Patent Publication Number: US-8989610-B2

Title: Image forming apparatus with power factor improvement section

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus provided with a power supply device having a power factor improvement section. 
     2. Description of the Related Art 
     Recently, an image forming apparatus is required to enhance the printing speed and reduce time from turning on of a commercial power supply to start of image formation, and an electric power of a heater deployed in a power supply device and a heat fixing device has been increased. In general, an input current supplied from a commercial power supply to the image forming apparatus has an upper limit of something like 15 A (ampere) in Japan, and particularly an image forming apparatus provided with a high-power power supply device and a high-power heater is required to be designed so as not to exceed this upper limit. 
     In order to satisfy the above requirement, there has been well known an image forming apparatus having a constitution in which electric power is effectively utilized by adding a power factor improvement section to a power supply device. Especially, the power supply device is provided with two DC/CD converters which are a DC/DC convertor supplying electric power mainly to a driving device and a DC/DC convertor supplying electric power mainly to a control device, and in many cases, the power factor improvement section is added only to the former DC/DC converter having a large supply power. As such a power factor improvement section used in a high-power power supply device, the pressure-rising type is often generally used. 
     However, the power factor improvement section has problems such as heat generation and reduction in efficiency due to switching loss and generation of noise, and it is preferable to operate while stopping switching of the power factor improvement section as much as possible. In order to address those problems, in Japanese Patent No. 3466351, for example, there is disclosed a constitution in which the switching of the power factor improvement section is stopped when an image forming apparatus is in a standby state. Further, in Japanese Patent Application Laid-Open No. 2007-101667, there is disclosed a constitution in which when a value of current flowing to a DC/DC converter which supplies electric power to a driving device and a control device is not more than a predetermined value, the switching of the power factor improvement section is stopped. Furthermore, in Japanese Patent Application Laid-Open No. H04-087565, there is disclosed a constitution in which the power factor improvement section is bypassed by a short circuit. 
     However, the above patent documents have the following problems. For example, the power factor improvement section disclosed in the Japanese Patent No. 3466351 always performs switching during a printing operation of the image forming apparatus, and there are effects of reduction of heat generation and improvement of the efficiency only when the image forming apparatus is in the standby state. In the first place, in consideration of variation in a commercial power supply voltage and a heater resistance, in order to suppress a value of current supplied from a commercial power supply to not more than a standard of current of 15 A under a condition in which the current value of the image forming apparatus is maximum, the power factor improvement section is provided in the image forming apparatus. Thus, the power factor improvement section is rarely required, and the power factor improvement section is required only during warm up at the time of turning on of the power supply of the image forming apparatus and during a period of time from several seconds to several ten seconds from start of printing at most, and the power factor improvement section may not be required according to the voltage value of the commercial power supply and the heater resistance of a fixing device. 
     In the constitutions disclosed in the Japanese Patent Application Laid-Opens Nos. 2007-101667 and H04-087565, although the load of the DC/DC converter is significantly different between the printing state and the standby state, a variation in the load of the DC/DC converter is small in the same operating state. Thus, in the printing state in which the load undergoes a transition while remaining large, the switching of the power factor improvement section can be hardly stopped. Accordingly, it is considered that it is less suitable to use the value of the current flowing to the DC/DC converter as a threshold value when whether or not the switching of the power factor improvement section is stopped is judged. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems, and provides an image forming apparatus in which an operation period of a power factor improvement section is made appropriate. 
     Another object of the present invention is to provide an image forming apparatus having a fixing section which heats and fixes an unfixed image, formed on a recording material, to a recording material, a power supply section which has a rectification section rectifying alternating current, a power factor improvement section receiving current output from the rectification section, and a DC/DC converter DC/DC converting current output from the power factor improvement section, a current detection section which detects current flowing to the heater, and a control section which controls operation of the power factor improvement section according to current detected by the current detection section. 
     Still another object of the present invention is to provide an image forming apparatus having a fixing section which has a heater and heats and fixes an unfixed image, formed on a recording material, to the recording material, a power supply section which has a rectification section rectifying alternating current, a power factor improvement section receiving current output from the rectification section, a DC/DC converter DC/DC converting current output from the power factor improvement section, and a bypassing switch connected in parallel to the power factor improvement section, a current detection section which detects current flowing to the heater, and a control section which controls the bypassing switch according to current detected by the current detection section. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an image forming apparatus in examples 1 and 2. 
         FIG. 2  is a schematic diagram showing a power supply device and a heater control section in the examples 1 and 2. 
         FIGS. 3A ,  3 B and  3 C are views for explaining phase control in the examples 1 and 2. 
         FIG. 4  is a circuit diagram of the power supply device in the embodiment 1. 
         FIGS. 5A ,  5 B,  5 C,  5 D,  5 E,  5 F,  5 G and  5 H are views for explaining a difference in apparent current according to presence of a power factor improvement section in the examples 1 and 2. 
         FIG. 6  is a view for explaining a relationship between the apparent current and a commercial power supply voltage in the examples 1 and 2. 
         FIG. 7  is a flow chart showing a processing sequence of on/off control of the power factor improvement section in the embodiment 1. 
         FIG. 8  is a view showing a change of heater current of the image forming apparatus in the examples 1 and 2. 
         FIG. 9  is a circuit diagram of the power supply device in the embodiment 2. 
         FIG. 10  is a flow chart showing a processing sequence of on/off control of the power factor improvement section in the embodiment 2. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
     Embodiment 1 
     (1) Image Forming Apparatus 
       FIG. 1  shows a schematic configuration diagram of a color image forming apparatus of this example. In the color image forming apparatus of this example, an electrophotographic system is used, and toner images with four colors of yellow (Y), magenta (M), cyan (C), and black (K) are superimposed, whereby a full-color image is formed. An image forming apparatus  100  is constituted of a sheet feeding section  121 , photosensitive drums  122  (Y, M, C, and K and hereinafter the description thereof is omitted), a charge sleeve  123 , a toner container  125 , a developing sleeve  126 , an intermediate transfer belt  127 , a transfer roller  128 , and a heat fixing device  130 . The photosensitive drum  122 , the charge sleeve  123 , the toner container  125 , and the developing sleeve  126  are collected in one container for each color of Y, M, C and K as an all-in-one cartridge  101 . 
     In the all-in-one cartridge  101  of each color, a light beam is irradiated onto the photosensitive drum  122 , charged by the charge sleeve  123 , from a scanner section  124  based on an exposure time converted by an image processing section (not shown), and an electrostatic latent image is formed on the photosensitive drum  122 . The developing sleeve  126  develops the electrostatic latent image with toner from the toner container  125  to form a monochrome toner image on the photosensitive drum  122 , and, thus, to superimpose four color toner images on the intermediate transfer belt  127 , whereby a multicolor toner image is formed. 
     A recording sheet  111  is fed from the sheet feeding section  121  by a feed roller  112  and conveyed along a conveying path  118  while being held by conveying rollers  113 ,  114 , and  115 . Then, the recording sheet  111  is sandwiched between the intermediate transfer belt  127  formed with the multicolor toner image and the transfer roller  128  and pressurized, so that the multicolor toner image on the intermediate transfer belt  127  is transferred to the recording sheet  111 . Toner remaining on the intermediate transfer belt  127  without being transferred to the recording sheet  111  is cleaned by the cleaner  129 , and cleaned waste toner is accumulated in a cleaner container  132 . 
     The recording sheet  111  transferred with the toner image is further conveyed along the conveying path  118 , and the toner image is fixed onto the recording sheet  111  by a heat fixing device  130 . The heat fixing device  130  of this example uses a film heat method and is constituted of a heater  136 , a fixing film (endless belt)  134 , a pressure roller  133 , a thermistor  135 , and so on. The pressure roller  133  is rotated and driven at a predetermined peripheral velocity by a fixing drive motor (not shown). By the rotational driving of the pressure roller  133 , the rotational force is directly applied to the fixing film  134  by a frictional force between the pressure roller  133  and an outer surface of the fixing film  134 , the fixing film  134  is rotated and driven while being in press contact and sliding with the heater  136 . The thermistor  135  is pressed against a rear surface of the heater  136  by a predetermined pressure and detects the temperature of the rear surface of the heater  136 . 
     The rotation of the fixing film  134  according to the rotation of the pressure roller  133  is stabilized, and when the heater  136  is in such a state that the temperature is increased to a predetermined temperature, the recording sheet  111  transferred with a toner image is conveyed to a nip portion formed by the fixing film  134  and the pressure roller  133 . The conveyed recording sheet  111  is conveyed while being pressurized in the nip portion, whereby the heat of the heater  136  is applied to the recording sheet  111  through the fixing film  134 , and the toner image is heat-fixed to the recording sheet  111 . After that the recording sheet  111  to which the toner image is heat-fixed passes through a discharge roller  137  and is discharged onto a discharge tray  131 . 
     Electric power required for executing the above-described image forming process is supplied to each section of the image forming apparatus  100  by a power supply device  138  receiving a supply of electric power from a commercial power supply  140  through an AC cable  139 . The details of the power supply device  138  will be described later. 
     (2) Power Supply Control to Heater 
     The power supply control the heater  136  in the heat fixing device  130  will be described using  FIG. 2 . FIG.  2  is a schematic diagram showing the power supply device  138  receiving the supply of electric power from the commercial power supply  140  in the image forming apparatus  100  and a heater control section of the heat fixing device  130 . A power supply line from the commercial power supply  140  is separated into two power supply lines, one to the heater  136  and the other to each load of other than the heater  136  (a driver system load  203   a  and a control system load  203   b ) through the power supply device  138 . 
     The heater  136  receives a supply of electric power from the commercial power supply  140  through a current transformer  205 , a relay  207 , a bidirectional three-terminal thyristor (hereinafter referred to as a triac)  209 . A thermoswitch  211  is disposed so as to be in contact with or adjacent to the heater  136  and used as a protection element which cuts off the power supply line from the commercial power supply  140  when the temperature of the heater  136  is abnormally high. A temperature fuse may be used as a protective element instead of the thermoswitch  211 . The triac  209  is an element for controlling a power supply/cutting off of the power supply to the heater  136 , and on/off control of the triac  209  is performed by phase control to be described later through a triac driver section  210 . 
     A zero-cross detection section  204  monitors a voltage of the commercial power supply  140  to detect a timing when the voltage passes through 0 V (zero-cross point), and, thus, to output a zero-cross signal to an engine controller  212 . A fixing current detection section  206  detects a value of current supplied to the heater  136  through the current transformer  205  and outputs a detection signal to the engine controller  212 . The thermistor  135  detects the temperature of the heater  136 . The engine controller  212  performs drive control of the relay  207  through a relay driver section  208  based on detection signals from the zero-cross detection section  204  and the fixing current detection section  206 , the temperature detected by the thermistor  135 , and so on. Further, the engine controller  212  performs control of the image forming operation of the image forming apparatus  100 , such as the on/off control of the triac  209 , through the triac driver section  210 . The engine controller  212  has ROM and RAM (not shown). The ROM holds a control program and data executed by the engine controller  212 , and the RAM is used for the control program executed by the engine controller  212  to temporally hold information. 
     In this example, the electric power is supplied to the heater  136  by the phase control. The phase control is a method of decomposing one half wave of the commercial power supply  140  into a plurality of waves as shown in  FIG. 3A  to turn on the triac  209  at a predetermined phase angle (hereinafter referred to as a “power feeding phase angle”), and, thus, to control the power supply to the heater  136 . A method of synchronizing with a voltage phase of the commercial power supply  140  is performed using the zero-cross signal output when the voltage of 0 V is detected by the zero-cross detection section  204 . 
       FIG. 3B  is a graph showing a relationship between the electric power supplied to the  136  and the power feeding phase angle. In  FIG. 3B , the vertical axis shows the electric power supplied to the heater  136  that is proportional to the square of the current value, and the horizontal axis shows the power feeding phase angle. From the waveform of  FIG. 3B , it can be shown that as the power feeding phase angle approaches nearer 0°, the electric power supplied to the heater  136  becomes large, and whereas, as the power feeding phase angle approaches nearer 180°, the electric power supplied to the heater  136  becomes small. Particularly, when the power feeding phase angle is 0°, the maximum electric power is supplied to the heater  136 , and when the power feeding phase angle is 180°, the electric power supplied to the heater  136  is zero. In  FIG. 3B , the upper waveform chart shows a relationship between the supplied electric power and the power feeding phase angle when the resistance value of the heater  136  is small or when the voltage of the commercial power supply  140  is large, and the lower waveform chart shows the relationship between the supplied electric power and the power feeding phase angle when the resistance value of the heater is large or when the voltage of the commercial power supply is small. From the two waveform charts, it can be shown that the larger the heater resistance, or the smaller the commercial power supply voltage, the smaller the electric power injected into the heater  136 , and whereas, the smaller the heater resistance, or the larger the commercial power supply voltage, the larger the electric power injected into the heater  136 . 
       FIG. 3C  shows an example of a power supply pattern during the phase control, and the power supply patterns in the cases where the power feeding phase angle is 90 degrees, 61 degrees, and 119 degrees are shown from the left side. In  FIG. 3C , a hatched portion shows that the electric power is injected, and a non-hatched portion shows that the electric power is not injected. 
     (3) Power Supply Device 
     The power supply device  138  which supplies the electric power to each section of the image forming apparatus  100  will be described using  FIG. 4 .  FIG. 4  is a schematic circuit diagram of the power supply device  138 . As shown in  FIG. 4 , the power supply device  138  shown in  FIG. 2  is constituted of a driving power supply device  431  supplying the electric power to the driver system load  203   a  and a controlling power supply device  432  supplying the electric power to the control system load  203   b . The driving power supply device  431  is constituted of a rectification section  421 , a power factor improvement section  422 , and a forward system DC/DC converter (direct current to direct current converter)  423  and outputs a direct voltage Vcc 1 . Meanwhile, the controlling power supply device  432  is constituted of a rectification smoothing section  424  and a DC/DC converter  425  and outputs a direct voltage Vcc 2 . A heater control section  433  of  FIG. 4  is constituted of the engine controller  212 , the zero-cross detection section  204 , the fixing current detection section  206 , the relay driver section  208 , the triac driver section  210 , the current transformer  205 , and the thermoswitch  211  shown in  FIG. 2 , and so on. 
     In the driving power supply device  431 , alternating current supplied from the commercial power supply  140  is first rectified by a rectifying diode  401  in the rectification section  421 , and a rectified direct current is input to the power factor improvement section  422 . The power factor improvement section  422  is constituted of a choke coil  402 , an FET (field-effect transistor)  403 , a diode  404 , a smoothing capacitor  405 , and a power factor improvement control section  441 . The power factor improvement control section  441  inputs a pulse signal (PWM signal) that controls turning on/off of the FET  403  to a gate terminal of the FET  403  based on the output of the diode  404  so that an input current waveform is close to a sine wave and duty-controls the FET  403 . Hereinafter, a state in which the power factor improvement control section  441  of the power factor improvement section  422  duty-controls the FET  403  by the on instruction of power factor control from the engine controller  212  is expressed as “a state in which the power factor improvement section  422  is turned on”. Meanwhile, a control state in which the power factor improvement control section  441  of the power factor improvement section  422  places the FET  403  in the turned-off state by the off instruction of the power factor control from the engine controller  212  is expressed as “a state in which the power factor improvement section  422  is turned off”. The DC/DC converter  423  is constituted of an FET  406 , a trans  407 , a rectifying diode  408 , a free-wheel diode  409 , a choke coil  410 , a capacitor  411 , and a Vcc 1  control section  442 . A primary winding wire and a secondary winding wire are wound around the trans  407 , and one terminal of the primary winding wire is connected to the power factor improvement section  422 , and the other terminal is connected to a drain terminal of the FET  406 . The secondary winding wire side of the trans  407  is constituted of the rectifying diode  408 , the free-wheel diode  409 , the choke coil  410 , the capacitor  411 , and so on and outputs the voltage Vcc 1 . The FET  406  is turned on/off by applying a pulse signal from the Vcc 1  control section  442  to a gate terminal. The Vcc 1  control section  442  controls the duty ratio of the pulse signal, whereby the DC/DC converter  423  outputs the stable voltage Vcc 1 . 
     Meanwhile, in the controlling power supply device  432 , the alternating current supplied from the commercial power supply  140  is rectified and smoothed by the rectification smoothing section  424  constituted of a rectifying diode  412  and a capacitor  413  and input to the DC/DC converter  425 . The DC/DC converter  425  is constituted of an FET  414 , a trans  415 , a rectifying diode  416 , a capacitor  417 , and a Vcc 2  control section  443 . One terminal of a primary winding wire of the trans  415  is directly connected to the output side of the rectifying diode  412  of the rectification smoothing section  424 , and the other terminal is connected to a drain terminal of the FET  414 . The secondary winding wire side of the trans  415  is constituted of the rectifying diode  416 , the capacitor  417 , and so on and outputs the voltage Vcc 2 . The Vcc 2  control section  443  duty-controls a pulse signal, which is input to a gate terminal of the FET  414  and controls the turning on/off of the FET  414 , in order to output the stable voltage Vcc 2 . 
     When the FET  406  of the DC/DC converter  423  is duty-controlled by the Vcc 1  control section  442  in such a state that the power factor improvement section  422  is turned on, the voltage Vcc 1  is output in such a state that the power factor of the current input to the rectification section  421  is approximately 1. Meanwhile, even though the FET  406  of the DC/DC converter  423  is duty-controlled by the Vcc 1  control section  442  in such a state that the power factor improvement section  422  is turned off, although the voltage Vcc 1  is output, the power factor of the current input to the rectification section  421  is not enhanced. 
     Next, the effect of adding the power factor improvement section  422  between the rectification section  421  and the DC/DC converter  423  will be described using a specific example shown in  FIGS. 5A to 5H . Regarding the load (electric power) during printing in the image forming apparatus  100  of this example, the load of the driving power supply device  431  is 300 W, the load of the controlling power supply device  432  is 80 W, and the load of the heater  136  is 1100 W. Further, the voltage of the commercial power supply  140  is set to 110 V. In the above conditions, the waveforms of the current flowing to each load when the image forming apparatus  100  performs the printing operation in such a state that the power factor improvement section  422  is turned off are shown in  FIGS. 5A ,  5 B, and  5 C.  FIG. 5A  shows the waveform of the current flowing to the driving power supply device  431 ,  FIG. 5B  shows the waveform of the current flowing to the controlling power supply device  432 , and  FIG. 5C  shows the waveform of the current flowing to the heater  136 . In  FIGS. 5A to 5H , the vertical axis shows a current value (unit: A), and the horizontal axis shows time (unit: msec).  FIG. 5D  shows the waveform of a total current obtained by adding the currents shown in  FIGS. 5A to 5C , that is, the total current flowing to the image forming apparatus  100 . 
     In  FIGS. 5A to 5C , an effective current value is calculated by dividing each load (electric power) of the driving power supply device  431 , the controlling power supply device  432 , and the heater  136  by a voltage of 110 V of the commercial power supply  140 , and an apparent current value is calculated based on each waveform charts. In  FIGS. 5A to 5C , the power factor is calculated by dividing the effective current value by the apparent current value. The effective current value of  FIG. 5D  is a total of the effective current values of  FIGS. 5A to 5C , the apparent current value is calculated based on the waveform chart, and the power factor is calculated by diving the effective current value by the apparent current value. As shown in  FIGS. 5A and 5B , the power factors of the driving power supply device  431  and the controlling power supply device  432  are low, such as approximately 0.61, and about 2 A is a reactive current in total. Although the heater  136  is a resistance load, since the engine controller  212  phase controls the power supply from the commercial power supply  140  to the heater  136 , the power factor is slightly reduced, such as not 1 but 0.93 as shown in  FIG. 5C . As shown in  FIG. 5D , when the currents flowing to all the loads are summed, the power factor is 0.89, and it can be shown that about 1.6 A (=15.07 A−13.45 A) that is a difference obtained by subtracting the effective current value from the apparent current value is a reactive current. 
     Meanwhile,  FIGS. 5E ,  5 F,  5 G, and  5 H are views showing a current waveform flowing to each load when the printing operation is performed in such a state that the power factor improvement section  422  added to the driving power supply device  431  is turned on and a waveform of a total current flowing to the image forming apparatus  100 .  FIG. 5E  is a view showing the waveform of current flowing to the driving power supply device  431 ,  FIG. 5F  is a view showing the waveform of current flowing to the controlling power supply device  432 , and  FIG. 5G  is a view showing the waveform of current flowing to the heater  136 . In  FIGS. 5A to 5H , the vertical axis shows the current value (unit: A), and the horizontal axis shows time (unit: msec).  FIG. 5H  shows the waveform of the total current obtained by adding the currents shown in  FIGS. 5E to 5G , that is, the total current flowing to the image forming apparatus  100 .  FIGS. 5E ,  5 F,  5 G, and  5 H correspond to  FIGS. 5A ,  5 B,  5 C, and  5 D, respectively, and since  FIGS. 5F and 5B  and  FIGS. 5G and 5C  are the currents flowing to a circuit without the power factor improvement section  422 , the same waveform is shown. Since the methods of calculating the effective current value, the apparent current value, and the power factor in  FIGS. 5E ,  5 F,  5 G, and  5 H are similar to those in the  FIGS. 5A ,  5 B,  5 C, and  5 D, description thereof will be omitted. 
       FIG. 5E  is a waveform chart showing the state in which the power factor improvement section  422  is turned on, and the power factor of the driving power supply device  431  is improved to 1 from 0.61 in  FIG. 5D  showing the state in which the power factor improvement section  422  is turned off. From  FIG. 5H , it can be shown that when all loads are summed, the power factor is enhanced to be 0.95, the reactive current value is 0.66 A (=14.11 A−13.45 A) and is reduced by about 1 A in comparison with  FIG. 5D  showing the state in which the power factor improvement section  422  is turned off. From this fact, it can be shown that in order to satisfy the standard of current of 15 A, provision of the power factor improvement section  422  is considerably effective. The reason that the power factor improvement section  422  is added to the driving power supply device  431  is that the load of the driving power supply device  431  is larger than the load of the controlling power supply device  432 , and a larger power factor improvement effect is obtained. 
     Next, the condition that the current flowing to the image forming apparatus  100  is maximum will be described using  FIG. 6 .  FIG. 6  is a view showing a relationship between the voltage of the commercial power supply  140  and the value of the apparent current flowing to the image forming apparatus  100 . In  FIG. 6 , the vertical axis shows a value of the apparent current (unit: A), and the horizontal axis shows the commercial power supply voltage (unit: V). In  FIG. 6 , the solid waveform shows the state in which the power factor improvement (PFC: Power Factor Correction) section  422  is turned on, and the dashed waveform shows the relationship between the commercial power supply voltage and the apparent current in the state in which the power factor improvement section  422  is turned off. As described above, regarding the load of the image forming apparatus  100  during printing, the load of the driving power supply device  431  is 300 W, the load of the controlling power supply device  432  is 80 W, and the load of the heater  136  is 1100 W. The heater resistance is set to 9.56Ω. 
     From  FIG. 6 , it can be shown that there is a point at which the apparent current is maximum near the commercial power supply  140  of 100 V regardless of the turned on/off state of the power factor improvement section  422 . 
     (4) Fixing Current Detection Section 
     The current supplied to the heater  136  is voltage-converted by the trans  205  shown in  FIG. 2 , converted into an effective value in the fixing current detection section  206 , and input as an analog signal to the engine controller  212 . The engine controller  212  performs the power supply control to the heater  136  based on the current value to the heater  136  converted from the input analog signal to a digital signal so that the current value does not exceed a rated current of 15 A of the commercial power supply  140 . 
     Since the current value output in the fixing current detection section  206  is an integrated value corresponding to a half period of a power supply frequency of a square waveform, the current value depends on the frequency, and the frequency of a power supply is required to be performed at the same time. In this example, the frequency of the power supply is calculated from an interval time at the falling of a zero-cross signal pulse detected by the zero-cross detection section  204 . The current detection timing is time corresponding to one period of the power supply. The fixing current detection section  206  is used as a protection circuit (not shown) which cuts off connection of the relay  207  when an abnormal current flows to the heater  136 . 
     (5) On/Off Control of Power Factor Improvement Section 
     Since the power factor improvement section  422  described above has problems such as heat generation and reduction in efficiency due to the switching loss of the FET  403  and generation of noise, it is preferable to hold the power factor improvement section  422  in the turned-off state as much as possible. Thus, the engine controller  212  performs control in which the power factor improvement section  422  is turned on when the current value detected by the fixing current detection section  206  is more than a predetermined value, and the power factor improvement section  422  is turned off when the current value is less than the predetermined value. The load of the heater  136  accounts for a large percentage of all loads of the image forming apparatus  100 . For example, in an image forming apparatus corresponding to A3 color with approximately 30 ppm (page per minutes), as described in “(3) Power supply device”, in comparison with the fact that the load of the power supply device  138  is approximately 380 W, the load of the heater  136  is approximately 1100 W. Moreover, in comparison with the power supply device  138 , the load of the heater  136  is always significantly varied. Thus, as the threshold value of the current used for judging turning on/off of the power factor improvement section  422 , it is suitable to use not the current value of the current flowing to the DC/DC converter  423  but the current value of the current flowing to the heater  136 . 
     Hereinafter, the on/off control of the power factor improvement section  422  will be described based on the current value detected by the fixing current detection section  206 , using the flow chart of  FIG. 7 . The processing shown in  FIG. 7  is executed by the engine controller  212  based on a control program stored in the ROM (not shown) of the engine controller  212 . The processing of the flow chart in the subsequent example is executed by the engine controller  212  as in the processing shown in  FIG. 7 . 
       FIG. 7  is a flow chart showing a processing sequence of the on/off control of the power factor improvement section  422  activated when the power supply of the image forming apparatus is turned on. First, when the power supply of the image forming apparatus  100  is turned on, in step  601  (hereinafter referred to as S 601 ), the engine controller  212  writes 0 in variables n and IF as memories provided in the RAM (not shown) of the engine controller  212 . The variable IF is a memory which stores the latest current value detected by the fixing current detection section  206 , and the current value is updated for each detection of the current value, that is, each one period of the commercial power supply  140 . In the processing of S 605  to be described later, the variable n is used as a memory which stores the number of times the current value detected by the fixing current detection section  206  is less than Ilimit 1  being a first threshold value. In S 602 , in preparation for warm-up of the image forming apparatus  100 , the engine controller  212  instructs the turned on state of the power factor improvement section  422  to the power factor improvement control section  441  so that the power factor improvement control section  441  performs duty control of the FET  403  based on the output of the diode  404 . 
     In S 603 , the engine controller  212  judges whether the detection signal of the current value to the heater  136  detected by the fixing current detection section  206  is input. In the engine controller  212 , when the detection signal is input, the operation proceeds to S 604 , and when detection signal is not input, the processing in S 603  is repeated. As described above, the timing at which the detection signal is input from the fixing current detection section  206  to the engine controller  212  is for each one period of the power supply. In S 604 , the engine controller  212  writes the current value detected in S 603  in the variables IF and updates the memory content of the variable IF. In S 605 , the engine controller  212  judges whether the current value stored in the variable IF is less than the threshold value Ilimit 1 , and when the current value stored in the variable IF is less than the threshold value Ilimit 1 , the operation proceeds to S 606 , or otherwise the operation proceeds to S 613 . The value of the threshold value Ilimit 1  is set so that the value of the current supplied from the commercial power supply  140  to the image forming apparatus  100  does not exceed the standard of current of 15 A (ampere) even in such a state that the engine controller  212  places the power factor improvement section  422  in the turned off state. Namely, when the current value detected by the fixing current detection section  206  is less than the threshold value Ilimit 1  (less than a first threshold value), the standard of current of 15 A of the commercial power supply can be satisfied even in the state in which the power factor improvement section  422  is turned off. In this example, the maximum current value assigned to the heater is set to 10 A so that the value of current supplied from the commercial power supply to the image forming apparatus does not exceed the maximum value of 15 A of the standard of current in such a state that the current value is the threshold value Ilimit 1 , that is, the operation of the power factor improvement section is stopped. 
     In S 606 , the engine controller  212  adds 1 as a stored value to the variable n and updates the value. In S 607 , the engine controller  212  judges whether the value of the variable n is more than a constant N. When the value of the variable n is more than the constant N, the operation proceeds to S 608 , and when the value of the variable n is not more than the constant N, the operation returns to S 603 . The constant N will be described later. In S 613 , the engine controller  212  writes 0 in the variable n, and the operation proceeds to S 603 . 
     In S 608 , the engine controller  212  places the power factor improvement section  422  in the turned off state and instructs the power factor improvement control section  441  to prevent the power factor improvement control section  441  from duty-controlling the FET  403 . In S 609 , the engine controller  212  writes 0 in the variable n and resets the value. In S 610 , the engine controller  212  judges whether the detection signal of the current value to the heater  136  detected by the fixing current detection section  206  is input. In the engine controller  212 , when the detection signal is input, the operation proceeds to S 611 , and when the detection signal is not input, the processing of S 610  is repeated. In S 611 , the engine controller  212  writes the current value detected in S 610  in the variable IF and updates the memory contents of the variable IF. In S 612 , the engine controller  212  judges whether the current value stored in the variable IF is not less than the threshold value Ilimit 1 . When the current value stored in the variable IF is not less than the threshold value Ilimit 1  (not less than a first threshold value), the operation proceeds to S 602 , and otherwise the operation returns to S 610 . 
     In the flow chart of  FIG. 7 , the variable n and the constant N are provided in order to prevent malfunctions of a circuit of the fixing current detection section  206 . Due to the malfunctions of the fixing current detection section  206 , the current value less than the threshold value is detected, and when the engine controller  212  immediately turns off the power factor improvement section  422 , the current flowing to the image forming apparatus  100  may exceed the standard of current value of 15 A. Thus, when the engine controller  212  instructs the power factor improvement control section  441  to place the power factor improvement section  422  in the turned off state, the instruction is delayed by the update time of the fixed current value IF. Namely, when the current values detected by the fixing current detection section  206  are less than the threshold value Ilimit 1  N times in a row, the power factor improvement section  422  is placed in the turned off state, and therefore, in order to prevent the malfunctions, a guard time corresponding to time obtained by multiplying one period of the commercial power supply  140  by the constant N is provided. 
     In accordance with the above-mentioned control flow of  FIG. 7 , a state in which the turned on/off state of the power factor improvement section  422  changes with the passage of the time when the power supply of the image forming apparatus  100  is actually turned on to perform the printing operation is shown in  FIG. 8 . In  FIG. 8 , the horizontal axis shows time, and the vertical axis shows the current value (the value of current flowing to the heater  136 ) detected by the fixing current detection section  206 . First, when the power supply of the image forming apparatus  100  is turned on, the warm-up operation of the image forming apparatus  100  is started to rapidly increase the heater temperature, and therefore, a large current more than the threshold value Ilimit 1  flows to the heater  136 . While the current more than the threshold value Ilimit 1  flows to the heater  136 , the engine controller  212  holds the power factor improvement section  422  in the turned on state. When the warm-up operation of the image forming apparatus  100  is terminated, the image forming apparatus  100  enters into the standby state. The current supplied from the commercial power supply  140  to the heater  136  is significantly reduced, and the current value is less than the threshold value Ilimit 1 ; therefore, the engine controller  212  places the power factor improvement section  422  in the turned off state. At this time, due to the reason as above, when the value of current supplied to the heater  136  is less than the threshold value Ilimit 1 , the engine controller  212  does not immediately place the power factor improvement section  422  in the turned off state but places the power factor improvement section  422  in the turned off state after a lapse of T×N time as a predetermined time. T represents the time of one period of the commercial power supply  140 , and N represents the constant described in  FIG. 7 . 
     Subsequently, when the image forming apparatus  100  receives a printing operation signal to start printing in the image forming apparatus  100 , the value of the current supplied to the heater  136  exceeds the threshold value Ilimit 1  again, and the engine controller  212  places the power factor improvement section  422  in the turned on state. At this time, when the value of the current supplied to the heater  136  is more than the threshold value Ilimit 1 , the engine controller  212  immediately places the power factor improvement section  422  in the turned on state. When the printing operation is continued, heat is gradually accumulated in the heat fixing device  130 , and the electric power supplied to the heater  136  is reduced. Consequently, the electric power supplied to the heater  136  is reduced, and when the value of the current supplied to the heater  136  detected for each one period of the commercial power supply  140  is less than the threshold value Ilimit 1  N times in a row, the engine controller  212  places the power factor improvement section  422  in the turned off state. 
     As described above, according to this example, the switching loss of the power factor improvement section can be suppressed. Especially, in this example, even during the image forming operation, the operation of the power factor improvement section is stopped in many times, whereby the switching loss of the power factor improvement section can be minimized while satisfying the standard of current of 15 A of the commercial power supply. 
     Embodiment 2 
     In the embodiment 1, the turning on/off of the power factor improvement section is controlled based on the value of current flowing to the heater, whereby the switching loss in the power factor improvement section can be suppressed. In the embodiment 2, a bypassing switch connected in parallel to the power factor improvement section is provided, whereby loss in the elements constituting the power factor improvement section is improved. Since the image forming apparatus, the control of the electric power supply to the heater, and the constitution of the fixing current detection section  206  in this example are the same as those in the embodiment 1, the descriptions thereof are omitted, and portions different from the embodiment 1 will be described hereinafter. 
     (1) Power Supply 
       FIG. 9  is a view showing a power supply device  138  of an image forming apparatus  100  of this example. In comparison with  FIG. 4  of the embodiment 1, the circuit configuration of  FIG. 9  is similar to that of  FIG. 4  with the exception of assigning different reference numerals to each element and adding a bypassing switch  934 , and therefore, only different points will be described hereinafter. 
     A driving power supply device  931  is constituted of a rectification section  921 , a power factor improvement section  922 , the bypassing switch  934 , and a DC/DC converter  923  and outputs a voltage Vcc 1 . The bypassing switch  934  is connected in parallel to the power factor improvement section  922  and turned on/off by a control signal (not shown) from the engine controller  212 . When the bypassing switch  934  is turned on, the current rectified by the rectification section  921  flows toward the bypassing switch  934  having a low impedance and is then input to the DC/DC converter  923  not through the power factor improvement section  922 . Meanwhile, when the bypassing switch  934  is turned off, the current rectified by the rectification section  921  flows toward the power factor improvement section  922 , and an output of the power factor improvement section  922  is then input to the DC/DC converter  923 . When the FET  906  of the DC/DC converter  923  is duty-controlled in such a state that the bypassing switch  934  is turned off, the voltage Vcc 1  is output in such a state that the power factor of the current input to the rectification section  921  is approximately 1. Meanwhile, When the FET  906  of the DC/DC converter  923  is duty-controlled in such a state that the bypassing switch  934  is turned on, although the voltage Vcc 1  is output, the power factor of the current input to the rectification section  921  is not enhanced. 
     The description of the effect obtained by adding the power factor improvement section  922  and the description of the condition that the current flowing to the image forming apparatus  100  is maximum are omitted because the contents are overlapped with the contents described in the embodiment 1. 
     (2) On/Off Control of Power Factor Improvement Section 
     Since the power factor improvement section  922  has problems such as heat generation and reduction in efficiency due to switching loss of the FET  903  and generation of noise, it is preferable to operate the DC/DC converter  923  not through the power factor improvement section  922  as much as possible. Further, regarding the loss generated in the power factor improvement section  922 , not only the switching loss in the FET  903  but also loss in the choke coil  902  and the diode  904  cannot be ignored. Accordingly, in order to achieve the above object, it is suitable to bypass the choke coil  902  and the diode  904  of the power factor improvement section  922  not only by stopping the switching of the FET  903  but also by turning on the bypassing switch  934 . Thus, in this example, when the current value of the current supplied to the heater  136  detected by the fixing current detection section  206  is more than a predetermined value, the bypassing switch  934  is turned off. At the same time, the power factor improvement section  922  is placed in the turned on state, and the FET  903  is duty-controlled. Meanwhile, when the current value of the current supplied to the heater  136  detected by the fixing current detection section  206  is less than a predetermined value, a control in which the bypassing switch  934  is turned on and the power factor improvement section  922  is bypassed is performed. 
     As the threshold value used for judging the turning on/off of the bypassing switch  934 , the current flowing the heater  136  is more suitably used than the current flowing to the DC/DC converter  923 , and the reason is as described in the embodiment 1. 
     Hereinafter, the on/off control of the bypassing switch  934  will be described based on the current value detected by the fixing current detection section  206 , using the flow chart of  FIG. 10 .  FIG. 10  is a flow chart showing a processing sequence of the on/off control of the bypassing switch  934  activated when the power supply of the image forming apparatus is turned on. First, when the power supply of the image forming apparatus  100  is turned on, the processing of S 1001  is executed. Since the processing of S 1001  is the same as the processing of S 601  of  FIG. 7 , the description here is omitted. The processing sequence in  FIG. 10  one-to-one corresponds to the processing sequence in  FIG. 7 , and the description of the subsequent processing in  FIG. 10  the same as the processing in  FIG. 7  is omitted. 
     In S 1002 , in preparation for warm-up of the image forming apparatus  100 , the engine controller  212  turns off the bypassing switch  934  and, at the same time, instructs the turned on state of the power factor improvement section  922  to the power factor improvement control section  941 , whereby the power factor improvement control section  941  performs duty control of the FET  903  based on an output of the diode  904 , and an output current from a rectification section  901  is input to the DC/DC converter  923  through the power factor improvement section  922 . Since the processing of S 1003  and S 1004  are the same as the processing of S 603  and S 604  in  FIG. 7 , the description thereof is omitted. 
     In S 1005 , the engine controller  212  judges whether the current value stored in the variable IF is less than a second threshold value Ilimit 2 , and when the current value stored in the variable IF is less than the threshold value Ilimit 2 , the operation proceeds to S 1006 , and otherwise the operation proceeds to S 1013 . The value of the threshold value Ilimit 2  is set so that the value of the current supplied from the commercial power supply  140  to the image forming apparatus  100  does not exceed the standard of current of 15 A even in such a state that the bypassing switch  934  is turned on and, at the same time, the power factor improvement section  922  is in the turned off state. Namely, when the current value detected by the fixing current detection section  206  is less than the second threshold value Ilimit 2  (less than the second threshold value), the standard of current of 15 A of the power supply can be satisfied even in such a state that the bypassing switch  934  is turned on and, at the same time, the power factor improvement section  922  is in the turned off state. Since the processing of S 1006 , S 1007 , and S 1013  are the same as the processing of S 603 , S 607 , and S 613  in  FIG. 7 , the description thereof is omitted. 
     In S 1008 , the engine controller  212  turns off the bypassing switch  934  and, at the same time, instructs the turned off state of the power factor improvement section  922  to a power factor improvement control section  941 . Consequently, the duty control of the FET  903  performed by the power factor improvement control section  941  is stopped, and the output current from the rectification section  901  is input to the DC/DC converter  923  through the bypassing switch  934 . Since the processing of S 1009 , S 1010 , and S 1011  are the same as the processing of S 609 , S 610 , and S 611 , the description thereof is omitted. In S 1009 , the engine controller  212  writes 0 in the variable n and resets the value. In S 1012 , the engine controller  212  judges whether the current value stored in the variable IF is not less than the threshold value Ilimit  2 , and when the current value stored in the variable IF is not less than the threshold value Ilimit  2  (not less than the second threshold value), the operation proceeds to S 1002 , and otherwise the operation returns to S 1010 . 
     As described above, according to this example, the switching loss of the power factor improvement section can be suppressed. Especially, in this example, even during the image forming operation, the power factor improvement section is bypassed by a bypassing switch in many times, whereby in addition to the effect in the embodiment 1, the loss in the choke coil and the diode can be minimized. 
     This application claims the benefit of Japanese Patent Application No. 2011-248778, filed Nov. 14, 2011, which is hereby incorporated by reference herein in its entirety.