Patent Publication Number: US-6664747-B2

Title: Dynamic focus regulation circuit of display apparatus

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for DYNAMIC FOCUS REGULATION CIRCUIT FOR DISPLAY DEVICE earlier filed in the Korean Industrial Property Office on Aug. 31, 2001 and there duly assigned Serial No. 2001-53306. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a display apparatus, and more particularly, to a dynamic focus regulation circuit for supplying a uniform parabolic waveform voltage regardless of variation of a horizontal frequency in a display mode. 
     2. Description of the Related Art 
     In a CRT (cathode ray tube) of a display apparatus, an electron beam is emitted from an electron gun unit to a panel coated with red/green/blue fluorescent material so as to form pixels, and thus a 2-dimensional picture is displayed on the panel by supplying a saw tooth waveform electric current to vertical and horizontal deflection coils. 
     In the conventional CRT, the panel is curved outwardly, and the distance between the electron gun unit and the panel is not uniform, and thus the sharpness of the picture becomes different according to the positions of pixels. That is, the sharpness on the edge of the panel is inferior to that on the center thereof. In the case of a high-resolution monitor, the above phenomenon is regulated by applying a regulation waveform voltage to the CRT focus voltage component by a focus regulation circuit. The regulation waveform is induced by an H-DY (Horizontal Deflection Yoke) generally outputting a horizontal deflection signal, and is synchronized with a deflection signal having a parabolic waveform. 
     FIG. 5 illustrates a conventional dynamic focus regulation circuit detecting a dynamic focus output waveform. As shown therein, the dynamic focus regulation circuit comprises a horizontal deflection circuit  110  oscillating a horizontal deflection signal, an S-regulation circuit  120  regulating an electric current outputted from the horizontal deflection circuit  110 , a dynamic focus output circuit  130  outputting a dynamic focus waveform voltage by amplifying the parabolic waveform voltage from the S-regulation circuit  120 , and a diode modulation circuit  140  horizontally modulating a raster. 
     The horizontal deflection circuit  110  includes a horizontal driving transistor  112 , a damper diode  114 , a resonance capacitor  116 , and an H-DY  118 . 
     The horizontal driving transistor  112  is switched on/off according to a horizontal driving signal generated by a video IC (not shown) or a horizontal oscillating IC (not shown). When the horizontal driving transistor  112  is switched on, B+ electric power from an FBT (Fly Back Transformer) is supplied to the H-DY  118 . 
     If the horizontal driving transistor  112  is rapidly switched on according to the horizontal driving signal, an electric current is induced to the H-DY  118 . On the other hand, if the horizontal driving transistor  112  is switched off, the electricity stored in the H-DY  118  is charged in the resonance capacitor  116 . Herein, when the resonance capacitor  116  is perfectly charged with the electricity, the resonance capacitor  116  discharges its electricity to the H-DY  118 , thereby re-storing the H-DY  118  with the electricity. Thereafter, according as the H-DY  118  is stored with electric energy, when the voltage of the H-DY  118  is so high that a forward bias can be applied to the damper diode  114 , the damper diode  114  is shorted and the electricity of the H-DY  118  vanishes. 
     Thus, when the electricity of the H-DY  118  vanishes into zero, the horizontal driving transistor  112  is switched on again by the horizontal driving signal, to thereby repeat the above described process. Thus, an electric current having a saw tooth waveform is generated for deflecting an electron beam horizontally. 
     The S-regulation circuit  120  regulates the saw tooth waveform electric current applied to the H-DY  118  so as to maintain the linearity of a screen. The S-regulation circuit  120  includes a basic S-regulation capacitor  119  connected with the H-DY  118  and the resonance capacitor  116  in parallel, an S-regulation capacitor  122 , and a switching transistor  124  switching on/off the S-regulation capacitor  122 . Herein, the switching transistor  124  is controlled by a microcomputer  150 . 
     With this configuration, if electric power is supplied to the S-regulation capacitor  122  by switching on/off the switching transistor  124  according to control of the microcomputer  150 , the voltage applied to the H-DY  118  is regulated by means of combination of the capacitance of the S-regulation capacitor  122  and the capacitance of the basic S-regulation capacitor  119 , thereby regulating the chopping waveform electric current. 
     Further, the dynamic focus output circuit  130  inverting-amplifies the convex parabolic waveform voltage across opposite ends of the basic S-regulation capacitor  119 , to thereby output the dynamic focus waveform voltage. 
     The dynamic focus output circuit  130  is connected with two ends of the basic S-regulation capacitor  119 , and includes a capacitor  134  applying only the AC signal of the convex parabolic waveform voltage through DC-coupling, a transformer (T)  132  inverting-boosts the convex parabolic waveform voltage outputted from the capacitor  134 , and a capacitor  136  outputting the parabolic waveform voltage induced in a secondary side of the transformer (T)  132  through noise-removal and DC-coupling. 
     That is, the convex parabolic waveform voltage outputted from two ends of the basic S-regulation capacitor  119  is inverting-amplified through the capacitor  134  and the transformer (T)  132 , and therefore changed into a convex parabolic waveform voltage to be outputted as the dynamic focus waveform voltage. 
     On the other hand, a monitor has various display modes. For example, a VGA mode has 640×480 resolution and a 31.5 KHz horizontal frequency, a SVGA mode has 1024*768 resolution and 35˜37 KHz horizontal frequencies, and a high-resolution mode has 1024×768 through 1280×1024 resolution and 64˜75 KHz horizontal frequencies. 
     To oscillate a horizontal deflection signal corresponding to the horizontal frequency, the capacitance of the S-regulation circuit  120  should be altered, i.e., the higher the resolution is, the higher the frequency of horizontal deflection signal should be. However, the higher the frequency of horizontal deflection is, the lower the parabolic waveform voltage generated from the S-regulation capacitor  122  is. 
     As described above, a frequency inputted to a display apparatus is altered according to setting up a display mode of the display apparatus, and therefore a parabolic waveform voltage corresponding to the horizontal frequency is altered. Thus, it is impossible to generate a precise focus voltage. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above-described shortcomings and user&#39;s need, and an object of the present invention is to provide a dynamic focus regulation circuit for supplying a uniform parabolic waveform voltage regardless of variation of a horizontal frequency in a display mode. 
     This and other objects of the present invention may be accomplished by the provision of a horizontal dynamic focus regulation circuit, which induces a horizontal dynamic focus waveform voltage by a horizontal deflection circuit generating a different frequency according to a display mode of a display apparatus, comprising a microcomputer outputting a plurality of control signals; a plurality of switching parts corresponding to the plurality of control signals outputted from the microcomputer, respectively; a plurality of S-regulation capacitors connecting with the plurality of switching parts in series, respectively; an auxiliary capacitor provided on a line diverged from a line connecting each switching part with each S-regulation capacitor, and being respectively connected with the S-regulation capacitor in parallel and series according to switching on and off of the switching part. 
     Preferably, each switching part is comprised of a transistor being switched on/off responsive to their respective control signals from the microcomputer according to the display mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 shows a dynamic focus regulation circuit according to the present invention; 
     FIG. 2 shows a dynamic focus regulation circuit according to one embodiment of the present invention; 
     FIG. 3 shows one equivalent circuit of the dynamic focus regulation circuit in FIG. 2; 
     FIG. 4 shows another equivalent circuit of the dynamic focus regulation circuit in FIG. 2; and 
     FIG. 5 shows a conventional dynamic focus regulation circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described in more detail with reference to the accompanying drawings. 
     As shown in FIG. 1, a dynamic focus regulation circuit comprises a horizontal deflection circuit  10  oscillating a horizontal deflection signal, an S-regulation circuit  20  regulating an electric current induced to the horizontal deflection circuit  10 , a dynamic focus output circuit  30  outputting a dynamic focus waveform voltage by amplifying a parabolic waveform voltage from the S-regulation circuit  20 , and a diode modulation circuit  40  horizontally modulating a raster. 
     The dynamic focus output circuit  30  outputs a dynamic focus waveform voltage by inverting-amplifying a convex parabolic waveform voltage received from the S-regulation circuit  20  regulating an electric current of the horizontal deflection circuit  10 , and adjusts the horizontal size of a screen. 
     The horizontal deflection circuit  10  includes a horizontal driving transistor  12 , a damper diode  14 , a resonance capacitor  16 , and an H-DY  18 . The horizontal driving transistor  12  is switched on/off so as to induce a saw tooth waveform electric current in the H-DY  18 , thereby deflecting an electron beam emitted from an electron gun unit (not shown). 
     The dynamic focus output circuit  30  includes a capacitor  34  applying only the AC signal of signals outputted from opposite ends of a basic S-regulation capacitor  19  through DC-coupling, a transformer (T)  32  inverting-boosts the convex parabolic waveform voltage outputted from the capacitor  34 , and a capacitor  36  outputting the parabolic waveform voltage induced in a secondary side of the transformer (T)  32  through noise-removal and DC-coupling. Thus, the dynamic focus waveform voltage is produced from the parabolic waveform voltage of the basic S-regulation capacitor  19 . 
     The S-regulation circuit  20  includes the basic S-regulating capacitor  19  connected with the H-DY  18  and the resonance capacitor  16  in parallel, an S-regulation capacitor (Cn)  22 , a switching transistor (Qn)  24  switching on/off the S-regulation capacitor  22 , and an auxiliary capacitor (Cfn)  26  provided across the line in which the S-regulation capacitor  22  is connected and the line in which the capacitor  34  is connected. Herein, the switching transistor  24  is switched on/off according to control of a microcomputer  50 . When the switching transistor  24  is switched on and off, the S-regulation capacitor  22  is connected with the auxiliary capacitor  26  in parallel and series, respectively. The switching transistor  24 , the S-regulation capacitor  22  and the auxiliary capacitor  26  form a dynamic focus regulation circuit Qn, Cn and Cfn. 
     FIG. 2 illustrates one embodiment of the dynamic focus regulation circuit of a display apparatus in more detail, the display apparatus having three display modes, concentrating on S-regulation. As shown therein, the S-regulation is controlled by three S-regulation capacitors C 1 , C 2 , and C 3  which are connected with each other in parallel via three switching transistors Q 1 , Q 2 , and Q 3  which are respectively connected with the S-regulation capacitors C 1 , C 2 , and C 3  in series, and three auxiliary capacitors Cf 1 , Cf 2 , and Cf 3  which are provided between the lines from the lines connecting the S-regulation capacitors C 1 , C 2 , and C 3  with the switching transistors Q 1 , Q 2 , and Q 3  and a line of a primary side of a transformer (T)  82 , respectively. 
     The switching transistors Q 1 , Q 2 , and Q 3  are switched on/off according to control of the microcomputer  50 . The microcomputer  50  selectively switches on/off the switching transistors Q 1 , Q 2 , and Q 3  according to a selected video signal mode such as VGA, SVGA, etc. so as to produce a capacitance appropriate to the selected video signal. 
     FIG. 3 illustrates an equivalent circuit, in which all the switching transistors Q 1 , Q 2 , and Q 3  are switched on, of the dynamic focus regulation circuit in FIG.  2 . As illustrated therein, when all the switching transistor Q 1 , Q 2 , and Q 3  are switched on, a parabolic waveform voltage supplied to the primary side of the transformer  82  is equivalent to an AC parabolic waveform voltage applied to the capacitance (Cm+C 1 +C 2 +C 3 ). Herein, a total capacitance of the auxiliary capacitors Cfm, Cf 1 , Cf 2 , and Cf 3  is far less than a total capacitance of the S-regulation capacitors Cm, C 1 , C 2 , and C 3 . Accordingly, the voltage applied to the auxiliary capacitors Cfm, Cf 1 , Cf 2 , and Cf 3  is near to zero, so that the parabolic waveform voltage supplied to the primary side of the transformer  82  is influenced by the S-regulation capacitors Cm, C 1 , C 2 , and C 3 . 
     Thus, when all the switching transistors Q 1 , Q 2 , and Q 3  are switched on, the finally outputted dynamic focus waveform voltage has the value such as the AC parabolic waveform voltage applied to the capacitance (Cm+C 1 +C 2 +C 3 ) inverting-amplified through the transformer  82 . At this time, because the voltage applied to the primary side of the transformer  82  has a minimum value, a turns ratio of the transformer  82  is set on the basis of the state that all the switching transistors Q 1 , Q 2 , and Q 3  are switched on. 
     FIG. 4 illustrates an equivalent circuit, in which only the switching transistor Q 1  is switched on, of the dynamic focus regulation circuit in FIG.  2 . As illustrated therein, when the switching transistor Q 1  is switched on, charging and discharging speed of the H-DY  18  is increased due to lowering the capacitance (Cm+C 1 ). Thus, because the voltage applied to the capacitance (Cm+C 1 ) gets higher, the convex parabolic waveform voltage is larger than the voltage in the case of the capacitance (Cm+C 1 +C 2 +C 3 ). Herein, although the parabolic waveform voltage applied to the primary side of the transformer  82  is divided at the ratio of the capacitances (C 3 //Cf 3 +C 2 //Cf 2 ) and (Cfm+Cf 1 ), it is equivalent to the parabolic waveform voltage applied to the capacitance (Cm+C 1 +C 2 +C 3 ). At this time, the capacitance of the auxiliary capacitors Cf 1 , Cf 2 , and Cf 3  is predetermined so as to obtain a preferable voltage-dividing ratio. 
     Herein below, the voltage-dividing ratio according to the on/off state of the switching transistors Q 1 , Q 2 , and Q 3  will be shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Approximate 
                   
               
               
                   
                   
                   
                   
                 Voltage dividing 
                 voltage dividing 
                 S-regulation 
               
               
                 Mode 
                 Q1 
                 Q2 
                 Q3 
                 ratio 
                 ratio 
                 capacitance 
               
               
                   
               
             
            
               
                 Mode 1 
                 Off 
                 Off 
                 Off 
                 Cfm:(C1//Cf1 + C2//Cf2 + C3//Cf3) 
                 Cfm:(Cf1 + Cf2 + Cf3) 
                 Cm 
               
               
                 Mode 2 
                 On 
                 Off 
                 Off 
                 (Cfm + Cf1):(C2//Cf2 + C3//Cf3) 
                 (Cfm + Cf1):(Cf2 + Cf3) 
                 Cm + C1 
               
               
                 Mode 3 
                 On 
                 On 
                 Off 
                 (Cfm + Cf1 + Cf2):(C3//Cf3) 
                 (Cfm + Cf1 + Cf2):Cf3 
                 Cm + C1 + C2 
               
               
                 Mode 4 
                 On 
                 On 
                 On 
                 (Cfm + Cf1 + Cf2 + Cf3):0 
                 (Cfm + Cf1 + Cf2 + Cf3):0 
                 Cm + C1 + C2 + C3 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, according to the on/off state of the switching transistor Q 1 , Q 2 , and Q 3 , the capacitance of the S-regulation circuit  20  is altered, thereby altering the parabolic waveform voltage outputted from the S-regulation circuit  20 . However, at the primary side of the transformer is are provided the auxiliary capacitors Cfm, Cf 1 , Cf 2 , and Cf 3  dividing the voltage applied to the S-regulation circuit  20 , thereby always outputting a uniform parabolic waveform voltage. 
     As described above, according to the present invention, in a dynamic focus regulation circuit, which regulates a dynamic focus waveform voltage through a horizontal deflection circuit including an S-regulation circuit, is added an auxiliary capacitor dividing a voltage according to the voltage applied to the S-regulation circuit, thereby outputting a uniform parabolic waveform voltage. Therefore, the dynamic focus regulation circuit supplies the uniform parabolic waveform voltage regardless of variation of a horizontal frequency in a display mode. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.