Abstract:
Conventionally, a thermal fixing apparatus contained in an image forming apparatus has a heater like a halogen heater or a film-heating type heater. Generally, the heater comprises a plurality of heat generating elements connected to an AC power supply. Since the thermal fixing apparatus has a plural of heat generating elements, it needs a switching control elements corresponding to the number of heaters. The switching control elements causes a need for being increased in size so as to cope with energizing of large current of driving the heater. For the purpose of solving the above problem, an apparatus requiring only a small number of semiconductor switching elements is provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image heating apparatus preferably for use as a fixing device in a copying machine, a laser beam printer or the like, and particularly to an image heating apparatus having a plurality of heat generating elements therein.  
           [0003]    2. Related Background Art  
           [0004]    Conventionally a thermal fixing apparatus contained in this type of image forming apparatus fixes an unfixed image (toner image) formed on a recording sheet by an electrophotographic process or other image forming means into the recording sheet, and there are well known types such as a thermal-roller type fixing apparatus having a halogen heater as a heat source or a film-heating type thermal fixing apparatus having a ceramic flexible heating sheet heater as a heat source.  
           [0005]    [0005]FIG. 8 shows an example of a general heater driver circuit adopted to this type of thermal fixing apparatus.  
           [0006]    As shown in FIG. 8, a heater  803  generally comprising a plurality of heat generating elements is connected to a commercial AC power supply  801  via a triac or other switching control elements  804  and  816  and power is supplied from this AC power supply  801 . The heater  803  is provided with a temperature detecting element, for example, a thermistor  814 , a temperature of the heater  803  is detected by the temperature detecting element  814 , a control circuit (power supply instruction means)  812  is turns on or off the switching control elements  804  and  816 , by which a power supply to the heater  803  is turned on or off to control a temperature of the thermal fixing apparatus to a certain temperature of a target.  
           [0007]    The on or off control of the power supply to the heater  803  is performed by a wave number control or a phase control of the commercial power supply  801 .  
           [0008]    The heater  803  has two generating elements, each having a length according to a width of a recording sheet, and therefore two heat generating elements are not concurrently energized. Filters  811  and  823  are provided to remove switching noises generated from the switching control elements  804  and  816  by turning on or off the heater  803 .  
           [0009]    The conventional apparatus set forth in the above requires switching control elements for controlling the heater by the number of the heat generating elements of the heater. In this condition, the switching control elements  804  and  816  for supplying power must turn on or off a power supply for large current to the heater  803 , thereby causing a need for being increased in size so as to cope with energizing of large current for driving the heater. This increase in size of the elements causes an increase of an amount of heat generated from the elements at switching or an increase of noises generated by the switching operation. Therefore, it is further required to take countermeasures against heat generation caused by the switching operation or to provide a filter for absorbing the noises.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of these problems, the present invention has been provided, and therefore it is an object of the present invention to provide an image heating apparatus requiring only a small number of semiconductor switching elements in spite of having a plurality of heat generating elements.  
           [0011]    It is another object of the present invention to provide an image heating apparatus, comprising:  
           [0012]    a heating member having a first heat generating element and a second heat generating element;  
           [0013]    relay means for relaying between a power supply and said heating member, said relay means connecting either said first heat generating element or said second heat generating element to the power supply;  
           [0014]    switching means arranged between the power supply and said heating member; and  
           [0015]    control means for controlling said switching means so that a temperature of said heating member is maintained at a set temperature.  
           [0016]    Other objects of the present invention will be apparent from the following detailed description by referring to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a sectional side elevation view schematically showing a main internal configuration of an image forming apparatus according to a first embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a diagram showing a circuitry of control and driver circuits of a thermal fixing apparatus according to the first embodiment;  
         [0019]    [0019]FIG. 3 is a schematic view showing a ceramic heater according to the first embodiment with a heat generating element contained therein;  
         [0020]    [0020]FIG. 4 is a diagram showing a circuitry of control and driver circuits in another example of a thermal fixing apparatus according to the first embodiment;  
         [0021]    [0021]FIG. 5 is a schematic view showing a ceramic heater in another example according to the first embodiment;  
         [0022]    [0022]FIG. 6 is a diagram showing a circuitry of control and driver circuits of a thermal fixing apparatus according to a second embodiment of the present invention;  
         [0023]    [0023]FIG. 7 is a schematic diagram showing a ceramic heater according to the second embodiment with a heat generating element contained therein; and  
         [0024]    [0024]FIG. 8 is a diagram showing a circuitry of control and driver circuits of a conventional thermal fixing apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    [First embodiment] 
         [0026]    An image forming apparatus of the present invention will be described below according to a first embodiment applied to a laser beam printer using an electrophotographic process.  
         [0027]    Referring to FIG. 1, there is shown a sectional side elevation view schematically showing a main internal configuration of the laser beam printer according to the first embodiment of the present invention.  
         [0028]    A laser beam printer  101  is provided with a cassette  102  for storing recording sheets S, a cassette presence/absence sensor  103  for detecting a presence or an absence of the recording sheets S in the cassette  102 , a cassette size sensor  104  for detecting a size of the recording sheets S in the cassette  102  (comprising a plurality of microswitches), and a feed roller  105  for feeding the recording sheets S from the cassette  102 .  
         [0029]    In a downstream of the feed roller  105  there is provided a registration roller pair  106  for synchronously conveying the recording sheets S. Additionally in a downstream of the registration roller pair  106  there is provided an image forming part  108  for forming a toner image on the recording sheet S on the basis of a laser light from a laser scanner part  107 .  
         [0030]    Furthermore, in a downstream of the image forming part  108  there is provided a thermal fixing apparatus  109  as a thermal fixing means for thermally fixing the toner image formed on the recording sheet S, and in a downstream of the thermal fixing apparatus  109  there are provided a sheet discharge sensor  110  for detecting a conveyance condition of a sheet discharging part, a discharging roller  111  for discharging the recording sheet S, and a stacking tray  112  to be stacked with the completed recording sheets S.  
         [0031]    The laser scanner  107  comprises a laser unit  113  for emitting a laser light modulated on the basis of an image signal (image signal VDO) transmitted from an external device  128  described later, a polygon motor  114  for scanning the laser light from the laser unit  113  on a photosensitive drum  117  described later, an imaging lens  115 , and a folded mirror  116 .  
         [0032]    The laser beam printer  101  comprises a photosensitive drum  117 , a primary charging roller  119 , a developing unit  120 , a transfer charging roller  121 , a cleaner  122  and the like needed for a known electrophotographic process, and the thermal fixing apparatus  109  comprises a fixing film  109   a , a pressure roller  109   b , a ceramic heater  203  arranged inside the fixing film  109   a , and a thermistor temperature detecting element (hereinafter referred to as a temperature detecting element)  214  as temperature detecting means for detecting a surface temperature of the ceramic heater.  
         [0033]    A main motor  123  supplies a driving force to the feed roller  105  via a feed roller clutch  124  and to the registration roller pair  106  via a registration roller  125  and further it supplies a driving force to respective units in the image forming part  108  including the photosensitive drum  117 , the thermal fixing apparatus  109 , and the discharging roller  111 .  
         [0034]    An engine controller  126  controls the laser scanner part  107  and the image forming part  108  as well as controlling the electrophotographic process with the thermal fixing apparatus  109  and the conveyance of the recording sheets S in the laser beam printer  101 .  
         [0035]    A video controller  127 , which is connected to an external device  131  such as a personal computer via a general-purpose interface (Centronics, RS232C, etc.)  130 , expands image information transmitted from the general-purpose interface to bit data and transmits the bit data as a VDO signal to the engine controller  126 .  
         [0036]    Referring to FIG. 2, there is shown driver and control circuit of the ceramic heater  203 .  
         [0037]    A commercial AC power supply  201  for supplying power is connected to the image forming apparatus  101  (See FIG. 1).  
         [0038]    The image forming apparatus  101  (See FIG. 1) causes the ceramic heater  203  to generate heat when the AC power supply  201  supplies power to the ceramic heater  203  via an AC filter  202 .  
         [0039]    The ceramic heater  203  contains two heat generating elements  203   a  and  203   b  as shown in an enlarged view in FIG. 3, with energizing appropriately switched between the heat generating elements in the heater according to a width of a recording sheet S for printing. Power supply to the ceramic heater  203  (the heat generating elements  203   a  and  203   b ) is performed by energizing or shutting down a triac  204 . Resistances  205  and  206  are bias resistances for the triac  204  and a photo triac coupler  207  is a device for securing a creepage distance for insulation between the primary and secondary resistances. A relay  213  is energized in response to a signal from a control circuit  212 . Relays  216  and  217  serve as switches (energizing heat generating element switching means) for switching a heat generating element for generating heat (for energizing) of the heat generating elements  203  and  203   b  contained in the ceramic heater  203  and they are turned on or off by the control circuit  212  according to a width of a recording sheet S. The triac  204  is turned on by energizing light-emitting diodes of the photo triac coupler  207 . A resistance  208  is used for restraining current of the photo triac coupler  207  and turned on or off by a transistor  209 . The transistor  209  is connected to the control circuit  212  via a resistance  210  and operates in response to an ON signal from the control circuit  212 . A filter  211  is arranged to restrain noises generated when the ceramic heater  203  is turned on or off.  
         [0040]    The AC power supply  201  is inputted to a zero-crossing detecting part of the control circuit  212  via the AC filter  202 . The zero-crossing detecting part of the control circuit  212  notifies the inside of the control circuit  212  that the AC power supply  201  is at a voltage of a certain threshold value or lower by means of a pulse signal. Hereinafter, this signal transmitted by the zero-crossing detecting part of the control circuit  212  is referred to as ZEROX signal.  
         [0041]    The control circuit  212  detects an edge of a pulse of the ZEROX signal and turns on or off the triac  204  by a phase control or a wave number control.  
         [0042]    A temperature detected by a temperature detecting element  214  (See FIG. 1, too) is detected as a shunt voltage between a resistance  215  and the temperature detecting element  214  and A/D-inputted to the control circuit  212  as a TH signal.  
         [0043]    In other words, a temperature of the ceramic heater  203  is monitored as a TH signal (digital signal) in the control circuit  212 . Then, it is compared with a preset temperature of the ceramic heater  203  set inside the control circuit  212 , by which power to be supplied to the ceramic heater  203  is calculated, the temperature is converted to a phase angle (phase control) or a wave number (wave number control) corresponding to the supplied power, and it is appropriately transmitted as an ON signal to the transistor  209 .  
         [0044]    Next, a heat generating operation will be described when using two heat generating elements arranged in the ceramic heater  203 .  
         [0045]    First, when a printing operation is started, the relay  213  is closed. With this, one of the two heat generating elements in the ceramic heater  203  is selected according to a width of the recording sheet S by closing the relay  216  or the relay  217 . The relay  216  and the relay  217  are used for selecting the heat generating element and therefore can be in a type including a relay switch as shown in FIG. 2 or a type including a triac as shown in FIG. 4. It is also possible to use other types of switching means. Furthermore, the relay  216  and the relay  217  are not turned on or off during energizing of the ceramic heater  203 , and therefore there is no need for arranging a noise removing means such as the filter  211  nor for making an allowance for a current capacity.  
         [0046]    Therefore, the triac  204  (a triac  404  in FIG. 4) is turned on or off while the temperature detecting element  214  is monitored, by which the temperature of the ceramic heater  203  is controlled to be an appropriate value.  
         [0047]    In this embodiment, as shown in FIG. 3, the heat generating elements of the ceramic heater indicated by black areas have different lengths, while it is possible to arrange a plurality of heat generating elements having the same length and to arrange respective heat elements as indicated by shaded areas shown in FIG. 5.  
         [0048]    In another example shown in FIG. 5, both of the relay  216  and the relay  217  are closed and two heat generating elements  203   a ′ and  203   b ′ are energized at a time so as to cope with a wide recording sheet. In addition, while two heat generating elements are specified as the number of heat generating elements contained the ceramic heater in this embodiment, it is possible to use three or more heat generating elements.  
         [0049]    [Second embodiment] 
         [0050]    Next, a second embodiment in which an image forming apparatus according to the present invention is applied to a laser beam printer will be described below with points different from the first embodiment focused on.  
         [0051]    In the laser beam printer according to this embodiment, a basic configuration and a mechanical operation mode in an image formation are almost the same as those of the first embodiment described above, and therefore the overlapped description will be omitted here.  
         [0052]    Referring to FIG. 6, there are shown driver and control circuits of a ceramic heater  603  of a thermal fixing apparatus arranged in the laser beam printer of this embodiment.  
         [0053]    The ceramic heater  603  contained in the thermal fixing apparatus according to this embodiment comprises two long heat generating elements  603   a  and a short heat generating element  603   b  as shown in FIG. 7. The two long heat generating elements  603   a  are energized if the recording sheet S is relatively wide. In this condition, a load variation caused by turning on or off the ceramic heater at a temperature control is minimized by an appropriate combination of a control of energizing respective heat generating elements  603   a . On the other hand, if the recording sheet S is relatively narrow, the heater is controlled so that only the short heat generating element  603   b  is energized. On its control, these two long heat generating elements are not energized concurrently with the short heat generating element.  
         [0054]    A relay  616  is a switching means used for a switching operation between one of the two long heat generating elements and the short heat generating element. This relay  616  does not performs the switching operation during energizing of the ceramic heater  603 . Therefore, there is no need for securing an excessive current capacity.  
         [0055]    When energizing the long heat generating elements for a wide recording sheet, the switching operation of the relay  616  is performed on the basis of an instruction from the control circuit  612 . Subsequently a triac  604  and a triac  617  are turned on or off to control the temperature of the ceramic heater  603 . At this point, with an appropriate distribution of a power supply control to the two heat generating elements, it becomes possible to reduce an adverse effect to the outside of the printer, particularly flickering, caused by a load variation of the ceramic heater  603 .  
         [0056]    While two heat generating elements can be selected out of the three heat generating elements in this embodiment, apparently it is also possible to use any selecting type as far as possible, including a plurality of heat generating elements selectable out of a plurality of ones and a single heat generating element selectable out of a plurality of heat generating elements such as, for example, one selectable out of two heat generating elements or some selectable out of three or more heat generating elements for the same control as for this embodiment.