Patent Publication Number: US-8530800-B2

Title: Heater control device, image forming apparatus, and computer program product

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2009-212610 filed in Japan on Sep. 15, 2009, Japanese Patent Application No. 2009-213792 filed in Japan on Sep. 15, 2009 and Japanese Patent Application No. 2010-203744 filed in Japan on Sep. 10, 2010. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heater control device for controlling turn-on of a heater, an image forming apparatus, and a computer program product. 
     2. Description of the Related Art 
     As a fixing heater used for an electrophotographic image forming apparatus, a halogen heater is widely used. The halogen heater among heaters has a characteristic that inrush current easily occurs particularly at a low temperature, and consumed current is continuously large. Therefore, a voltage drop occurs at a commercial power supply in synchronization with a turn-on timing of the heater, which causes a lighting device such as a fluorescent light to flicker. 
     There is known a technology for controlling a turn-on pattern on a half-wave cycle basis in a period of 10 half-wavelengths (100 ms) close to a frequency caused to flicker so that turn-on control of the heater is prevented from switching at about 10 Hz band with respect to a flickering frequency band (8.8 to 10 Hz) to which human eyes are sensitive, or so that a frequency component becomes as small as possible (see, e.g., Japanese Patent No. 3316170). This deals with problems such as flicker and control of a harmonic current and of a noise terminal voltage. 
     Moreover, as a purpose of preventing the inrush current, there is proposed a technology for introducing phase control (soft start) that a heater is turned on for only a part of a half-wavelength right before the high-frequency turn-on pattern and the on-time is made gradually longer (see, e.g., Japanese Patent Application Laid-open No. 2004-212510). 
     However, in the heater control using the turn-on pattern on the half-wave cycle basis in the period of 10 half-wavelengths, a difference occurs in a flicker level caused by a turn-on duty (turn-on ratio), and it is understood that frequency characteristics are worse in low duty (around 10 to 30%) as compared with that in medium duty (around 40 to 60%) and that the flicker level tends to be bad. 
     Therefore, there is a problem that inrush current caused by a very short turn-off period (a few 10 msec) during the turn-on cannot be prevented and thus improvement of the flicker level is not expected so much in the low duty in which the turn-off period continues for several half waves or more. Moreover, there is a problem that the number of turn-ons is small in the low duty and the frequency component cannot be improved (frequency cannot be made higher). There is also a problem that when the phase control is used during the turn-on, the characteristics of the harmonic current and the noise terminal voltage may be extremely worsened. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, there is provided a heater control device that includes a temperature detector that detects a temperature of a heated object heated by a heater; an alternating-current power supply for applying an alternating current voltage to the heater; a turn-on ratio decision unit that determines a turn-on ratio of the heater based on the temperature and a target temperature; a turn-on pattern decision unit that determines a partial turn-on pattern, as a turn-on pattern of the heater, which is a pattern of a turn-on ratio higher than determined turn-on ratio in terms of a control period, and to which a partial turn-on instead of a full turn-on is allocated on a half-wavelength basis of the alternating current voltage within the control period, based on the turn-on ratio of the heater; and a turn-on controller that controls turn-on of the heater based on determined turn-on pattern. 
     According to another aspect of the present invention, there is provided an image forming apparatus that includes a fixing unit that includes a heater, and a temperature detector for detecting a temperature of a heated object heated by the heater; an alternating-current power supply for applying an alternating current voltage to the heater; a turn-on ratio decision unit that determines a turn-on ratio of the heater based on the temperature and a target temperature; a turn-on pattern decision unit that determines a partial turn-on pattern, as a turn-on pattern of the heater, which is a pattern of a turn-on ratio higher than determined turn-on ratio in terms of a control-period, and to which a partial turn-on instead of a full turn-on is allocated on a half-wavelength basis of the alternating current voltage within the control period, based on the turn-on ratio of the heater; and a turn-on controller that controls turn-on of the heater based on determined turn-on pattern. 
     According to still another aspect of the present invention, there is provided a computer program product that includes a computer-readable recording medium containing instructions for controlling a heater. The instructions, when executed by a computer, cause the computer to perform determining a turn-on ratio of the heater based on a temperature of a heated object heated by the heater and a target temperature; determining a partial turn-on pattern, as a turn-on pattern of the heater, which is a pattern of a turn-on ratio higher than determined turn-on ratio in terms of a control-period, and to which a partial turn-on instead of a full turn-on is allocated on a half-wavelength basis of the alternating current voltage within the control period, based on the turn-on ratio of the heater; and controlling turn-on of the heater based on determined turn-on pattern. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an entire configuration of an image forming apparatus according to a first embodiment; 
         FIG. 2  is a schematic diagram representing turn-on patterns; 
         FIG. 3  is a flowchart of a heater control process performed by the image forming apparatus according to the first embodiment; 
         FIG. 4  is a diagram representing one example of a 40% turn-on pattern to which partial turn-ons are allocated by a partial turn-on allocation unit according to the first embodiment; 
         FIG. 5A  is a diagram for explaining how to calculate a ratio of a partial turn-on to a half-wavelength; 
         FIG. 5B  is a diagram for explaining how to calculate a ratio of a partial turn-on to a half-wavelength; 
         FIG. 6  is a diagram representing one example of a 60% turn-on pattern to which partial turn-ons are allocated; 
         FIG. 7  is a diagram representing one example of a turn-on pattern to which partial turn-ons are allocated; 
         FIG. 8  is a diagram representing one example of the turn-on pattern to which partial turn-ons are allocated; 
         FIG. 9  is a diagram representing one example of the turn-on pattern to which partial turn-ons are allocated; 
         FIG. 10  is a diagram representing one example of the turn-on pattern to which partial turn-ons are allocated; 
         FIG. 11  is a block diagram of an entire configuration of an image forming apparatus according to a second embodiment; 
         FIG. 12  is a flowchart of a heater control process performed by the image forming apparatus according to the second embodiment; 
         FIG. 13  is a diagram representing one example of a 60% turn-on pattern to which partial turn-ons are allocated by a partial turn-on allocation unit according to the second embodiment; 
         FIG. 14  is a diagram representing one example of turn-on patterns to which partial turn-ons are allocated by the partial turn-on allocation unit according to the second embodiment; and 
         FIG. 15  is a flowchart of a heater control process according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of a heater control device, an image forming apparatus, and a program according to the present invention will be explained in detail below with reference to the accompanying drawings. 
     (First Embodiment) 
       FIG. 1  is a block diagram of an entire configuration of an image forming apparatus  10  according to an embodiment of the present invention. The image forming apparatus  10  includes the heater control device for controlling a heater in a fixing unit or the like provided in the image forming apparatus  10 . More specifically, the image forming apparatus  10  mainly includes a main power supply  100  and a control board  110 . The image forming apparatus  10  further includes a fixing unit  120 , a power supply SW  141 , and a door SW  142 . 
     The control board  110  controls the entire image forming apparatus  10 . The control board  110  is implemented as a computer including a CPU, a RAM, a ROM, a NVRAM, an ASIC (Application Specific Integrated Circuit) (not shown), and an input-output interface, which are connected to each other via a bus. 
     The control board  110  controls on/off of a triac (TRI)  143  and an electromagnetic relay  106  internally provided in the main power supply  100 , to control temperature and on/off of a halogen heater  121  of the fixing unit  120 . Any other heater such as a ceramic heater may be used instead of the halogen heater  121 . 
     A thermistor  122  provided near the halogen heater  121  of the fixing unit  120  detects a surface temperature of a heated object of the halogen heater  121 . The control board  110  performs A/D conversion on the surface temperature of the heated object detected by the thermistor  122 , to detect the surface temperature of the heated object of the halogen heater  121 . The control board  110  controls on/off of the TRI  143  and the electromagnetic relay  106  so that the surface temperature is stabilized. 
     When the power supply SW  141  of the image forming apparatus  10  is turned on, noise of a current supplied from an AC power supply  101  is removed by a filter  102 , and is then smoothed by a rectifier diode  103  and a smoothing capacitor  104 , to be supplied to a DDC (Digital Down Converter)  105 . The DDC  105  is a switching type DC to DC converter, and supplies a constant voltage Vcc to the control board  110  and 24 volts to the electromagnetic relay  106 . 
     The electromagnetic relay  106  can turn on a switch  107  and also can turn off the fixing unit  120  via the control board  110  in response to turning on of the door SW  142  of the image forming apparatus  10 . That is, the electromagnetic relay  106  functions as a safety device of the fixing unit  120 . 
     A zero-cross detection circuit  108  detects a zero cross point of the AC power supply  101 . The control board  110  turns on/off the TRI  143  according to the zero cross point. If the switch  107  is on, then the voltage of an alternating current supplied to the zero-cross detection circuit  108  approaches zero at each half-wavelength. Therefore, a transistor of the zero-cross detection circuit  108  cannot hold the on-voltage. The zero-cross detection circuit  108  detects this state of the transistor and outputs a zero-cross signal to the control board  110 . 
     The control board  110  includes a turn-on pattern storage unit  111  and a control unit  112 . The control unit  112  performs thinning control for controlling on/off of energization to the halogen heater  121  using a half-wavelength of an alternating current voltage as one unit. The control unit  112  also performs thinning-phase control as a combination of phase control for turning on only a part of the half-wavelength and the thinning control. The thinning-phase control will be explained later. The control unit  112  controls, specifically, turn-on of the halogen heater  121  according to the turn-on pattern stored in the turn-on pattern storage unit  111 . 
     The turn-on pattern storage unit  111  stores therein turn-on patterns. The turn-on pattern is a turn-on pattern of the halogen heater  121  on a control-period basis. The control period is a voltage period of the AC power supply  101  controlled by the control board  110 , and is a period of a preset length. In the present embodiment, the control period is set to 10 half-wavelengths. The turn-on pattern stored in the turn-on pattern storage unit  111  is set in terms of 10 half-wavelengths corresponding to the above-mentioned wavelengths. 
       FIG. 2  is a schematic diagram representing turn-on patterns. The turn-on pattern storage unit  111  stores therein turn-on patterns associated with turn-on duties respectively. Here, the turn-on duty is a turn-on ratio of the halogen heater  121 . In the present embodiment, 10 turn-on patterns are stored therein at 10% turn-on duty intervals. In  FIG. 2 , half-wavelengths indicated by diagonal lines are areas equivalent to turn-on of the halogen heater  121 . For example, when the turn-on duty is 30%, turn-on of the halogen heater  121  is set to predetermined 3 half-wavelengths of the 10 half-wavelengths. Thus, the turn-on patterns stored in the turn-on pattern storage unit  111  are thinning patterns used to thin out a turn-on period of the halogen heater  121  on the half-wavelength basis. The turn-on control of the halogen heater  121  using the thinning pattern is called “thinning control”. 
     The turn-on patterns stored in the turn-on pattern storage unit  111  according to the present embodiment are turn-on patterns with which frequency bands of around 10 Hz are avoided. That is, the turn-on patterns are allocated with full turn-ons or full turn-offs so as to avoid flicker. In the present embodiment, when the turn-on duty is 10%, a half-wave control pattern based on 20 half-wavelengths set as the control period is stored therein. 
     The control unit  112  in  FIG. 1  includes a turn-on duty decision unit  113 , a turn-on pattern extraction unit  114 , a partial turn-on allocation unit  115 , and a turn-on controller  116 . The turn-on duty decision unit  113  determines a turn-on duty based on a surface temperature of a heated object of the halogen heater  121  detected by the thermistor  122  and also based on a target temperature. 
     The turn-on pattern extraction unit  114 , when the turn-on duty determined by the turn-on duty decision unit  113  is equal to or less than a preset threshold, extracts a turn-on pattern associated with a turn-on duty that is higher than the turn-on duty determined by the turn-on duty decision unit  113 , from the turn-on pattern storage unit  111 . In the present embodiment, the threshold is set to 30%. The threshold should be an arbitrary value and is therefore not limited to the embodiment. 
     The turn-on pattern to be extracted when the turn-on duty is less than the threshold is previously set. In the present embodiment, when the turn-on duty is determined as 10%, a 40% turn-on pattern which is excellent in frequency characteristics is to be extracted. However, the turn-on pattern to be extracted is not limited to 40%, and thus a turn-on pattern of any other turn-on duty which is equal to or higher than the threshold may be extracted. 
     Meanwhile, when the turn-on duty determined by the turn-on duty decision unit  113  is higher than the threshold, the turn-on pattern extraction unit  114  extracts a turn-on pattern associated with the turn-on pattern determined by the turn-on duty decision unit  113 . 
     The partial turn-on allocation unit  115 , when the turn-on pattern extraction unit  114  extracts a turn-on pattern associated with a turn-on duty that is higher than the turn-on duty determined by the turn-on duty decision unit  113  from the turn-on pattern storage unit  111 , allocates a partial turn-on, instead of a full turn-on, to a half-wavelength allocated with the full turn-on in the turn-on pattern extracted by the turn-on pattern extraction unit  114 . As a result, the partial turn-on pattern of the halogen heater  121  is determined as the turn-on pattern of the halogen heater  121 . Here, the partial turn-on represents that the halogen heater  121  is turned on for only a part of period of the half-wavelength. That is, the partial turn-on is a control to change the phase of an alternating current voltage supplied to the halogen heater  121 . 
     The turn-on pattern obtained by the partial turn-on allocation unit  115  is a pattern in which a partial turn-on is allocated to a thinning pattern, and turn-on control of the halogen heater  121  using this turn-on pattern is called “thinning-phase control”. 
     The turn-on controller  116  controls turn-on of the halogen heater  121  based on the turn-on pattern extracted by the turn-on pattern extraction unit  114  or based on the turn-on pattern allocated with the partial turn-on by the partial turn-on allocation unit  115 . 
       FIG. 3  is a flowchart of a heater control process performed by the image forming apparatus  10 . First, the turn-on duty decision unit  113  determines a turn-on duty (Step S 100 ). If the turn-on duty determined by the turn-on duty decision unit  113  is equal to or less than the threshold (30%) (Yes at Step S 102 ), the turn-on duty decision unit  113  extracts a turn-on pattern associated with a turn-on duty which is higher than the determined turn-on duty, from the turn-on pattern storage unit  111  (Step S 104 ). In the present embodiment, the 40% turn-on pattern is extracted. Next, the partial turn-on allocation unit  115  allocates a partial turn-on to a half-wavelength, of the turn-on pattern extracted by the turn-on pattern extraction unit  114 , to which the full turn-on is allocated (Step S 106 ). Next, the turn-on controller  116  controls turn-on of the halogen heater  121  according to the turn-on pattern to which the partial turn-on is allocated by the partial turn-on allocation unit  115  (Step S 120 ). 
     Meanwhile, at Step S 102 , when the turn-on duty is higher than the threshold (No at Step S 102 ), the turn-on pattern extraction unit  114  extracts the turn-on pattern associated with the turn-on duty determined by the turn-on duty decision unit  113 , from the turn-on pattern storage unit  111  (Step S 110 ). Then, the turn-on controller  116  controls turn-on of the halogen heater  121  according to the turn-on pattern extracted by the turn-on pattern extraction unit  114  (Step S 120 ). At this point, the heater control process is completed. 
       FIG. 4  is a diagram representing one example of a turn-on pattern to which partial turn-ons are allocated by the partial turn-on allocation unit  115 . If the turn-on duty is determined as 10% by the turn-on duty decision unit  113 , the turn-on pattern extraction unit  114  extracts the  40 % turn-on pattern from the turn-on pattern storage unit  111 . The partial turn-on allocation unit  115 , as shown in  FIG. 4 , allocates partial turn-ons instead of full turn-ons to half-wavelengths, of the 40% turn-on pattern, to which the full turn-ons are allocated. 
     When the turn-on duty is low, a period during which the halogen heater  121  is on becomes short within the control period. Therefore, if the turn-on control is performed using the turn-on pattern stored in the turn-on pattern storage unit  111 , there arises a problem that inrush current increases caused by continuation of a turn-off period. 
     On the other hand, the image forming apparatus  10  according to the present embodiment is configured to reduce the turn-off period by dividing a turn-on of 1 or 2 or more of half-wavelengths set by the turn-on duty into partial turn-ons in a plurality of half-wavelengths and allocating the partial turn-ons within the control period. Moreover, it is configured that the half-wavelength allocated with the partial turn-on follows the turn-on pattern stored in the turn-on pattern storage unit  111 . This enables an increase in inrush current to be prevented and flicker to be reduced. 
     Next, a partial turn-on allocation process will be explained below. The partial turn-on allocation unit  115  according to the present embodiment sets a ratio of turn-ons in half-wavelengths so that the sum of turn-on power required for a plurality of partial turn-ons allocated to the turn-on pattern becomes equal to the sum of turn-on power set from the turn-on duty determined by the turn-on duty decision unit  113 . Furthermore, it is set so that the values of turn-on power in the partial turn-ons are equal to each other. 
     Referring to  FIGS. 5A and 5B , an example of allocating a 10% turn-on duty to a 40% turn-on pattern will be explained below. The amount of power required in the full turn-on for 1 half-wavelength equivalent to the 10% turn-on duty, as shown in  FIG. 5A , is calculated by Equation (1). In Equation (1), v represents an effective value of AC power supply voltage, and a, b represent zero-cross timing. θ (rad) is a phase angle at which the sine wave crosses through zero at time a and b.
 
∫ a   b v×√{square root over (2)}×sin θdθ  (1)
 
     When a partial turn-on is allocated to the 40% turn-on pattern, as shown in  FIG. 5B , four partial turn-ons are allocated to the turn-on pattern. If periods of the four partial turn-ons are set as c-d, e-f, g-h, and i-j, then the sum of turn-on power of the four partial turn-ons is calculated by Equation (2). Therefore, in order to make the sum of amounts of turn-on power set from the turn-on duty determined by the turn-on duty decision unit  113  equal to the sum of turn-on power required for a plurality of partial turn-ons allocated to the turn-on pattern, the periods are simply set so that the amount of power in Equation (1) and the amount of power in Equation (2) become equal to each other.
 
∫ c   d v×√{square root over (2)}×sin θdθ+∫ e   f v×√{square root over (2)}×sin θdθ+∫ g   h v×√{square root over (2)}×sin θdθ+∫ i   j v×√{square root over (2)}×sin θdθ  (2)
 
     Moreover, in the present embodiment, each of the periods is set so that the four partial turn-on periods shown in  FIG. 5B  are equal to each other. 
     As explained above, the image forming apparatus  10  according to the present embodiment achieves the turn-on power set by the turn-on duty determined by the turn-on duty decision unit  113  while increasing the number of turn-ons per control period, and thus, a desired power supply can be maintained. 
     Furthermore, in the present embodiment, if the partial turn-ons of which periods are equal to each other or uniform partial turn-ons are allocated, and if the amount of turn-on power of each of the partial turn-ons is made constant, then there does not occur a difference in voltage drop during continuous turn-on. Therefore, voltage fluctuation can be prevented, and a flicker level can be improved. Moreover, by making the periods of the partial turn-ons equal to each other, the control can be simplified. 
     As explained above, according to the image forming apparatus  10  of the first embodiment, when the determined turn-on duty is low, by allocating a turn-on required for the turn-on duty, as a partial turn-on, to the turn-on pattern which is set so as to avoid flicker, the flicker level can be improved while preventing the level of the harmonic current and the noise terminal voltage from being lowered. 
     In the phase control in which a turn-on is allocated to a part of the half-wavelength, there arises a problem that the level of the harmonic current and the noise terminal voltage is worsened. On the other hand, in the present embodiment, the number of phase controls is thinned out so as to perform phase control on, for example, only periods of the full turn-ons in the 40% turn-on pattern. Therefore, it is possible to prevent the level of the harmonic current and the noise terminal voltage from being worsened. 
     A first modification of the present embodiment will be explained below. In the image forming apparatus  10  according to the present embodiment, if the turn-on duty determined by the turn-on duty decision unit  113  is 30% or less, the turn-on pattern allocated with the partial turn-on is set to the 40% turn-on pattern, however, it may be set to a 60% turn-on pattern instead.  FIG. 6  is a diagram of a turn-on pattern in which partial turn-ons equivalent to a 10% turn-on duty per 1 half-wavelength is allocated to a 60% turn-on pattern. 
     The 60% turn-on pattern has a high ratio of the turn-on as compared with that of the 40% turn-on pattern. Therefore, the inrush current can be reduced although the level of the harmonic current and the noise terminal voltage is worsened. Therefore, it is preferable to set which of 40% and 60% turn-on patterns is to be extracted, based on characteristics of the apparatus and the like. 
     For example, if the rated power of the halogen heater  121  is relatively large and improvement of the flicker level is preferred, then it is preferable that the number of turn-ons be increased and a power supply per turn-on be reduced. Therefore, in this case, it is effective to use the 60% turn-on pattern. 
     Meanwhile, if the rated power of the halogen heater  121  is relatively small and prevention of level worsening of the harmonic current and the noise terminal voltage is preferred, then it is preferable that the number of turn-ons be decreased and the number of phase controls be reduced. Therefore, in this case, it is effective to use the 40% turn-on pattern. 
     As a second modification, if the turn-on duty determined by the turn-on duty decision unit  113  is 30% or less which is the threshold, the turn-on pattern extraction unit  114  may extract different turn-on patterns to be allocated with partial turn-ons depending on the determined turn-on duty. For example, if the turn-on duty is determined as 10%, the turn-on pattern extraction unit  114  may extract the 40% turn-on pattern, while if the turn-on duty is determined as 20%, the turn-on pattern extraction unit  114  may extract the 60% turn-on pattern. Conversely, if the turn-on duties are determined as 10% and 20%, then the 60% and 40% turn-on patterns may be extracted respectively. 
     As a third modification, the partial turn-on allocation unit  115  may make amounts of power of a plurality of partial turn-ons allocated to the determined turn-on pattern different from each other. For example, as shown in  FIG. 7 , the partial turn-on allocation unit  115  may allocate a plurality of partial turn-ons so that the turn-on power is getting large from the head to the end of the turn-on pattern. Thus, voltage fluctuations upon turn-on of the halogen heater  121  can be prevented. 
     As another example, as shown in  FIG. 8 , the partial turn-on allocation unit  115  may allocate a plurality of partial turn-ons so that the turn-on power is getting small from the head to the end of the turn-on pattern. Thus, voltage fluctuations upon turn-off of the halogen heater  121  can be prevented. 
     As another example, as shown in  FIG. 9  or  FIG. 10 , the partial turn-on allocation unit  115  may allocate not only the partial turn-on but also the full turn-on. Thus, even when the amounts of power of the partial turn-ons are made different from each other, each period of the partial turn-ons is determined so that the sum of the amounts of turn-on power of the partial turn-ons allocated to the turn-on pattern becomes equal to the amount of power set by the determined duty. 
     As a fourth modification, the partial turn-on allocation unit  115  allocates a turn-on pattern such that the turn-on power gradually increases, as shown in  FIG. 7  in a fixed period after the turn-on of the halogen heater  121  is started, as a comparatively long-term control. Thereafter, the partial turn-on allocation unit  115  may allocate a turn-on pattern of fixed turn-on power as shown in  FIG. 4  and  FIG. 6 , and may allocate a turn-on pattern such that the turn-on power gradually decreases, as shown in  FIG. 8 , in a fixed period before the halogen heater  121  is turned off. 
     (Second Embodiment) 
       FIG. 11  is a block diagram of an entire configuration of an image forming apparatus  1100  according to a second embodiment. The image forming apparatus  1100  includes the heater control device for controlling a heater in a fixing unit or the like provided in the image forming apparatus  1100 . More specifically, the image forming apparatus  1100  mainly includes the main power supply  100  and a control board  1110 . The image forming apparatus  1100  further includes a fixing unit  1120 , the power supply SW  141 , and the door SW  142 . 
     The fixing unit  1120  includes two halogen heaters: a first halogen heater  121 A and a second halogen heater  121 B. The fixing unit  1120  further includes a first thermistor  122 A and a second thermistor  122 B provided near the first halogen heater  121 A and the second halogen heater  121 B, respectively. The amount of turn-on power of the first halogen heater  121 A is equal to that of the second halogen heater  121 B. 
     The control board  1110  controls the entire image forming apparatus  1100 . The control board  1110  is implemented as a computer including a CPU, a RAM, a ROM, a NVRAM, an ASIC (Application Specific Integrated Circuit) (not shown), and an I/O interface, which are connected to each other via a bus. 
     The control board  1110  controls on/off of two triacs: first triac (TRI)  143 A and a second TRI  143 B, and the electromagnetic relay  106  internally provided in the main power supply  100 , to control temperature and on/off of the first halogen heater  121 A and the second halogen heater  121 B of the fixing unit  1120 . Any other heaters such as a ceramic heater may be used instead of the first halogen heater  121 A and the second halogen heater  121 B. 
     The first thermistor  122 A provided near the first halogen heater  121 A detects a surface temperature of a heated object of the first halogen heater  121 A. Likewise, the second thermistor  122 B provided near the second halogen heater  121 B detects a surface temperature of a heated object of the second halogen heater  121 B. The control board  1110  performs A/D conversion on the surface temperature detected by the first thermistor  122 A, to detect the surface temperature of the heated object of the first halogen heater  121 A. Likewise, the control board  1110  performs A/D conversion on the surface temperature detected by the second thermistor  122 B, to detect the surface temperature of the heated object of the second halogen heater  121 B. The control board  1110  controls on/off of the first TRI  143 A and the second TRI  143 B and of the electromagnetic relay  106  so that the surface temperature of the heated object of the first halogen heater  121 A and the surface temperature of the heated object of the second halogen heater  121 B are stabilized. 
     Here, the functions and operations of the power supply SW  141 , the AC power supply  101 , the filter  102 , the rectifier diode  103 , the smoothing capacitor  104 , the DDC  105 , the electromagnetic relay  106 , the switch  107 , and the door SW  142  are the same as these of the first embodiment. 
     The zero-cross detection circuit  108 , similarly to the first embodiment, detects a zero cross point of the AC power supply  101 , and the control board  1110  turns on/off the first TRI  143 A and the second TRI  143 B according to the zero cross point. 
     The control board  1110  includes the turn-on pattern storage unit  111  and a control unit  1112 . The control unit  1112  performs thinning control for controlling on/off of energization to the first halogen heater  121 A and the second halogen heater  121 B using a half-wavelength of the alternating current voltage as one unit. The control unit  1112  also performs thinning-phase control as a combination of phase control, in which only a part of the half-wavelength is turned on, and the thinning control. The thinning-phase control will be explained later. The control unit  1112  controls specifically turn-on of the first halogen heater  121 A and of the second halogen heater  121 B according to the turn-on patterns stored in the turn-on pattern storage unit  111 . 
     The turn-on pattern storage unit  111 , similarly to the first embodiment, stores therein turn-on patterns. The turn-on patterns are those of the first halogen heater  121 A and the second halogen heater  121 B on a control-period basis. That is, the turn-on patterns stored in the turn-on pattern storage unit  111  are used for heater control of both the first halogen heater  121 A and the second halogen heater  121 B. The turn-on patterns of the present embodiment are associated with turn-on duties which are turn-on ratios of the first halogen heater  121 A and the second halogen heater  121 B, and are similar to these of the first embodiment as shown in  FIG. 2 . 
     The control unit  1112  in  FIG. 11  includes a turn-on duty decision unit  1113 , a turn-on pattern extraction unit  1114 , a partial turn-on allocation unit  1115 , and a turn-on controller  1116 . The turn-on duty decision unit  1113  determines a turn-on duty of the first halogen heater  121 A based on a surface temperature of the heated object of the first halogen heater  121 A detected by the first thermistor  122 A and also based on a target temperature. The turn-on duty decision unit  1113  also determines a turn-on duty of the second halogen heater  121 B based on a surface temperature of the heated object of the second halogen heater  121 B detected by the second thermistor  122 B and also based on a target temperature. 
     Here, the turn-on pattern extraction unit  1114 , the partial turn-on allocation unit  1115 , and the turn-on pattern storage unit  111  function as a turn-on pattern decision unit. 
     The turn-on pattern extraction unit  1114  determines whether the sum of turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is equal to or less than a first threshold. If it is equal to or less than the first threshold, then it is further determined whether one of the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is 0%. When the sum of turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is equal to or less than the first threshold and both of them are not 0%, then the turn-on pattern extraction unit  1114  extracts two turn-on patterns, from the turn-on pattern storage unit  111 , which are associated with turn-on duties higher than the turn-on duties determined by the turn-on duty decision unit  1113  and in which the second halogen heater  121 B is turned off when the first halogen heater  121 A is turned on. 
     The turn-on pattern extraction unit  1114 , when the sum of the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is higher than the first threshold, further determines whether the sum of the turn-on duties is equal to or less than a second threshold. When the sum of the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is higher than the first threshold and is equal to or less than the second threshold, the turn-on pattern extraction unit  1114  extracts two turn-on patterns, from the turn-on pattern storage unit  111 , in which the total of the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is 100%, which are associated with turn-on duties higher than the turn-on duties determined by the turn-on duty decision unit  1113 , and in which the second halogen heater  121 B is turned off when the first halogen heater  121 A is turned on. 
     Furthermore, the turn-on pattern extraction unit  1114 , when the sum of the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is higher than the first threshold and is higher than the second threshold, extracts two turn-on patterns corresponding to the turn-on duties determined by the turn-on duty decision unit  1113  from the turn-on pattern storage unit  111 , and thereby determines the turn-on patterns corresponding to the determined turn-on duties using the control period as a unit, as the turn-on patterns of the first halogen heater  121 A and of the second halogen heater  121 B. 
     The partial turn-on allocation unit  1115 , when the turn-on pattern extraction unit  1114  extracts turn-on patterns associated with turn-on duties higher than the turn-on duties determined by the turn-on duty decision unit  1113  from the turn-on pattern storage unit  111 , allocates partial turn-ons of the first halogen heater  121 A and the second halogen heater  121 B, instead of the full turn-ons, to the half-wavelengths allocated with the full turn-ons in the turn-on patterns extracted by the turn-on pattern extraction unit  1114 . As a result, the partial turn-ons are determined as the turn-on patterns of the first halogen heater  121 A and the second halogen heater  121 B. Here, the partial turn-on is used to turn on the first halogen heater  121 A and the second halogen heater  121 B for only a part of the period of the half-wavelength. That is, the partial turn-on is a control to change a phase of the alternating current voltage supplied to each of the first halogen heater  121 A and the second halogen heater  121 B. 
     The turn-on patterns obtained by the partial turn-on allocation unit  1115  are patterns (partial turn-on patterns) in which the partial turn-ons are allocated to the thinning patterns, and the turn-on control of the first halogen heater  121 A and the second halogen heater  121 B using these turn-on patterns is referred to as “thinning-phase control”. 
     The turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B based on the turn-on patterns extracted by the turn-on pattern extraction unit  1114  or based on the turn-on patterns allocated with the partial turn-ons by the partial turn-on allocation unit  1115 . 
       FIG. 12  is a flowchart of a heater control process performed by the image forming apparatus  1100 . First, the turn-on duty decision unit  1113  determines turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B (Step S 1200 ). 
     Next, the turn-on pattern extraction unit  1114  determines whether a turn-on duty obtained by summing the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B determined by the turn-on duty decision unit  1113  is equal to or less than the first threshold (Step S 1202 ). 
     Then, if the summed turn-on duty is equal to or less than the first threshold (Yes at Step S 1202 ), then the turn-on pattern extraction unit  1114  further determines whether one of the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B is 0% (Step S 1203 ). 
     When both the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B are not 0% (No at Step S 1203 ), then the turn-on pattern extraction unit  1114  extracts a turn-on pattern of a turn-on duty higher than the determined turn-on duties (Step S 1204 ). 
     For example, in a case where the first threshold is 20%, if both the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B determined at Step S 1200  are 10%, then the turn-on pattern extraction unit  1114  extracts a turn-on pattern associated with a turn-on duty of 60% from the turn-on pattern storage unit  111 . 
     Next, the partial turn-on allocation unit  1115  allocates partial turn-ons of the first halogen heater  121 A and the second halogen heater  121 B, instead of full turn-ons, to the half-wavelengths allocated with the full turn-ons of the turn-on pattern extracted by the turn-on pattern extraction unit  1114  (Step S 1206 ). Next, the turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B according to the turn-on pattern to which the partial turn-ons are allocated by the partial turn-on allocation unit  1115  (Step S 1208 ). 
       FIG. 13  is a diagram representing a 60% turn-on pattern to which partial turn-ons are allocated by the partial turn-on allocation unit  1115  in a partial allocation process (Step S 1206 ). The partial turn-on allocation unit  1115  alternately allocates partial turn-ons of the first halogen heater  121 A and partial turn-ons of the second halogen heater  121 B, instead of the full turn-ons, to the half-wavelengths of the full turn-ons included in the 60% turn-on pattern. As a result, the partial turn-ons of the first halogen heater  121 A and the second halogen heater  121 B are allocated to 3 half-wavelengths in one control period, respectively. 
     Moreover, the partial turn-on allocation unit  1115  sets a ratio of turn-ons in half-wavelengths so that the sum of turn-on power required for a plurality of partial turn-ons of the first halogen heater  121 A allocated to the turn-on pattern becomes equal to the sum of turn-on power set by the turn-on duty of the first halogen heater  121 A determined by the turn-on duty decision unit  1113 . Furthermore, it is set so that the values of turn-on power in the partial turn-ons of the first halogen heater  121 A are equal to each other. The same goes for the partial turn-ons of the second halogen heater  121 B. 
     As another example, setting may be made so that the values of turn-on power in the partial turn-ons are different from each other. In this case, also, the sum of the amounts of turn-on power required for the partial turn-ons is preferably set so as to be equal to the sum of the turn-on power set by the turn-on duties. 
     When the turn-on duty is low, each period during which the first halogen heater  121 A and the second halogen heater  121 B are on within the control period becomes short. Therefore, if the turn-on control is performed using the turn-on pattern stored in the turn-on pattern storage unit  111 , there arises a problem that inrush current caused by continuation of the turn-off period is increased. 
     On the other hand, the image forming apparatus  1100  according to the present embodiment is configured to reduce the turn-off period by dividing a turn-on for 1 or 2 or more of half-wavelengths set by the turn-on duty into partial turn-ons in a plurality of half-wavelengths and by allocating the partial turn-ons within the control period. Moreover, it is configured that the half-wavelength allocated with the partial turn-on follows the turn-on pattern stored in the turn-on pattern storage unit  111 . Thus, an increase in the inrush current can be prevented and flicker can be reduced. 
     In the phase control that allocates a turn-on to a part of the half-wavelength, there arises a problem that the level of the harmonic current and the noise terminal voltage is worsened. In contrast, in the present embodiment, the number of phase controls is thinned out so as to perform the phase control only on a period of full turn-on in the 60% turn-on pattern, for example. Therefore, it is possible to suppress worsening of the level of the harmonic current and the noise terminal voltage. 
     In the present embodiment, the turn-on pattern extracted by the turn-on pattern extraction unit  1114  at Step S 1204  is set to the 60% turn-on pattern which is relatively excellent in frequency characteristics, however, it is not limited to the 60% turn-on pattern. Thus, it may be a turn-on pattern of other turn-on duty. However, it is preferably a turn-on pattern being three or more times as high as the determined turn-on duty. 
     Referring back to Step S 1203 , if one of the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B is 0% (Yes at Step S 1203 ), the turn-on pattern extraction unit  1114  extracts a turn-on pattern of a turn-on duty higher than the determined turn-on duty of the halogen heater in which the turn-on duty is not 0% (Step S 1241 ). For example, in the present embodiment, the 40% turn-on pattern is extracted. Next, the partial turn-on allocation unit  1115  allocates partial turn-ons to half-wavelengths allocated with the full turn-ons of the turn-on pattern extracted by the turn-on pattern extraction unit  1114  (Step S 1242 ). Next, the turn-on controller  1116  controls the turn-ons of the halogen heaters according to the turn-on pattern to which the partial turn-ons are allocated by the partial turn-on allocation unit  1115  (Step S 1208 ). 
     Referring back to Step S 1202 , if the turn-on duty obtained by summing the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B determined by the turn-on duty decision unit  1113  is higher than the first threshold (No at Step S 1202 ), then the turn-on pattern extraction unit  1114  further determines whether the summed turn-on duty is equal to or less than the second threshold (Step S 1220 ). 
     If the summed turn-on duty is equal to or less than the second threshold (Yes at Step S 1220 ), then the turn-on pattern extraction unit  1114  extracts turn-on patterns of the turn-on duties that total 100% from the turn-on pattern storage unit  111  (Step S 1222 ). The partial turn-on allocation unit  1115  allocates partial turn-ons of the first halogen heater  121 A and the second halogen heater  121 B to the turn-on patterns, respectively (Step S 1224 ). Next, the turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B according to the turn-on patterns to which the partial turn-ons are allocated by the partial turn-on allocation unit  1115  (Step S 1208 ). 
     For example, it is assumed that the second threshold is set to 30% and, at Step  51200 , one of the turn-on duties of the first halogen heater  121 A and the second halogen heater  121 B is determined as 10% and the other turn-on duty is determined as 20%. In this case (No at Step S 1202 , Yes at Step S 1220 ), at Step S 1222 , the turn-on pattern extraction unit  1114  extracts a turn-on pattern associated with the turn-on duty of 40% and a turn-on pattern associated with the turn-on duty of 60% as turn-on duties that total 100% from the turn-on pattern storage unit  111 . Then, at Step S 1224 , the partial turn-on allocation unit  1115  allocates partial turn-ons of the halogen heater (the first halogen heater  121 A or the second halogen heater  121 B), for which the turn-on duty of 20% is determined instead of the full turn-ons, to the half-wavelengths of the full turn-ons included in the 40% turn-on pattern. Furthermore, the partial turn-on allocation unit  1115  allocates partial turn-ons of the halogen heater (the first halogen heater  121 A or the second halogen heater  121 B), for which the turn-on duty of 10% is determined instead of the full turn-ons, to the half-wavelengths of the full turn-ons included in the 60% turn-on pattern (Step S 1224 ). 
       FIG. 14  is a diagram representing turn-on patterns to which partial turn-ons are allocated by the partial turn-on allocation unit  1115  in a partial turn-on allocation process (Step S 1224 ). The turn-on patterns shown in  FIG. 14  are those when the turn-on duty of the first halogen heater  121 A is determined as 10% and the turn-on duty of the second halogen heater  121 B is determined as 20%. 
     As shown in the upper and middle patterns of  FIG. 14 , the partial turn-on allocation unit  1115  first allocates the partial turn-ons of the first halogen heater  121 A to the 40% turn-on pattern and then allocates the partial turn-ons of the second halogen heater  121 B to the 60% turn-on pattern. In this case, also, as already explained, the partial turn-on allocation unit  1115  allocates the partial turn-ons to the 40% turn-on pattern so that the sum of the amounts of turn-on power required for a plurality of partial turn-ons within the control period of the first halogen heater  121 A becomes equal to the amount of turn-on power set from the turn-on duty (10%) of the first halogen heater  121 A determined by the turn-on duty decision unit  1113 , and so that the amounts of power of the partial turn-ons are equal to each other. The same goes for allocation of the partial turn-ons of the second halogen heater  121 B to the 60% turn-on pattern. 
     Thus, the partial turn-ons of the first halogen heater  121 A are allocated to 4 half-wavelengths included in the 40% turn-on pattern. Also, the partial turn-ons of the second halogen heater  121 B are allocated to 6 half-wavelengths included in the 60% turn-on pattern. 
     At Step S 1208  in  FIG. 12 , the turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B using the turn-on pattern in which the two turn-on patterns obtained through the partial turn-on allocation process (Step S 1224 ) are superimposed on each other, as shown in the lower pattern of  FIG. 14  (Step S 1208 ). 
     The turn-on patterns stored in the turn-on pattern storage unit  111  according to the present embodiment have a relationship such that the full turn-ons and full turn-offs of the two turn-on patterns, which are added to become 100% like 40% and 60%, and 30% and 70%, are opposite to each other at the turn-on duty of 50%. Therefore, in the turn-on control of the first halogen heater  121 A and the second halogen heater  121 B using the turn-on pattern in which the 40% and 60% turn-on patterns are superimposed on each other, the second halogen heater  121 B is turned off when the first halogen heater  121 A is turned on. 
     Therefore, as explained above, by using the 40% and 60% turn-on patterns for the two first halogen heater  121 A and second halogen heater  121 B, it is possible to avoid simultaneous turn-ons of the first halogen heater  121 A and the second halogen heater  121 B, which allows an increase in inrush current to be prevented and flicker to be reduced. 
     When the sum of the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B is 30% and if the partial turn-ons of the two first halogen heater  121 A and second halogen heater  121 B are allocated to the  60 % turn-on pattern as shown in  FIG. 13 , the phase angle is 90 degrees, which becomes so large that the inrush current is not negligible. Therefore, in the present embodiment, when the sum of the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B is 30%, the number of phase controls is increased by using the two 40% and 60% turn-on patterns in the above manner, so that the phase angle is made small. This allows two kinds of power for execution of the half wave cycles in the turn-on pattern used for turn-on control, and thus it is possible to perform control favorable to flicker with a small potential difference as compared with the case in which only one halogen heater is turned on. 
     Moreover, when the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B are different from each other in the above manner, by allocating a turn-on pattern of a higher turn-on duty to the halogen heater having a high turn-on duty, the amounts of power of the partial turn-ons can be made constant, which allows voltage fluctuations during continuous turn-on to be prevented and the flicker level to be improved. 
     As another example, the turn-on pattern selected by the turn-on pattern extraction unit  1114  is not limited to the 40% and 60% turn-on patterns, and thus may be a turn-on pattern of other turn-on duty. However, as explained above, it is preferable to extract two turn-on patterns having different turn-on timings in terms of prevention of the inrush current from being increased. 
     Referring back to Step S 1220 , if the turn-on duty obtained by summing the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B determined by the turn-on duty decision unit  1113  is higher than the second threshold (No at Step S 1220 ), then the turn-on pattern extraction unit  1114  reads the turn-on patterns of the turn-on duties determined for the first halogen heater  121 A and second halogen heater  121 B from the turn-on pattern storage unit  111  (Step S 1230 ). The turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B according to the turn-on patterns extracted by the turn-on pattern extraction unit  1114  (Step S 1208 ). 
     For example, when the sum of the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B determined at Step S 1200  is higher than 30%, specifically, when it is obtained from any combination other than a combination of 10% and 10% or of 10% and 20% (No at Step S 1202  and No at Step S 1220 ), at Step S 1230 , the turn-on pattern extraction unit  1114  extracts a turn-on pattern, for the first halogen heater  121 A, associated with the turn-on duty of the first halogen heater  121 A determined by the turn-on duty decision unit  1113  from the turn-on pattern storage unit  111 . The turn-on pattern extraction unit  1114  further extracts a turn-on pattern, for the second halogen heater  121 B, associated with the turn-on duty of the second halogen heater  121 B determined by the turn-on duty decision unit  1113  from the turn-on pattern storage unit  111 . 
     As explained above, the image forming apparatus  1100  according to the present embodiment, when the sum of the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B is equal to or less than the threshold, performs the thinning-phase control, and the flicker level can thereby be improved while preventing the level of the harmonic current and the noise terminal voltage from being worsened. Moreover, it is possible to control so that turn-on timings of the two first halogen heater  121 A and second halogen heater  121 B do not synchronize, and thus an increase in the inrush current can be prevented. 
     A first modification of the present embodiment will be explained. In the present embodiment, if both the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B are 10%, the partial turn-ons of the two first halogen heater  121 A and second halogen heater  121 B are allocated to one turn-on pattern, and if the turn-on duty of one of the two first halogen heater  121 A and second halogen heater  121 B is 10% and the turn-on duty of the other halogen heater is 20%, the partial turn-ons of the halogen heaters are allocated to the two turn-on patterns respectively. However, instead of this, even if one of the turn-on duties is 10% and the other is 20%, the partial turn-ons of the two first halogen heater  121 A and second halogen heater  121 B may be allocated to one turn-on pattern. As another example, even if both of the turn-on duties are 10%, two turn-on patterns may be extracted and the partial turn-ons of each of the halogen heaters may be allocated to each of the turn-on patterns. 
     Thus, when the sum of the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B is equal to or less than the threshold, the turn-ons of the two halogen heaters are simply controlled based on the turn-on pattern in which the partial turn-ons of the two first halogen heater  121 A and second halogen heater  121 B are allocated to the half-wavelengths so as to avoid flicker during the control period. Therefore, the turn-on pattern decision method is not limited to the embodiment. 
     Furthermore, as a second modification, there is no need to include the turn-on pattern storage unit  111  and the turn-on pattern extraction unit  1114 . In this case, the turn-on controller  1116  generates a turn-on pattern of each of the turn-on duties so that flicker is avoided, and simply allocates partial turn-ons to the turn-on pattern. 
     As a third modification, the image forming apparatus  1100  may be provided with only one halogen heater. In this case, only one thermistor and TRI are simply provided accordingly. Thus, when only one halogen heater is provided, the turn-on duty decision unit  1113  determines a turn-on duty of only one halogen heater. The turn-on pattern extraction unit  1114 , when the determined turn-on duty is equal to or less than the threshold, extracts a turn-on pattern associated with a turn-on duty higher than the determined turn-on duty. The partial turn-on allocation unit  1115  allocates the partial turn-ons instead of the full turn-ons to the half-wavelengths of the full turn-ons included in the turn-on pattern extracted by the turn-on pattern extraction unit  1114 . The turn-on controller  1116  controls turn-on of the halogen heater according to the turn-on pattern allocated with the partial turn-ons by the partial turn-on allocation unit  1115 . 
     (Third Embodiment) 
     Next, an image forming apparatus  1100  according to a third embodiment will be explained below. The configuration of the image forming apparatus  1100  according to the third embodiment is the same as that of the image forming apparatus  1100  according to the second embodiment. However, the amounts of turn-on power of the two first halogen heater  121 A and second halogen heater  121 B in the image forming apparatus  1100  according to the third embodiment are different from each other. In the present embodiment, the amount of turn-on power of the first halogen heater  121 A is set to 500 watts, and the amount of turn-on power of the second halogen heater  121 B is set to 700 watts. 
       FIG. 15  is a flowchart representing a heater control process performed by the image forming apparatus  1100  according to the third embodiment. First, the turn-on duty decision unit  1113  determines turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B (Step S 1500 ). 
     Next, the turn-on pattern extraction unit  1114  determines whether a turn-on duty obtained by summing the turn-on duty of the first halogen heater  121 A and the turn-on duty of the second halogen heater  121 B determined by the turn-on duty decision unit  1113  is equal to or less than a first threshold (Step S 1502 ). 
     If the summed turn-on duty is equal to or less than the first threshold (Yes at Step S 1502 ), then the turn-on pattern extraction unit  1114  extracts turn-on patterns of the turn-on duties that total 100% and in which the halogen heater with a large amount of turn-on power has a high turn-on duty, from the turn-on pattern storage unit  111  (Step S 1504 ). The partial turn-on allocation unit  1115  allocates partial turn-ons of the first halogen heater  121 A and the second halogen heater  121 B to the turn-on patterns respectively (Step S 1506 ). Next, the turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B according to the turn-on patterns to which the partial turn-ons are allocated by the partial turn-on allocation unit  1115  (Step S 1508 ). 
     For example, in a case where the first threshold is 20%, if both the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B are 10% (Yes at Step S 1502 ), then the turn-on pattern extraction unit  1114  extracts the 40% turn-on pattern for the first halogen heater  121 A from the turn-on pattern storage unit  111 , and extracts the 60% turn-on pattern for the second halogen heater  121 B which has a larger amount of turn-on power than that of the first halogen heater  121 A, from the turn-on pattern storage unit  111 . Thus, the turn-on pattern with a high turn-on duty is allocated to the halogen heater with the large amount of power. 
     It should be noted that the turn-on patterns obtained at Step S 1506  are the same as the turn-on patterns shown in  FIG. 14 . That is, the 10% turn-on duty of the first halogen heater  121 A is allocated to four partial turn-ons in one control period according to the 40% turn-on pattern as shown in  FIG. 14 . The 10% turn-on duty of the second halogen heater  121 B with a large amount of turn-on power as compared with that of the first halogen heater  121 A is allocated to six partial turn-ons in one control period according to the 60% turn-on pattern as shown in  FIG. 14 . 
     Thus, the turn-on pattern extraction unit  1114  allocates the turn-on pattern with a higher turn-on duty to the halogen heater with a larger amount of turn-on power. This allows achievement of control of high-frequency turn-on so as to prevent inrush current. Moreover, because the amounts of turn-on power of the partial turn-ons can be made constant, voltage fluctuations during continuous turn-on can be prevented and the flicker level can be improved. 
     Referring back to Step S 1502 , if the value obtained by summing the turn-on duties of the two first halogen heater  121 A and second halogen heater  121 B determined at Step S 1500  is higher than the first threshold (for example, the summed value is 30% or more in the example that the first threshold is 20%) (No at Step S 1502 ), the turn-on pattern extraction unit  1114  extracts a turn-on pattern, for the first halogen heater  121 A, associated with the turn-on duty of the first halogen heater  121 A determined by the turn-on duty decision unit  1113 , and extracts a turn-on pattern, for second halogen heater  121 B, associated with the turn-on duty of the second halogen heater  121 B determined by the turn-on duty decision unit  1113  (Step S 1520 ). The turn-on controller  1116  controls the turn-ons of the first halogen heater  121 A and the second halogen heater  121 B according to the respective turn-on patterns of the first halogen heater  121 A and the second halogen heater  121 B (Step S 1508 ). At this point, the heater control process is completed. 
     The other configurations and processes of the image forming apparatus  1100  according to the third embodiment are the same as the configurations and operations of the image forming apparatus  1100  according to the second embodiment. 
     As a first modification of the third embodiment, a turn-on pattern allocated with partial turn-ons of each halogen heater may be selected not only based on a difference in amounts of power in the halogen heaters but also by adding the turn-on duties determined for the halogen heaters as explained in the second embodiment. 
     The first to third embodiments are configured to determine turn-on patterns by previously storing turn-on patterns indicating full turn-ons and full turn-offs on the half-wavelength basis in the turn-on pattern storage unit  111 , extracting a turn-on pattern from the turn-on pattern storage unit  111  based on the determined turn-on duty, and allocating the partial turn-ons to the full turn-ons of the extracted turn-on pattern, however, the way to determine is not limited thereto. For example, it can be configured that the turn-on patterns indicating full turn-ons and full turn-offs on the half-wavelength basis are not stored previously, and that a turn-on pattern in which the partial turn-ons are allocated to the full turn-ons based on the determined turn-on duty is determined. 
     The image forming apparatuses  10  and  1100  according to the first to third embodiments include a control unit such as a CPU, a storage unit such as ROM and RAM, and an external storage device such as a HDD and a CD drive device, and implement a hardware configuration using an ordinary computer. 
     A heater control program executed in the image forming apparatuses  10  and  1100  according to the first to third embodiments is provided as a computer program product by being recorded in a computer-readable recording medium such as a CD-ROM, a flexible disk (FD), a CD-R, and a DVD (Digital Versatile Disk) in an installable format file or in an executable format file. 
     Moreover, the heater control program executed in the image forming apparatuses  10  and  1100  according to the first to third embodiments may be configured so that it is provided by storing the heater control program on a computer connected to a network such as the Internet and causing it to be downloaded through the network. Furthermore, the heater control program executed in the image forming apparatuses  10  and  1100  according to the first to third embodiments may be configured so that it is provided or distributed through the network such as the Internet. In addition, the heater control program executed in the image forming apparatuses  10  and  1100  according to the first to third embodiments may be configured so that it is provided by being previously installed in ROM or the like. 
     The heater control program executed in the image forming apparatuses  10  and  1100  according to the first to third embodiments is a module configuration including the units (the turn-on duty decision unit, the turn-on pattern extraction unit, the partial turn-on allocation unit, and the turn-on controller). As actual hardware, the CPU (processor) reads the heater control program from the recording medium and executes it, so that the units are loaded on a main storage unit and are generated on the main storage unit. 
     It should be noted that in the embodiments, the image forming apparatuses  10  and  1100  according to the present invention may be a multifunction product having at least two functions among a copy function, a printer function, a scanner function, and a facsimile function, and can be applied to any one of the image forming apparatuses such as a copier, a printer, a scanner device, a facsimile device, and the like. 
     Although the invention has been described with respect to specific embodiments 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 that fairly fall within the basic teaching herein set forth.