Patent Publication Number: US-6987936-B2

Title: Image forming apparatus, image forming method, and fixing unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present document incorporates by reference the entire contents of Japanese priority documents, 2003-161331 filed in Japan on Jun. 5, 2003 and 2004-161777 filed in Japan on May 31, 2004. 
   BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to an image forming apparatus, an image forming method, and a fixing unit, and more particularly, to an image forming apparatus like a copying machine, a digital multifunction product, and a printer that apply heat to a fixing member of a fixing unit using an auxiliary power supply, an image forming method, and a fixing unit that is used in the image forming apparatus. 
   2) Description of the Related Art 
   The image forming apparatus such as the copying machine or the printer forms an image on a recording medium like plain paper or an OHP transparency. In this image formation, an electrophotographic system is adopted taking into account high speed of the image formation, an image quality, cost, and the like. The electrophotographic system is a system in which a toner image is formed on a recording medium and fixed on the recording medium with heat and pressure. As a fixing system, a heat roll system is adopted most frequently at present in terms of safety and the like. The heat roll system is a system in which a heating roller, which is heated by a heat generation member like a halogen heater, and a pressurizing roller, which is arranged to be opposed to the heating roller, are brought into pressed contact with each other to form a mutual press-contact section called a nip section, and the recording medium having the toner image transferred thereon through is passed through this nip section and heated. 
   In recent years, environmental problems have increased in importance, and efforts have been made to reduce energy consumption in the image forming apparatus such as the copying machine or the printer. What cannot be neglected in considering the reduction of energy consumption in the image forming apparatus is power saving in the fixing unit that fixes toner on a recording medium. 
   In realizing power saving, it is effective to reduce energy consumption when the fixing unit is on standby. Thus, it is desirable to reduce power supply to zero when the fixing unit is not used. However, in a conventional structure for the fixing unit, if power is reduced to zero when the fixing unit is on standby, it takes time to warm up a heating roller thereof when the fixing unit is used again. This makes waiting time longer and deteriorates convenience for a user. Consequently, a structure for increasing temperature of the heating roller rapidly has been required. 
   To reduce the warm-up time for the heating roller, it is effective to increase input energy per a unit time, that is, rated power. Actually, in some high-speed image forming apparatuses with high print speed, a supply voltage is increased to 200 volts to increase the rated power. However, in typical offices in Japan, in general, a power supply has an upper limit of 100 V/15 A (1500 W), and it is necessary to apply special work to power supply related facilities in a place where an image forming apparatus is set. Thus, it cannot be said that this is a general solution. Therefore, the fact of the matter is that, even if it is attempted to warm up the heating roller in a short time, the upper limit of the input energy cannot be lifted. 
   In addition, there is a method of reducing thickness of the heating roller to warm up the heating roller in a short time. However, since a thermal capacity of the heating roller decreases, in the case of an image forming apparatus with high print speed, temperature may fall if continuous printing is performed even in a state in which temperature on the surface of the heating roller has risen to a set temperature. 
   To improve the warm-up of the heating roller, power is accumulated in an auxiliary power supply in advance and the power is supplied from the auxiliary power supply at the time of power shortage. This makes it possible to solve the problem of the fall in temperature at the time of a print operation. 
   For example, Japanese Patent Application Laid-Open Publication No. Hei 10-282821 proposes a technique for increasing maximum supply power using an auxiliary power supply in a fixing unit to thereby realize reduction in energy consumption. Such a fixing unit disclosed in Japanese Patent Application Laid-Open Publication No. Hei 10-282821 supplies power from a main power supply unit and a secondary battery or a primary battery and uses a nickel-cadmium battery or a lead-acid battery as a source of the secondary battery. Such a secondary battery has a characteristic that a capacity thereof deteriorates and decreases when charge and discharge are repeated many times and a useful life thereof is reduced when discharge is performed with a larger current. In general, even in the nickel-cadmium battery, which is said to have a long useful life with a large current, the number of times of repetition of charge and discharge is about 500 to 1000. If charge and discharge are repeated twenty times a day, the battery comes to the end of its life in about one month. Therefore, the secondary battery has a disadvantage that time and labor are required for replacement of the battery, and running cost such as cost for the battery to be replaced increases. Moreover, a lead-acid storage battery is not preferable as office equipment because, for example, the battery uses liquid sulfuric acid. 
   Consequently, for example, Japanese Patent Application Laid-Open Publication No. 2002-184554 discloses an image forming apparatus that uses a large-capacitance capacitor like an electric double layer capacitor as an auxiliary power supply for a fixing unit. Such an electric double layer capacitor has a characteristic that the number of times of repetition of charge and discharge is several tens thousand to several hundreds thousand, and a useful life depending on the number of times of charge and discharge is far longer than that of a battery. 
   However, when the auxiliary power supply is used, power must be charged in the auxiliary power supply. If a print operation is performed in a state in which sufficient power is not charged in the auxiliary power supply, charge in the auxiliary power supply is fully consumed in the midstream of the print operation. As a result, print speed falls from the midstream of the print operation, and if power of the auxiliary power supply runs short, an image with a poor fixing property is obtained unless fall in temperature in the heating roller is prevented by taking measures such as increasing an interval of print operations. On the other hand, it is preferable for a user that time until completion of an image forming job is shorter. 
   There is also a technique for changing a linear velocity or a sheet feeding time according to a state of a charging voltage at an auxiliary power supply. However, in any event, longer time is required until completion of a print operation compared with a case in which a fully charged auxiliary power supply is used. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve at least the above problems in the conventional technology. 
   The image forming apparatus according to one aspect of the present invention includes a fixing unit that thermally fixes a toner image using a fixing member, where the fixing unit is heated by a heat generating member that generates heat by power supply from a chargeable auxiliary power supply; a power control unit that controls the power supply from the auxiliary power supply to the heat generating member based on a charging voltage of the auxiliary power supply; a job-turnaround-time predicting unit that predicts a job turnaround time required for executing an image forming job; a charging-time predicting unit that predicts a charging time for charging the auxiliary power supply to a predetermined charging voltage based on the charging voltage of the auxiliary power supply; and a control unit that controls a charging operation for the auxiliary power supply and execution of the image forming job based on the job turnaround time predicted and the charging time predicted, such that the job turnaround time for the image forming job is minimized. 
   The method of forming an image with a fixing unit that thermally fixes a toner image using a fixing member, where the fixing unit is heated by a heat generating member that generates heat by power supply from a chargeable auxiliary power supply, according to another aspect of the present invention includes steps of controlling the power supply from the auxiliary power supply to the heat generating member based on a charging voltage of the auxiliary power supply; predicting a job turnaround time required for executing an image forming job; predicting a charging time for charging the auxiliary power supply to a predetermined charging voltage based on the charging voltage of the auxiliary power supply; and controlling a charging operation for the auxiliary power supply and execution of the image forming job based on the job turnaround time predicted and the charging time predicted, such that the job turnaround time for the image forming job is minimized. The fixing unit that thermally fixes a toner image using a fixing member, where the fixing unit is heated by a heat generating member that generates heat by power supply from a chargeable auxiliary power supply, according to still another aspect of the present invention includes a power control unit that controls the power supply from the auxiliary power supply to the heat generating member based on a charging voltage of the auxiliary power supply; a job-turnaround-time predicting unit that predicts a job turnaround time required for executing an image forming job; a charging-time predicting unit that predicts a charging time for charging the auxiliary power supply to a predetermined charging voltage based on the charging voltage of the auxiliary power supply; and a control unit that controls a charging operation for the auxiliary power supply and execution of the image forming job based on the job turnaround time predicted and the charging time predicted, such that the job turnaround time, for the image forming job is minimized. 
   The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of an image forming apparatus to which the present invention is applied; 
       FIG. 2  is a schematic of an ADF shown in  FIG. 1 ; 
       FIG. 3  is a schematic for illustrating a detailed mechanism around a sheet feeding tray and a sheet feeding bottom plate shown in  FIG. 2 ; 
       FIG. 4  is a schematic of an operation unit of the image forming apparatus; 
       FIG. 5  is a schematic for illustrating an internal structure of a fixing unit shown in  FIG. 1 ; 
       FIG. 6  is a schematic of a power control system for the fixing unit; 
       FIG. 7  is a schematic of an AC heater drive circuit shown in  FIG. 6 ; 
       FIG. 8  is a schematic of a capacitor charger shown in  FIG. 6 ; 
       FIG. 9  is a schematic of a capacitor charge and discharge circuit shown in  FIG. 6 ; 
       FIG. 10  is a schematic of a control unit shown in  FIG. 6 ; 
       FIG. 11  is a timing chart for explaining temperature control of a fixing roller; 
       FIG. 12  is a flowchart of a process procedure for ON/OFF control of a fixing heater; 
       FIG. 13  is a flowchart of a process procedure for a copy job in a control unit of an image forming apparatus according to a first embodiment of the present invention; 
       FIG. 14  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at a capacitor CP 1  when Tc+T 1 HIGH is smaller than T 1 LOW; 
       FIG. 15  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at the capacitor CP 1  when Tc+T 1 HIGH is not smaller than T 1 LOW; 
       FIG. 16  is a flowchart of a process procedure for a copy job in a control unit of an image forming apparatus according to a second embodiment of the present invention; 
       FIG. 17  is a graph for illustrating control for a copy job and a completion state of the job in the second embodiment; 
       FIG. 18  is a flowchart of a process procedure for a copy job in a control unit of an image forming apparatus according to a third embodiment of the present invention; 
       FIG. 19  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at the capacitor CP 1  when Tc+T 1 HIGH 2 +T 1 LOW 2  is smaller than T 1 LOW; and 
       FIG. 20  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at the capacitor CP 1  when Tc+T 1 HIGH 2 +T 1 LOW 2  is not smaller than T 1 LOW. 
   

   DETAILED DESCRIPTION 
   Exemplary embodiments of an image forming apparatus, an image forming method, and a fixing unit according to the present invention will be explained in detail with reference to the accompanying drawings. In this specification, an “image forming job” indicates a photocopying job and a printing job. 
     FIG. 1  is a schematic of an image forming apparatus to which the present invention is applied. An image forming apparatus  1  is, for example, a digital multifunction product, and has of a copying function, a printing function, and a facsimile function. The image forming apparatus  1  is made capable of sequentially switching and selecting the copying function, the printing function, and the facsimile function with an application switching key of an operation unit (see  FIG. 2 ). Selection of the copying function, the printing function, and the facsimile function puts the image forming apparatus in a copier mode, a printer mode, and a facsimile mode, respectively. 
   The image forming apparatus  1  includes an automatic document feeder (ADF)  10  that automatically conveys a document to a document reading position, an image reader  20  that optically reads image information on the document conveyed to the document reading position, a writing unit  30  that writes the read image information of the document in a printer unit  40 , and a printer unit  40  that forms a toner image of the image information on the document written by the writing unit  30 , transfers the toner image onto a material to have an image transferred thereon like transfer paper, and discharges the material to have an image transferred thereon. 
   In the copier mode, in the ADF  10 , when a start key on an operation unit  150  is depressed, a drawing-out roller  103 , a sheet feeding roller  104 , a separation roller  105 , and a feeding belt  112  feed a document at the top of a document stack, which is formed by stacking documents on a sheet feeding tray  101  and a sheet feeding bottom plate  102  with image surfaces thereof facing upward, to a predetermined position on a contact glass  16 . 
   The ADF  10  has a count function for counting up the number of documents every time feeding for one document is completed. The image reader  20  serving as image input means reads image information from the document on the contact glass  116 . Then, the feeding belt  12  and a discharging roller  114  discharge the document onto a sheet discharge stand  115 . 
   Similarly, the drawing-out roller  103 , the sheet feeding roller  104 , the separation roller  105 , and the feeding belt  112  feed a second document (a document at the top now) on the sheet feeding tray  101  and the sheet feeding bottom plate  102  to the predetermined position on the contact glass  116 . The image reader  20  reads image information from this document on the contact glass  116 . Then, the feeding belt  112  and the discharging roller  114  discharge the document on the sheet discharge stand  115 . A conveying motor (not shown) drives the drawing-out roller  103 , the sheet feeding roller  104 , the separation roller  105 , the feeding belt  112 , and the discharging roller  114 . 
   A first sheet feeder  140 , a second sheet feeder  141 , and a third sheet feeder  142 , which serve as sheet feeding means, feed sheets consisting of transfer paper serving as materials to have an image transferred thereon stacked on a first tray  143 , a second tray  144 , and a third tray  145 , respectively, when the sheet feeders are selected. A vertical conveyance unit  146  conveys this transfer paper to a position where the transfer paper comes into abutment against a photosensitive body  118  serving as an image bearing body. For example, a photosensitive drum is used as the photosensitive body  118 , which is driven to rotate by a main motor (not shown). 
   The writing unit  30  serving as writing means converts the image information on the document read by the image reader  20  into optical information via a not-shown image processing unit. A charger (not show) uniformly charges the photosensitive body  118 . Then, the photosensitive body  118  is exposed to light according to the optical information from the writing unit  30 , whereby an electrostatic latent image is formed on the photosensitive body  118 . A developing device  119  develops the electrostatic latent image on the photosensitive body  18  to change the electrostatic latent image to a toner image. 
   A conveyor belt  120  serves as both sheet conveying means and transfer means. When a transfer bias is applied from a power supply, the conveyor belt  120  transfers the toner image on the photosensitive body  118  onto a transfer paper, which is conveyed from the vertical conveyance unit  146 , while conveying the transfer paper at the same velocity as the photosensitive body  118 . A fixing unit  121  fixes the toner image on this transfer paper, and a sheet discharging unit  122  discharges the transfer paper to a sheet discharge tray  123 . After the toner image is transferred, a cleaning device (not shown) cleans the photosensitive body  118 . The photosensitive body  118 , the charger (not shown), the wiring unit  30 , the developing device  119 , and the conveyor belt  120  constitute an image forming unit that forms an image on a transfer paper according to image information. 
   The operations described above are operations at the time when an image is copied to one side of a sheet in a normal mode. However, when images are copied to both sides of a transfer paper in a duplex mode, the sheet discharging unit  122  switches the transfer paper, which is fed from any one of sheet feeding trays  143  to  145  and has the image formed on the surface thereof as described above, to a duplex paper receiving and conveying path  124  side rather than to the paper discharge tray  123 . Then, a reversing unit  125  switches back to reverse the transfer paper, which is conveyed to a duplex conveyance unit  126 . 
   The duplex conveyance unit  126  conveys the transfer paper, which is conveyed to this duplex conveyance unit  126 , to a vertical conveyance unit  146 . The vertical conveyance unit  146  conveys the transfer paper to a position where the transfer paper comes into abutment against the photosensitive body  118 . Then, the toner image formed on the photosensitive body  118  as described above is transferred onto a back of the transfer paper and fixed by the fixing unit  121 , whereby a duplex copy is obtained. The sheet discharging unit  122  discharges this duplex copy to the sheet discharge tray  123 . 
   When the transfer paper is reversed and discharged, the sheet discharging unit  122  discharges the transfer paper, which is switched back and reversed by the reversing unit  125 , to the sheet discharge tray  123  through a reversed sheet discharge and conveyance path  127  rather than conveying the transfer paper to the duplex conveyance unit  126 . 
   In the printer mode, image information is inputted to the writing unit  30  instead of the image information from an image processing unit, and an image forming unit forms an image on a transfer paper. Moreover, in the facsimile mode, a not-shown facsimile transmission and reception unit sends image information from an image reading unit to a party on the other end, and the facsimile transmission and reception unit receives image information from the party on the other end. The image information is inputted to the writing unit  30  instead of the image information from the image processing unit, whereby the image forming unit forms an image on the transfer paper. 
   The image forming apparatus with the above-mentioned structure has a high-speed mode, in which the image forming apparatus executes a copy job (printer job) at high speed, and a low-speed mode, in which the image forming apparatus executes a copy job (printer job) at low speed. The numbers of revolutions of a drive motor necessary for conveying transfer paper, a main motor that drives the photosensitive body  118 , a conveyance drive motor that conveys transfer paper, a drive motor for that rotates the fixing roller, a polygon motor that writes an electrostatic latent image in a photosensitive body, and the like are made variable. A not-shown control unit (see  FIG. 6 ) changes the numbers of revolutions of these motors to execute the high-speed mode and the low-speed mode. 
     FIG. 2  is a schematic of an ADF shown in  FIG. 1 .  FIG. 3  is a schematic for explaining a mechanism for detecting the number of documents set in the ADF  10  in  FIG. 2 . 
   The ADF  10  includes a drawing-out roller home position sensor  106 , a document set sensor  107 , a bottom plate lifting motor  108 , a bottom plate lifting motor  109 , a slit encoder  110 , and a slit sensor  111 . 
   In the ADF  10 , when a document is set on the sheet feeding tray  101  and the sheet feeding bottom plate  102 , the document set sensor  107  detects the document (is turned ON) to output a detection signal to the control unit (see  FIG. 6 ). In response to the detection signal, the control unit (see  FIG. 6 ) rotates the bottom plate lifting motor  108  and lifts the sheet feeding bottom plate  102  together with the document using the bottom plate lifting lever  109  via a gear, a belt, or the like. Consequently, the drawing-out roller  103  for delivering the document to the sheet feeding roller  104  rises together with the document. Upon detecting the drawing-out roller  103 , the drawing-out roller home position sensor  106  outputs a detection signal to the control unit (see  FIG. 6 ). The control unit (see  FIG. 6 ) stops the bottom plate lifting motor  108 . 
   The slit encoder  110  is fixed to a rotation shaft of the bottom plate lifting motor  108  and rotates following rotation of the bottom plate lifting motor  108 . The slit sensor  111  detects a rotation angle of the slit encoder  110  and outputs a detection signal to the control unit (see  FIG. 6 ). This rotation angle of the slit encoder  110  is proportional to an amount of lifting of the bottom plate lifting lever  109 . The control unit (see  FIG. 6 ) assumes the number of documents set in the ADF  10  based upon the detection signal inputted from the slit encoder  110 . 
     FIG. 4  is a schematic of an operation unit of the image forming apparatus. The operation unit  150  is a unit for giving an instruction for execution of a copy job and the like to the control unit (see  FIG. 6 ). As shown in  FIG. 4 , the operation unit  150  includes: an LCD  161  with a touch panel that displays necessary information and function keys; a KEY  163  including mechanical keys like a start key and a ten key; an LED; and the like. 
     FIG. 5  is a schematic for illustrating an internal structure of a fixing unit shown in  FIG. 1 . The fixing unit  121  includes: a fixing roller  151  serving as a fixing member; a pressurizing roller  152  serving as a pressurizing member for pressurizing the fixing roller  151 ; an AC fixing heater HT 2  serving as a main heating unit and a fixing heater HT 1  serving as an auxiliary heating unit that are arranged inside the fixing roller  151  and heat the fixing roller  151  from the inside thereof; and a thermistor TH 11  serving as temperature detecting means that is in abutment against the surface of the fixing roller  151  and detects a surface temperature (fixing temperature) of the fixing roller  151 . 
   The fixing roller  151  consists of an elastic member like silicon rubber and thermally fixes a toner image transferred onto transfer paper. The pressurizing roller  152  consists of an elastic member like silicon rubber and is pressed against the fixing roller  151  at a fixed pressurizing force by a not-shown pressurizing unit. In general, a halogen heater is used for the fixing heater HT 1  and the AC fixing heater HT 2 . However, other resistors may be used. The AC fixing heater HT 2  generates heat (turns on) when AC power is supplied from an AC heater drive circuit (see  FIG. 7 ), and the fixing heater HT 1  generates heat (turns on) when a voltage is supplied from a capacitor (see  FIG. 6 ) serving as an auxiliary power supply. 
   In the fixing unit  121  with the above-mentioned structure, the fixing roller  151  and the pressurizing roller  152  are driven to rotate by a not-shown driving mechanism. A toner carried on a sheet P such as transfer paper is fixed on the sheet P according to heating and pressurization by the fixing roller  151  and the pressurizing roller  152  when the sheet P passes through a nip section between the fixing roller  151  and the pressurizing roller  152 . 
   Note that, in this context, the fixing member and the pressurizing member are generally rollers as shown in  FIG. 5  but are not limited to the rollers. An endless belt or the like may be used for one or both of the fixing member and the pressurizing member. In addition, the fixing heater HT 1  and the AC fixing heater HT 2  are arranged inside the fixing roller  151 . However, the fixing heater HT 1  and the AC fixing heater HT 2  may be arranged in any position as long as the fixing heater HT 1  and the AC fixing heater HT 2  are capable of heating the fixing roller  151 . 
     FIG. 6  is a schematic of a power control system for the fixing unit. The power control system for the fixing unit includes a main power SW  201  that turns ON/OFF supply of AC power, a control unit  202  that controls respective units of the image forming apparatus and a power supply circuit  200 , a capacitor charger  203  that charges a capacitor CP, a DC power generation circuit  204  that generates DC power of the image forming apparatus; an AC heater drive circuit  205  that supplies AC power to the AC fixing heater HT 2 , an input current detection circuit  206  that detects an input current inputted from an AC power supply; an interlock switch  207 , a capacitor charge and discharge circuit  208  that discharges the capacitor CP 1  and supplies DC power to the fixing heater HT 1 , and a capacitor CP 1  serving as an auxiliary power supply for the fixing heater HT 1 . 
   The AC power supply supplies AC power to the AC heater drive circuit  205 , the DC power generation circuit  205 , and the capacitor charger  203  via the main power SW  201  and the input current detection circuit  206 . 
   The control unit  202  is a unit for controlling the respective units of the power supply circuit  200 . The control unit  202  controls operations of the capacitor charger  203 , the AC heater drive circuit  205 , and the capacitor charge and discharge circuit  208 . More specifically, the control unit  202  sends a control signal S 1  to the capacitor charger  203  to control an operation for charging the capacitor CP 1  by the capacitor charger  203 . In addition, the control unit  202  sends control signals S 3  and S 4  to the capacitor charge and discharge circuit  208  to control an operation for turning ON/OFF the fixing heater HT 1  by the capacitor charge and discharge circuit  208 . Further, the control unit  202  sends control signals S 8  and S 9  to the AC heater drive circuit  208  to control an operation for turning ON/OFF the fixing heater HT 2  by the AC heater drive circuit  205 . Moreover, the control unit  202  estimates the number of documents set in the ADF  10  based on a detection signal inputted from the slit sensor  111  and predicts a turnaround time for a copy job for each of the operation modes (the high-speed mode and the low-speed mode) based on the estimated number of documents, the number set by the operation unit  150 , and a time necessary for printing one sheet in each of the operation modes. 
   The input current detection circuit  206  is provided among the main power SW  201 , the AC heater drive circuit  205 , the DC power generation circuit  204 , and the capacitor charger  202 . The input current detection circuit  206  detects an input current of AC power inputted via the main power SW  201 , and outputs a detection current S 7  to the control unit  202 . This input current fluctuates according to operation states of the AC heater drive circuit  205 , the DC power generation circuit  204 , the capacitor charger  202 , and the image forming apparatus. 
   The DC power generation circuit  204  generates power Vcc, which is used mainly in a control system inside the image forming apparatus, and power Vaa, which is used mainly in a drive system and a medium/high voltage power supply, base on the AC power inputted via the main power SW  201 , and outputs the power Vcc and the power Vaa to the respective units. 
   The interlock switch  207  is a switch to be turned ON/OFF in association with not-shown covers of the image forming apparatus. When the image forming apparatus has a drive member or a medium/high voltage application member that can be touched by the covers when the covers are opened, the interlock switch  207  interrupts a power supply such that an operation of the drive member or application of a voltage to the application member is stopped when the covers are opened. A part of the power Vaa generated by the DC power generation circuit  204  is inputted to the interlock switch  207  and is inputted to the capacitor charge and discharge circuit  208  and the AC heater drive circuit  205  via this interlock switch  207 . 
   The AC heater drive circuit  205  turns ON/OFF the AC fixing heater HT 2  according to the control signals S 8  and S 9  inputted from the control unit  202 . 
   The capacitor charger  203  is connected to the capacitor CP 1  and charges the capacitor CP 1  based on the control signal S 1  inputted from the control unit  202 . 
   The capacitor CP 1  is constituted by a large-capacitance capacitor such as the electric double layer capacitor. The capacitor CP 1  is connected to the capacitor charger  203  and the capacitor charge and discharge circuit  208  and charged by the capacitor charger  203 . Charged power of the capacitor CP 1  is supplied to the fixing heater HT 1  according to ON/OFF control by the capacitor charge and discharge circuit  208 . 
   The capacitor charge and discharge circuit  208  discharges the power accumulated in the capacitor CP 1  according to the control signals S 3  and S 4  inputted from the control unit  202  and turns ON/OFF the fixing heater HT 1 . 
   The thermistor TH 11  is provided near the fixing roller  151  and outputs a detection signal S 6  corresponding to a surface temperature of the fixing roller S 151  to the control unit  202 . Since a resistance of the thermistor TH 11  changes according to temperature, the control unit  202  utilizes temperature change in the resistance to detect the surface temperature of the fixing roller  151  based on the detection signal S 6 . 
     FIG. 7  is a schematic of the AC heater drive circuit  205  shown in  FIG. 6 . The AC heater drive circuit  205  includes a filter FIL 21  that removes noise of AC power to be inputted, a fixing relay for safety RL 21  that is turned ON/OFF according to the control signal S 9  inputted from the control unit  202 , a diode D 21  for preventing counter-electromotive force in the fixing relay for safety RL 21 , and a heater ON/OFF circuit  220  that turns ON/OFF the AC fixing heater HT 2  based on the control signal S 8  inputted from the control unit  202 . 
   The AC power supply is connected to one end side of the fixing heater HT 2  via the filter FIL 21  and the fixing relay for safety RL 21 . The other end side of the fixing heater HT 2  is connected to the heater ON/OFF circuit  220 . 
   The heater ON/OFF circuit  220  includes a triac TRI 21  for turning ON/OFF the AC power supply, a photocoupler PC 21  for turning ON a gate of the triac TRI 21  and insulating a signal from the control unit  202  that is a secondary side, a transistor TR 21  for driving a light-emitting side LED of the photocoupler PC 21 , a noise absorbing snubber circuit including a capacitor CP 1  and a resistor R 21 ; a noise absorbing inductor L 21 , a resistor R 22  serving a dynamic current prevention resistor, and resistors R 23  and R 24  serving as current limiting resistors for the photocoupler PC  21 . 
   In the AC heater drive circuit  205  with the above-mentioned structure, the AC fixing heater HT 2  turns on when power is supplied in a state in which both the fixing relay for safety RL 21  and a gate of the transistor TR 21  are turned ON. 
   In a state in which the control signal S 9  to be supplied to the fixing relay for safety RL 21  is turned ON, the control unit  202  turns ON/OFF the control signal S 8  to be supplied to the gate of the transistor TR 21  of the heater ON/OFF circuit  220  to control turning on/off of the AC fixing heater HT 2 . 
     FIG. 8  is a schematic of the capacitor charger  203  shown in  FIG. 6 . The capacitor charger  203  includes a noise filter (NF)  211  that removes noise of an AC voltage to be inputted, an inrush prevention circuit  212  for preventing an inrush current, a diode bridge DB that rectifies AC power PS to be inputted via the inrush prevention circuit  212 , a capacitor CP 100  that smoothes the rectified AC voltage, an FET control unit  213  that controls switching of an FET  214  to control a charging operation of the capacitor CP 1  (see  FIG. 6 ), an FET  214  that turns ON/OFF a transformer T 100 ; a transformer T 100  that boosts an input voltage, a rectifying and smoothing circuit  215  that rectifies and smoothes an output on a secondary side of the transformer T 100  to convert the output into a DC output, a constant current detection unit  126  that detects a current, a constant voltage detection unit  217  that detects a voltage, an overvoltage detection unit  218  that detects an overvoltage to prevent the overvoltage from being applied to the capacitor CP 1 , a diode D 100  for preventing a reverse current from the capacitor CP 1 , and an insulator  219 . 
   An AC voltage inputted from the AC power supply PS is rectified by the diode bridge DB via the inrush current prevention circuit after noise is removed from the AC voltage by the noise filter NF. A DC voltage obtained by smoothing the AC voltage with the capacitor CP 1  is inputted to a primary side of the transformer T 100 . The FET control unit  213  starts switching control for the FET  214  to charge the capacitor CP 1  when the control signal A 1  inputted from the control unit  202  (see  FIG. 5 ) is turned ON. The FET control unit  203  subjects the FET  214  to switching control based on respective detection signals inputted from the current detection unit  216 , the voltage detection unit  217 , and the overvoltage detection unit  218  to perform constant current control, constant voltage control, or constant power control for charging the capacitor CP 1 . In general, it is desirable to charge the capacitor CP 1  at a constant current. However, a charging time can be reduced by charging the capacitor CP 1  according to the constant power control. 
   The transformer T 100  is turned ON/OFF by the FET  214  to boost a primary side input thereof and outputs the input from the secondary side. The rectifying and smoothing circuit  215  rectifies and smoothes the secondary side output of the transformer T 100 , and the secondary side output is outputted to the capacitor CP 1  via the diode D 100 . The current detection unit  216 , the voltage detection unit  217 , and the overvoltage detection unit  218  detects a current, a voltage, and an overvoltage at the secondary side output of the transformer T 100  after the rectification and smoothing. Respective detection signals of the current, the voltage, and the overvoltage are inputted to the FET control unit  213 . 
     FIG. 9  is a schematic of the capacitor charge and discharge circuit  208  shown in  FIG. 6 . The capacitor charge and discharge circuit  208  includes: a charging and discharging switch  231 ; a fixing relay for safety RL 11 ; a diode D 11  for preventing counter-electromotive force in the fixing relay RL 11 ; and a both-end voltage detection circuit  232  that detects a voltage across the capacitor CP 1 . 
   The charging and discharging switch  231  and the fixing relay for safety RL 11  are connected to both the ends of the capacitor CP 1 . The charging and discharging switch  231  is turned ON/OFF according to the control signal S 3  inputted from the control unit  202 . Similarly, the fixing relay for safety RL 11  is turned ON/OFF according to the control signal S 4  inputted from the control unit  202 . 
   When both the charging and discharging switch  231  and the fixing relay for safety RL 11  are turned ON, charge accumulated in the capacitor CP 1  is discharged, and power is supplied to the fixing heater HT 1 . 
   The both-end voltage detection circuit  232  detects a voltage across the capacitor CP 1  and outputs a voltage signal S 5  of the voltage to the control unit  202 . The control unit  202  always monitors this voltage signal S 5  to monitor a charging state of the capacitor CP 1 . 
     FIG. 10  is a schematic of the control unit  202  shown in  FIG. 6 . The control unit  202  includes a CPU  241 , a memory  242 , and the like. 
   The CPU  241  is connected to a memory  242  for storing programs and data for controlling the image forming apparatus and controls the image forming apparatus and the power supply circuit  200  based on the programs stored in the memory  242 . 
   The voltage signal (analog signal) S 5  that represents the voltage across the capacitor CP 1  detected by the both-end voltage detection circuit  232  of the capacitor charge and discharge circuit  208 , the detection signal (analog signal) S 6  that is divided according to resistances of the thermistor TH 11  for detecting a surface temperature of the fixing roller  151  and the resistor R 41 , and the detection current (analog signal) S 7  that is obtained by detecting an input current of the image forming apparatus in the input current detection circuit  206 , and the like are inputted to the CPU  241 . 
   The CPU  241  outputs, via an IO port, the control signal S 1  for turning ON/OFF charging for the capacitor CP 1 , the control signal S 3  for turning ON/OF the charging and discharging switch  231 , the control signal S 4  for turning ON/OFF the fixing relay for safety RL 11 , the control signal S 8  for turning ON/OFF the heat ON/OFF circuit  220 , the control signal s 9  for turning ON/OFF the fixing relay for safety R 21 , and the like. 
   In addition, the CPU  241  is constituted to control the operation unit  150  and monitors an input of the KEY  163  provided on the operation unit  150 . A DRV  243  is a driver that drives an LCD  11 , and a DRV  244  is a driver that drives an LED  162 . The CPU  241  controls and drives the DRV  243  and the DRV  244 . 
     FIG. 11  is a timing chart for explaining temperature control of the fixing roller  151 : (a) shows a surface temperature T of the fixing roller  151 ; (b) shows a detection current (input current) I detected by the input current detection circuit  206 ; (c) shows timing of ON/OFF of the AC fixing heater HT 2 ; and (d) shows timing of ON/OFF of the fixing heater HT 1 . 
   A period t 1  indicates a warm-up period of the image forming apparatus (fixing roller  151 ), a period t 2  indicates a standby period of the image forming apparatus, and periods t 3  to t 6  indicate periods after a copy operation is started. 
   In the period t 1 , the control unit  202  raises a surface temperature of the fixing roller  151  to a predetermined temperature Tt. Usually, the control unit  202  supplies power to the AC fixing heater HT 2  serving as a heat generating member from an AC power supply to heat the fixing roller  151  as shown in (a). In this period t 1 , although the AC fixing heater HT 2  is at full duty, since a copy operation is not performed, the input current I is equal to or less than a maximum input current Imax as shown in (b). 
   When the surface temperature of the fixing roller  151  detected by the thermistor TH 11  reaches a target temperature Tt that is temperature allowing a copy operation, the control unit  202  stops power supply to the AC fixing heater HT 2 , the warm-up period t 1  ends, and the image forming apparatus shifts to a copy standby state (t 2  period). 
   In the period t 2 , the control unit  202  turns ON the AC fixing heater HT 2  when the surface temperature of the fixing roller  151  falls to be lower than the target temperature Tt and turns OFF the AC fixing heater HT 2  when the surface temperature reaches the target temperature Tt while monitoring the surface temperature. The control unit  202  repeats the turning ON/OFF of the AC fixing heater HT 2 . In the period t 2 , even if the surface temperature of the fixing roller  151  has reached the target temperature Tt, when a copy operation is started in a state in which the periphery of the fixing unit  121  is not warmed sufficiently, as in the period t 3 , the surface temperature of the fixing roller  151  may fall immediately after the copy operation is started even in a state in which the AC fixing heater HT 2  is on. Actually, a copy operation is possible even if the surface temperature of the fixing roller  151  falls to be lower than the target temperature Tt. However, since a fixing property cannot be secured if the surface temperature of the fixing roller  151  falls to be lower than Tmin, the copy operation has to be stopped. 
   During the copy operation in the period t 3 , it is likely that the input current I of the DC power generation circuit  204  increases following an increase in a load current on a DC power supply side, and as shown in (b), an input current of the entire apparatus also increases, power consumption of the entire apparatus also increases, and the input current I reaches the maximum input current Imax. The input current I should not exceed this maximum input current Imax from a viewpoint of apparatus specifications. Thus, in the period t 3 , a turning-on rate of the AC fixing heater HT 2  cannot be further increased as shown in (c). Therefore, as in the period t 4 , the control unit  202  discharges and supplies power charged in the capacitor (CP 1 ) in advance to the fixing heater HT 1 , which is provided separately from the AC fixing heater HT 2 , to thereby raise the surface temperature of the fixing roller  151  such that the surface temperature of the fixing roller  151  does not fall to be lower than Tmin. 
   In period t 4 , as in the period t 2 , the control unit  202  discharges the capacitor CP 1  as shown in  FIG. 11D  to supply power to the fixing heater HT 1 . When the surface temperature of the fixing roller  151  reaches the target temperature Tt as shown in (a), the control unit  202  stops the discharge. 
   In the period t 5 , even if the discharge and supply of power from the capacitor CP 1  is stopped, when the surface temperature of the fixing roller  151  can be maintained at the target temperature Tt only by the AC fixing heater HT 2  as in the period t 6 , the control unit  202  controls ON/OFF of the AC fixing heater HT 2  to thereby perform temperature control for the fixing roller  151 . 
     FIG. 12  is a flowchart of a process procedure for ON/OFF control for the fixing heater HT 1  and the AC fixing heater HT 2 . The ON/OFF control for the fixing heater TH 1  and the AC fixing heater TH 2  by the control unit  202  will be described with reference to  FIG. 12 . 
   The control unit  202  detects a surface temperature of the fixing roller  151  according to the detection signal S 6  inputted from the temperature detection thermistor TH 11  and judges whether the surface temperature of the fixing roller  151  is equal to or higher than the target temperature Tt (step S 1 ). As a result of this judgment, if the surface temperature of the fixing roller  151  is equal to or higher than the target temperature Tt (“Yes” in step S 1 ), the control unit  202  turns OFF the control signals S 3  and S 4 , which are outputted to the capacitor charge and discharge circuit  208 , to turn OFF the fixing heater HT 1  (step S 2 ), and turns OFF the control signals S 8  and S 9 , which are outputted to the AC heater drive circuit  205 , to turn OFF the AC fixing heater HT 2  (step S 3 ). 
   On the other hand, if the surface temperature of the fixing roller  151  is not equal to or higher than the target temperature Tt (“No” in step S 1 ), first, the control unit  202  judges whether the AC fixing heater HT 2  is ON (step S 4 ). As a result of this judgment, if the AC fixing heater HT 2  is not ON (“No” in step S 4 ), the control unit  202  turns ON the control signals S 8  and S 9 , which are outputted to the AC heater drive circuit  205 , to turn ON the AC fixing heater HT 2  and control the AC fixing heater HT 2  such that the surface temperature of the fixing roller  151  reaches the target temperature Tt (step S 6 ). In addition, if the AC fixing heater HT 2  is ON (“Yes” in step S 4 ), the control unit  202  turns ON the control signals S 8  and S 9 , which are outputted to the capacitor charge and discharge circuit  208 , to turn ON the fixing heater HT 1  before the surface temperature of the fixing roller  151  falls to be lower than Tmin (step S 5 ). 
   A method of reducing a copy job time will be described with reference to  FIG. 13 . As described above, the control unit  202  assumes the number of documents set in the ADF  10  based on a detection signal for the slit encoder  110  to be inputted from the slit sensor  111 . 
   The control unit  202  calculates a copy time (job completing time) T 1  in the respective operation modes (the high-speed mode and the low-speed mode) based on the estimated number of documents and the number set by the operation unit  150 . A time required for printing one sheet in the respective modes (the high-speed mode and the low-speed mode) varies depending upon a size of transfer paper, a type of transfer paper (thickness of transfer paper such as thick paper or plain paper, etc.), selection of enlargement/reduction printing, setting of simplex/duplex printing or the like, a time period in which printing is performed, temperature of a fixing roller (high-speed printing is impossible at the time of low temperature), and the like. 
   The control unit calculates (predicts) a job completing time in the high-speed mode (time for completing a job at high speed) T 1 HIGH and a job completing time in the low-speed mode (time for completing a job at high speed) T 1 LOW according to following expressions (1) and (2), respectively.
 
 T   1 HIGH= T   2 HIGH+{( nd×ns )−1 }×T   3 HIGH  (1)
 
 T   1 LOW= T   2 LOW+{( nd×ns )−1 }×T   3 LOW  (2)
 
where, nd is number of documents, ns is set number of sheets, T 2 HIGH is a first copy time in the high-speed mode, T 3 HIGH is a time required for printing per one sheet in the high-speed mode, T 2 LOW is a first copy time in the low-speed mode, and T 3 LOW is a time required for printing per one sheet in the low-speed mode.
 
   In the expressions (1) and (2), in a first copy (copy of a first sheet), a time for reading a document and a time until first transfer paper is fed are required, and a turnaround time is different from that in copying second and subsequent sheets. Thus, the job completing time is calculated by adding a copy time for the second and subsequent sheets {(nd×ns)−1}×T 3  to a first copy time T 2 . 
   The control unit  202  calculates (predicts) a charging time that is required when the capacitor charger  203  charges the capacitor CP 1 . Following expression (3) represents an example of an expression for calculating a charging time Tc that is required when the capacitor charger  203  charges the capacitor CP 1  at a constant current
 
 Tc=Cx ( V   1 − V   2 )/ I  [second]  (3)
 
where, Tc is a charging time, C is an electrostatic capacitance of the capacitor, V 2  is a target charging voltage, V 2  is a present charging voltage, and I is a charging current.
 
   Following expression (4) represents an example of an expression for calculating the charging time Tc that is required when the capacitor charger  203  charges the capacitor CP 1  at constant power
 
 Tc=Cx ( V   h **2− V   i **2)/2 W  [second]  (4)
 
where, Tc is a charging time, C is an electrostatic capacitance of the capacitor, Vh is charging completion power, Vi is a charging start voltage, and W is charging power.
 
   When the operation unit  150  instructs execution of a copy job, the control unit  202  calculates job completing times for the copy job in the high-speed mode and the low-speed mode, and calculates a charging time required for charging the capacitor CP 1  to a target charging voltage. Then, the control unit  202  controls an operation for charging the capacitor CP 1  and an operation for executing the copy job based on the calculated job completing times for the copy job in the high-speed mode and the low-speed mode and the calculated charging time for the capacitor CP 1  such that a turnaround time of the copy job is reduced. 
   Here, the explanation is on the premise that printing in the low-speed mode (e.g., 25 cpm (copy/min)) is possible even if the capacitor CP 1  is not charged sufficiently, and printing in the high-speed mode (e.g., 50 cpm (copy/min)) is impossible unless the capacitor CP 1  is charged sufficiently. In addition, it is considered that, as an example, the capacitor CP 1  is in a fully charged state when a charging voltage is 44 volts, and a threshold charging voltage decided in advance (target charging voltage) is 32 volts. 
     FIG. 13  is a flowchart of a process procedure for a copy job in the control unit  202 . The control for a copy job by the control unit  202  will be explained with reference to  FIG. 13 . When the operation unit  150  requests copying (step S 1301 ), the control unit  202  checks a charging voltage at the capacitor CP 1  (step S 1302 ) and judges whether the charging voltage is equal to or higher than the threshold charging voltage decided in advance (e.g., 32 volts) (step S 1303 ). Here, if the charging voltage at the time of the print operation request is lower than the threshold charging voltage, as shown in  FIG. 11 , it is likely that the capacitor CP 1  having a small charging capacitance cannot supply power to the fixing heater HT 1  sufficiently at the time when temperature of the fixing roller  151  falls, and fixing property defect occurs. 
   If the charging voltage at the capacitor CP 1  is equal to or higher than the threshold charging voltage (“Yes” in step S 1303 ), the control unit  202  executes the copy job in the high-speed mode (step S 1304 ). Then, the control unit  202  continues the copy job in the high-speed mode until the copy job is completed (step S 1305 ). 
   On the other hand, if the charging voltage at the capacitor CP 1  is not equal to or higher than the threshold charging voltage (“No” in step S 1303 ), the control unit  202  calculates the charging time Tc (step S 1306 ). Then, the control unit  202  calculates the job completing time T 1 LOW in the low-speed mode and the job completing time T 1 HIGH in the high-speed mode according to the expressions (1) and (2) (step S 1307 ). 
   Then, the control unit  202  judges whether Tc+T 1 HIGH is smaller than T 1 LOW (step S 1308 ). Here, the control unit  202  judges in which of the following cases a turnaround time is shorter: a case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1  or a case in which the control unit  202  executes the copy job in the low-speed mode without charging the capacitor CP 1 . For example, when a large amount of copying is performed, a copy job may be completed faster if the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1 . 
   If Tc+T 1 HIGH is smaller than T 1 LOW (“Yes” in step S 1308 ), the control unit  202  charges the capacitor CP 1  until the charging voltage at the capacitor CP 1  reaches the full charging voltage (e.g., 44 volts) (step S 1309 ). Thereafter, the control unit  202  executes the copy job in the high-speed mode (step S 1310 ). Then, the control unit  202  continues the copy job until the copy job is completed (step S 1311 ). Note that, in this embodiment, the control unit  202  charges the capacitor CP 1  until the charging voltage at the capacitor CP 1  reaches the full charging voltage (e.g., 44 volts) in step S 1309 . However, the control unit  202  may control the copy job in the high-speed mode to be started before the charging voltage reaches the full charging voltage, for example, at a point when the charging voltage has reached the threshold voltage (e.g., 32 volts). 
   On the other hand, if Tc+T 1 HIGH is not smaller than T 1 LOW (“No” in step S 1308 ), the control unit  202  executes the copy job in the low-speed mode without charging the capacitor CP 1  (step S 1312 ). Then, the control unit  202  continues the copy job in the low-speed mode until the copy job is completed (step S 1313 ). 
     FIG. 14  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at a capacitor CP 1  when Tc+T 1 HIGH is smaller than T 1 LOW.  FIG. 15  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at the capacitor CP 1  when Tc+T 1 HIGH is not smaller than T 1 LOW. The horizontal axis represents an elapsed time from start of the job, and the vertical axis represents a completion state of the job (e.g., the number of copied sheets). 
   The graph A shown n  FIG. 14  indicates a case in which the control unit  202  executes the copy job in the high-speed mode when it is judged in step S 1303  that the charging voltage at the capacitor CP 1  is equal to or higher than the threshold voltage 32 volts. The graph B indicates a case in which the control unit  202  executes the copy job in the low-speed mode, and a graph C indicates a case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1 . In the example shown in  FIG. 14 , the job completing time in the case in which the control unit  202  executes the copy job in the low-speed mode from the beginning (graph B) (T 1 LOW) is longer than the job completing time in the case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1  (graph C) (Tc+T 1 HIGH). Thus, the control unit  202  judges in step S 1308  that Tc+T 1 HIGH is smaller than T 1 LOW and controls the copy job to be executed in the high-speed mode after charging the capacitor CP 1 . 
   On the other hand, in the example shown in  FIG. 15 , the job completing time in the case in which the control unit  202  executes the copy job in the low-speed mode from the beginning (graph B) (T 1 LOW) is shorter than the job completing time in the case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1  (graph C) (Tc+T 1 HIGH). Thus, the control unit  202  judges in step S 1308  that Tc+T 1 HIGH is not smaller than T 1 LOW and controls the copy job to be executed in the low-speed mode from the beginning. 
   According to the first embodiment, the control unit  202  calculates a charging time for charging the capacitor CP 1  to a predetermined charging voltage and also calculates a turnaround time of a set copy job, and then controls execution of a charging operation for the capacitor CP 1  and execution of the copy job based on the calculated charging time and the calculated turnaround time of the copy job such that the turnaround time of the copy job is minimized. Thus, when the capacitor CP 1  is used as a power supply source for a fixing unit, even in a state in which the capacitor CP 1  is not charged sufficiently, a completion time for the copy job can be minimized under the constraints. 
   Furthermore, according to the first embodiment, the image forming apparatus has the high-speed mode and the low-speed mode, and the control unit  202  calculates a turnaround time of a copy job set for each of the modes, executes the copy job in the high-speed mode after charging the capacitor CP 1  when a sum of a charging time and the turnaround time of the copy job in the high-speed mode is smaller than the turnaround time of the copy job in the low-speed mode, and executes the copy job in the low-speed mode without charging the capacitor CP 1  when the sum is not smaller than the turnaround time of the copy job in the low-speed mode. Thus, the control unit  202  can execute the copy job in one of the high-speed mode after charging the capacitor CP 1  or the low-speed mode without charging the capacitor CP 1  that requires a shorter time. This makes it possible to further reduce a time for the copy job. 
   Moreover, according to the first embodiment, the control unit  202  estimates the number of documents set in the ADF  10  and calculates a turnaround time of a copy job based on the estimated number of documents. This makes it possible to calculate the turnaround time of the copy job with a simple method. 
   The image forming apparatus uses the electric double layer capacitor as an auxiliary power supply. This makes it possible to reduce running cost for the image forming apparatus. 
   In the image forming apparatus according to the first embodiment, when copying is requested and a charging voltage at the capacitor CP 1  is equal to or higher than a threshold voltage decided in advance, the control unit  202  executes a job in the high-speed mode until the job is completed. However, it is likely that, even if the control unit  202  executes a job in the high-speed mode from the beginning, fixing property defect occurs if a charging voltage at the capacitor CP 1  falls to the threshold voltage during the execution of the job and power is not supplied from the capacitor CP 1  to the fixing heater HT 1  at the time when temperature of the fixing roller  151  falls. 
   Therefore, in the image forming apparatus according to a second embodiment of the present invention, a job completing time is further minimized by, when a voltage at the capacitor CP 1  has decreased to a threshold voltage while the job is executed in the high-speed mode, switching the high-speed mode to the low-speed mode. 
   A structure of the image forming apparatus according to the second embodiment is the same as that in the first embodiment. 
     FIG. 16  is a flowchart of a process procedure for a copy job in the control unit  202  of an image forming apparatus according to the second embodiment. The control for the copy job in the control unit  202  will be explained with reference to  FIG. 16 . When the operation unit  150  requests copying (step S 1601 ), the control unit  202  checks a charging voltage at the capacitor CP 1  (step S 1602 ) and judges whether the charging voltage is equal to or higher than a threshold charging voltage decided in advance (e.g., 32 volts) (step S 1603 ). 
   Processing at the time when the charging voltage at the capacitor CP 1  is lower than the threshold charging voltage (steps S 1609  to S 1604  and S 1616 ) is performed in the same manner as the processing for copy job control in the image forming apparatus according to the first embodiment (steps S 1306  to S 1311  and S 1313 ). 
   On the other hand, in step S 1603 , if the charging voltage at the capacitor CP 1  is equal to or higher than the threshold charging voltage (“Yes” in step S 1603 ), the control unit  202  executes the copy job in the high-speed mode (step S 1604 ). While the copy job is executed, the control unit  202  checks the charging voltage at the capacitor CP 1  every fixed time (step S 1605 ) and judges whether the charging voltage has decreased to the threshold charging voltage decided in advance (e.g., 32 volts) (step S 1606 ). Then, if the charging voltage has not decreased to the threshold charging voltage (“No” in step S 1606 ), the control unit  202  continues to execute the copy job in the high-speed mode. 
   On the other hand, in step S 1606 , if the charging voltage at the capacitor CP 1  has decreased to the threshold charging voltage (“Yes” in step S 1606 ), since it is likely that fixing property defect occurs unless power is supplied from the capacitor CP 1  to the fixing heater HT 1  at the time when temperature of the fixing roller  151  falls, the control unit switches the high-speed mode to the low-speed mode to execute the copy job (step S 1607 ). Then, the control unit  202  continues the copy job until the copy job is completed (step S 1608 ). 
     FIG. 17  is a graph for illustrating control for a copy job and a completion state of the job in the second embodiment. The horizontal axis represents an elapsed time from start of the job, and the vertical axis represents a completion state of the job (e.g., the number of copied sheets). 
   The graph A indicates a case in which the control unit  202  executes the copy job in the high-speed mode when it is judged in step S 1603  that the charging voltage at the capacitor CP 1  is equal to or higher than the threshold voltage 32 volts. The graph B indicates a case in which the control unit  202  executes the copy job in the low-speed mode, and a graph C indicates a case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1 . 
   As shown in the example of  FIG. 17 , even if it is judged in step S 1603  that the charging voltage at the capacitor CP 1  is equal to or higher than the threshold voltage 32 volts, at a point when the charging voltage at the capacitor CP 1  has decreased to the threshold charging voltage (32 volts) (step S 1606 ) while the control unit  202  is executing the job in the high-speed mode, the control unit  202  switches the high-speed mode to the low-speed mode to execute the copy job. 
   In this way, in the image forming apparatus according to the second embodiment, the control unit  202  performs job control to switch the high-speed mode to the low-speed mode at a point when the charging voltage at the capacitor CP 1  has decreased to the threshold charging voltage while the control unit  202  is executing the job. Thus, when the capacitor CP 1  is used as a power supply source for a fixing unit, even if the capacitor CP 1  is charged sufficiently and the charging voltage decreases gradually, a completing time for a copy job can be further minimized by controlling the job more appropriately. 
   In the image forming apparatus according to the first and the second embodiments, when copying is requested and a charging voltage at the capacitor CP 1  is lower than a threshold voltage decided in advance, the control unit  202  predicts a job completing time in the low-speed mode, a charging time, and a job completing time in the high-speed mode to control the job such that the job completing time is minimized. However, it is likely that, even if the control unit  202  executes a job in the high-speed mode after charging the capacitor CP 1 , fixing property defect occurs if a charging voltage at the capacitor CP 1  falls to the threshold voltage during the execution of the job and power is not supplied from the capacitor CP 1  to the fixing heater HT 1  at the time when temperature of the fixing roller  151  falls. 
   Therefore, in the image forming apparatus according to a third embodiment of the present invention, a job completing time is further minimized by, when the control unit  202  executes a job in the high-speed mode after charging the capacitor CP 1 , predicting a job completing time at the time when a charging voltage at the capacitor CP 1  falls to the threshold charging voltage during execution of the job and the control unit  202  switches the high-speed mode to the low-speed mode to execute the job. 
   A structure of the image forming apparatus according to the third embodiment is the same as that in the first embodiment. 
     FIG. 18  is a flowchart of a process procedure for a copy job in the control unit  202  of an image forming apparatus according to the third embodiment. The control for the copy job in the control unit  202  will be explained with reference to  FIG. 18 . When the operation unit  150  requests copying (step S 1801 ), the control unit  202  checks a charging voltage at the capacitor CP 1  (step S 1802 ) and judges whether the charging voltage is equal to or higher than a threshold charging voltage decided in advance (e.g., 32 volts) (step S 1803 ). 
   Then, if the charging voltage at the capacitor CP 1  is equal to or higher than the threshold charging voltage (“Yes” in step S 1803 ), the control unit  202  executes the copy job in the high-speed mode (step S 1804 ). Since control processing of the job in this step and subsequent steps (steps S 1804  to S 1808 ) is the same as the job control processing of the image forming apparatus according to the second embodiment (steps S 1604  to S 1608 ), the control processing won&#39;t be explained again. 
   On the other hand, if the charging voltage at the capacitor CP 1  is not equal to or higher than the threshold charging voltage (“N” in step S 1803 ), the control unit  202  calculates a charging time Tc (step S 1809 ). Here, the charging time Tc is calculated according to the expression (3) or (4) as in the first embodiment. 
   Subsequently, the control unit  202  calculates a job completing time T 1 LOW in the low-speed mode, a job execution time T 1 HIGH 2  from start of job execution in the high-speed mode until a charging voltage at the capacitor CP 1  falls to the threshold charging voltage (e.g., 32 volts), and a completing time T 1 LOW 2  from start of a job in the low-speed mode in a state in which a charging voltage at the capacitor CP 1  is the threshold charging voltage until completion of the job (step S 1810 ). 
   Here, T 1 LOW is calculated according to the expression (2) as in the first embodiment. T 1 HIGH 2  and T 1 LOW are calculated according to following expressions (5) and (6), respectively.
 
 T   1 HIGH 2 = nc/sh  ( CPM )  (5)
 
 T   1 LOW 2 =( ns−nc )/ sl  ( CPM )  (6)
 
where n c  is predicted number of copied sheets up to a point when the charging voltage at the capacitor CP 1  falls to the threshold charging voltage, sh is speed at the time of the high-speed mode, ns is set number of sheets, and sl is speed at the time of the low-speed mode.
 
   Note that the predicted number of copied sheets up to a point when the charging voltage at the capacitor CP 1  falls to the threshold charging voltage only has to be found in advance by, for example, a method of executing a job in the high-speed mode plural times and calculating an average of the number of copied sheets in the plural times of execution of the job. In addition, the job is executed plural times because the number of copied sheets changes depending on conditions such as an environment in which the image forming apparatus is used, a type of paper, and a type of an image pattern or the like. 
   Then, the control unit  202  judges whether Tc+T 1 HIGH 2 +T 1 LOW 2  is smaller than T 1 LOW (step s 1811 ). Here, the control unit  202  judges in which of the following cases a turnaround time is short: a case in which the control unit  202  executes a copy job in the high-speed mode after charging the capacitor CP 1  and switches the high-speed mode to the low-speed mode to execute the job at a point when the charging voltage at the capacitor CP 1  has fallen to the threshold charging voltage, and a case in which the control unit  202  executes the copy job in the low-speed mode without charging the capacitor CP 1 . 
   If Tc+T 1 HIGH 2 +T 1 LOW 2  is smaller than T 1 LOW (“Yes” in step S 1811 , the control unit  202  charges the capacitor CP 1  until the charging voltage at the capacitor CP 1  reaches a full charging voltage (e.g., 44 volts) (step S 1812 ), and then executes the copy job in the high-speed mode (step S 1813 ). During the execution of the copy job, the control unit  202  checks the charging voltage at the capacitor CP 1  every fixed time (step S 1814 ) and judges whether the charging voltage has decreased to the threshold charging voltage decided in advance (e.g., 32 volts) (step S 1815 ). Then, if the charging voltage has not decreased to the threshold charging voltage (“No” in step S 1815 ), the control unit  202  continues to execute the copy job in the high-speed mode. 
   On the other hand, if the charging voltage at the capacitor CP 1  has decreased to the threshold charging voltage (“Yes” in step S 1815 ), it is likely that fixing property defect occurs unless power is supplied form the capacitor CP 1  to the fixing heater HT 1  at the time when temperature of the fixing roller  151  falls. Thus, the control unit  202  switches the high-speed mode to the low-speed mode to execute the copy job (step S 1816 ). Then, the control unit  202  continues the copy job until the copy job is completed (step S 1817 ). 
   On the other hand, if Tc+T 1 HIGH 2 +T 1 LOW 2  is not smaller than T 1 LOW (“N” in step S 1811 ), the control unit  202  executes the copy job in the low-speed mode without charging the capacitor CP 1  (step S 1818 ). Then, the control unit  202  continues the copy job in the low-speed mode until the copy job is completed (step S 1819 ). 
     FIG. 19  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at the capacitor CP 1  when Tc+T 1 HIGH 2 +T 1 LOW 2  is smaller than T 1 LOW.  FIG. 20  is a graph for illustrating control for a copy job and a completion state of the job according to a state of a charging voltage at the capacitor CP 1  when Tc+T 1 HIGH 2 +T 1 LOW 2  is not smaller than T 1 LOW. The horizontal axis represents an elapsed time from start of the job, and the vertical axis represents a completion state of the job (e.g., the number of copied sheets). 
   The graph A shown in  FIG. 19  indicates a case in which the control unit  202  executes the copy job in the high-speed mode when it is judged in step S 1803  that the charging voltage at the capacitor CP 1  is equal to or higher than the threshold voltage 32 volts. The graph B indicates a case in which the control unit  202  executes the copy job in the low-speed mode. A graph C indicates a case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1  and switches the high-speed mode to execute the job at a point when the charging voltage at the capacitor CP 1  has fallen to the threshold charging voltage. In the example shown in  FIG. 19 , the job completing time in the case in which the control unit  202  executes the copy job in the low-speed mode from the beginning (graph B) (T 1 LOW) is longer than the job completing time in the case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1  and switches the high-speed mode to execute the job at a point when the charging voltage at the capacitor CP 1  has fallen to the threshold charging voltage (graph C) (Tc+T 1 HIGH 2 +T 1 LOW 2 ). Thus, the control unit  202  judges in step S 1811  that Tc+T 1 HIGH 2 +T 1 LOW 2  is smaller than T 1 LOW and controls the copy job to be executed in the high-speed mode after charging the capacitor CP 1 . 
   On the other hand, in the example shown in  FIG. 20 , the job completing time in the case in which the control unit  202  executes the copy job in the low-speed mode from the beginning (graph B) (T 1 LOW) is shorter than the job completing time in the case in which the control unit  202  executes the copy job in the high-speed mode after charging the capacitor CP 1  and switches the high-speed mode to execute the job at a point when the charging voltage at the capacitor CP 1  has fallen to the threshold charging voltage (graph C) (Tc+T 1 HIGH 2 +T 1 LOW 2 ). Thus, the control unit  202  judges in step S 1811  that Tc+T 1 HIGH 2 +T 1 LOW 2  is not smaller than T 1 LOW and controls the copy job to be executed in the low-speed mode from the beginning. 
   In this way, in the image forming apparatus according to the third embodiment, when the control unit  202  executes a job in the high-speed mode after charging the capacitor CP 1 , the control unit  202  performs job control to switch the high-speed mode to the low-speed mode at a point when the charging voltage at the capacitor CP 1  has decreased to the threshold charging voltage while the control unit  202  is executing the job. Thus, when the capacitor CP 1  is used as a power supply source for a fixing unit, even if the capacitor CP 1  is charged sufficiently and the charging voltage decreases gradually, a completing time for a copy job can be further minimized by controlling the job more appropriately. 
   Note that, although the image forming apparatuses according to the first to the third embodiments have the high-speed mode and the low-speed mode as operation modes, the image forming apparatuses may have modes of three stages, namely, a high-speed mode, a medium-speed mode, and a low-speed mode. 
   In the first to the third embodiments, the reduction of a turnaround time of a copy job at the time when the copy function of the image formation apparatus in  FIG. 1  is used is explained. However, it is also possible to reduce a turnaround time of a printer job when the printing function of the image forming apparatus is used. In this case, the image forming apparatus interprets a print instruction inputted from an external personal computer or the like, calculates the number of pages of print data and the number of sheets to be printed, and calculates a turnaround time of the printer job according to the following expression: the number of pages×the number of sheets to be printed×print time per one sheet. Since other operations are the same as those in the first embodiment, the operations won&#39;t be explained again. 
   Note that, in the second and the third embodiments, a predetermined voltage, with which a charging voltage at the capacitor CP 1  is checked and compared in the beginning of job control, and a threshold voltage, with which the charging voltage is checked and compared during execution of a job, are set to be an identical value. However, the threshold voltage may be set to a different value. 
   In addition, the present invention is not limited to the above-mentioned embodiments but may be modified and executed appropriately as long as such a modification does not depart from the scope of the present invention. 
   Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.