  110  and the sheet post-processor  120  is not to be operated (NO in S 504 ); (iii) when the operation mode is the print mode and at least one of the first additional 5 V load components  310  and the second additional 5 V load components  320 , in addition to the main 5 V load components, are to be operated (NO in S 506 ); and (iv) when the operation mode is the standby mode or the scan mode and at least the second addition 5 V load components  320 , among the 5 V loads, are not to be operated (NO in S 523 ). In the embodiment, the intermediate target voltage value is, for example, 5.0 V. 
         [0071]    Following processing in either S 507 , S 510 , or S 524 , the power supply device  100  refers to the operation mode signal once again (S 508 ) and, when there has been no change in operation mode (NO in S 509 ), proceeds to S 508  and continues monitoring the operation mode. Meanwhile, when there has been a change in operation mode (YES in S 509 ), the power supply device  100  proceeds to S 502  to repeat the processing described above once again. 
         [0072]    Through the processing described above, the target voltage value for the 5 V loads is changed as the operation mode of the image forming device  1  changes, as illustrated in  FIG. 6 . Specifically, when the operation mode changes from the sleep mode to the standby mode or the scan mode, the target voltage value for the 5 V loads is set to either the intermediate value (5.0 V) or the minimum value (4.9 V), depending upon the operation state of the 5 V loads. 
         [0073]    Further, when the operation mode changes from the standby mode or the scan mode to the print mode, the target voltage value for the 5 V loads is set to either the maximum value (5.1 V) or the intermediate value, depending upon the operation state of the 5 V loads. Contrariwise, when the operation mode changes from the print mode to the standby mode or the scan mode, the target voltage value for the 5 V loads is set to either the intermediate value or the minimum value, depending upon the operation state of the 5 V loads. 
         [0074]    Further, when the operation mode changes from the standby mode or the scan mode to the sleep mode, the target voltage value for the 5 V loads is set to the intermediate value. 
       [6] Control of Voltage Supplied to 24 V Loads 
       [0075]    The combination of the upper limiter circuit  481  and the control described above achieves limiting the voltage supplied to the 24 V loads to be within ±10% of the 24 V rated voltage. 
         [0076]      FIG. 7  shows graphs illustrating examples of current-voltage characteristics of the 24 V loads. In  FIG. 7 , graph  701  illustrates current-voltage characteristics of the 24 V loads when the target voltage value for the 5 V loads is 5.1 V and the amount of current output to the 5 V loads is 10 A. When all 5 V loads are operating, or that is, when the main 5 V load components, the first additional 5 V load components  310 , and the second additional 5 V load components  320  are all operating at the same time, the amount of current output to the 5 V loads reaches 10 A. 
         [0077]    Further, graph  702  illustrates current-voltage characteristics of the 24 V loads when the target voltage value for the 5 V loads is 5.1 V and when among the 5 V loads, the main 5 V load components and the first additional 5 V load components  310  are operating but the second additional 5 V load components  320  are not operating, in which case the amount of current output to the 5 V loads is 8 A. From graphs  701  and  702 , it can be seen that the greater the amount of current output to the 5 V loads, the greater the voltage supplied to the 24 V loads. 
         [0078]    Further, graph  702  shows, for example, that when the amount of current output to the 24 V loads is 1 A, the voltage supplied to the 24 V loads is as high as 28.0 V. In the present embodiment, the maximum voltage set to the upper limiter circuit  481  is 26.4 V. Thus, electric power loss occurring at the upper limiter circuit  481  in this case is calculated as follows. 
         [0000]      (28.0 V−26.4 V)×1 A=1.6 W
 
         [0079]    A Typical Electricity Consumption (TEC) value calculated based on this electric power loss indicates that in an image forming device performing printing at the speed of 28 sheets per minute, this electric power loss amounts to 54 Wh. In an image forming device model having a TEC value of 1350 Wh, this electric power loss corresponds to a power efficiency decrease of approximately 4%. 
         [0080]    Meanwhile, in the embodiment, the target voltage value is set to 4.9 V when among the 5 V loads, the main 5 V load components and the first additional 5 V load components  310  are to be operated but the second additional 5 V load components  320  are not to be operated (i.e., when the amount of current to be output to the 5 V loads is 8 A). This reduces the voltage supplied to the 24 V loads compared to when the target voltage value is 5.1 V, as shown by graph  703 . By making this configuration, when the amount of current to be output to the 24 V loads is 1 A, the voltage supplied to the 24 V loads decreases from the above-described value of 28.0 V to 26.5 V, in which case the power loss occurring at the upper limiter circuit  481  is as low as around 0.1 W. This corresponds to a power efficiency improvement of 51 Wh (TEC value). 
         [0081]    More specifically, without this configuration, the voltage supplied to the 24 V loads would be too high (as shown by graph  701 ) when YES in S 523  in  FIG. 5  (i.e., when the operation mode is the standby mode and the main 5 V load components, the first additional 5 V load components  310 , and the second additional 5 V load components  320  are all to be operated). Meanwhile, since the target voltage value for the 5 V loads is decreased to 4.9 V in the embodiment (S 524 ), the voltage supplied to the 24 V loads decreases compared to when the target voltage value for the 5 V loads is not decreased. Due to this, the configuration pertaining to the embodiment reduces electric power loss occurring at the upper limiter circuit  481 . 
         [0082]      FIG. 8  shows graphs illustrating relationships between electric power loss and the current amount for the 24 V loads. The electric power loss at the upper limiter circuit  481  is small when the operation mode is the sleep mode or the print mode, regardless of whether the target voltage value for the 5 V loads is set to the maximum value (shown by graph  801  in  FIG. 8 ) or to the minimum value (shown by graph  802  in  FIG. 8 ). 
         [0083]    Meanwhile, the electric power loss at the upper limiter circuit  481  reaches 3 W or greater (shown by graph  801  in  FIG. 8 ) when the target voltage value for the 5 V loads is set to the maximum value 5.1 V, the amount of current to be supplied to the 5 V loads is 10 A, and the operation mode is the standby mode. 
         [0084]    However, even when the amount of current to be output from the 5 V loads is 10 A and the operation mode is the standby mode, the electric power loss occurring can be reduced to as low as around 0.1 W (shown by graph  802  in  FIG. 8 ) by setting the target voltage value for the 5 V loads to 4.9 V. As such, the electric power loss occurring at the upper limiter circuit  481  can be reduced by setting a low target voltage value for the 5 V loads. 
         [0085]    Meanwhile, when only the main 5 V load components among the 5 V loads are to be operated, the amount of current to be output from the 5 V loads is as low as 1 A. When setting the target voltage value for the 5 V loads to 4.9 V in such a case, the voltage supplied to the 24 V loads becomes low, as shown by graph  706  in  FIG. 7 . In particular, when the operation mode is the print mode and the image reader  110  is to be operated or both the image reader  110  and the sheet post-processor  120  are to be operated, the voltage supplied to the 24 V loads falls below the rated voltage range of 24 V±10%. 
         [0086]    Meanwhile, in the embodiment, the target voltage value for the 5 V loads is set to 5.0 V in such a case. This increases the voltage supplied to the 24 V loads compared to when the target voltage value is 4.9 V, as shown by graph  705 . Also, further increasing the target voltage value for the 5 V loads to 5.1 V results in a further increase in the voltage supplied to the 24 V loads, as shown by graph  704 . As a result, the voltage supplied to the 24 V loads becomes close to exactly 24.0 V. 
         [0087]    More specifically, without this configuration, the voltage supplied to the 24 V loads would be too low (as shown by graph  706 ) when YES in S 506  in  FIG. 5  (i.e., when the operation mode of the image forming device  1  is the print mode, both the image reader  110  and the sheet post-processor  120  are to be operated among the 24 V loads, and only the main 5 V load components is to be operated among the 5 V loads). Meanwhile, since the target voltage value for the 5 V loads is increased to 5.1 V in the embodiment (S 507  in  FIG. 5 ), the voltage supplied to the 24 V loads increases compared to when the target voltage value for the 5 V loads is not increased, to be within the rated voltage range. 
       [7] Modifications 
       [0088]    Up to this point, description has been provided of the technology pertaining to the present disclosure based on an embodiment thereof. However, the technology pertaining to the present disclosure shall not be construed as being limited to the embodiment, and modifications including those described in the following can be made without departing from the spirit and scope thereof. 
         [0089]    (1) The embodiment provides description based on an example where the upper limiter circuit  481  is used to limit the voltage supplied to the 24 V loads to a predetermined maximum voltage. As long as the voltage supplied to the 24 V loads can be decreased by decreasing the voltage supplied to the 5 V loads, electric power loss at the upper limiter circuit  481  can be reduced regardless of the circuit structure of the upper limiter circuit  481 . 
         [0090]    (2) The embodiment provides description based on an example where the output voltage monitor  471  sets the target voltage value for the 5 V loads based on the operation mode signal and the load signal. However, the following modification may be made, for example. 
         [0091]    For example, the control unit  200  may set the target voltage value to the output voltage monitor  471  by referring to the operation mode and the load state of the image forming device  1 . This modification simplifies the structure of the output voltage monitor  471 , and thus reduces the cost and size of the power supply device  100 . 
         [0092]    (3) The embodiment provides description based on an example where, in S 522  in  FIG. 5 , the operation state of the 5 V loads is checked and the target voltage value for the 5 V loads is changed. However, the target voltage value for the 5 V loads may be changed depending upon the device structures of the 5 V loads, instead of the operation state of the 5 V loads. When making this modification, the decision of YES may be made in S 523  when the main 5 V load components, the first additional 5 V load components  310 , and the second additional 5 V load components  320  are all mounted on or attached to the image forming device  1 , and the decision of NO may be made in S 523  when at least one of the first additional 5 V load components  310  and the second additional 5 V load components  320  is not mounted on or attached to the image forming device  1 . 
         [0093]    Further, in S 503  of  FIG. 5 , instead of checking the operation state of the 24 V loads to determine whether or not to change the target voltage value for the 5 V loads, a determination may be made of whether or not the image reader  110  and the sheet post-processor  120  are mounted onto the image forming device  1 . When making this modification, the decision of YES may be made in S 504  when both the image reader  110  and the sheet post-processor  120  are mounted onto the image forming device  1 , and the decision of NO may be made in S 504  when at least one of the image reader  110  and the sheet post-processor  120  is not mounted onto the image forming device  1 . Making such modifications similarly achieves the effects described in the embodiment. 
         [0094]    (4) The embodiment describes an example where the power supply device  100  outputs 5 V DC as the low voltage and 24 V DC as the high voltage. However, the low voltage need not be 5 V, and the high voltage need not be 24 V. Further, the maximum and minimum of the target voltage value for the 5 V loads need not be 5.1 V and 4.9 V, respectively. Any value within the rated voltage range of the 5 V loads may be set to each of the maximum and the minimum of the target voltage value for the 5 V loads. Further, the rated voltage range of the voltage supplied to the 24 V loads need not be the ±10% range described in the embodiment, and may be replaced with another range. 
         [0095]    (5) The embodiment describes an example where the image forming device  1  is a MFP having the tandem system. However, the image forming device  1  may for example be a MFP not having the tandem system, or may be a monochrome MFP. Further, the technology pertaining to the present disclosure achieves similar effects also when applied to a MFP not having a facsimile communication function. 
         [0096]    Although the technology pertaining to the present disclosure has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the technology pertaining to the present disclosure, they should be construed as being included therein.