Abstract:
An apparatus and method for driving an ink-jet printhead in which current is applied to a heater to heat ink to be supplied in an ink chamber to generate a bubble to eject ink from the ink chamber, the apparatus including a circuit that alternately applies current to the heater to alternate a direction of current flowing through the heater.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an apparatus and method for driving an ink-jet printhead. More particularly, the present invention relates to an apparatus and method for driving a thermal ink-jet printhead that is able to extend a lifespan of a heater by alternately applying current pulses to the heater.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of ink at a desired position on a recording sheet. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is a thermal ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink to cause an ink droplet to be ejected due to the expansive force of the formed bubble. A second type is a piezoelectric ink-jet printhead, in which an ink droplet is ejected by a pressure applied to the ink due to a deformation of a piezoelectric element.  
         [0005]     An ink droplet ejection mechanism of a thermal ink-jet printhead will now be explained in detail. When a current pulse is supplied to a heater, which includes a heating resistor, the heater generates heat and ink near the heater is instantaneously heated to approximately 700° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated and exert pressure on ink filling an ink chamber. As a result, ink around a nozzle is ejected from the ink chamber in the form of a droplet through the nozzle.  
         [0006]     A thermal inkjet printhead is classified into a top-shooting type, a side-shooting type, and a back-shooting type depending on a bubble growing direction and a droplet ejection direction. In a top-shooting type of printhead, bubbles grow in the same direction in which an ink droplet is ejected. In a side-shooting type of printhead, bubbles grow in a direction perpendicular to a direction in which an ink droplet is ejected. In a back-shooting type of printhead, bubbles grow in a direction opposite to a direction in which an ink droplet is ejected.  
         [0007]     An ink-jet printhead using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after being ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase an operating frequency.  
         [0008]      FIG. 1  illustrates an exploded perspective view of a conventional thermal ink-jet printhead.  FIG. 2  illustrates a cross-sectional view for explaining a process of ejecting an ink droplet using the conventional thermal ink-jet printhead of  FIG. 1 .  
         [0009]     Referring to  FIGS. 1 and 2 , the conventional thermal ink-jet printhead includes a substrate  10 , an ink chamber  26 , which is formed on the substrate  10  and stores ink therein, partition walls  14 , which define the ink chamber  26 , a heater  12 , which is disposed within the ink chamber  26 , a nozzle  16 , through which an ink droplet  29 ′ is ejected, and a nozzle plate  18 , through which the nozzle  16  is formed. In operation, a current pulse is supplied to the heater  12  to generate heat, such that ink  29  filled in the ink chamber  26  is heated, thereby generating a bubble  28 . The generated bubble  28  is continuously expanded such that pressure is applied to the ink  29  filled in the ink chamber  26 , thereby ejecting the ink droplet  29 ′ out of the printhead through the nozzle  16 . Subsequently, ink  29  from a manifold  22  is introduced into the ink chamber  26  through an ink channel  24 . Resultantly, the ink chamber  26  is refilled with ink  29 .  
         [0010]      FIG. 3  is a circuit diagram of a first conventional circuit for driving a thermal ink-jet printhead.  FIG. 4  is a diagram illustrating pulses of the first conventional circuit of  FIG. 3 .  
         [0011]     Referring to  FIGS. 3 and 4 , in a circuit to which a positive voltage V 1  is constantly applied as a supply voltage pulse V CC  to drive an ink-jet printhead, a current pulse I H  is supplied to a thin film heater  30  using a drive signal S DR  and a field effect transistor (FET). According to the conventional circuit, since a current flows in a constant direction through the heater  30 , damage to the heater  30  may occur due to electromigration. Recently, attempts to reduce an amount of energy applied to a high-density printhead by reducing a thickness of a heater therein have been made. As the heater becomes thinner, however, damage to the heater due to electromigration becomes a more serious problem.  
         [0012]      FIG. 5  is a circuit diagram of a second conventional circuit for driving an ink-jet printhead.  FIG. 6  is a diagram illustrating pulses of the second conventional circuit of  FIG. 5 .  
         [0013]     Referring to  FIGS. 5 and 6 , in a circuit to which a supply voltage pulse V CC  is supplied to drive an ink-jet printhead, a current pulse I H  is supplied to a heater  50  using a drive signal S DR  and a driving electric FET. A current waveform is controlled by means of a pull down resistor and two electric FETs. According to the second conventional circuit, current waveform distortion, such as overshoot, may be reduced, and thus maximum current amplitude is lowered, which results in a decrease in damage to the heater  50  due to electromigration. As mentioned above, the second conventional circuit similarly has a similar in reducing the possibility of damage to the heater  50  that is caused by a decrease in a thickness of the heater  50 .  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention is therefore directed to an apparatus and method for driving a thermal ink-jet printhead, which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art.  
         [0015]     It is a feature of an embodiment of the present invention to provide an apparatus and a method for driving a thermal ink-jet printhead that are able to extend a lifespan of a heater by alternately applying current pulses to the heater.  
         [0016]     It is another feature of an embodiment of the present invention to provide an apparatus and a method for driving a thermal ink-jet printhead that improve reliability of the performance of the ink-jet printhead.  
         [0017]     At least one of the above features and other advantages may be provided by an apparatus for driving an ink-jet printhead in which current is applied to a heater to heat ink to be supplied in an ink chamber to generate a bubble to eject ink from the ink chamber, the apparatus including a circuit that alternately applies current to the heater to alternate a direction of current flowing through the heater.  
         [0018]     The heater may have a first and a second end and the circuit may include a first switch selectively connecting a positive voltage terminal to the first end of the heater and a second switch selectively connecting a negative voltage terminal to the first end of the heater, wherein the first switch and the second switch are alternately turned on.  
         [0019]     The first switch may be an N-channel electric field effect transistor (FET) having a source, a drain, and a gate. The source of the N-channel electric FET may be connected to the first end of the heater. The drain and the gate of the N-channel electric FET may be connected together.  
         [0020]     The second switch may be a P-channel electric field effect transistor (FET) having a source, a drain, and a gate. The source of the P-channel electric FET may be connected to the first end of the heater. The drain and the gate of the P-channel electric FET may be connected together.  
         [0021]     The circuit may further include a third switch selectively connecting the second end of the heater to a ground terminal. The third switch may be an electric field effect transistor (FET) selectively connecting or disconnecting the second end of the heater to or from the ground terminal in response to a drive signal applied to a gate of the third switch.  
         [0022]     The circuit may alternately connect a positive voltage to the heater to flow current through the heater in a first direction and a negative voltage to the heater to flow current through the heater in a second direction, which is opposite to the first direction.  
         [0023]     At least one of the above features and other advantages may be provided by a method for driving and ejecting ink from an ink-jet printhead including flowing current in a first direction through a heater for heating ink to be supplied to an ink chamber to generate a bubble to eject ink from the ink chamber, flowing current in a second direction through the heater to generate a bubble to eject ink from the ink chamber, wherein the first direction and the second direction are opposite.  
         [0024]     Applying current in the first direction may include connecting a positive voltage terminal to a first end of the heater using a first switch and connecting a ground terminal to a second end of the heater using a third switch. Applying current in the second direction may include connecting a negative voltage terminal to a first end of the heater using a second switch and connecting a ground terminal to a second end of the heater using a third switch. The ground terminal may be selectively connected to the second end of the heater in response to a drive signal applied to the third switch.  
         [0025]     At least one of the above features and other advantages may be provided by a method for driving and ejecting ink from an ink-jet printhead including periodically applying a positive voltage to a positive voltage terminal and selectively connecting the positive voltage terminal to the heater, periodically applying a negative voltage to a negative voltage terminal and selectively connecting the negative voltage terminal to the heater through a second switch, periodically applying a positive drive signal to a switch for connecting the heater to a ground terminal whenever either the positive voltage or negative voltage is applied.  
         [0026]     Application of the positive voltage to the positive voltage terminal and application of the positive drive signal to the switch may flow current through the heater in a first direction. Application of the negative voltage to the negative voltage terminal and application of the positive drive signal to the switch may flow current through the heater in a second direction, which is opposite to the first direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0028]      FIG. 1  illustrates an exploded perspective view of a conventional thermal ink-jet printhead;  
         [0029]      FIG. 2  illustrates a cross-sectional view for explaining a process of ejecting an ink droplet using the conventional thermal ink-jet printhead of  FIG. 1 ;  
         [0030]      FIG. 3  is a circuit diagram of a first conventional circuit for driving a thermal ink-jet printhead;  
         [0031]      FIG. 4  is a diagram illustrating pulses of the first conventional circuit of  FIG. 3 ;  
         [0032]      FIG. 5  is a circuit diagram of a second conventional circuit for driving a thermal ink-jet printhead;  
         [0033]      FIG. 6  is a diagram illustrating pulses of the second conventional circuit of  FIG. 5 ;  
         [0034]      FIG. 7  is a circuit diagram of a circuit for driving a thermal ink-jet printhead according to an embodiment of the present invention; and  
         [0035]      FIG. 8  is a diagram illustrating pulses of the circuit according to an embodiment of the present invention shown in  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     Korean Patent Application No. 2003-52472, filed on Jul. 29, 2003, in the Korean Intellectual Property Office, and entitled: “Apparatus for Driving an Ink-Jet Printhead,” is incorporated by reference herein in its entirety.  
         [0037]     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the figures, the dimensions of layers and regions are exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout.  
         [0038]      FIG. 7  is a circuit diagram of a circuit for driving a thermal ink-jet printhead according to an embodiment of the present invention.  FIG. 8  is a diagram illustrating pulses of the circuit according to an embodiment of the present invention shown in  FIG. 7 .  
         [0039]     Referring to  FIG. 7 , in a circuit for driving an ink-jet printhead, a first end of a heater  70  is connected both to a positive voltage terminal  72  and a negative voltage terminal  74 . A high voltage, which is higher than a reference voltage, is applied to the positive voltage terminal  72 , and a low voltage, which is lower than the reference voltage, is applied to the negative voltage terminal  74 . For convenience of description, a ground voltage is referred to as the reference voltage in connection with  FIG. 7 . Resultantly, a positive voltage pulse V PP  is supplied to the positive voltage terminal  72 , and a negative voltage pulse V NP  is supplied to the negative voltage terminal  74 .  
         [0040]     To alternately apply current pulses to the heater  70 , a first switch S 1  is disposed between the positive voltage terminal  72  and the first end of the heater  70 , and a second switch S 2  is disposed between the negative voltage terminal  74  and the first end of the heater  70 .  
         [0041]     In this exemplary embodiment of the present invention, the first switch S 1  is an N-channel electric FET. A source S of the N-channel electric FET is connected to the first end of the heater  70 . A drain D and a gate G of the N-channel electric FET are connected together. Therefore, when a predetermined positive voltage is supplied to the positive voltage terminal  72 , the first switch S 1  allows the positive voltage terminal  72  to be connected to the first end of the heater  70 , causing a current to flow through the heater  70 . However, the N-channel electric FET may be driven by an external drive signal other than the positive voltage.  
         [0042]     In this exemplary embodiment of the present invention, the second switch S 2  is a P-channel electric FET. A source S of the P-channel electric FET is connected to the first end of the heater  70 . A drain D and a gate G of the P-channel electric FET are connected together. Therefore, when a predetermined negative voltage is supplied to the negative voltage terminal  74 , the second switch S 2  allows the negative voltage terminal  74  to be connected to the first end of the terminal  70 , causing current to flow through the heater  70 . However, the P-channel electric FET may be driven by an external drive signal other than the negative voltage.  
         [0043]     In addition to the first and second switches S 1  and S 2 , a third switch S 3  may be disposed between a second end of the heater  70  and a ground terminal GND to selectively connect or disconnect the second end of the heater  70  to or from a ground terminal GND.  
         [0044]     In this embodiment, the third switch S 3  is an electric FET. The electric FET selectively connects or disconnects the second end of the heater  70  to or from the ground terminal GND in response to a drive signal S DR  applied to a gate of the third switch S 3 . Although the third switch S 3  is illustrated as an N-channel electric FET in  FIG. 7 , the third switch S 3  may alternatively be a P-channel electric FET.  
         [0045]      FIG. 8  is a diagram illustrating the positive voltage pulse V PP  that is supplied to the positive voltage terminal  72 , the negative voltage pulse V NP  that is supplied to the negative voltage terminal  74 , and the drive signal S DR  that is applied to the electric FET acting as the third switch S 3 .  
         [0046]     Referring to  FIG. 8 , a predetermined positive voltage V 1  is periodically applied to the positive voltage terminal  72 , and a predetermined negative voltage −V 1  is periodically applied to the negative voltage terminal  74 . The negative voltage −V 1  is applied at a time t 2  that is halfway between a time t 1  when a first positive voltage V 1  is applied and a time t 3  when a second positive voltage V 1  is applied. A positive drive signal voltage V 2  is periodically applied to the electric FET acting as the third switch S 3  whenever either the positive voltage V 1  or the negative voltage −V 1  is applied.  
         [0047]     A principle of alternately applying current pulses to the heater  70  in the ink-jet printhead driving circuit according to the exemplary embodiment of the present invention will now be explained.  
         [0048]     When a first positive voltage V 1  is supplied to the positive voltage terminal  72  at a time t 1 , the N-channel electric FET acting as the first switch S 1  connects the positive voltage terminal  72  to the first end of the heater  70 . At this time, since no voltage is supplied to the negative voltage terminal  74 , the P-channel electric FET acting as the second switch S 2  disconnects the negative voltage terminal  74  from the first end of the heater  70 . If a positive drive signal voltage V 2  is applied to the electric FET acting as the third switch S 3  at time t 1 , the electric FET acting as the third switch S 3  connects the second end of the heater  70  to the ground terminal GND. Accordingly, a current flows from the positive voltage terminal  72  through the heater  70  toward the ground terminal GND at time t 1 . Hence, current flows in a forward direction, i.e., downwardly, through the heater  70  at time t 1 .  
         [0049]     When a negative voltage −V 1  is supplied to the negative voltage terminal  74  at a time t 2 , the P-channel electric FET acting as the second switch S 2  connects the negative voltage terminal  74  to the first end of the heater  70 . At this time, since no voltage is supplied to the positive voltage terminal  72 , the N-channel electric FET acting as the first switch S 1  disconnects the positive voltage terminal  72  from the first end of the heater  70 . If a positive drive signal voltage V 2  is applied to the electric FET acting as the third switch S 3  at time t 2 , the electric FET acting as the third switch S 3  connects the second end of the heater  70  to the ground terminal GND. Accordingly, a current flows from the ground terminal GND through the heater  70  toward the negative voltage terminal  74  at time t 2 . Hence, current flows in a reverse direction, i.e., upwardly, through the heater  70  at time t 2 . Thus, a direction in which current flows through the heater  70  at time t 2  is opposite to a direction in which current flows through the heater  70  at time t 1 .  
         [0050]     Subsequently, when a second positive voltage V 1  is supplied to the positive voltage terminal  72  at a time t 3  and a positive drive signal voltage V 2  is applied to the electric FET acting as the third switch S 3 , a current flows through the heater  70  in the same forward direction as that at time t 1 .  
         [0051]     If the above procedures are repeated, current pulses are alternately applied to the heater  70  at periodic intervals, thereby alternating a direction of current flow through the heater  70 .  
         [0052]     When a current is alternately applied to the heater  70  of the ink-jet printhead at periodic intervals, the possibility of causing a defect in an atomic structure by an electron wind force, which is generated by current flow, is reduced. This reduction occurs because a possibility of damage at a position where electron flow starts is reduced to half when current flows alternately through the heater  70  as compared to when current flows in only one direction. Thus, if current flows periodically and alternately through the heater  70 , the possibility of damage to the heater  70  is reduced as compared to when current flows in a single direction.  
         [0053]     As described above, an apparatus for driving an ink-jet printhead according to an embodiment of the present invention may have the following advantages.  
         [0054]     First, since current can alternately flow through the heater, the possibility of damage to the heater due to electromigration is reduced to half of that when a current flows in only one direction. Accordingly, a time until the heater becomes damaged is delayed, thereby extending a lifespan of the heater.  
         [0055]     Second, since a direction of current flowing through the heater is not related to an amount of thermal energy generated by the heater, a circuit for driving an ink-jet printhead according to an embodiment of the present invention is able to provide the same performance as a conventional circuit. Consequently, the reliability of the ink-jet printhead may be improved by modifying the drive circuit without specifically enhancing a quality of the heater.  
         [0056]     Exemplary embodiments of the present invention have been disclosed herein and, although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. For example, each element of the ink-jet printhead may be made of a material other than those mentioned, and the specific figures suggested in each step are variable within a range where the manufactured ink-jet printhead can normally operate. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.