Patent Publication Number: US-6907203-B2

Title: Method of controlling a fusing temperature of an electrophotographic imaging apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Patent Application No. 2002-62255 filed with the Korea Industrial Property Office on Oct. 12, 2002, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of controlling a fusing temperature of an electrophotographic imaging apparatus, such as a printer, a copy machine, or a facsimile, and more particularly, to a method of controlling a fusing temperature of a fusing apparatus having a rubber layer thereon. 
   2. Description of the Related Art 
   Electrophotographic imaging apparatuses include a fusing apparatus that heats a sheet of paper to which a toner image is transferred, to instantaneously fuse and fix the toner image on the paper. The fusing apparatus includes a fusing roller that is heated to fuse the toner image on the paper, and a pressing roller that pushes the paper against the fusing roller to tightly support the fusing roller while the paper is fed therebetween. 
     FIG. 1  is a cross-sectional view of a conventional fusing roller  10  in which a halogen lamp (heater)  12  is installed as a heating source.  FIG. 2  is a cross-sectional view of a fusing apparatus provided with the fusing roller  10  of FIG.  1 . 
   Referring to  FIG. 1 , the fusing roller  10  includes a cylindrical roller  11  and the halogen lamp  12  installed at a core of the roller  11 . A toner-releasing coating layer  11   a  made of Teflon is formed on a surface of the roller  11 . The halogen lamp  12  generates heat to heat the fusing roller  10 . 
   Referring to  FIG. 2 , a pressing roller  13  is positioned below the fusing roller  10 , and a sheet of paper  14  is fed between the pressing roller  13  and the fusing roller  10 . The pressing roller  13  is elastically supported by a spring  13   a  to contact the fusing roller  10  and to apply a predetermined pressure to push the paper  14  toward the fusing roller  11 . While the paper  14  to which an unstable toner image has been transferred passes between the fusing roller  10  and the pressing roller  13 , the toner image formed of toner particles  14   a  is fused onto the paper  14  by pressure and heat. 
   A thermistor  15  that measures a surface (fusing) temperature of the fusing roller  11 , and a thermostat  16  that cuts off a power supply to the halogen lamp  12  from an external power source when the surface temperature of the fusing roller  10  exceeds a predetermined set value, are installed adjacent to the fusing roller  10 . The thermistor  15  measures the surface temperature of the fusing roller  10  to transmit an electrical signal corresponding to the measured surface temperature to a controller (not shown) of a printer (not shown). The controller controls the external power source to supply the power supply to the halogen lamp  12  based on the measured temperature to keep the surface temperature of the fusing roller  10  within a given range. When the measured temperature of the fusing roller  10  exceeds the predetermined set value as a result of failure in a temperature control by the thermistor  15  and the controller, a contact (not shown) of the thermostat  16  becomes open to cut off the power supply supplied to the halogen lamp  12  from the external power source. 
   In the above-described fusing roller  10  having the halogen lamp  12  as the heating source, only the toner-releasing coating layer  11   a  having a thickness of 20-30 μm is formed on the cylindrical roller  11 . Accordingly, there is rarely a difference in surface temperatures between the roller  11  and the toner-releasing coating layer  11   a , so that the surface temperature of the fusing roller  10  can be measured from the toner image-releasing coating layer  11   a  to control the external power source or the halogen lamp  12  to supply the power supply to the halogen lamp  12  by an on-off control. 
     FIG. 3  is a flowchart showing the on-off control of the fusing apparatus in the electrophotographic imaging apparatus. Referring to  FIGS. 2 and 3 , the surface temperature of the fusing roller  10  is measured at a predetermined interval, for example, at 100 ms in operation  40 . 
   The measured temperature of the fusing roller  10  is compared with a target fusing temperature in operation  42 . If the measured temperature of the fusing roller  10  is lower than the target fusing temperature, the halogen lamp  12  is turned onin operation  44 . If the measured temperature of the fusing roller  10  is higher than or equal to the target fusing temperature, the halogen lamp  12  is turned off in operation  46 . After operation  44  or  46 , operation  40  of measuring the surface temperature of the fusing roller  10  is repeated. In other words, a temperature of the fusing roller  10  can be simply controlled to be constant by measuring the surface temperature at intervals and controlling the halogen lamp  12  or the external power source to supply the power supply to the halogen lamp  12  by the on-off control. 
   However, the fusing apparatus used in a high-speed printer capable of printing  25  sheets of paper a minute or in a color printer requires a greater fusing nip between the fusing roller  10  and the pressing roller  13  to obtain a longer fusing duration and a higher fusing efficiency. To this end, a method of disposing a rubber layer having a predetermined thickness between the toner-releasing coating layer  11   a  and the cylindrical roller  11  of the fusing roller  10  has been suggested. 
   Referring to  FIG. 4 , a fusing roller  50  includes a cylindrical roller  51  and a halogen lamp  52  installed at the core of the cylindrical roller  51 . The cylindrical roller  51  is formed of aluminum with a thickness of 1.5 mm, a rubber layer  53  having a thickness of 1.5 mm is formed on the cylindrical roller  51 , and a Teflon coating layer  53   a  having a thickness of 20-30 μm is formed on the rubber layer  53 . The halogen lamp  52  generates the heat in the cylindrical roller  51 , and the cylindrical roller  51  is heated by the heat radiated from the halogen lamp  52  and transfers the heat to the rubber layer  53  and the coating layer  53   a.    
     FIG. 5  is a graph of temperature profiles with respect to time at various positions in a radial direction of the fusing roller  50  when a predetermined power is supplied to the halogen lamp  52  of the fusing roller  50  of FIG.  4 . Referring to  FIG. 5 , in the cylindrical roller  51  having a thickness ranging from a radial distance of 13 mm, which is measured from a core (center) of the fusing roller  50  in a radial direction, to 14.5 mm, i.e., an outer circumference of the cylindrical roller  51 , the temperature of the cylindrical roller  51  is constant throughout its thickness when being heated and measured because of a high thermal conductivity of the cylindrical roller  51  made of aluminum. 
   In the rubber layer  53  having a thickness ranging from the radial distance of 14.5 mm to 16 mm, the measured temperature of the rubber layer  53  tends to drop greatly with the increase of the radial distance from the core of the cylindrical roller  51 . This is because the thermal conductivity of the rubber layer  53  is so low that a heat transfer rate (speed) from the cylindrical roller  51  to a surface of the rubber layer  53  is very slow. For example, for a heating duration of 90 seconds, the temperature of the cylindrical roller  51  reaches 230° C. due to a thickness of the rubber layer  53  while the surface temperature of the fusing roller  50  is as low as 180° C. 
   When a temperature control to the fusing roller  50  having the thick rubber layer  53  is performed by a general on-off control method, the following problems occur. When the surface temperature of the fusing roller  50  reaches the target temperature, for example, 180° C., the temperature of the cylindrical roller  51  is 230° C. If the halogen lamp  52  is turned off at this time, the temperature of the cylindrical roller  51  immediately drops while the surface temperature of the rubber layer  53  continues to rise when the cylindrical roller  51  has a higher temperature than that of the rubber layer  53 . As a result, the surface temperature of the fusing roller  50  rises above the target fusing temperature. 
   When the surface temperature of the fusing roller  50  is lower than the target fusing temperature in a print mode, the halogen lamp  52  is turned on to heat the fusing roller  50 . At this time, if the surface temperature of the fusing roller  50  is maintained below the target fusing temperature during the printing mode, the temperature of the cylindrical roller  51  may increase to a certain temperature—higher than the target temperature. As a result, the rubber layer  53  may be thermally deformed. 
   Furthermore, since a power control period is too short according to this method, the halogen lamp  52  must be turned on and off frequently, thereby causing flicker problems. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of controlling a fusing temperature of a fusing roller having a thick rubber layer in an electrophotographic imaging apparatus to improve a quality of an image fused onto a recording medium by minimizing deviation of a surface temperature of the fusing roller and by increasing a control period of a power supplied to a heater of the fusing roller. 
   In accordance with an aspect of the present invention, there is provided a method of controlling a fusing temperature of a fusing roller in an electrophotographic imaging apparatus, the fusing roller having a cylindrical roller, a heater heating the cylindrical roller, and a rubber layer formed on a surface of the cylindrical roller with a predetermined thickness, the method comprising: determining whether a predetermined new power control period starts; if the new power control period starts, calculating a power supply ratio corresponding to a power to be supplied to the heater during the new power control period with respect to a maximum power that can be supplied to the heater; if the calculated power supply ratio is greater than zero, supplying the power corresponding to the power supply ratio to the heater during the new power control period; and if the calculated power supply ratio is not greater than zero, repeating the above operations, wherein the power supply ratio is calculated by adding a predetermined offset value β to a control value that is a product of a predetermined coefficient α and a subtraction of a measured temperature of the fusing roller from a target fusing temperature. The offset value β is smaller than or equal to a ratio of a power supply with respect to the maximum power that is supplied to the heater for the new power control period to maintain the measured temperature of the fusing roller at the target fusing temperature when the measured temperature is around the target fusing temperature. 
   In the above method, the offset value β may be determined according to the target fusing temperature of the fusing roller. The coefficient α may be determined according to at least one of a quality of a sheet of paper, a printing speed, and whether a printing mode is color printing. The supplying of the power corresponding to the power supply ratio to the heater during the new power control period may be performed according to a duty control. 
   The present invention provides another method of controlling a fusing temperature of a fusing roller in an electrophotographic imaging apparatus, the fusing roller having a cylindrical roller, a heater heating the cylindrical roller, and a rubber layer formed on a surface of the cylindrical roller with a predetermined thickness, the method comprising: determining whether a predetermined new power control period starts; if the new power control period starts, determining whether a measured temperature of the fusing roller is lower than a target fusing temperature; if the measured temperature is lower than the target fusing temperature, calculating a power supply ratio corresponding to a power to be supplied to the heater during the new power control period with respect to a maximum power that can be supplied to the heater; supplying the power corresponding to the power supply ratio to the heater during the new power control period; if the new power control period does not start yet or if the measured temperature is not lower than the target fusing temperature, determining whether a predetermined new offset control period starts; and if the new offset control period starts, calculating an offset power supply ratio corresponding to the power to be supplied to the heater during the new offset control period and supplying the power corresponding to the calculated offset power supply ratio to the heater. 
   The present invention also provides another method of controlling a fusing temperature of a fusing roller in an electrophotographic imaging apparatus, the fusing roller having a cylindrical roller, a heater heating the cylindrical roller, and a rubber layer formed on a surface of the cylindrical roller with a predetermined thickness, the method comprising: determining whether a predetermined new power control period starts; if the new power control period starts, calculating a power supply ratio corresponding to a power to be supplied to the heater during the new power control period with respect to a maximum power that can be supplied to the heater; determining whether the calculated power supply ratio is greater than zero; if the calculated power supply ratio is greater than zero, supplying the power corresponding to the power supply ratio to the heater during the new power control period; if the new power control period does not start yet or if the calculated power supply ratio is smaller than or equal to zero, determining whether a predetermined new offset control period starts; and if the new offset control period starts, calculating an offset power supply ratio corresponding to the power to be supplied to the heater during the new offset control period and supplying the power corresponding to the calculated offset power supply ratio to the heater. 
   The present invention also provides another method of controlling a fusing temperature of a fusing roller in an electrophotographic imaging apparatus, the fusing roller having a cylindrical roller, a heater heating the cylindrical roller, and a rubber layer formed on a surface of the cylindrical roller with a predetermined thickness, the method comprising: determining whether a predetermined new power control period starts; if the new power control period starts, determining whether a measured temperature of the fusing roller is lower than a target fusing temperature; turning on the heater during the new power control period if the measured temperature is lower than the target fusing temperature and turning off the heater during the new power control period if the measured temperature is greater than or equal to the target fusing temperature; if the new power control period does not start yet, determining whether a predetermined new offset control period starts; and if the new offset control periodstarts, calculating an offset power supply ratio corresponding to a power to be supplied to the heater during the new offset control period and supplying the power corresponding to the calculated offset power supply ratio to the heater. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and/or other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a cross-sectional view of a conventional fusing roller in which a halogen lamp is installed as a heating source; 
       FIG. 2  is a cross-sectional view of a fusing apparatus with the fusing roller of FIG.  1 . 
       FIG. 3  is a flowchart showing an on-off control of the fusing apparatus of  FIG. 2  in an electrophotographic imaging apparatus; 
       FIG. 4  is a cross-sectional view of a fusing roller with a rubber layer between a cylindrical roller and a toner-releasing coating layer; 
       FIG. 5  is a graph of temperature profiles with respect to time at various positions in a radial direction of the fusing roller of  FIG. 4  when a predetermined power is applied to a heater of the fusing roller; 
       FIG. 6  is a block diagram of a power control apparatus for controlling a fusing temperature of an electrophotographic imaging apparatus according to an embodiment of the present invention; 
       FIG. 7  is a flowchart illustrating a method of controlling a fusing temperature according to another embodiment of the present invention; 
       FIG. 8  is a graph explaining a duty control based on an offset value β; 
       FIG. 9  is a flowchart illustrating a method of controlling a fusing temperature according to another embodiment of the present invention; 
       FIG. 10  illustrates a phase control according to power supply ratios; 
       FIG. 11  is a flowchart illustrating a method of controlling a fusing temperature according to another embodiment of the present invention; and 
       FIG. 12  is a flowchart illustrating a method of controlling a fusing temperature according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
   Exemplary embodiments of a method of controlling a fusing temperature of an electrophotographic imaging apparatus according to an embodiment of the present invention will be described in detail with reference to the appended drawings. In the drawings, thickness of layers and regions are exaggerated for clarity. 
     FIG. 6  is a block diagram of a power control apparatus to control the fusing temperature of the electrophotographic imaging apparatus according to an embodiment of the present invention. A fusing apparatus of  FIG. 2  or  4  will be referred to in the following description. 
   In the power control apparatus of  FIG. 6 , a fusing temperature measuring unit  101  measures a surface (fusing) temperature of a fusing roller  50  of  FIG. 4  at a predetermined interval of, for example, 100 ms, using a thermal sensor, such as a thermistor  15 . An analog value (measured surface temperature) measured by the fusing temperature measuring unit  101  is converted to a digital value by an analog-to-digital converter (ADC)  103  to be input to a controller  105 . The controller  105 , which performs computations required to control the electrophotographic imaging apparatus, compares the measured surface temperature with a predetermined target fusing temperature and outputs a control signal to an alternating current (AC) power supply unit  107  to control a heater (halogen lamp)  109 . The AC power supply unit  107  controls a power supplied to the heater  109  according to the control signal received from the controller  105 . 
   The fusing temperature measuring unit  101  and the heater  109  correspond to the thermistor  15  and the halogen lamp  12  or  52  of  FIG. 2  or  4 , respectively. 
     FIG. 7  is a flowchart illustrating a method of controlling the fusing (surface) temperature according to an embodiment of the present invention. Referring to  FIGS. 4 ,  6 , and  7 , the fusing temperature measuring unit  101  measures the surface temperature of the fusing roller  50  at a predetermined interval, for example, of 100 ms, and transmits the measured surface temperature (analog signal) to the ADC  103 . The ADC  103  converts the received analog signal into a digital signal and outputs the digital signal to the controller  105  in operation  110 . 
   The controller  105  determines whether a predetermined (previous) power control period, for example, of 30 seconds, has been terminated and whether a new power control period starts in operation  111 . If the new power control period does not start yet, the operation  110  is repeated. 
   If it is determined in operation  111  that the new power control period starts, the controller  105  calculates a power supply ratio (PSR) corresponding to a power to be supplied to the heater  109  for the new power control period in operation  112 . 
   Next, the controller  105  determines whether the calculated PSR is greater than zero in operation  113 . 
   If the power supply ratio (PSR) is determined to be a positive value in operation  113 , the controller  105  outputs a control signal to the AC power supply unit  107  to enable the AC power supply unit  107  to supply the power corresponding to the PSR to the heater  105  according to a duty control, which will be described later, in operation  114 . When the new power control period is as short as less than a few seconds, the heater  109  may be turned on all the time during the new power control period before a next power control period starts. 
   If the PSR is determined to be less than or equal to zero in operation  113 , the process returns to the operation  110 . 
   Equation (1) below is an exemplary equation to calculate the PSR according to a proportional (P) control. However, the present invention is not limited thereto. Other equations according to a proportional-integral (PI) control, a proportional-integral-derivative (PID) control, etc., can be applied to calculate the PSR.
 
 PSR =α( Tt−Tm )+β  (1)
 
where Tt denotes the target fusing temperature of the fusing roller  50  that varies depending on a type and a thickness of a sheet of paper, the number of paper sheets to be printed, and whether it is color printing or not, and Tm denotes the measured temperature of the fusing roller  50 . PSR represents a percentage of the power supplied to the heater  109  for the new power control period with respect to a maximum power that can be supplied to the heater  109 , and is a sum of a predetermined offset value β and a control value, which is the product of a subtraction of the measured temperature from the target fusing temperature by a predetermined coefficient α. For example, when 10% of the maximum power is supplied to the heater  109  for the new power control period if the measured temperature is 5° C. lower than the target fusing temperature, and when 15% of the maximum power is supplied to the heater  109  for the new power control period if the measured temperature is 10° C. lower than the target fusing temperature, the coefficient α equals 1, and the offset value β equals 5. The coefficient α is determined by a quality of the paper, a printing speed, whether it is color printing or not, etc. The offset value β is a ratio of the power supplied to the heater  109  during the new power control period with respect to the maximum power that can be supplied to the heater  109  to keep the surface temperature of the fusing roller  50  constant at the target fusing temperature when the surface temperature is maintained at a predetermined level. The power corresponding to the offset value β is supplied to the fusing roller for each control period, which includes a plurality of duty periods (cycles), according to the duty control even after the surface temperature of the fusing apparatus has reached the target fusing temperature, to maintain the surface temperature of the fusing apparatus constant.
 
   If the PSR calculated in operation  112  is lower than zero, the measured temperature of the fusing roller  50  is too high to be compensated by the addition of the offset value β to the control value α(Tt−Tm). Thus, when the measured temperature of the fusing roller  50  is higher than the target fusing temperature by a predetermined amount, the power is not supplied to the heater  109  to prevent the surface temperature of the fusing roller  50  from rising high above the target fusing temperature. 
     FIG. 8  is a graph explaining the duty control based on the offset value β. The duty control includes turning on the heater  109  only for a sub-period T 2  in a main period T 1  and turning off the heater  109  for a remaining time of the main period T 1 .
 β(%)= T   2   /T   1 ×100  (2) 
   The duty control of turning on the heater  109  for the sub-period T 2  in each main period to supply just a required amount of the power to the heater  109  is based on a fact that a surface temperature increase of the fusing roller  50  in response to heating is very slow. 
   The offset value β maintains the surface temperature of the fusing roller  50  at a predetermined target temperature, is determined according to the target fusing temperature of the fusing roller  50 , and can be expressed as equation (3) below.
 
β=γ Tt +δ  (3)
 
where γ and δ are constants.
 
   According to a duty control method, when the fusing apparatus is in a no-load state, the power corresponding to the offset value β is supplied to the heater  109  for a period of time to keep the surface temperature of the fusing apparatus at a predetermined target fusing temperature. In a case that there is a rapid drop in the surface temperature due to some factors, such as an ambient temperature, the drop in the surface temperature is also compensated during the duty control so that the surface temperature can be maintained constant during the duty control. Furthermore, when there is a drop in the surface temperature as a result of a printing operation of the fusing apparatus, in addition to the compensation for the temperature drop through the duty control, the target fusing temperature itself and the offset value β may be raised to maintain the surface temperature constant so that print quality enhancement increases. 
     FIG. 9  is a flowchart illustrating a method of controlling the fusing temperature according to another embodiment of the present invention. Referring to  FIGS. 4 ,  6 , and  9 , the fusing temperature measuring unit  101  measures the surface temperature of the fusing roller  50  at the predetermined interval, for example, of 100 ms, and transmits the measured surface temperature (analog signal) to the ADC  103 . The ADC  103  converts the received analog signal into the digital signal and outputs the digital signal to the controller  105  in operation  120 . 
   The controller  105  determines whether the predetermined power control period, for example, of 30 seconds, has terminated and the new power control period starts in operation  121 . If it is determined that the new power control period starts, it is determined whether the measured temperature is lower than the target fusing temperature in operation  122 . 
   If the measured temperature is determined to be lower than the target fusing temperature, the controller  105  calculates a PSR′ using equation (4) in operation  123 . Equation (4) below is an exemplary equation to calculate the PSR′ according to the P control. However, the present invention is not limited thereto. Other equations according to the PI control, the PID control, etc., can be applied to calculate the PSR′.
 
 PSR ′=α′( Tt−Tm )+β′  (4)
 
where Tt denotes the target fusing temperature of the fusing roller  50  that varies depending on the type and the thickness of the paper, the number of paper sheets to be printed, and whether it is color printing or not, and Tm denotes the measured temperature of the fusing roller  50 . PSR′ represents the percentage of the power supplied to the heater  109  for a predetermined period with respect to the maximum power that can be supplied to the heater  109 , and is the sum of a predetermined offset value β′ and the control value, which is the product of the subtraction of the measured temperature from the target temperature by a predetermined coefficient α′. The coefficient α′ is determined by the quality of the paper, the printing speed, whether it is color printing or not, etc. The offset value β′ of equation (4) as a common constant may be the same as the offset value β in equation (1).
 
   Next, the controller  105  controls the power supplied to the heater  109  in operation  127  according to the PSR′ calculated in operation  123 . 
   If it is determined in operation  121  that the new power control period does not start yet or if the measured temperature is determined to be greater than or equal to the target fusing temperature in operation  122 , it is determined whether a new offset control period starts in operation  125 . 
   If it is determined in operation  125  that the new offset control period starts, an offset power supply ratio (Offset PSR) for the new offset control period is calculated using equation (5) in operation  126 . The Offset PSR is determined by the target fusing temperature, as expressed in equation (5) below.
 
Offset  PSR=εTt+ζ   (5)
 
where Tt denotes the target fusing temperature of the fusing roller  50  that varies depending on the type and the thickness of the paper, the number of paper sheets to be printed, and whether a printing mode is color printing or not. The Offset PSR is the percentage of the power supplied to the heater  109  for the offset supply period with respect to the maximum power that can be supplied to the heater  109 . ε and ζ are constants determined by a structure of the fusing roller  50 , for example, a diameter and a thickness of a cylindrical roller  51 , a thicknesses of the rubber layer  53  and a thickness of the toner-releasing coating layer  53   a  (refer to FIG.  4 ), etc., and the performance of the heater  109 .
 
   It is an aspect that the Offset PSR expressed as equation (5) is determined to be smaller than or equal to the power supply ratio with which the power is supplied to the heater  109  for the single new power control period with respect to the maximum power supplied to the heater  109  to keep the fusing (surface) temperature constant when the fusing roller is operated in a no-load state after the surface temperature of the fusing roller  50  reaches the target fusing temperature. A phase control is preferable to control the heater  109  based on the Offset PSR. However, the duty control corresponding to the Offset PSR is also applicable to control the fusing roller  50 , which has a rubber layer  53  and is subject to a delayed response to heating. 
   Next, the heater  109  is controlled to heat the fusing roller  50  for the new power control period in operation  127  according to the power supply ratio PSR′ calculated in operation  123  or for the offset control period according to the Offset PSR calculated in operations  26 . Next, the process returns to the operation  120 . 
   If it is determined in operation  125  that the new offset control period does not start, the process returns to operation  120 . 
   In the embodiment of the fusing temperature control method according to  FIG. 9 , as described above, when the new power control period does not start and the new offset control period starts, the power corresponding to the offset power supply ratio is supplied to the heater  109  to maintain the surface temperature constant. In addition, when there is a drop in the surface temperature as a result of the printing operation of the fusing apparatus, the surface temperature can be kept constant by compensating for the drop. In this embodiment, various controls can be achieved by varying the power control period and the offset control period. It is possible that the new power control period is a multiplication of an integer and the offset control period, or the offset control period is a multiplication of an integer and the power control period. 
     FIG. 10  shows waveforms of pulse voltages applied to the heater  109  corresponding to 10%, 20%, 25%, 33%, and 50% of a source voltage, respectively. In the waveforms of  FIG. 10 , dark half waves of a half period (T/2) represent a period of time for which the source voltage is supplied to the heater. As can be inferred from  FIG. 10 , an equal amount of the power can be supplied periodically to the heater  109  for the new power control period according to the phase control. 
     FIG. 11  is a flowchart illustrating a method of controlling the fusing temperature according to another embodiment of the present invention. Referring to  FIGS. 4 ,  6 , and  11 , the fusing temperature measuring unit  101  measures the surface temperature of the fusing roller  50  at a predetermined interval, for example, of 100 ms, and transmits the measured surface temperature (analog signal) to the ADC  103 . The ADC  103  converts the received analog signal into the digital signal and outputs the digital signal to the controller  105  in operation  130 . 
   The controller  105  determines whether the power control period, for example, a period of 30 seconds, has terminated and the new power control period starts in operation  131 . If it is determined that the new power control period starts, the controller  105  calculates a PSR″ using equation (6) below in operation  132 . Equation (6) is an exemplary equation used to calculate the PSR″ according to the P control. However, the present invention is not limited thereto. Other equations according to the PI control, the PID control, etc., can be applied to calculate the PSR″.
 
 PSR ″=α″( Tt−Tm )+β″  (6)
 
where Tt denotes the target fusing temperature of the fusing roller  50  that varies depending on the type and the thickness of the paper, the number of paper sheets to be printed, and whether it is color printing or not, and Tm denotes the measured surface temperature of the fusing roller  50 . PSR″ represents the percentage of the power supplied to the heater  109  for the new power control period with respect to the maximum power that can be supplied to the heater  109 . α″ is a coefficient used to compensate for a difference between the target fusing temperature and the measured temperature, and β″ is a common constant.
 
   Next, it is determined whether the power supply ratio PSR″ calculated in operation  132  is greater than zero in operation  133 . If the power supply ratio PSR″ is determined to be a positive value in operation  133 , the heater  109  is controlled to heat the fusing roller  50  for the new power control period in operation  136  according to the power supply ratio PSR″ calculated in operation  132 . 
   If it is determined in operation  131  that the new power control period does not start yet or if the power supply ratio PSR″ is determined to be less than or equal to zero in operation  133 , it is determined whether the new offset control period starts in operation  134 . 
   If it is determined in operation  134  that the new offset control period starts, an offset power supply ratio (Offset PSR′) for the new offset control period is calculated using equation (7) below in operation  135 . The Offset PSR′ is determined by the target fusing temperature, as expressed in equation (7) below.
 
Offset  PSR′=ε′Tt+ζ′   (7)
 
where ε′ and ζ′ are constants determined by the structure of the fusing roller  50 , for example, the diameter and the thickness of the cylindrical roller  51 , the thicknesses of the rubber layer  53 , the thickness of the toner-releasing coating layer  53   a  (refer to FIG.  4 ), etc., and the performance of the heater  109 .
 
   It is possible that the Offset PSR expressed as equation (7) is determined to be smaller than or equal to the power supply ratio with which the power is supplied for the single new power control period with respect to the maximum power supply to the heater  109  to keep the fusing temperature constant when the fusing roller  50  is operated in the no-load state after the surface temperature of the fusing roller  50  reaches the target fusing temperature. The phase control is used to control the heater  109  based on the Offset PSR. However, the duty control corresponding to the Offset PSR is also applicable to control the fusing roller  50  which has the rubber layer  53  and is subject to the delayed response to heating. 
   Next, the heater  109  is controlled to heat the fusing roller  50  for the power control period in operation  136  according to the power supply ratio PSR″ calculated in operation  132  or for the offset control period according to the Offset PSR′ calculated in operation  135 . Next, the process returns to operation  130 . 
   If it is determined in operation  134  that the new offset control period does not start yet, the process returns to the operation  130 . 
   In the fusing temperature control method according to the present embodiment, when the new power control period does not start and the new offset control period starts, the power corresponding to the offset power supply ratio is supplied to the heater  109  to maintain the surface temperature constant. In addition, when there is a drop in the surface temperature as a result of the printing operation of the fusing apparatus, the surface temperature can be kept constant by compensating for the drop. 
   The present embodiment shown in  FIG. 11  is substantially the same as the embodiment of  FIG. 9 , except that the heater  109  is controlled according to the power supply ratio PSR″ as far as the power supply ratio PSR″ calculated in operation  132  is the positive value in operation  133  even when the measured temperature of the fusing roller  30  is greater than the target fusing temperature. 
     FIG. 12  is a flowchart illustrating a method of controlling the fusing temperature according to another embodiment of the present invention. Referring to  FIGS. 4 ,  6 , and  12 , the fusing temperature measuring unit  101  measures the surface temperature of the fusing roller  50  at a predetermined interval, for example, of 100 ms, and transmits the measured surface temperature (analog signal) to the ADC  103 . The ADC  103  converts the received analog signal to a digital signal and outputs the digital signal to the controller  105  in operation  140 . 
   The controller  105  determines whether the predetermined power control period, for example, a period of 1-2 seconds, has terminated and the new power control period starts in operation  141 . If it is determined that the new power control period starts, it is determined whether the measured temperature is lower than the target fusing temperature in operation  142 . 
   If the measured temperature is determined to be lower than the target fusing temperature, the heater  109  is turned on during the new power control period in operation  145 , and the process returns to the operation  140 . 
   If the measured temperature is determined to be greater than or equal to the target fusing temperature, the heater  109  is turned off in operation  146 ), and the process goes to the operation  143 . 
   If it is determined in operation  141  that the new power control period does not start yet after the surface temperature of the fusing roller  50  has been measured or after the heater  109  is turned off in operation  146 , it is determined whether the new offset control period starts in operation  143 . 
   If it is determined in operation  143  that the new offset control period starts, the offset power supply ratio, Offset PSR, for the new offset control period is calculated using equation (5) illustrated supra. The offset power supply ratio used in this embodiment is substantially the same as the Offset PSR of  FIG. 5 , and thus a detailed description thereon is omitted. 
   Next, the heater  109  is controlled to heat the fusing roller  50  for the offset control period according to the Offset PSR in operation  147 . Next, the process returns to the operation  140 . 
   If it is determined in operation  143  that the new offset control period does not start, the process returns to operation  140 . 
   In the fusing temperature control method according to the present embodiment, the surface temperature of the fusing roller  50  is controlled by the on-off control when the new power control period starts. When the new power control period does not start yet or when the new power control period starts and the heater  109  turned off due to the measured temperature of the fusing roller  50  that is greater than the target fusing temperature, the above-described offset control is performed to keep the temperature of the fusing roller constant. 
   The fusing temperature control methods described in the above embodiments may be used individually or in combination depending on the printing circumstances. For example, the fusing temperature control method according to the embodiment of  FIG. 7  may be applied in a print standby mode. The fusing temperature control method according to the embodiment of  FIG. 9  may be applied for a slower color printing mode. The fusing temperature control method according to the embodiment of  FIG. 12  may be applied for a relatively speedy monochromic printing mode. 
   As described above, in the method of controlling the fusing temperature of the electrophotographic imaging apparatus according to the embodiments of the present invention, a thermal loss during a print mode is compensated by conventional power control methods, and the thermal loss in the fusing apparatus in the no-load state is compensated by periodically supplying the power to the heater, so that the fusing temperature is maintained nearly constant, and a quality of images fused onto a recording medium is improved. 
   In addition, according to the present invention, since the power control is performed for a predetermined cycle that is longer than usual, ranging from a few to tens of seconds, there is no serious flicker problem. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents.