Abstract:
An image forming apparatus is provided with: a heating unit which generates heat to fuse a toner on a printing medium; a switch which selectively supplies alternating current (AC) power to the heating unit; a first switching driver which drives the switch to supply the AC power to the heating unit; and a first supply limiter which allows the AC power to be supplied to the heating unit by the first switching driver if a polarity of the AC power is the same as a preset polarity, and cuts off the AC power supplied to the heating unit if the polarity of the AC power is opposite to the preset polarity.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims all benefits accruing under 35 U.S.C. §119 from Korean Patent Application No. 2007-81421, filed on Aug. 13, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Aspects of the present invention relate to an image forming apparatus, and more particularly, to an image forming apparatus which fuses a toner on a printing medium to form an image, and a control method thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    An image forming apparatus, such as, a laser printer, a photo-copier, a facsimile machine and a multi-functional product, typically utilizes a heating unit and a fusing circuit to fuse a toner on a printing medium, such as, paper to form an image. 
         [0006]      FIG. 1  illustrates a typical fusing circuit for driving operation of a heating unit in an image forming apparatus. As shown in  FIG. 1 , the image forming apparatus may include a heating unit  11 , such as, a lamp which heats a toner to be fused on a printing medium, and a fusing circuit  10  arranged to drive the heating unit  11 . The fusing circuit  10  includes a triac Q 1  (i.e., bidirectional triode thyristor) disposed between an alternating current (AC) source and the heating unit  101 , a transistor Q 2  coupled to receive a control signal, and a photo-coupler PC 1  including a light emitter PC 1   a  and a light receiver PC 1   b  to control operation the triac Q 1  based upon receipt of the control signal, via the transistor Q 2 . 
         [0007]    The detailed operation of the fusing circuit  10  is as follows. If a level of a control signal is low, the transistor Q 2  is turned OFF. Then, a current does not flow through the light emitter PC 1   a , and the light receiver PC 1   b  is turned OFF. In this case, a trigger signal is not generated by the light receiver PC 1   b  of the photo-coupler PC 1 . If the triac Q 1  is turned OFF, alternating current (AC) power is not supplied to the heating unit  11 . 
         [0008]    Meanwhile, if a level of a control signal is high, the transistor Q 2  is turned ON. Then, a current corresponding to DC power Vcc flows through the light emitter PC 1   a , and the light receiver PC 1   b  is turned ON. If the light receiver PC 1   b  is turned ON, the triac Q 1  is also turned ON, thereby establishing a supply path of AC power to the heating unit  11 . Then, the AC power is supplied to the heating unit  11 , thereby heating the heating unit  11  to a preset temperature for a fusing operation. However, if a polarity of the current flowing through the triac Q 1  is reverse, the triac Q 1  is turned OFF to cut off the supply path of the AC power to the heating unit  11 . 
         [0009]    According to the foregoing operation principle, the image forming apparatus may adjust the level of the control signal so as to supply only a half-wave range of the AC power to the heating unit  11 . As a result, the heating temperature of the heating unit  11  can be controlled by adjusting the number of half-wave ranges of the AC power supplied to the heating unit  11  for a predetermined time. 
         [0010]    In addition, the photo-coupler PC 1  may be used to adjust the level of the control signal according to the half-wave range of the AC power. The photo-coupler PC 1  is designed to operate by detecting whether a phase of the AC power is reverse. The photo-coupler PC 1  remains turned OFF even if the level of the inputted control signal is high. The photo-coupler PC 1  may also be designed to be turned ON only after the phase of the AC power is reverse. 
         [0011]    However, the photo-coupler PC 1  may not accurately detect when the phase of the AC power is reverse due to its own characteristics. The problem will be described with reference to  FIG. 2  which illustrates a waveform of the current supplied to the heating unit  11  herein below. 
         [0012]    As shown in  FIG. 2 , an input voltage Vin is inputted to the photo-coupler PC 1 , and the phase thereof is the same as the phase of the AC power. An output current Iout is supplied to the heating unit  11 . As shown therein, if an absolute value of the input voltage Vin is smaller than a preset reference voltage Vth during a range A 1  in which the level of the control signal is high, the light receiver PC 1   b  of the photo-coupler PC 1  is turned ON. 
         [0013]    As described above, the light receiver PC 1   b  is not turned ON when the input voltage Vin becomes zero, i.e. precisely when the phase of the AC power is reverse, but is turned ON a time, which corresponds to the reference voltage Vth, before the phase of the AC power is reverse. Then, the triac Q 1  is also turned ON before the phase of the AC power is reverse. The waveform of the output current Iout supplied to the heating unit  11  is not a complete half wave, and a current having an opposite polarity flows in advance (refer to B 1  in  FIG. 2 ). If the waveform of the output current Iout supplied to the heating unit  11  is not the complete half wave, noises, such as an electromagnetic interference (EMI) or harmonics, occur. 
         [0014]      FIG. 3  illustrates a graph which shows noises occurring during the operation of the conventional image forming apparatus  10 . As shown in  FIG. 3 , reference numerals D 1  and E 1  represent the magnitude of the noises measured when the current has the maximum value and the minimum value, respectively. Reference numerals F 1  and G 1  represent acceptable limits of the noises of D 1  and E 1 , respectively. 
         [0015]    As shown therein, noises which exceed the acceptable limits F 1  and G 1  occur in a low frequency band (approximately 150 KHz to 200 KHz). The noises which exceed the acceptable limits F 1  and G 1  may cause drastic changes in voltages supplied to electronic devices near the image forming apparatus  10 . Such electronic devices may be adversely affected, e.g. may flicker or malfunction due to the noises. 
       SUMMARY OF THE INVENTION 
       [0016]    Several aspects and example embodiments of the present invention provide an image forming apparatus which minimizes noise occurring during a fusing operation and improves reliability, and a control method thereof. 
         [0017]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
         [0018]    In accordance with an example embodiment of the present invention, an image forming apparatus comprising: a heating unit which generates heat to fuse a toner on a printing medium; a fusing circuit which drives operation of the heating unit, and comprises a switch which selectively supplies alternating current (AC) power to the heating unit; a first switching driver which drives the switch to supply the AC power to the heating unit; and a first supply limiter which allows the AC power to be supplied to the heating unit by the first switching driver if a polarity of the AC power is the same as a preset first polarity, and which cuts off the AC power supplied to the heating unit if the polarity of the AC power is opposite to the first polarity. 
         [0019]    According to an aspect of the present invention, the switch may include a triac, and the first switching driver triggers a gate of the triac to supply the AC power to the heating unit. 
         [0020]    According to an aspect of the present invention, the first switching driver may include a first photo-coupler which is turned ON upon receipt of a control signal when a polarity of the AC power is reverse. 
         [0021]    According to another aspect of the present invention, the first switching driver may further include a first transistor which is turned ON to allow the first photo coupler to operate if the control signal is received. 
         [0022]    According to an aspect of the present invention, the first supply limiter may include a first diode which is disposed between the gate of the triac and an output terminal of the first photo-coupler. 
         [0023]    According to another aspect of the present invention, the fusing circuit may further include: a second switching driver which drives the switch to supply AC power having a polarity opposite to that of the AC power supplied by the first switching driver, to the heating unit; and a second supply limiter which allows the AC power to be supplied to the heating unit by the second switching driver if the polarity of the AC power is the same as a second polarity opposite to the first polarity, and which cuts off the AC power supplied to the heating unit if the polarity of the AC power is opposite to the second polarity. 
         [0024]    According to a further aspect of the present invention, the second switching driver may include a second photo-coupler which is turned ON upon receipt of a control signal when a polarity of the AC power is reverse, and a second transistor which is turned ON to allow the second photo-coupler to operate if the control signal is received. 
         [0025]    According to a further aspect of the present invention, the second supply limiter may include a second diode which is disposed between the gate of the triac and an output terminal of the second photo-coupler. 
         [0026]    In accordance with another example embodiment of the present invention, a control method of an image forming apparatus which has a heating unit generating heat to fuse a toner on a printing medium and form an image, the control method including: attempting to supply alternating current (AC) power to the heating unit if a control signal is generated to supply the AC power to the heating unit; and supplying the AC power to the heating unit if a polarity of the AC power to be supplied to the heating unit is the same as a preset polarity, and cutting off the AC power supplied to the heating unit if the polarity of the AC power is opposite to the preset polarity. 
         [0027]    According to an aspect of the present invention, the cutting off the AC power may include preventing a trigger signal from being transmitted from an output terminal of a photo coupler turned ON by the control signal, to a gate of a triac selectively supplying the AC power to the heating unit. 
         [0028]    In accordance with yet another example of the present invention, an image forming apparatus is provided with a heating unit which generates heat to fuse a toner on a printing medium during a fusing operation to form an image; and a fusing circuit which drives operation of the heating unit to minimize noise from occurring during the fusing operation, the fusing circuit comprising: a switch disposed between a power source and the heating unit, to selectively supply power to the heating unit to generate heat for the fusing operation; a first switching driver arranged to activate the switch to supply power to the heating unit; and a first supply limiter arranged to enable the power to be supplied to the heating unit, via the first switching driver, if a polarity of the power is the same as a preset polarity, and to disable the power supplied to the heating unit if the polarity of the power is opposite to the preset polarity. 
         [0029]    According to an aspect of the present invention, the switch corresponds to a triac having a first input terminal connected to the power source, a second input terminal connected to the heating unit, and a gate driven by the first switching driver. 
         [0030]    According to another aspect of the present invention, the first switching driver comprises: a first photo-coupler including a light emitter connected to a voltage terminal to emit light upon receipt of a first control signal, and a light receiver connected to the switch to activate the switch when the polarity of the power is reverse; and a first transistor including a first electrode electrically connected to the voltage terminal, via the light emitter of the first photo-coupler, a second electrode connected to ground, and a gate electrode driven upon receipt of the first control signal. 
         [0031]    According to another aspect of the present invention, the fusing circuit further comprises: a second switching driver arranged in parallel with the first switching driver, to activate the switch to supply power having a polarity opposite to that of the power supplied by the first switching driver, to the heating unit; and a second supply limiter arranged in parallel with the first supply limiter, to enable the power to be supplied to the heating unit, via the second switching driver, if the polarity of the power is the same as a second polarity opposite to the first polarity, and to disable the power supplied to the heating unit if the polarity of the power is opposite to the second polarity. 
         [0032]    According to yet another aspect of the present invention, the second switching driver comprises: a second photo-coupler arranged in parallel with the first photo-coupler, including a light emitter connected to a voltage terminal to emit light upon receipt of a second control signal, and a light receiver connected to the switch to activate the switch when the polarity of the power is reverse; and a second transistor arranged in parallel with the first transistor, including a first electrode electrically connected to the voltage terminal, via the light emitter of the second photo-coupler, a second electrode connected to ground, and a gate electrode driven upon receipt of the second control signal. 
         [0033]    According to an aspect of the present invention, the first and second supply limiters comprise first and second diodes arranged in parallel and disposed between the gate of the triac and an output terminal of the first and second photo-couplers. 
         [0034]    According to yet another aspect of the present invention, the fusing circuit further comprises: an inductance connected to the power source to remove noise occurring when the power is supplied to the heating unit; a first resistor and a first capacitor connected in series, and disposed between the inductance and the output terminals of the first and second photo-couplers, to remove noise occurring when the switch is turned ON; a second resistor and a second capacitor arranged in parallel with the first and second photo-couplers to remove noise occurring when the light receiver of the first and second photo-couplers are turned ON; and a third resistor disposed between the inductance and the light receiver of the first and second photo-couplers. 
         [0035]    According to a further aspect of the present invention, the first and second switching drivers further comprise first and second resistors connected to the gate electrode of the first and second transistors. In addition to the example embodiments and aspects as described above, further aspects and embodiments will be apparent by reference to the drawings and by study of the following descriptions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    A better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the following written and illustrated disclosure focuses on disclosing example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and that the invention is not limited thereto. The spirit and scope of the present invention are limited only by the terms of the appended claims. The following represents brief descriptions of the drawings, wherein: 
           [0037]      FIG. 1  is a circuit diagram of a typical fusing circuit for driving operation of a heating unit in an image forming apparatus; 
           [0038]      FIG. 2  illustrates a waveform of a current supplied to a heating unit in an image forming apparatus; 
           [0039]      FIG. 3  illustrates a graph which shows noises occurring during the operation of an image forming apparatus; 
           [0040]      FIG. 4  is a circuit diagram of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to an example embodiment of the present invention; 
           [0041]      FIG. 5  illustrates a waveform of a current supplied to a heating unit in an image forming apparatus according to an example embodiment of the present invention; 
           [0042]      FIG. 6  is a circuit diagram of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to another example embodiment of the present invention; 
           [0043]      FIG. 7  is a circuit diagram of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to yet another example embodiment of the present invention; 
           [0044]      FIG. 8  illustrates a graph which shows noises occurring during an operation of an image forming apparatus according to an example embodiment of the present invention; and 
           [0045]      FIG. 9  is a flowchart which describes a control method of an image forming apparatus according to an example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0046]    Reference will now be made in detail to the present 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. 
         [0047]      FIG. 4  is a circuit diagram of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to an example embodiment of the present invention. The image forming apparatus may correspond to a laser printer, a photo-copier, a facsimile machine and a multi-functional product, which fuses a toner on a printing medium, such as, paper to form an image. 
         [0048]    The image forming apparatus may include an image processor (not shown) which processes image data to be printed on a printing medium, a laser scanning unit (not shown) which scans laser to the processed image data, a photosensitive drum (not shown) which forms a latent image thereon by the laser scanning unit, a cartridge (not shown) which accommodates a toner therein to be developed on the latent image formed, a transfer roller (not shown) which transfers the developed toner to the printing medium, a fusing unit (not shown) which fuses the transferred toner on the printing medium by heat and pressure, a feeding unit (not shown) which feeds the printing medium, and a power supply (not shown) which supplies operating power to the foregoing elements. 
         [0049]    As shown in  FIG. 4 , the image forming apparatus includes a heating unit  101  such as a lamp to supply heat for a fusing operation, and a fusing circuit  100  arranged to drive operation of the heating unit  101 . The heating unit  101  may be included in the fusing unit. The fusing circuit includes a triac Q 10  (i.e., a bidirectional triode thyristor) disposed between an alternating current (AC) source and the heating unit  101 , which connects or disconnects a supply path of alternating current (AC) power to the heating unit  101 ; a photo-coupler PC  0  which controls the connection or disconnection of the triac Q 10 ; and a transistor Q 20  which controls the operation of the photo-coupler PC 10  according to a control signal. 
         [0050]    The photo-coupler PC 10  includes a light emitter PC 10   a  which emits light if a current flows therethrough, i.e., a diode that converts electrical power into light, and a light receiver PC 10   b  which is turned ON and OFF according to light emitted by the light emitter PC 10   a . With the foregoing configuration, the fusing circuit  100  allows the AC power to be selectively supplied to the heating unit  101 . 
         [0051]    The transistor Q 20  is disposed between a power terminal Vcc and ground, and includes a collector connected to the power terminal Vcc, via the light emitter PC 10   a  of the photo-coupler PC 10 , an emitter connected to the ground, and a gate electrode coupled to receive a control signal. As shown in  FIG. 4 , the transistor Q 20  is a NPN transistor; however, PNP transistor may also be utilized as well as other IC circuits. 
         [0052]    The fusing circuit  100  further includes a diode  102  which is disposed between a gate of the triac Q 10  and an output terminal (of the light receiver PC 10   b ) of the photo-coupler PC 10 . An anode of the diode  102  is connected to the light receiver PC 10   b , while a cathode thereof is connected to the gate of the triac Q 10 . As the diode  102  is disposed in such a direction, a trigger signal is supplied to the gate of the triac Q 10  if the polarity of the trigger signal applied to the gate of the triac Q 10  is positive (i.e. if a voltage of the anode of the diode  102  is higher than that of the cathode thereof. However, if the polarity of the trigger signal is negative (i.e. if the voltage of the anode of the diode  102  is lower than that of the cathode thereof), the trigger signal is cut off. The operation of the fusing circuit  100  in an image forming apparatus having the foregoing configuration will be described as follows. 
         [0053]    First, if a level of the control signal is low, the transistor Q 20  is turned OFF. Then, a current does not flow through the light emitter PC 10   a . As the light is not emitted, the light receiver PC 10   b  is turned OFF. As the current does not flow through the light receiver PC 10   b , a trigger signal is not generated. If the triac Q 10  is turned OFF, the gate of the triac Q 10  is not triggered. Then, the triac Q 10  remains turned OFF. While the triac Q 10  is turned OFF, the AC power is not supplied to the heating unit  101 . 
         [0054]    If the level of the control signal is high, the transistor Q 20  is turned ON. Then, the current corresponding to DC power Vcc flows through the light emitter PC 10   a , and the light receiver PC 10   b  is turned ON. 
         [0055]    The phase of the current flowing through the light receiver PC 10   b  is substantially the same as that of the AC power. Thus, the polarity of the trigger signal generated by the light receiver PC 10   b  is the same as that of the AC power. If the polarity of the AC power is positive, i.e. if the polarity of the trigger signal is positive, the trigger signal is supplied to the gate of the triac Q 1  through the diode  102 . If the triac Q 1  is triggered to turn ON, the supply path of the AC power from the AC power source to the heating unit  102  is established. In this case, the AC power from the AC power source is supplied to the heating unit  101  to generate heat. If the polarity of the AC power is turned negative, the triac Q 10  is turned OFF to disconnect the supply path of the AC power from the AC power source to the heating unit  101 . Then, the AC power from the AC power source is not supplied to the heating unit  101 . 
         [0056]    If the level of the control signal is high while the polarity of the AC power is negative, the polarity of the trigger signal is also negative. In this case, the trigger signal is blocked by the diode  102  so as not to be supplied to the gate of the triac Q 10 . If the triac Q 10  is turned OFF, the gate of the triac Q 10  is not triggered. Thus, the triac Q 10  remains turned OFF. While the triac Q 10  is turned OFF, the supply path of the AC power to the heating unit  101  is disconnected. Thus, the AC power is not supplied to the heating unit  101 . 
         [0057]    The photo-coupler PC 10  may be designed to operate by detecting whether the polarity of the AC power is reverse. That is, the photo-coupler PC 10  remains turned OFF even if the level of the input control signal is high, and may be turned ON only when the phase of the AC power is reverse. 
         [0058]    As shown in  FIG. 5 , if the control signal is high while the polarity of the input voltage Vin having the same phase as the AC power is negative (refer to A 10 ), the photo-coupler PC 10  may determine that the polarity of the input voltage Vin is reverse from negative to positive in case that an absolute value of the input voltage Vin is smaller than a preset reference voltage Vth. As a result, the light receiver PC 10   b  of the photo-coupler PC 10  is turned ON. 
         [0059]    However, the polarity of the trigger signal generated by the light receiver PC 10   b  is still negative. Thus, the trigger signal is blocked by the diode  102  so as not to be supplied to the gate of the triac Q 10 . As the triac Q 10  remains turned OFF, the AC power is not supplied to the heating unit  101 . 
         [0060]    Then, the diode  102  operates to allow the AC power to be supplied to the heating unit  101 , if a high control signal is applied and the polarity of the AC power supplied to the heating unit  101  is positive. However, if the polarity of the AC power is negative, the diode  102  cuts off the AC power supplied to the heating unit  101 . 
         [0061]    According to an example embodiment of the present invention, even if the photo-coupler PC 10  does not accurately detect when the phase of the AC power is reverse due to its own properties, the AC power having the polarity opposite to the desired polarity is prevented from being supplied to the heating unit  101 . Then, noises, such as EMI, are minimized and reliability of the image forming apparatus may improve. 
         [0062]    The image forming apparatus may adjust the level of the control signal to supply only a half-wave range of the AC power to the heating unit  101 . By adjusting the number of the half wave ranges of the AC power supplied to the heating unit  101  for a predetermined time, the heating temperature of the heating unit  101  may be controlled. 
         [0063]    Hereinafter, an image forming apparatus according to another example embodiment of the present invention will be described.  FIG. 6  is a circuit diagram of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to another example embodiment of the present invention. Configurations and functions of the fusing circuit  100   a  and the image forming apparatus equivalent to those of the fusing circuit  100  and the image forming apparatus, shown in  FIG. 4 , will be omitted for the sake of brevity. 
         [0064]    As shown in  FIG. 6 , the fusing circuit  100   a  includes the same circuit elements, shown in  FIG. 4 , that is, a triac Q 10  (i.e., a bidirectional triode thyristor) disposed between an alternating current (AC) source and the heating unit  101 , which connects or disconnects a supply path of alternating current (AC) power to the heating unit  101 ; first photo-couplers PC 10  including a first light emitter PC 10   a  and a first light receiver PC 10   b , which control the connection or disconnection of the triac Q 10 ; a first diode  102  disposed between the first light receiver PC 10   b  of the first photo-couplers PC 10  and the triac Q 10 , and a first transistor Q 20  which controls the operation of the photo-coupler PC 10  according to a first control signal. 
         [0065]    In addition, the fusing circuit  100   a  may further include second photo-couplers PC 12   a  and PC 12   b , which turn ON or turn OFF a triac Q 10 , and a second transistor Q 22  which controls an operation of the second photo-couplers PC 12   a  and PC 12   b  according to a second control signal. 
         [0066]    The second photo-couplers PC 12   a  and PC 12   b  include a second light emitter PC 12   a  which emits light if a current flows therethrough, and a second light receiver PC 12   b  which is turned ON or OFF according to light emitted by the second light emitter PC 12   a . For purposes of convenience, marks of the first photo-couplers PC 10   a  and PC 10   b  and the second photo-couplers PC 12   a  and PC 12   b  are omitted from  FIG. 6 . However, the second photo-couplers PC 12   a  and PC 12   b  are disposed in parallel with the first photo-couplers PC 10   a  and PC 10   b.    
         [0067]    The fusing circuit  100   a  further includes a second diode  122  which is disposed between a gate of the triac Q 10  and an output terminal (of the second light receiver PC 12   b ) of the second photo-couplers PC 12   a  and PC 12   b . An anode of the second diode  122  is connected to the gate of the triac Q 10 , while a cathode thereof is connected to the second light receiver PC 12   b . As the second diode  122  is disposed in such a direction, the trigger signal is supplied to the gate of the triac Q 10  if a polarity of a trigger signal applied to the gate of the triac Q 10  is negative (i.e. if a voltage of the anode of the second diode  122  is higher than that of the cathode thereof. However, if the polarity of the trigger signal is positive (i.e., if the voltage of the anode of the second diode  122  is lower than that of the cathode thereof), the trigger signal is not transmitted. 
         [0068]    Hereinafter, the detailed operation of the fusing circuit  100   a , shown in  FIG. 6 , will be described. The first control signal of the fusing circuit  100   a  is equivalent or similar to the control signal of the fusing circuit  100 , shown in  FIG. 4 . Thus, the description of the first control signal will be omitted for the sake of brevity. 
         [0069]    If a level of the second control signal is low, the second transistor Q 22 , the second light emitter PC 12   a , the second light receiver PC 12   b  and the triac Q 10  are not turned ON. In this case, the AC power is not supplied to the heating unit  101 . 
         [0070]    If a level of the second control signal is high, the second transistor Q 22  is turned ON. Then, a current corresponding to DC power Vcc flows through the second light emitter PC 12   a , and the second light receiver PC 12   b  is turned ON. 
         [0071]    If a polarity of the AC power is negative, i.e. if the polarity of the trigger signal generated by the second light receiver PC 12   b  is negative, the trigger signal is supplied to the gate of the triac Q 10  through the second diode  122 . The triac Q 10  is triggered to be turned ON, and the AC power is supplied to the heating unit  101 . If the polarity of the AC power is turned positive, the triac Q 10  is turned OFF. In this case, the AC power is not supplied to the heating unit  101 . 
         [0072]    If the level of the second control signal is high while the polarity of the AC power is positive, the polarity of the trigger signal also is positive. Then, the trigger signal is blocked by the second diode  122  so as not to be supplied to the gate of the triac Q 10 . If the triac Q 10  is turned OFF, the gate of the triac Q 10  is not triggered. The triac Q 10  remains turned OFF. The supply path of the AC power to the heating unit  101  is not established, while the triac Q 1  is turned OFF. Thus, the AC power is not supplied to the heating unit  101 . 
         [0073]    Similarly to the first photocouplers PC 10   a  and PC 10   b , if the second control signal is high while the polarity of an input voltage Vin is positive and if an absolute value of the input voltage Vin is smaller than a preset reference voltage Vth, the second photo-couplers PC 12   a  and PC 12   b  may determine that the polarity of the input voltage Vin is reverse from positive to negative, and may turn ON the light receiver PC 12   b.    
         [0074]    However, when the second light receiver PC 12   b  is turned ON, the polarity of the trigger signal generated by the second light receiver PC 12   b  is still positive. Thus, the trigger signal is blocked by the second diode  122  so as not to be supplied to the gate of the triac Q 10 . As the triac Q 10  remains turned OFF, the AC power is not supplied to the heating unit  101 . 
         [0075]    If a high-level control signal is applied and the polarity of the AC power supplied to the heating unit  101  is negative, the second diode  122  allows the AC power to be supplied to the heating unit  101 . If the polarity of the AC power is negative, the second diode  122  cuts off the AC power supplied to the heating unit  101 . 
         [0076]    The second control signal may be opposite to the first control signal input to the first transistor Q 20 , with respect to the polarity of the AC power to be supplied to the heating unit  101 . For example, if the first control signal is designed to supply the AC power having a positive polarity to the heating unit  101 , the second control signal may be designed to supply the AC power having a negative polarity to the heating unit  101 . By adjusting the first and second control signals, one of the positive half-wave range and the negative half-wave range of the AC power may be supplied to the heating unit  101 . The first and second control signals may be generated by a control signal generator (not shown) or a main controller of an image forming apparatus. 
         [0077]    Turning now to  FIG. 7 , a circuit diagram of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to yet another example embodiment of the present invention is illustrated. Configurations and functions of the fusing circuit  100   b  and the image forming apparatus equivalent or similar to those of the fusing circuit  100  and the image forming apparatus, shown in  FIG. 4 , and the fusing circuit  100   a  and the image forming apparatus, shown in  FIG. 6 , will be omitted herein for the sake of brevity. 
         [0078]    The fusing circuit  100   b  may further include a first resistor R 1 , a first capacitor C 1 , a second resistor R 2 , a second capacitor C 2 , an inductor L, a third resistor R 3 , a fourth resistor R 4  and a fifth resistor R 5 . 
         [0079]    The first resistor R 1  and the first capacitor C 1  remove noises occurring when a triac Q 10  is switched. The second resistor R 2  and the second capacitor C 2  remove noises occurring when a first light receiver PC 10   b  and a second light receiver PC 12   b  are switched, to stabilize the fusing circuit  100   b . The inductor L removes noises occurring when the AC power is switched. 
         [0080]    The third resistor R 3  determines a level of a current flowing through the first and second light receivers PC 10   b  and PC 12   b . A resistance value of the third resistor R 3  is set to trigger a gate of the triac Q 10 . The fourth and fifth resistors R 4  and R 5  determine levels of first and second control signals supplied to bases of the first transistor Q 20  and the second transistor Q 22 , respectively. 
         [0081]      FIG. 8  illustrates a graph which shows noises occurring during the operation of the fusing circuit  100   b  according to an example embodiment of the present invention. As shown in  FIG. 8 , reference numerals D 10  and E 10  refer to the magnitude of noises measured when a current has the maximum value and the minimum value. Reference numerals F 1  and G 1  refer to acceptable limits of the noises as shown in  FIG. 3 . As shown therein, noises drastically decrease in a low frequency band (approximately 150 KHz to 200 KHz) according to an example embodiment of the present invention, compared with C 1 , shown in  FIG. 3 . 
         [0082]      FIG. 9  is a flowchart which describes a control method of a fusing circuit for driving operation of a heating unit in an image forming apparatus according to an example embodiment of the present invention. The fusing circuit according to an example embodiment of the present invention may include a fusing circuit  100 ,  100   a  or  100   b  which is shown in  FIG. 4 ,  6  or  7 . 
         [0083]    If the control signal is generated to supply the AC power to the heating unit  101 , an attempt to supply the AC power to the heating unit  101  is made at operation S 101 . The control signal generated at operation S 101  to supply the AC power to the heating unit  101  may include the control signal, the first control signal or the second control signal in  FIG. 4 ,  6  or  7 . The process of attempting to supply the AC power to the heating unit  101  may include a process of transmitting the trigger signal to the gate of the triac Q 10  according to the control signal, the first control signal or the second control signal by the first transistor Q 20  and the first photo-couplers PC 10   a  and PC 10   b , or by the second transistor Q 22  and the second photo couplers PC 12   a  and PC 12   b.    
         [0084]    If the polarity of the AC power to be supplied to the heating unit  101  is to the same as the preset polarity, the AC power is supplied to the heating unit  101  at operation S 102 . If the polarity of the AC power is opposite to the preset polarity, the AC power supplied to the heating unit  101  is cut off at operation S 102 . At operation S 102 , the polarity of the AC power and the arrangement directions of the first diode  102  or the second diode  122  may determine whether the polarity of the AC power is equivalent to the preset polarity. The process of supplying the AC power to the heating unit  101  may include a process of transmitting the trigger signal generated by the first light receiver PC 10   b  or the second light receiver PC 12   b  to the gate of the triac Q 10  through the first diode  102  or the second diode  122  and turning on the triac Q 10 . 
         [0085]    The process of cutting off the AC power supplied to the heating unit  101  may include a process of cutting off the trigger signal generated by the first light receiver PC 10   b  or the second light receiver PC 12   b  by the first diode  102  or the second diode  122  so as not to be supplied to the gate of the triac Q 10 , and not turning on the triac Q 10 . 
         [0086]    As described above, the present invention provides an image forming apparatus which minimizes noises during a fusing operation, prevents from adversely affecting electronic devices near or around the image forming apparatus and improves reliability, and a control method thereof. 
         [0087]    While there have been illustrated and described what are considered to be example embodiments of the present invention, it will be understood by those skilled in the art and as technology develops that various changes and modifications, may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. Many modifications, permutations, additions and sub-combinations may be made to adapt the teachings of the present invention to a particular situation without departing from the scope thereof. For example, the fusing circuit, shown in  FIG. 4 ,  FIG. 6  and  FIG. 7 , may be incorporated into the main controller of an image forming apparatus. Individual circuit components of the fusing circuit, shown in  FIG. 4 ,  FIG. 6  and  FIG. 7 , can be replaced by equivalent IC, as long as noises can be contained in substantially the same way. Accordingly, it is intended, therefore, that the present invention not be limited to the various example embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims.