Patent Publication Number: US-10775732-B2

Title: Power supply circuit and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/943,658, filed Apr. 2, 2018, which application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-120608, filed Jun. 20, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a power supply circuit and an image forming apparatus. 
     BACKGROUND 
     In the related art, there is a technology in which heat is generated from a heating element such as a heater using electric power from an AC power supply, and processing of a component, melting of a wax, and the like are performed using the generated heat of the heating element. For example, in an image forming apparatus that performs printing in accordance with a print request, a dye material (toner) is melted so as to be fixed on a print medium by a fixing roller which is heated by a high temperature heating element. In this manner, the image forming apparatus forms an image on the print medium. 
     High electric power of about thousands of watts is required for heating the heating element to a high temperature to melt the toner. In addition, the heat of the heating element after the processing is ended is generally discharged into the air. A charging control device which includes a thermoelectric conversion element and a storage battery can be provided. The thermoelectric conversion element generates electric power when heated. A storage battery stores the electric power generated by the thermoelectric conversion element. 
     In such a charging control device, driving a DC/DC converter is required for drawing electric power from the thermoelectric conversion element. The current which can be drawn from the thermoelectric conversion element is limited depending on the characteristics and a temperature difference across the thermoelectric conversion element. Thus, a situation in which electric power to be drawn from the thermoelectric conversion element is not increased, even though a control of increasing an output of the DC/DC converter is performed, may occur. In such a case, there is a problem in that efficiency of drawing electric power from the thermoelectric conversion element is decreased. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram illustrating an example of a configuration of an image forming apparatus according to an embodiment. 
         FIG. 1B  is a diagram illustrating an example of an overview of an entire circuit configuration according to the embodiment. 
         FIG. 2  is a diagram illustrating an example of a configuration of a power supply circuit according to the embodiment. 
         FIG. 3  is a diagram illustrating an example of a configuration of an insulating DC-DC circuit according to the embodiment. 
         FIG. 4  is a diagram illustrating an example of a configuration of a heater control circuit according to the embodiment. 
         FIG. 5  is a diagram illustrating an example of a configuration of an A-CC circuit according to the embodiment. 
         FIG. 6  is a diagram illustrating an example of a configuration of a DC-DC circuit according to the embodiment. 
         FIG. 7  is a diagram illustrating a relationship between a paper discharge timing, a temperature, an A-CC driving pulse, and a generated power current. 
         FIG. 8  is a diagram illustrating an example of characteristics of a thermoelectric conversion element. 
         FIGS. 9A to 9C  are diagrams illustrating examples of a change of the generated power current when ON duty cycle of the A-CC driving pulse is switched. 
         FIG. 10  is a diagram illustrating a relationship between the generated power current and the ON duty cycle. 
         FIG. 11  is a diagram illustrating the relationship between the generated power current and the ON duty cycle. 
         FIG. 12  is a diagram illustrating an example of an operation of a control circuit when electric power is taken out from the thermoelectric conversion element. 
         FIG. 13  is a diagram illustrating an example of an operation of the control circuit when the electric power is supplied to a load circuit. 
         FIG. 14  is a diagram illustrating an operation of each of the circuits in the power supply circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide a power supply circuit having high efficiency and an image forming apparatus. 
     According to an embodiment, a power supply circuit includes a thermoelectric conversion element configured to generate electric power when it is differentially heated, an adjustable current circuit configured to draw a current from the thermoelectric conversion element and resultantly output a constant current over a period of time, a voltage conversion circuit configured to output a voltage based on the current output by the adjustable current circuit, and a control circuit configured to control the adjustable current circuit to change a target value of the constant current output by the adjustable current circuit. 
     Hereinafter, an embodiment will be described with reference to the drawings. 
       FIG. 1A  is a diagram illustrating a configuration example of an image forming apparatus  1  according to an embodiment. 
     The image forming apparatus  1  is, for example, a multifunction peripheral (MFP) that performs various types of processing such as image forming while transporting a recording medium such as a print medium therein. The image forming apparatus  1  charges a photoconductive drum and irradiates the photoconductive drum with light in accordance with image data (print data) for printing, so as to form a latent image (electrostatic latent image) on the photoconductive drum. The image forming apparatus  1  causes a toner (developer) to adhere to the latent image formed on the photoconductive drum, and transfers the toner which adheres to the latent image, onto a print medium, so as to form a toner image on the print medium. The image forming apparatus  1  nips the print medium on which the toner image is formed, by one of a pair of fixing rollers  46  (which will be described later herein) and a thermal fixing belt BT which is heated to a high temperature (which will be described later herein), so as to fix the adhered toner image onto the print medium. 
     The image forming apparatus  1  acquires an image present on a medium in a manner that imaging is performed in an image sensor by using the reflected light of light with which the medium was irradiated, and charges accumulated in the image sensor are read and the read charges are converted into a digital signal representative of the image. 
     The image forming apparatus  1  includes a housing  11 , a document stand  12 , a scanner unit  13 , an automatic document feeder (ADF)  14 , a paper feeding cassette  15 , a paper discharge tray  16 , an image forming unit  17 , a transporting unit  18 , a thermal fixing unit F, and a thermoelectric conversion element  74 . 
     The housing  11  is the main body for holding the document stand  12 , the scanner unit  13 , the ADF  14 , the paper feeding cassette  15 , the paper discharge tray  16 , the image forming unit  17 , the transporting unit  18 , the thermal fixing unit F, and the thermoelectric conversion element  74 . 
     The document stand  12  is a part on which a medium P as an original document to be scanned is placed. The document stand  12  includes a glass plate  31  and a space  33  therein. The medium P as the original document is placed on the glass plate  31 . The space  33  is positioned on a surface on an opposite side of a placement surface  32  of the glass plate  31 , on which the medium P as the original document is placed. 
     The scanner unit  13  acquires an image from the medium P in accordance with a control signal of a main controller  19  (which will be described later herein). The scanner unit  13  is disposed in the space  33  on the document stand  12  of the placement surface  32 , which is an opposite side of the placement surface  32 . The scanner unit  13  includes an image sensor, an optical element, an illumination equipment, and the like. 
     The image sensor is an imaging element in which pixels, in which light is converted into an electric signal (image signal), are arranged along a generally straight line. The image sensor is configured, for example, by a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS), or other imaging elements. 
     The optical element forms an image in the pixels of the image sensor using light received within a predetermined reading range. The reading range of the optical element is a line-like region on the placement surface  32  of the document stand  12 . The optical element forms an image in the pixels of the image sensor with light which is reflected by the medium P placed on the placement surface  32  of the document stand  12  and transmitted through the glass plate  31 . 
     Illumination equipment irradiates the medium P with light. The illumination equipment includes a light source and a light guide for irradiating the medium P with light from the light source. The illumination equipment irradiates a region including the reading region of the optical element, with light emitted from the light source. The irradiation is performed by the light guide. 
     When the medium P is placed on the placement surface  32  of the document stand  12 , the scanner unit  13  is driven by a driving mechanism (not illustrated) in a sub-scanning direction which is perpendicular to an arrangement direction (main scanning direction) of the pixels in the image sensor and is parallel to the placement surface  32 . The scanner unit  13  continuously acquires an image for each line by the image sensor, while being driven in the sub-scanning direction. Thus, the scanner unit  13  acquires image data (original document image data) of the entirety of the medium P placed on the placement surface  32  of the document stand  12 . 
     The ADF  14  is a mechanism for transporting the medium P. The ADF  14  is provided on the document stand  12  to be freely closed or opened. The ADF  14  takes a medium P disposed on a tray, in accordance with the control of the main controller  19  (which will be described later). The ADF  14  transports the taken medium P to a scanning location on the glass plate  31  of the document stand  12 . 
     If the medium P is transported by the ADF  14 , the scanner unit  13  is driven to a position facing a position of the glass plate  31  at which the medium P is located by the ADF  14 . The scanner unit  13  continuously acquires an image from the medium P transported by the ADF  14 , for each line by the image sensor, and thus acquires image data (original document image data) of the entirety of the medium P transported by the ADF  14 . 
     The paper feeding cassette  15  is a cassette for accommodating a print medium P to be printed upon. The paper feeding cassette  15  is configured so as to allow for a supply of the print medium P to be located therein from the outside of the housing  11 . For example, the paper feeding cassette  15  is configured so as to be allowed to be withdrawn from the housing  11 . 
     The paper discharge tray  16  is a tray for supporting the print medium P discharged from the image forming apparatus  1 . 
     The image forming unit  17  is a printer that forms an image on a print medium P under the control of the main controller  19  (which will be described later herein). For example, the image forming unit  17  charges the drum, and forms a latent image depending on image data (print data) for printing, on the charged drum. The image forming unit  17  causes a toner to adhere to the latent image formed on the drum and transfers the toner adhering to the latent image, onto the print medium P. Thus, the image forming unit  17  forms an image on the print medium P. The image forming unit  17  includes, for example, a drum  41 , an exposure machine  42 , a developing machine  43 , a transfer belt  44 , a pair of transfer rollers  45 , and the thermal fixing unit F, as illustrated in  FIG. 1A . 
     The drum  41  is a photoconductive drum which is formed to have a cylindrical shape. The drum  41  is provided to come into contact with the transfer belt  44 . The surface of the drum  41  is uniformly charged by a charging charger (not illustrated). The drum  41  rotates at a constant speed by a driving mechanism (not illustrated). 
     The exposure machine  42  forms an electrostatic latent image on the charged drum  41 . The exposure machine  42  irradiates the surface of the drum  41  with a laser beam by a light emitting element or the like in accordance with the print data, and thus forms an electrostatic latent image on the surface of the drum  41 . The exposure machine  42  includes a light emitting unit and an optical element. 
     The light emitting unit has a configuration in which light emitting elements for emitting light in accordance with an electric signal (image signal) are arranged in line. Each of the light emitting elements in the light emitting unit emits light having a wavelength which allows a latent image to be formed on the charged drum  41 . The light emitted from the light emitting unit forms an image on the surface of the drum  41  by the optical element. 
     The developing machine  43  causes the toner (developer) to adhere to the electrostatic latent image formed on the drum  41 . Thus, the developing machine  43  forms a toner image on the surface of the drum  41 . 
     The drum  41 , the exposure machine  42 , and the developing machine  43  of the image forming unit  17  are provided for each of colors of cyan, magenta, yellow, and black, for example. In this case, each of a plurality of developing machines  43  holds a different color toner. 
     The transfer belt  44  is a member that receives a toner image formed on the surface of the drum  41  and causes the toner image to be transferred to a print medium P. The transfer belt  44  is moved by the rotation of the roller. The transfer belt  44  receives the toner image formed on the drum  41  at a position in contact with the drum  41 , and moves the received toner image to the pair of transfer rollers  45 . 
     The pair of transfer rollers  45  is configured to interpose the transfer belt  44  and a print medium P between them. The pair of transfer rollers  45  cause the toner image on the transfer belt  44  to be transferred to the print medium P. 
     The thermal fixing unit F includes the pair of fixing rollers  46 , a thermal fixing belt BT which is formed in an endless shape, a first inductor L 1 , a heater control circuit  73 . In this example, one of the pair of fixing rollers  46  and the first inductor L 1  are arranged within the perimeter of the thermal fixing belt BT as shown in  FIG. 1A . The outer peripheral surface of the other of the pair of fixing rollers  46  comes into contact with the outer peripheral surface of the thermal fixing belt BT and presses the thermal fixing belt BT toward the one of the pair of fixing rollers  46 . The print medium P on which a toner image is transferred passes between the outer peripheral surface of the thermal fixing belt BT and the outer peripheral surface of the other of the pair of the fixing rollers  46 . The thermal fixing belt BT is heated by induction heating using the first inductor L 1  under control of the heater control circuit  73 . The outer peripheral surface of the thermal fixing belt BT and the outer peripheral surface of the other of the pair of the fixing rollers  46  press on the print medium P nipped therebetween in the heated state, and thus fixes the transferred toner image onto the print medium P. That is, the thermal fixing unit F fixes the toner image, and thus causes a fixed image to be formed on the print medium P under control of the heater control circuit  73 . 
     The transporting unit  18  transports a print medium P. The transporting unit  18  includes a transporting path and a sensor. The transporting path is configured by a plurality of guides and a plurality of rollers. The sensor detects a transportation position of a print medium P on the transporting path. The transporting path is a path through which a print medium P is transported. Transporting rollers, which are arranged along the transporting path are rotated by a motor which operates under the control of the main controller  19  (which will be described later). Thus, the print medium P is transported along the transporting path. Some of the plurality of guides are moved by a motor which operates under the control of the main controller  19  (which will be described later), and thus causes the transporting path for transporting a print medium P to be switched. 
     For example, as illustrated in  FIG. 1A , the transporting unit  18  includes a feeding roller  51 , a fed paper transporting path  52 , a paper discharging path  53 , and a reversal transporting path  54 . 
     The feeding roller  51  picks up an upper most print medium P among print media stacked in the paper feeding cassette  15  and feeds it to the fed paper transporting path  52 . 
     The fed paper transporting path  52  is a transporting path for transporting the print medium P which is taken out from the paper feeding cassette  15  by the feeding roller  51  to the image forming unit  17 . 
     The paper discharging path  53  is a transporting path for discharging a print medium P on which an image is formed by the image forming unit  17 , from the housing  11 . The print medium P discharged on the paper discharging path  53  is discharged to the paper discharge tray  16 . 
     The reversal transporting path  54  is a transporting path for supplying a print medium P to the image forming unit  17  again in a state where, for example, the front and the back, or the front and the rear of the print medium P on which an image is formed by the image forming unit  17  are reversed. 
       FIG. 1B  is a schematic diagram of an entire circuit configuration of the image forming apparatus  1  according to the embodiment. As shown in  FIG. 1B , the image forming apparatus  1  includes, for example, a first power supply circuit  81 , a second power supply circuit  82 , a third power supply circuit  83 , a stand-by power supply circuit  20 , the heater control circuit  73 , the main controller  19 , and the thermoelectric conversion element  74 . 
     The thermoelectric conversion element  74  is provided at a position on a downstream side and in the vicinity of the thermal fixing unit F in the paper transporting direction of the paper discharging path  53 . The thermoelectric conversion element  74  generates electric power upon being heated. The thermoelectric conversion element  74  is, for example, thermocouple in which two different kinds of metals or semiconductors are bonded to each other. The thermoelectric conversion element  74  is heated by heat of a thermal fixing load  47  such as a paper, toner and heated air around the paper, passing through the thermoelectric conversion element  74 , after being heated by the thermal fixing unit F and heat transmitted through the air from the thermal fixing unit F, and resultantly generates electric power. That is, the thermoelectric conversion element  74  generates electric power from the heat. 
     The first power supply circuit  81  is configured to receive a power supply from an AC power supply E and supply power by 1200 W to the heater control unit  73 , for example. 
     The second power supply circuit  82  is configured to receive a power supply from the AC power supply E and supply power by 300 W to the transport unit  18 , for example. 
     The third power supply circuit  83  is configured to receive a power supply from the AC power supply E and supply power by 100 W to the main controller  19 , for example. 
     In the main controller  19 , a stand-by module SB is provided and controls power supply to the main controller  19  during the image forming apparatus is in a sleep mode in which the supply of the electric power to the heater control circuit  73  and a transport unit  18  is suspended for energy saving. 
     The stand-by power supply circuit  20  is configured to receive a power supply from either one of the AC power supply E and the thermoelectric conversion element  74  and supply power by 1 W to the stand-by module SB in the main controller  19 , for example. 
     The heater control circuit  73  is configured to control the induction heating by the thermal fixing unit F in collaboration with the main controller  19 . 
     In addition, the main controller  19  entirely controls the image forming apparatus  1 . For example, the main controller  19  entirely performs control of the scanner unit  13 , the ADF  14 , the image forming unit  17 , the transporting unit  18 , and an operation input unit (not shown) of the image forming apparatus  1 . 
     The main controller  19  includes, for example, a CPU, a ROM, a RAM, and a nonvolatile memory. 
     The CPU is a computing element (for example, a processor) that performs computing processing. The CPU performs various types of processing based on data such as a program, which is stored in the ROM. The CPU functions as a control unit which can perform various operations, by executing a program stored in the ROM. The CPU inputs print data for forming an image on a print medium P to the image forming unit  17 . The CPU inputs a transporting control signal for an instruction to transport a print medium P, to the transporting unit  18 . 
     The ROM is a nonvolatile read only memory. The ROM stores a program and data used in the program, for example. 
     The RAM is a volatile memory functioning as a working memory which can be read from and written to. The RAM temporarily stores data during the processing of the CPU. The RAM temporarily stores a program executed by the CPU. 
     The nonvolatile memory is a storage medium (storage unit) which is capable of storing various types of information. The nonvolatile memory stores a program and data used in the program, for example. As the nonvolatile memory, for example, a solid state drive (SSD), a hard disk drive (HDD), or another storage device is provided. Instead of the nonvolatile memory, a memory IF such as a card slot into which a storage medium such as a memory card can be inserted may be provided. 
     The stand-by power supply circuit  20  is configured to supply electric power to the stand-by module SB.  FIG. 2  is a circuit diagram illustrating a configuration of the stand-by power supply circuit  20 . The stand-by power supply circuit  20  receives AC power supplied from the external AC power supply E. The stand-by power supply circuit  20  converts the supplied AC power into electric power having a voltage in accordance with a stand-by module SB, and supplies the electric power obtained by the conversion to the stand-by module SB. 
     The stand-by power supply circuit  20  includes a full-wave rectifying circuit  71 , an insulating DC-DC circuit  72 , an adjustable constant current (A-CC) circuit  75 , a secondary battery  76 , a residual amount detection circuit  77 , a DC-DC circuit  78 , and a control circuit  79 . 
     The full-wave rectifying circuit  71  is a circuit configured to perform full-wave rectification of the AC power input from the AC power supply E and to supply a ripple voltage to a circuit at a subsequent stage of the stand-by power supply circuit  20 . For example, the full-wave rectifying circuit  71  is configured by a plurality of diodes and includes a rectifying bridge configured to receive the input AC power. 
     The insulating DC-DC circuit  72  is a converter that supplies DC electric power to the stand-by module SB using the ripple voltage from the full-wave rectifying circuit  71 . 
       FIG. 3  is a diagram illustrating a configuration example of the insulating DC-DC circuit  72 . The insulating DC-DC circuit  72  is, for example, a flyback converter. The insulating DC-DC circuit  72  supplies electric power to the secondary side thereof which is insulated from the primary side thereof, to which electric power is supplied from the power source. The insulating DC-DC circuit  72  includes a first capacitor C 1 , a transformer T 1 , a first switching element SW 1 , a rectifying diode D, a second capacitor C 2 , a first pulse width modulation (PWM) pulse generator  81 , a first voltage detection circuit V 1 , and a photocoupler  82 . The transformer T 1  includes a primary winding W 1 , a secondary winding W 2  and a ferrite core FC. 
     The first capacitor C 1  smoothes the input ripple voltage. The primary winding W 1  of the transformer T 1  functions as a primary transformer winding. The secondary winding W 2  of the transformer T 1  is electromagnetically coupled with the primary winding W 1  and functions as a secondary transformer winding. The first switching element SW 1  performs switching between a conduction state (ON or closed) and a non-conduction state (OFF or open) in accordance with a pulse signal input from the first PWM pulse generator  81 , and thus causes the current flowing in the primary winding W 1  to switch between ON and OFF. The rectifying diode D rectifies the current generated in the secondary winding W 2 . The second capacitor C 2  smoothes the voltage generated in the secondary winding W 2 . 
     The first PWM pulse generator  81  inputs a pulse signal to the first switching element SW 1 , under the control of the control circuit  79 . Thus, the first PWM pulse generator  81  performs switching of the first switching element SW 1  between ON and OFF states. The first voltage detection circuit V 1  detects the voltage of the second capacitor C 2 . The photocoupler  82  applies feedback to the first PWM pulse generator  81  in accordance with a detection result of the first voltage detection circuit V 1 . 
     In the above configuration, if the first switching element SW 1  turns ON, a current flows in the primary winding W 1  and electric energy is converted into magnetic energy. At this time, a reverse voltage is applied to the rectifying diode D on the secondary side of the circuit, and thus electric power is not delivered to the secondary side of the circuit. Then, if the first switching element SW 1  turns OFF, a current flows into the second capacitor C 2  via the rectifying diode D by accumulated magnetic energy, and electric power is stored in the second capacitor C 2 . Electric power stored in the second capacitor C 2  is supplied to the stand-by module SB at the subsequent stage of the stand-by power supply circuit  20 , as DC electric power. 
     The electric power delivered to the secondary side is determined based on the current flowing in the primary winding W 1 . That is, the electric power delivered to the secondary side is determined by the time period when the first switching element SW 1  is in an ON state. The first PWM pulse generator  81  increases the current flowing in the primary winding W 1  and increases the electric power delivered to the secondary side by widening the pulse width, i.e., the duration of the first pulse, to thereby increase the “ON” duty cycle. The first voltage detection circuit V 1  and the photocoupler  82  perform feedback of an output voltage to the first PWM pulse generator  81 . The first PWM pulse generator  81  controls the pulse width of the pulse signal for driving the first switching element SW 1 , based on feedback from the first voltage detection circuit V 1  and the photocoupler  82 , and thus holds the output voltage constant. 
     The insulating DC-DC circuit  72  may also be a DC-DC converter having an insulating function. For example, the insulating DC-DC circuit  72  may also be configured by an LLC circuit, an insulating forward circuit, and an insulating double forward circuit. 
     The heater control circuit  73  is connected to the AC power supply E. The heater control circuit  73  is a circuit configured to generate an electromagnetic wave using AC power from the AC power supply E and to induce a current in the first inductor L 1  and thereby heat the thermal fixing belt BT by induction heating. 
       FIG. 4  is a diagram illustrating a configuration example of the heater control circuit  73 . The heater control circuit  73  is, for example, a half bridge circuit. The heater control circuit  73  includes a full-wave rectifying circuit  83 , a third capacitor C 3 , a fourth capacitor C 4 , a second switching element SW 2 , a third switching element SW 3 , the first inductor L 1 , and a half bridge alternating pulse generator  84 . 
     The full-wave rectifying circuit  83  is a circuit configured to perform full-wave rectification of the AC power input from the AC power supply E and to supply a ripple voltage to a circuit at the subsequent stage. For example, the full-wave rectifying circuit  83  is configured by a plurality of diodes and includes a rectifying bridge configured to receive the input AC power. 
     The third capacitor C 3  and the fourth capacitor C 4  are connected in series to a DC terminal of the full-wave rectifying circuit  83 . The second switching element SW 2  and the third switching element SW 3  are connected to the DC terminal of the full-wave rectifying circuit  83  so as to be in parallel with the series connection of the third capacitor C 3  and the fourth capacitor C 4 . The first inductor L 1  is connected between a connection point of the second switching element SW 2  and the third switching element SW 3  and a connection point of the third capacitor C 3  and the fourth capacitor C 4 . 
     The half bridge alternating pulse generator  84  inputs a pulse signal to the second switching element SW 2  and inputs a pulse signal having a logical value reversed to that of the pulse signal input to the second switching element SW 2 , to the third switching element SW 3  under the control of the control circuit  79 . Thus, the half bridge alternating pulse generator  84  performs switching of the second switching element SW 2  and the third switching element SW 3  between the conduction state (ON) and the non-conduction state (OFF). 
     In the above configuration, the second switching element SW 2  and the third switching element SW 3  alternately turn ON and OFF. Thus, electric power of a DC voltage supplied from the full-wave rectifying circuit  83  is converted into high-frequency AC power. The heater control circuit  73  performs induction heating using the high-frequency AC power. In induction heating, an eddy-current is generated in a conductor of the thermal fixing belt BT, and heat is generated by virtue of the resistance of the conductor in the thermal fixing belt BT. Thus, the heater control circuit  73  causes the first inductor L 1  to generate the electromagnetic field and heats the thermal fixing belt BT using the generated electromagnetic field. 
     The half bridge alternating pulse generator  84  generates a square wave (pulse signal) corresponding to 50% of the pulses of a predetermined frequency, by using a timer, a CR time constant, and the like. The half bridge alternating pulse generator  84  inputs the generated pulse signal to a gate of one switching element and inputs a pulse signal obtained by reversing a logical value, to a gate of the other switching element. The half bridge alternating pulse generator  84  provides a dead time when a pulse signal for OFF is input to the two switching elements, so as to cause the two switching elements not to simultaneously turn ON. 
     The half bridge alternating pulse generator  84  changes a frequency generated by the timer, under the control of the control circuit  79 . If the frequency (transmission frequency) generated by the timer is increased, a high-frequency current flowing in the first inductor L 1  is decreased and the quantity of heat generated in the thermal fixing belt BT is decreased. If the frequency (transmission frequency) generated by the timer is decreased, the high-frequency current flowing in the first inductor L 1  is increased and the quantity of heat generated in the thermal fixing belt BT is increased. That is, the half bridge alternating pulse generator  84  controls heat generation of the thermal fixing belt BT under the control of the control circuit  79 . 
     The heater control circuit  73  is configured to be capable of heating the thermal fixing belt BT. For example, the heater control circuit  73  may also be configured by a full bridge inverter, a quasi-E class inverter, and a push-pull inverter. 
     The A-CC circuit  75  draws electric power generated in the thermoelectric conversion element  74 . The A-CC circuit  75  is a variable constant current circuit configured to draw a constant current having a value in accordance with the control of the control circuit  79 , from the thermoelectric conversion element  74 . 
       FIG. 5  is a diagram illustrating a configuration example of the A-CC circuit  75 . The A-CC circuit  75  includes a second inductor L 2 , a fourth switching element SW 4 , a fifth switching element SW 5 , a fifth capacitor C 5 , a second PWM pulse generator  85 , a first synchronous rectification pulse generator  86 , and a current detection circuit A. In the A-CC circuit  75 , the second PWM pulse generator  85  generates a pulse signal for a basic switching control of the fourth switching element SW 4  and the first synchronous rectification pulse generator  86  generates a pulse signal based on the pulse signal generated by the second PWM pulse generator  85 . 
     The fourth switching element SW 4  and the fifth switching element SW 5  are connected in series between a pair of output terminals. The second inductor is connected between a connection point of the fourth switching element SW 4  and the fifth switching element SW 5 , and one of a pair of input terminals. The fifth capacitor C 5  is connected in parallel with a series connection of the fourth switching element SW 4  and the fifth switching element SW 5 , when viewed from the pair of output terminals. 
     The second PWM pulse generator  85  inputs a pulse signal to the fourth switching element SW 4  under the control of the control circuit  79 . Thus, the second PWM pulse generator  85  performs switching of the fourth switching element SW 4  between the conduction state (ON) and the non-conduction state (OFF). 
     The first synchronous rectification pulse generator  86  inputs a pulse signal to the fifth switching element SW 5  in accordance with a signal supplied from the second PWM pulse generator  85 . Thus, the first synchronous rectification pulse generator  86  switches the fifth switching element SW 5  between the conduction state (ON) and the conduction state (OFF) through a body diode. The fifth switching element SW 5  normally causes a current to intermittently flow in a reverse direction from its source toward its drain. Thus, the fifth switching element SW 5  is in a state where a current flows via the body diode, not in the OFF state. A circuit operation is established by causing a current to flow in the body diode, but a 0.7 V loss occurs which is the forward voltage of the body diode. Therefore, if the fifth switching element SW 5  is caused to be in the ON state so as to allow conduction without current passing through the body diode, it is possible to reduce loss in the fifth switching element SW 5 . This is generally referred to as synchronous rectification control. 
     The second PWM pulse generator  85  supplies a signal to the first synchronous rectification pulse generator  86  such that the first synchronous rectification pulse generator  86  inputs a pulse signal having a logical value reversed to that of the pulse signal input to the fourth switching element SW 4 , to the fifth switching element SW 5 . Thus, the second PWM pulse generator  85  causes the fourth switching element SW 4  and the fifth switching element SW 5  to alternately turn ON and OFF, i.e., allow current to pass therethrough in the ON state, and prevent current passing therethrough during the OFF state. The second PWM pulse generator  85  provides a dead time when all of the fourth switching element SW 4  and the fifth switching element SW 5  are in the OFF state, in the pulse signal so as to cause the fourth switching element SW 4  and the fifth switching element SW 5  not to simultaneously turn ON. 
     The current detection circuit A is connected in series with the second inductor L 2  and the fourth switching element SW 4 , at a location between the pair of the input terminals. The current detection circuit A detects a current value of a current generated by electric power which is generated by the thermoelectric conversion element  74 . 
     In the above-described configuration, if the fourth switching element SW 4  is in the ON state, a current flows along the path of one input terminal, the second inductor L 2 , the fourth switching element SW 4 , the current detection circuit A, and the other input terminal. The current in this path causes magnetic energy to be stored in the second inductor. 
     Then, if the fourth switching element SW 4  turns OFF and the fifth switching element turns ON in a state where magnetic energy is stored in the second inductor, a current flows along the path of the second inductor L 2 , the fifth capacitor C 5 , and the current detection circuit A. The current in this path causes magnetic energy in the second inductor L 2  to be converted into charge energy in the fifth capacitor C 5 . The charge energy stored in the fifth capacitor C 5  is charged in the secondary battery  76 . 
     When the fourth switching element SW 4  is in the ON state, the current detection circuit A detects a value of a current (generated power current) flowing through, in the order of, the one input terminal, the second inductor L 2 , the fourth switching element SW 4 , and the other input terminal. The current detection circuit A supplies the detection result to the control circuit  79 . When the fourth switching element SW 4  is in the OFF state, the current detection circuit A detects a value of a current (generated power current) flowing through in the order of the one input terminal, the second inductor L 2 , the fifth switching element SW 5 , the fifth capacitor C 5 , and the other input terminal. The current detection circuit A supplies the detection result to the control circuit  79 . The control circuit  79  supplies a control signal to the second PWM pulse generator  85  in accordance with the detection result from the current detection circuit A. 
     The second PWM pulse generator  85  adjusts the pulse width of a pulse signal input to the fourth switching element SW 4 , based on the control signal supplied from the control circuit  79 . Thus, it is possible to perform constant current control to cause a constant current value selected by the control circuit  79  to flow. That is, the second PWM pulse generator  85  adjusts the pulse width of a pulse signal input to the fourth switching element SW 4  such that a constant current having a target value in accordance with the control signal supplied from the control circuit  79  flows. 
     The secondary battery  76  is a storage battery that stores electric power supplied from the A-CC circuit  75  and supplies the stored electric power to other circuits. For example, the secondary battery  76  supplies electric power to the DC-DC circuit  78  at a subsequent stage of the control circuit  20 . 
     The residual amount detection circuit  77  detects the residual amount of the electric power stored in the secondary battery  76  and supplies the detection result to the control circuit  79 . 
     The DC-DC circuit  78  is a converter that converts a voltage of electric power supplied from the secondary battery  76  into a voltage required by the stand-by module SB and supplies the DC electric power to the stand-by module SB. 
       FIG. 6  is a diagram illustrating a configuration example of the DC-DC circuit  78 . The DC-DC circuit  78  includes a third inductor L 3 , a sixth switching element SW 6 , a seventh switching element SW 7 , a sixth capacitor C 6 , a third PWM pulse generator  87 , a second synchronous rectification pulse generator  88 , and a second voltage detection circuit V 2 . 
     The sixth switching element SW 6  and the seventh switching element SW 7  are connected in series between the pair of output terminals. The third inductor is connected at a location between a connection point of the sixth switching element SW 6  and the seventh switching element SW 7  and one of the pair of the input terminals. The sixth capacitor C 6  is connected in parallel with a series connection of the sixth switching element SW 6  and the seventh switching element SW 7 , when viewed from the pair of output terminals. 
     The third PWM pulse generator  87  inputs a pulse signal to the sixth switching element SW 6  under the control of the control circuit  79 . Thus, the third PWM pulse generator  87  switches the sixth switching element SW 6  between the conduction state (ON) and the non-conduction state (OFF). 
     The second synchronous rectification pulse generator  88  inputs a pulse signal to the sixth switching element SW 6  in accordance with a signal supplied from the third PWM pulse generator  87 . Thus, the second synchronous rectification pulse generator  88  switches the seventh switching element SW 7  between the conduction state (ON) and the conduction state (OFF) through a body diode. 
     The third PWM pulse generator  87  supplies a signal to the second synchronous rectification pulse generator  88  such that the second synchronous rectification pulse generator  88  inputs a pulse signal having a logical value reversed to that of the pulse signal input to the sixth switching element SW 6 , to the seventh switching element SW 7 . Thus, the third PWM pulse generator  87  causes the sixth switching element SW 6  and the seventh switching element SW 7  to alternately turn ON and OFF. The third PWM pulse generator  87  provides a dead time when all of the sixth switching element SW 6  and the seventh switching element SW 7  are in the OFF state in the pulse signal so as to cause the sixth switching element SW 6  and the seventh switching element SW 7  not to simultaneously turn ON. 
     The second voltage detection circuit V 2  is connected in parallel with the sixth capacitor at a portion between the pair of output terminals. 
     In the above-described configuration, if the sixth switching element SW 6  is in the ON state, a current flows in the path of one input terminal, the third inductor L 3 , the sixth switching element SW 6 , and the other input terminal. The current in this path causes magnetic energy to be stored in the third inductor. 
     Then, if the sixth switching element SW 6  turns OFF and the seventh switching element turns ON in a state where magnetic energy is stored in the third inductor, a current flows in the path of the third inductor L 3  and the sixth capacitor C 6 . The current in this path causes magnetic energy in the third inductor L 3  to be converted into charge energy in the sixth capacitor C 6 . The charge energy stored in the sixth capacitor C 6  is supplied to the stand-by module SB. 
     The second voltage detection circuit V 2  detects a voltage of the sixth capacitor, and supplies the detection result to the third PWM pulse generator  87 . The third PWM pulse generator  87  performs a control based on the detection result of the voltage in the second voltage detection circuit V 2 , such that the voltage in the sixth capacitor is equal to a voltage selected in accordance with the control of the control circuit  79 . Thus, the DC-DC circuit  78  supplies DC electric power at the voltage selected in accordance with the control of the control circuit  79  to the stand-by module SB using the electric power supplied from the secondary battery  76 . 
     The seventh switching element SW 7  is, for example, an N-type MOSFET. When the N-type MOSFET is in the ON state, a current flows from the drain thereof toward the source thereof. When the N-type MOSFET is in the OFF state, the N-type MOSFET operates as a body diode in which a current flows from the source thereof toward the drain thereof. If the N-type MOSFET turns ON in this state, the N-type MOSFET operates as a switch at a threshold voltage which is lower than a voltage when the N-type MOSFET operates as the diode. 
     For example, when a voltage applied to the body diode of the seventh switching element SW 7  is 1.5 V, conduction resistance when the seventh switching element SW 7  is in the ON state is 0.01Ω, and if a current flowing at this time is 1 A, the resulting potential difference satisfies V=1 Aλ0.01Ω=0.01 V. That is, the voltage is lower than the voltage generated in the body diode. As a result, it is possible to decrease conduction loss. As described above, a case where a MOSFET is connected in a reverse direction and is used instead of a diode in order to decrease the conduction loss is generally referred to as synchronous rectification. 
     The control circuit  79  controls an operation of the insulating DC-DC circuit  72 , an operation of the heater control circuit  73 , an operation of the A-CC circuit  75 , and an operation of the DC-DC circuit  78 . Specifically, the control circuit  79  controls an output voltage of the insulating DC-DC circuit  72  by inputting a control signal to the first PWM pulse generator  81 . The control circuit  79  controls the quantity of heat to be generated in the thermal fixing belt BT by the heater control circuit  73 , by inputting a control signal to the half bridge alternating pulse generator  84  of the heater control circuit  73 . The control circuit  79  controls a current flowing in the second inductor L 2  of the A-CC circuit  75 , by inputting a control signal to the second PWM pulse generator  85  of the A-CC circuit  75 . The control circuit  79  controls an output voltage of the DC-DC circuit  78  by inputting a control signal to the third PWM pulse generator  87  of the DC-DC circuit  78 . 
     Next, a control of the operation of the A-CC circuit  75  by the control circuit  79  will be described in detail. 
       FIG. 7  is a diagram illustrating a relationship between a paper discharge timing, a temperature, an A-CC driving pulse, and a generated power current. The horizontal axis indicates time. Vertical axes respectively indicate the paper discharge timing, the temperature, the A-CC driving pulse, and the generated power current in that order from the top. Here, an example in which the electrophotographic image forming apparatus  1  discharges three sheets of print media after printing thereon is used. 
     The paper discharge timing indicates a timing when the print medium is discharged from the thermal fixing unit F in the image forming apparatus  1 . A speed at which the print medium is discharged is, for example, one sheet per second. The temperature indicates a temperature detected by a temperature sensor (not illustrated) that detects the temperature of a paper, toner and heated air around the paper, passing through the temperature sensor, after being heated by the thermal fixing unit F. The A-CC driving pulse is a signal used when the second PWM pulse generator  85  in the A-CC circuit  75  controls the fourth switching element SW 4 . The generated power current indicates a value of a current detected by the current detection circuit A in the A-CC circuit  75 . 
     For example, if the thermal capacity of the thermoelectric conversion element  74  is very small, a temperature difference occurs between the opposed ends of the thermoelectric conversion element  74  caused by the air which is discharged along with the print medium. The temperature difference occurring between the opposed ends of the thermoelectric conversion element  74  is increased as the print medium is passing, and is slowly decreased after the print medium passes. The temperature difference occurring between the opposed ends of the thermoelectric conversion element  74  is increased whenever the number of discharged print media is increased. 
     The control circuit  79  controls the A-CC driving pulse to aim at obtaining a current which is substantially proportional to the temperature difference occurring between or across the opposed ends of the thermoelectric conversion element  74 , from the thermoelectric conversion element  74  as the generated power current. The control circuit  79  controls the timing for switching fourth switching element SW 4  of the A-CC circuit  75  between ON and OFF, by adjusting the pulse width of the A-CC driving pulse. That is, the control circuit  79  adjusts the pulse width of the A-CC driving pulse, and thus the A-CC circuit  75  controls the current value of the generated power current from the thermoelectric conversion element  74 . The control circuit  79  receives the current value of the generated power current from the A-CC circuit, and adjusts the pulse width of the A-CC driving pulse in accordance with the received current value. Thus, the control circuit  79  controls the A-CC circuit  75  to receive a current having a target or desired current value, from the thermoelectric conversion element  74 . 
     If the temperature difference occurring between the opposed ends of the thermoelectric conversion element  74  is small, the obtained generated power current is reduced. Thus, electric power required for driving the A-CC circuit  75  may be greater than electric power that can be obtained from the thermoelectric conversion element  74 . When it is not possible to expect that electric power of a predetermined quantity or greater can be generated by the thermoelectric conversion element  74 , the control circuit  79  controls the A-CC circuit  75  to cause the A-CC driving pulse to indicate the OFF state. When the A-CC driving pulse indicates the OFF state, the current which flows via the body diode of the fourth switching element SW 4  in the A-CC circuit  75  is also zero. As a result, the generated current is zero. 
     For example, when a temperature which is equal to or higher than a predetermined temperature is not detected by the temperature sensor, the control circuit  79  controls the A-CC circuit  75  to suspend generation of the A-CC driving pulse. The control circuit  79  may have a configuration of suspending generation of the A-CC driving pulse when the control circuit  79  recognizes that the operation of the heater control circuit  73  is suspended, in order to recognize the operation of the heater control circuit  73 . That is, if the control circuit  79  operates the heater control circuit  73 , and thus discharging of a print medium is started, the control circuit  79  generates the A-CC driving pulse so as to operate the A-CC circuit  75 . For example, the control circuit  79  operates the A-CC circuit  75  with a pulse having a frequency of 100 kHz and a pulse period of about 10 μsec. That is, the control circuit  79  supplies pulses of about the sixth power of ten to the A-CC circuit  75  during a period when one print medium is discharged. Thus, it is possible to control the A-CC circuit  75  at a very small resolution. 
     Next, characteristics of the thermoelectric conversion element  74  will be described. 
       FIG. 8  is a diagram illustrating an example of the characteristics of the thermoelectric conversion element  74 . The graph in  FIG. 8  indicates a value of a current which can be taken out, i.e., drawn from, the thermoelectric conversion element  74 , with respect to the pulse width, that is, the ON duty cycle of the A-CC driving pulse. The horizontal axis in the graph in  FIG. 8  indicates the length of the ON duty cycle of the A-CC driving pulse. The vertical axis in the graph indicates the value of the current which can be taken out from the thermoelectric conversion element  74 . 
     The second PWM pulse generator  85  in the A-CC circuit  75  generates the A-CC driving pulse of a desired amplitude for a selectable duration based on the ON duty cycle designated by the control circuit  79 . If the ON duty cycle of the A-CC driving pulse is small, the duration of the pulse is of a first relatively short time period, and the current taken out from the thermoelectric conversion element  74  is small. If the ON duty cycle of the A-CC driving pulse is long, the duration of the pulse is of a second time period, which is longer than that of the relatively short time period, and the current taken out from the thermoelectric conversion element  74  is large. However, the current taken out from the thermoelectric conversion element  74  is determined based on the maximum value of a current which can be taken out in accordance with the temperature difference between the opposed ends of the thermoelectric conversion element  74 . Thus, if the current taken out from the thermoelectric conversion element  74  reaches the maximum value, the current taken out from the thermoelectric conversion element  74  is increased no more even if the ON duty cycle of the A-CC driving pulse is increased. 
     In the example in  FIG. 8 , when the temperature difference between the opposed ends of the thermoelectric conversion element  74  is 80 degrees C., the maximum value of the current that can be taken out from the thermoelectric conversion element  74  is 0.1 A. When the temperature difference between the opposed ends of the thermoelectric conversion element  74  is 100 degrees C., the maximum value of the current that can be taken out from the thermoelectric conversion element  74  is 0.18 A. When the temperature difference between the opposed ends of the thermoelectric conversion element  74  is 120 degrees C., the maximum value of the current that can be taken out from the thermoelectric conversion element  74  is 0.26 A. That is, the maximum value of the current that can be taken out from the thermoelectric conversion element  74  is increased in proportion to the temperature difference between the opposed ends of the thermoelectric conversion element  74  when this difference is increased. 
     As described above, because the maximum value of the current, that can, or is to be, taken out depends on the temperature difference between the opposed ends of the thermoelectric conversion element  74  and the frequency of the fluctuation of the temperature difference between the opposed ends of the thermoelectric conversion element  74  is high, the current to be taken out may be decreased or an operation may be unstably performed even though a circuit (constant current circuit) configured to take a constant current out is connected to the thermoelectric conversion element  74 . Thus, as illustrated in  FIGS. 2 and 5 , the A-CC circuit  75  which can adjust the current value of the current taken out from the thermoelectric conversion element  74  is connected to the thermoelectric conversion element  74 . 
     Next, a method of controlling the A-CC circuit  75  by the control circuit  79  will be described. 
     The control circuit  79  controls the ON duty cycle of the A-CC driving pulse based on a change of the current value (generated power current) detected by the current detection circuit A of the A-CC circuit  75  when the ON duty cycle of the A-CC driving pulse for driving the A-CC circuit  75  is changed. More specifically, the control circuit  79  performs switching at three stages in an order of the duration of the ON duty cycle of the A-CC driving pulse. The control circuit  79  determines whether the ON duty cycle of the A-CC driving pulse should be increased, reduced, or unchanged, based on determination of whether the current of the generated power has increased, not changed, or decreased. 
       FIGS. 9A to 9C  illustrate examples of a change of the generated power current when the ON duty cycle of the A-CC driving pulse is switched. The example will be described on the assumption that a generated power current when the ON duty cycle, i.e., the ON pulse duration, of the A-CC driving pulse is the smallest is the generated power current I 1 , a generated power current when the ON duty cycle, i.e., the ON pulse duration, of the A-CC driving pulse is the next smallest is the generated power current I 2 , and a generated power current when the ON duty cycle, i.e., the ON pulse duration, of the A-CC driving pulse is the largest is the generated power current I 3 . 
     For example, as illustrated in  FIG. 9A , when the generated power current is slowly increased with an increase of the ON duty cycle of the A-CC driving pulse, it is estimated that generated power current which can be taken out from the thermoelectric conversion element  74  yet remains. Thus, when I 1 &lt;I 2 &lt;I 3  is satisfied (Case 1), the control circuit  79  increases the ON duty cycle, i.e., the ON pulse duration of the A-CC driving pulse. 
     For example, as illustrated in  FIG. 9B , when an increase of the generated power current with the increase of the ON duty cycle of the A-CC driving pulse is stopped in the middle of the increase, it is estimated that generated power current which can be taken out from the thermoelectric conversion element  74  reaches the maximum value. Thus, when I 1 &lt;I 2 =I 3  is satisfied (Case 2), the control circuit  79  determines that the ON duty cycle of the A-CC driving pulse is adequate, and maintains the ON duty cycle of the A-CC driving pulse. 
     For example, as illustrated in  FIG. 9C , when the generated power current is not changed with the increase of the ON duty cycle of the A-CC driving pulse, it is estimated that generated power current that can be taken out from the thermoelectric conversion element  74  has reached the maximum value and the ON duty cycle is too long. Thus, when I 1 =I 2 =I 3  is satisfied (Case 3), the control circuit  79  decreases the ON duty cycle of the A-CC driving pulse. 
     When the current ON duty cycle is set as D 1 , the control circuit  79  controls the ON duty cycle of the A-CC driving pulse, for example, by a program as follows. 
     If (I 1 &lt;I 2  and I 2 &lt;I 3 ) {A=1;} 
     Else if(I 1 &lt;I 2  and I 2 ==I 3 ) {A=2:} 
     Else if(I 1 ==I 2  and I 2 ==I 3 ) {A=3;} 
     Else {A=999;} // error 
     Case A: 
     A=1{D 2 =D 2 +d;} 
     A=2{D 2 =D 2 ;} 
     A=3{D 2 =D 2 - d;}   
     End case; 
       FIGS. 10 and 11  are diagrams illustrating a relationship between the generated power current and the ON duty cycle when the A-CC circuit  75  is controlled by the control method of the A-CC circuit  75  illustrated in  FIGS. 9A to 9C .  FIG. 10  illustrates an example in which the temperature difference between the opposed ends of the thermoelectric conversion element  74  is slowly increased.  FIG. 11  illustrates an example in which the temperature difference between the opposed ends of the thermoelectric conversion element  74  is slowly decreased. Horizontal axes in  FIGS. 10 and 11  indicate time. Vertical axes in  FIGS. 10 and 11  respectively indicate an ideal current curve, the generated power current, and the A-CC driving pulse. The ideal current curve shows characteristics of the temperature difference-generated power current of the thermoelectric conversion element  74 . When the horizontal axis is set to be in a (millisecond) ms range, it is not possible to illustrate ON and OFF of the A-CC driving pulse having a frequency of 100 kHz and a period of 10 μS, in the drawings. Thus, in the examples in  FIGS. 10 and 11 , a coarse pulse is illustrated as the A-CC driving pulse for convenience. 
     The control circuit  79  acquires the current value of the generated power current while changing the ON duty cycle of the A-CC driving pulse by ±Δd. The control circuit  79  determines whether the ON duty cycle of the A-CC driving pulse has increased, been maintained, or decreased in accordance with any change of the acquired current value, while changing the ON duty cycle of the A-CC driving pulse. A mode in which the control circuit  79  acquires the current value of the generated power current is referred to as “a current detection mode”. A mode in which determination of whether the control circuit  79  has increased, maintained, or decreased the ON duty cycle of the A-CC driving pulse is performed, and the ON duty cycle is changed based on the determination result is referred to as “a microcomputer processing determination-and-setting change mode”. The control circuit  79  alternately performs “the current detection mode” and “the microcomputer processing determination-and-setting change mode”, and thus sequentially changes the ON duty cycle. 
     For example, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 1 −Δd to D 1 +Δd, during a period of a time t 0  to a time t 1 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 1  to a time t 2 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 2 . In the example in  FIG. 10 , the control circuit  79  determines to increase the ON duty cycle during the period of the time t 1  to the time t 2 . The control circuit  79  changes the ON duty cycle from ON duty cycle D 1  to ON duty cycle D 2  which is greater than the ON duty cycle D 1 , at the time t 2 . 
     Then, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle a range of D 2 −Δd to D 2 +Δd, during a period of the time t 2  to a time t 3 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of time t 3  to a time t 4 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 4 . In the example in  FIG. 10 , the control circuit  79  determines to maintain the ON duty cycle during the period of the time t 3  to the time t 4 . In this case, the control circuit  79  maintains the ON duty cycle D 2  even after the time t 4 . 
     Then, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 2 −Δd to D 2 +Δd, during a period of the time t 4  to a time t 5 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 5  to a time t 6 , based on the acquired current value. The control circuit  79  changes the ON duty based on the determination result, at the timing t 6 . In the example in  FIG. 10 , the control circuit  79  determines to increase the ON duty cycle during the period of the time t 5  to the time t 6 . The control circuit  79  changes the ON duty cycle from ON duty cycle D 2  to ON duty cycle D 3  which is greater than the ON duty cycle D 2 , at the time t 6 . 
     Then, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 3 −Δd to D 3 +Δd, during a period of the time t 6  to a time t 7 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 7  to a time t 8 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 8 . In the example in  FIG. 10 , the control circuit  79  determines to increase the ON duty cycle during the period of the time t 7  to the timing t 8 . The control circuit  79  changes the ON duty cycle from ON duty cycle D 3  to ON duty cycle D 4  which is greater than the ON duty cycle D 3 , at the time t 8 . 
     Next, as illustrated in  FIG. 11 , an example in which the temperature difference between the opposed ends of the thermoelectric conversion element  74  is slowly decreased will be described. 
     The control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 4 −Δd to D 4 +Δd, during a period of a time t 9  to a time t 10 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 10  to a time t 11 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 11 . In the example in  FIG. 11 , the control circuit  79  determines to maintain the ON duty cycle during a period of the time t 9  to the time t 11 . In this case, the control circuit  79  maintains the ON duty D 4  cycle even after the time t 11 . 
     Then, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 4 −Δd to D 4 +Δd, during a period of the time t 11  to a time t 12 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 12  to a time t 13 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 13 . In the example in  FIG. 11 , the control circuit  79  determines to decrease the ON duty cycle during the period of the time t 12  to the time t 13 . The control circuit  79  changes the ON duty cycle from ON duty cycle D 4  to ON duty cycle D 5  which is smaller than the ON duty cycle D 4 , at the time t 13 . 
     Then, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 5 −Δd to D 5 +Δd, during a period of the time t 13  to a time t 14 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 14  to a time t 15 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 15 . In the example in  FIG. 11 , the control circuit  79  determines to maintain the ON duty cycle during a period of the time t 14  to the time t 15 . In this case, the control circuit  79  maintains the ON duty cycle D 5  even after the time t 15 . 
     Then, the control circuit  79  acquires the current value of the generated power current from the current detection circuit A of the A-CC circuit  75  while changing the ON duty cycle in a range of D 5 −Δd to D 5 +Δd, during a period of the time t 15  to a time t 16 . The control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle during a period of the time t 16  to a time t 17 , based on the acquired current value. The control circuit  79  changes the ON duty cycle based on the determination result, at the time t 17 . In the example in  FIG. 11 , the control circuit  79  determines to decrease the ON duty cycle during the period of the timing t 16  to the timing t 17 . The control circuit  79  changes the ON duty cycle from ON duty cycle D 5  to ON duty cycle D 6  which is smaller than the ON duty cycle D 5 , at the time t 17 . 
     With the above processing, the control circuit  79  can control the current value of the generated power current to generally follow the ideal current curve. 
     Next, the control of the stand-by power supply circuit  20  by the control circuit  79  will be described.  FIG. 12  is a diagram illustrating an example of an operation of taking electric power from the thermoelectric conversion element  74 . 
     When image is formed on a print medium, the control circuit  79  performs initial settings of various registers (ACT 11 ). 
     The control circuit  79  inputs a control signal to the insulating DC-DC circuit  72  so as to operate the insulating DC-DC circuit  72  (ACT 12 ). The control circuit  79  controls the pulse width of the pulse signal which is input to the first switching element SW 1  by the first PWM pulse generator  81 , and thus holds the output voltage of the insulating DC-DC circuit  72  to be constant. 
     The control circuit  79  inputs a control signal to the DC-DC circuit  78  so as to suspend an operation of the DC-DC circuit  78  (ACT 13 ). Thus, the control circuit  79  performs a control such that electric power for the stand-by module SB is supplied from the insulating DC-DC circuit  72  and not from the DC-DC circuit  78 . 
     The control circuit  79  inputs a control signal to the heater control circuit  73  so as to operate the heater control circuit  73  (ACT 14 ). The control circuit  79  controls heat generation of the thermal fixing belt BT. Then, the control circuit  79  causes the print medium to be discharged. 
     The control circuit  79  inputs a control signal to the A-CC circuit  75  so as to operate the A-CC circuit  75  (ACT 15 ). The control circuit  79  designates the ON duty cycle of an initial A-CC driving pulse to the A-CC circuit  75 . 
     The control circuit  79  acquires the current value of the generated power current detected by the current detection circuit A of the A-CC circuit  75 , while changing the ON duty cycle of the A-CC driving pulse in the A-CC circuit  75  (ACT 16 ). 
     The control circuit  79  determines whether this corresponds to Case 1, Case 2, or Case 3, based on a change of the acquired current value of the generated power current (ACT 17 ). That is, the control circuit  79  determines whether to increase, maintain, or decrease the ON duty cycle of the A-CC driving pulse. 
     When the control circuit  79  determines that conditions correspond to Case 1, the control circuit  79  increases the ON duty cycle of the A-CC driving pulse (ACT 18 ). Specifically, the control circuit  79  calculates ON duty cycle after update Dr (Dr=D+Δd), based on the current ON duty D. 
     When the control circuit  79  determines that conditions correspond to Case 2, the control circuit  79  maintains the ON duty cycle of the A-CC driving pulse (ACT 19 ). Specifically, the control circuit  79  sets the ON duty cycle after the update Dr, to be D. 
     When the control circuit  79  determines that conditions correspond to Case 3, the control circuit  79  decreases the ON duty cycle of the A-CC driving pulse (ACT 20 ). Specifically, the control circuit  79  calculates ON duty cycle after update Dr (Dr=D−Δd), based on the current ON duty D cycle. 
     If the control circuit  79  calculates the ON duty cycle after the update Dr, the control circuit  79  determines whether or not to suspend the operation of the heater control circuit  73  (ACT 21 ). For example, when printing on a print medium is ended, the control circuit  79  makes a determination to suspend the operation of the heater control circuit  73 . 
     When the control circuit  79  makes a determination to not suspend the operation of the heater control circuit  73  (YES in ACT 21 ), the control circuit  79  transmits the ON duty cycle after the update Dr to the A-CC circuit  75  (ACT 22 ), and then causes the process to proceed to ACT 16 . The control circuit  79  repeats the processes of ACT 16  to ACT 22  during a period until the operation of the heater control circuit  73  is suspended. Thus, the control circuit  79  performs the processing illustrated in  FIGS. 10 and 11 . As a result, the control circuit  79  can control the current value of the generated power current to follow the ideal current curve. 
     When the control circuit  79  determines to suspend the operation of the heater control circuit  73  (NO in ACT 21 ), the control circuit  79  suspends the operation of the A-CC circuit  75  (ACT 23 ), and ends the processing. 
       FIG. 13  is a diagram illustrating an example of the operation of the control circuit  79  when electric power is supplied to the stand-by module SB. It is assumed that, for example, a DC voltage of 5 V is supplied to the stand-by module SB. When the image forming apparatus  1  operates, electric power is supplied in the path of the full-wave rectifying circuit  71 , the insulating DC-DC circuit  72 , and the stand-by module SB from the AC power supply E. At this time, the generated power current by the thermoelectric conversion element  74  is taken out by the A-CC circuit  75  and is stored in the secondary battery  76 . If a predetermined time elapses from when image forming on a print medium is completed, the image forming apparatus  1  is put into the sleep mode. 
     When the image forming apparatus  1  is in the sleep mode, the control circuit  79  suspends the operation of the heater control circuit  73  (ACT 31 ). 
     The control circuit  79  determines whether or not the residual amount of the electric power in the secondary battery  76  is equal to or greater than a preset threshold, based on the detection result supplied from the residual amount detection circuit  77  (ACT 32 ). 
     When the control circuit  79  determines that the residual amount of the electric power in the secondary battery  76  is equal to or greater than the preset threshold (YES in ACT 32 ), the control circuit  79  suspends the operation of the insulating DC-DC circuit  72  (ACT 33 ). The control circuit  79  suspends the operation of the A-CC circuit  75  (ACT 34 ). The control circuit  79  operates the DC-DC circuit  78  so as to supply the electric power stored in the secondary battery  76  to the stand-by module SB (ACT 35 ). Then, the process proceeds to ACT 39 . In this manner, when the residual amount of power remaining in the secondary battery  76  is equal to or greater than the threshold, the stand-by power supply circuit  20  controls the circuits to supply the electric power from the secondary battery  76  to the stand-by module SB. 
     When the control circuit  79  determines that the residual amount of the electric power in the secondary battery  76  is smaller than the preset threshold (NO in ACT 32 ), the control circuit  79  operates the insulating DC-DC circuit  72  to supply the electric power from the AC power supply E to the stand-by module SB (ACT 36 ). The control circuit  79  suspends the operation of the A-CC circuit  75  (ACT 37 ). The control circuit  79  suspends the operation of the DC-DC circuit  78  (ACT 38 ) and causes the process to proceed to ACT 39 . In this manner, when the residual amount of power in the secondary battery  76  is smaller than the threshold, the stand-by power supply circuit  20  controls the circuits to supply the electric power from the AC power supply E to the stand-by module SB. 
     The control circuit  79  determines whether or not the sleep mode continues (ACT 39 ). When the control circuit  79  determines that the sleep mode continues (Yes in ACT 36 ), the control circuit  79  causes the process to proceed to ACT 32 . Thus, the control circuit  79  switches a power source for supplying the electric power to the stand-by module SB, between the secondary battery  76  and the AC power supply while the residual amount of the secondary battery  76  is continuously monitored. When the control circuit  79  determines that the sleep mode has ended (NO in ACT 36 ), the control circuit  79  ends the processing in  FIG. 13 . 
       FIG. 14  is a diagram illustrating a control of the units in the power supply circuit. The horizontal axis indicates time. Vertical axes respectively indicate a printing timing by the image forming unit  17 , a control timing of the heater control circuit  73 , a control timing of the insulating DC-DC circuit  72 , a control timing of the A-CC circuit  75 , a control timing of the DC-DC circuit  78 , a change of the residual amount of charge or power in the secondary battery  76 , and a supply source for supplying the electric power to the stand-by module SB. 
     In this example, it is assumed that printing is not performed in a period from a time t 20  to a time t 21  and printing is performed on six sheets in a period from the time t 21  to a time t 22 . In addition, it is assumed that printing is not performed in a period from the time t 22  to a time t 24  and printing is performed on three sheets in a period from the time t 24  to a time t 25 . 
     The stand-by power supply circuit  20  operates in the sleep mode in the period from the time t 20  to the time t 21 . 
     If a printing instruction is input, the control circuit  79  starts an operation of the heater control circuit  73  at the time t 21  and starts preparation for printing. If the temperature of the thermal fixing belt BT is equal to or higher than a predetermined temperature, printing is performed in the period from the time t 21  to the time t 22 . If the temperature difference in the thermoelectric conversion element  74  is equal to or greater than a predetermined value, the control circuit  79  causes the A-CC circuit  75  to store power generated by the thermoelectric conversion element  74  in the period from the time t 21  to the time t 22 . That is, the control circuit  79  operates the A-CC circuit  75  in accordance with the operation of the heater control circuit  73 . If this state continues, electric power is charged into the secondary battery  76 , and then electric power charged in the secondary battery  76  becomes equal to or greater than a threshold value. If the printing is completed and generated power is not sufficiently taken from the thermoelectric conversion element  74 , the control circuit  79  suspends the operation of the A-CC circuit  75 . 
     The stand-by power supply circuit  20  switches an operation mode of the MFP to the sleep mode from the time t 22  when a predetermined time elapses from when the printing is completed. Thus, the control circuit  79  suspends the operation of the heater control circuit  73  and suspends the operation of the insulating DC-DC circuit  72 . In the example in  FIG. 14 , the control circuit  79  operates the DC-DC circuit  78  to cause the residual amount of the electric power in the secondary battery  76  to be equal to or greater than a threshold. Thus, the supply source for supplying electric power to the stand-by module SB is switched from the AC power supply E to the secondary battery  76  at the time t 22 . 
     The control circuit  79  sequentially confirms whether or not the residual amount of power in the secondary battery  76  is equal to or greater than the threshold. In the example in  FIG. 14 , the control circuit  79  determines that the residual amount of power in the secondary battery  76  is smaller than the threshold, at the time t 23 . In this case, the control circuit  79  starts the operation of the insulating DC-DC circuit  72  and suspends the operation of the DC-DC circuit  78 . Thus, the supply source for supplying electric power to the stand-by module SB is switched from the secondary battery  76  to the AC power supply E at the time t 23 . 
     If a printing instruction is input again, the control circuit  79  starts the operation of the heater control circuit  73  at the time t 24  and starts preparation for printing. If the temperature of the thermal fixing belt BT is equal to or higher than a predetermined temperature, printing is performed in the period from the time t 24  to the time t 25 . If the temperature difference in the thermoelectric conversion element  74  is equal to or greater than a predetermined value, the control circuit  79  causes the A-CC circuit  75  to take power generated by the thermoelectric conversion element  74  during the period from the time t 24  to the time t 25 . Thus, electric power is charged into the secondary battery  76 , and then the electric power charged into the secondary battery  76  becomes equal to or greater than the threshold. If the printing is completed and the generated power is not sufficiently taken out from the thermoelectric conversion element  74 , the control circuit  79  suspends the operation of the A-CC circuit  75 . 
     The stand-by power supply circuit  20  switches an operation mode of the MFP to the sleep mode from the time t 25  when a predetermined time elapses from when the printing is completed. Thus, the control circuit  79  suspends the operation of the heater control circuit  73  and suspends the operation of the insulating DC-DC circuit  72 . In the example in  FIG. 14 , the control circuit  79  operates the DC-DC circuit  78  to cause the residual amount of the electric power in the secondary battery  76  to be equal to or greater than a threshold, i.e., the secondary battery is charged. Thus, the supply source for supplying electric power to the stand-by module SB is switched from the AC power supply E to the secondary battery  76  again at the time t 25 . 
     According to the stand-by power supply circuit  20  configured as described above, the thermoelectric conversion element  74  generates electric power from the heat of a paper and toner heated by the thermal fixing unit F when printing is performed. The power generated using the heat of the thermoelectric conversion element  74  is taken out by the A-CC circuit  75 . Thus, the stand-by power supply circuit  20  can supply electric power generated by heat to the stand-by module SB. The control circuit  79  of the stand-by power supply circuit  20  controls the A-CC circuit  75  that takes current out from the thermoelectric conversion element  74 , based on the change of the current taken out from the thermoelectric conversion element  74 . That is, the control circuit  79  controls the target value of the constant current taken out by the A-CC circuit  75 , based on the change of the value of the current flowing from the thermoelectric conversion element  74  into the A-CC circuit  75  when the control circuit  79  controls the A-CC circuit  75  so as to change the target value of the constant current to be taken out from the thermoelectric conversion element  74 . Thus, the stand-by power supply circuit  20  can take electric power out from the thermoelectric conversion element  74  with high efficiency. 
     The A-CC circuit  75  includes the second inductor L 2  which is connected in series to the thermoelectric conversion element  74 , the fourth switching element SW 4  which is connected in series to the thermoelectric conversion element  74 , and the current detection circuit A which is connected in series to the thermoelectric conversion element  74  and detects the current value of the current flowing from the thermoelectric conversion element  74  into the A-CC circuit  75 . The A-CC circuit  75  includes the fifth switching element SW 5  and the fifth capacitor C 5  which are connected in series at a portion between the connection point of the second inductor L 2  and the fourth switching element SW 4  and the other terminal of the fourth switching element SW 4 . Further, the A-CC circuit  75  includes the second PWM pulse generator  85  and the first synchronous rectification pulse generator  86  which are driver circuits configured to input pulse signals for causing the fourth switching element SW 4  and the fifth switching element SW 5  to turn ON and OFF, to the fourth switching element SW 4  and the fifth switching element SW 5 , respectively. In this configuration, the control circuit  79  maintains the current ON state when the current value detected by the current detection circuit A is decreased with the decrease of the ON duty cycle of the pulse signal and the current value detected by the current detection circuit A is not changed with the increase of the ON duty cycle of the pulse signal. When the current value detected by the current detection circuit A is decreased with the decrease of the ON duty cycle of the pulse signal and the current value detected by the current detection circuit A is increased with the increase of the ON duty cycle of the pulse signal, the control circuit  79  increases the ON duty cycle. When the current value detected by the current detection circuit A is not changed with the decrease of the ON duty cycle of the pulse signal and the current value detected by the current detection circuit A is not changed with the increase of the ON duty cycle of the pulse signal, the control circuit  79  decreases the ON duty cycle. Thus, the stand-by power supply circuit  20  can take electric power out from the thermoelectric conversion element  74  along the ideal current curve determined by the characteristics and the temperature of the thermoelectric conversion element  74 . 
     If electric power generated by heat is consumed by the stand-by module SB, the stand-by power supply circuit  20  switches the supply source of the electric power so as to supply electric power to the stand-by module SB from the AC power supply E. Thus, the stand-by power supply circuit  20  can use the electric power of the secondary battery  76  when electric power remains in the secondary battery  76 , and can use electric power from the AC power supply E when electric power does not remain in the secondary battery  76 . Thus, in the stand-by power supply circuit  20 , it is possible to improve efficiency of electric power consumption. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.