Abstract:
A circuit for regulating an anode voltage in an information display apparatus. A horizontal deflection circuit has trace and retrace modes of operation. The horizontal deflection circuit further has a fixed retrace capacitance; and a series combination of a switched retrace capacitance and a switch element, which combination is coupled in parallel with the fixed retrace capacitance. The switch element is in a closed position as the horizontal deflection circuit enters the retrace mode of operation. A voltage divider network samples the anode voltage. A control circuit is responsive to the sample voltage for controlling the switch element, such that the control circuit effects regulation of the anode voltage by selecting a time during the retrace period at which to place the switch element in an open position. The control circuit may also provide a raster width compensation voltage for an east-west pincushion correction circuit.

Description:
BACKGROUND 
     Field Of The Invention 
     This invention relates generally to the field of high voltage regulation in an information display apparatus, and, in particular, to regulating the anode voltage in an information display apparatus that utilizes a cathode ray tube. 
     Background Information 
     In today&#39;s multimedia environment, it becomes increasingly desirable to use a single information display apparatus for several different applications. 
     For example, in addition to displaying video information, a television receiver can currently be used to display teletext information. Most recently, manufacturers of computers and consumer electronics have begun to offer products wherein the television receiver is also used to display information provided by a computer, such as text pages, high resolution graphics, and the like. 
     These different applications demand increased performance from the deflection and high voltage circuits of the information display apparatus. For instance, in the display of video information, the information display apparatus is required to provide a dark screen and must be capable of a high beam current and a high anode voltage. The use of the same information display apparatus to display high resolution graphics of the type typically associated with a color display monitor additionally requires that the information display apparatus provide a stable raster size, good focus, and a small spot size. These additional requirements, in turn, depend upon the presence of a well-regulated anode voltage for the cathode ray tube. 
     One approach is provided by U.S. Pat. No. 5,357,175, which discloses regulating the anode voltage by varying the B+ voltage responsive to variations in the anode voltage. This document also discloses providing an error signal generated from the anode voltage to a horizontal amplitude correction circuit  54  for regulating a pincushion correction voltage. Another approach to regulating the anode voltage for the cathode ray tube is provided by European Patent Application 0 128 223 A1, which discloses a flyback period control circuit  1  that switches a resonating capacitor Ct&#39; into and out of a horizontal deflection circuit only during the flyback period TR. In this way, the flyback period control circuit  1  controls the flyback period TR to control the high voltage HV. 
     SUMMARY 
     The present invention is directed to a high voltage regulation circuit that provides a well-regulated anode voltage for the cathode ray tube of the information display device. 
     A circuit for regulating an anode voltage in an information display apparatus comprises: a horizontal deflection circuit having trace and retrace modes of operation; the horizontal deflection circuit further having a fixed retrace capacitance and a series combination of a switched retrace capacitance and a switch element, the series combination coupled in parallel with the fixed retrace capacitance, wherein the switch element is in a closed position as the horizontal deflection circuit enters the retrace mode of operation; a voltage divider network for providing a sample voltage indicative of a magnitude of the anode voltage; and a control circuit responsive to the sample voltage for controlling the switch element, such that the control circuit effects regulation of the anode voltage by selecting a time during the retrace period at which to place the switch element in an open position. 
     The above, and other features, aspects, and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram of a high voltage regulation circuit that embodies the present invention; 
     FIG. 2 is a schematic diagram of the high voltage regulation circuit of FIG. 1; 
     FIGS. 3 a - 3   d  show voltage and current waveforms useful for explaining the operation of the high voltage regulation circuit of FIGS. 1 and 2; and 
     FIGS. 4 a  and  4   b  show, in block and schematic form, the high voltage regulation circuit in combination with different raster correction circuits of the information display apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates the principle of operation of the inventive high voltage regulation circuit  100 . A horizontal deflection circuit  10  deflects the electron beam across the screen of the cathode ray tube to form a raster. A retrace voltage VR from the horizontal deflection circuit  10  is used to produce the high-voltage anode voltage EHT required by the cathode ray tube. The retrace voltage VR is shown in FIG. 3 a ; the retrace voltage VR typically has a substantially sinusoidal shape at a low load, or low beam current, condition. This substantially sinusoidal shape is slightly deformed when the retrace voltage VR is subjected to a high load, or high beam current, condition. In FIGS. 3 a - 3   d  a dashed line indicates a high load, or high beam current, condition and a solid line indicates a low load, or low beam current, condition. 
     A sample of the anode voltage EHT is used by the control circuit  20  to determine the precise instant at which the switch element SW 2  should be placed in the open position. For example, the control circuit  20  opens the switch element SW 2  sooner if the anode voltage EHT drops below a predetermined level; conversely, the control circuit  20  opens the switch element SW 2  later if the anode voltage EHT exceeds the predetermined level. In this manner the anode voltage EHT can be precisely regulated. 
     FIG. 2 shows a schematic representation of an exemplary embodiment of the high voltage regulation circuit  100 . The horizontal deflection circuit  10  is formed by the horizontal drive  11 , the switch element SW 1 , the S-shaping capacitance CS, the horizontal deflection yoke LH, the fixed retrace capacitance CR 1 , and the retrace capacitance CR 2  switched by switch element TR 1  (shown as SW 2  in FIG.  1 ). For purposes of the present description the switch element TR 1  is initially closed so that the retrace capacitances CR 1  and CR 2  are in parallel; thus, the effective retrace capacitance is the sum of the two retrace capacitances. The horizontal driver circuit  11  may be a conventional type that is well-known to those having ordinary skill in the art and will not be described further. 
     The horizontal driver circuit  11  controls the operation of the switch element SW 1  at a horizontal-rate frequency to periodically deflect an electron beam from a left edge of the screen to a right edge of the screen (from the perspective of one viewing the screen). The switch element SW 1  typically consists of an npn-type bipolar junction transistor coupled in parallel with a diode, such that the cathode of the diode is coupled to the collector electrode of the transistor, and the emitter electrode of the transistor and the anode of the diode are coupled to the ground, or reference, potential. During the period of time that switch SW 1  is in a closed position, also referred to as the “trace” period, the electron beam is deflected from the left edge of the screen to the right edge of the screen by virtue of the resonant interaction of the S-shaping capacitance CS and the horizontal deflection yoke LH. During this trace period the retrace voltage VR at the junction J 1  of the S-shaping capacitor CS and the retrace capacitances CR 1  and CR 2  is substantially equal to the ground, or reference, potential. 
     In the period between subsequent trace periods, also referred to as the “retrace” period, the switch element SW 1  is in an open position and the electron beam is returned from the right edge of the screen to the left edge of the screen by virtue of the resonant interaction of the horizontal deflection yoke LH and the retrace capacitances CR 1  and CR 2 . During this retrace period the retrace voltage VR at the junction has a substantially sinusoidal shape, as shown in FIG. 3 a , and a peak magnitude that is equal to approximately 1200 V for the exemplary embodiment of FIG.  2 . 
     The retrace voltage VR is coupled through a primary winding  30  of transformer T 1  to a secondary winding  40 , where it is rectified by the diode D 1  to produce an anode voltage EHT for the cathode ray tube. The anode voltage is a high voltage and is equal to approximately 30 kV for the exemplary high voltage regulation circuit  100  of FIG.  2 . 
     The regulation of the anode voltage EHT occurs as follows. The anode voltage EHT is sampled by a sampling network to provide a sample voltage V 1 . In FIG. 2, the sampling network is implemented by a voltage divider network. The anode voltage EST is divided by the voltage divider formed by the resistors R 1  and R 2  to provide the sample voltage V 1 , which is applied to the non-inverting input  51  of the error amplifier  50 . The sample voltage V 1  is compared against a reference voltage Vref that is applied to the inverting input  52  of the error amplifier  50 . The error voltage Verr obtained by subtracting the reference voltage Vref from the sample voltage V 1  is amplified and provided at the output  53  of the error amplifier  50 . The gain of the error amplifier  50  is determined by the ratio of the resistor R 4  to the resistor R 5 . This ratio also affects the output impedance of the high voltage regulation circuit, which, for the exemplary embodiment of FIG. 2, is equal to approximately  150  kΩ. 
     This error voltage Verr, which directly tracks the anode voltage EHT, is low-pass filtered by the combination of the resistor R 3  and the capacitor C 3  to produce a control voltage Vctrl, which is applied to the non-inverting input  61  of a comparator  60 . In the exemplary embodiment shown in FIG. 2, it has been experimentally determined that the high voltage regulation circuit  100  performs best when the low-pass filter formed by the resistor R 3  and the capacitor C 3  has a cut-off frequency equal to approximately 20 Hz. The choice of a particular cut-off frequency for this low-pass filter is specific to the component values used in each embodiment of the particular high voltage regulation circuit  100  and involves a trade-off between two competing demands placed on the filter. Typically, a higher cut-off frequency for this filter, and thus a faster response for the control loop of the high voltage regulation circuit, is undesirable. Because of coupling between the transformer T 1 , which is usually of the diode-split type, and the horizontal deflection circuit  10 , there are typically phase and amplitude errors in a horizontal deflection current flowing through the deflection coil LH. These phase and amplitude errors cannot be effectively compensated if they occur too quickly. However, the high voltage regulation circuit should react quickly when the picture changes from dark to bright and vice versa. An acceptable choice for the cut-off frequency for the low-pass filter appropriately balances these competing requirements. 
     The inverting input  62  of the comparator  60  accepts a horizontal frequency ramp voltage Vramp which is generated by charging capacitor C 1  through resistor R 3 . Capacitor C 1  is discharged to ground through transistor TR 2  by using a horizontal-rate waveform  70  to turn transistor TR 2  on at a horizontal-rate frequency. The horizontal-rate waveform  70  may be provided by a secondary winding (not shown) of the transformer T 1  and may thus have the same general shape, although inverted, as the retrace voltage VR at junction J 1 . In the exemplary embodiment of the high voltage regulation circuit  100  shown in FIG. 2, the horizontal-rate waveform  70  may have a peak-to-peak voltage equal to approximately 200 V. The waveform  70  is coupled to the base electrode of the transistor TR 2  by a differentiator formed by a capacitor C 4  and resistors R 8  and R 9 . The values for the components of the differentiator are selected such that transistor TR 2  turns on and discharges the capacitor C 1  near the end of the retrace period. 
     The comparator  60  functions as a pulse-width modulator. The switch drive signal  64  at the output  63  of the comparator  60  remains at a high level until the ramp voltage Vramp exceeds the control voltage Vctrl; at that time, the switch drive signal  64  goes to a low level, as shown in FIG.  2 . The control voltage Vctrl thus determines when the output signal transitions from a high state to a low state. For example, imagine that the anode voltage EHT exceeds the predetermined level, as represented by the reference voltage Vref. The error voltage Verr, and thus the control voltage Vctrl, will assume a level nearer the power supply voltage of the error amplifier  50 , and the switch drive signal  64  will remain at a high state for a longer period of time, because of the relatively high threshold level set by the control voltage Vctrl. If, on the other hand, the anode voltage EHT is below the predetermined level, the error voltage Verr, and thus the control voltage Vctrl, will assume a level nearer ground, or the reference potential, of the error amplifier  50 , and the output signal will remain at a high state for a shorter period of time, because of the now relatively low threshold level set by the control voltage Vctrl. 
     The switch drive signal  64  uses a low-current, high voltage driver circuit  80  to control the operation of the switch element TR 1 . For instance, at the start of the trace period, the switch drive signal  64  at the output  63  of the comparator  60  is at a high, as indicated by the waveform shown in FIGS. 2 and 3 c . The magnitude of the high state of the switch drive signal  64  is equal to approximately the power supply voltage to the comparator  60 . The transistor TR 3 , the base electrode of which is coupled to the comparator  60  and controlled by the switch drive signal  64 , is thus off, and the capacitor C 2  charges to the supply voltage potential through the path defined by the resistors R 5  and R 6 , the capacitor C 2 , and the diode D 3 . The diode D 2  is off, and the switch element TR 1  is on by virtue of being coupled at its base electrode to the supply voltage potential through the resistor R 7 . The switched retrace capacitance CR 2  is thus coupled in parallel to the fixed retrace capacitance CR 1 , so that the effective retrace capacitance is the sum of the fixed CR 1  and switched CR 2  retrace capacitances. Note that minimal power is dissipated by the switch element TR 1  because the retrace voltage VR at junction J 1  and a current I 1  through the switch element TR 1  are both equal to approximately zero during the trace period. The switch element TR 1  may be a transistor having an industry part number BU506DF, which has an integrated body diode that is labeled D 4  in FIG.  2 . The switch element TR 1  must have a voltage rating that is sufficiently high to enable switch element TR 1  to withstand the peak magnitude of the retrace voltage VR. 
     As the retrace period begins, the switch element TR 1  is still on; the current I 1  flows to ground through the switch element TR 1 , as indicated by the waveform shown in FIG. 3 d , thereby charging the switched retrace capacitor CR 2 ; and the ramp voltage Vramp approaches the control voltage Vctrl. The peak magnitude of the current I 1  flowing through the switch element TR 1  during the retrace period is only approximately 0.5 A, so power dissipation by the transistor is minimized. Once the ramp voltage Vramp exceeds the control voltage Vctrl, the switch drive signal  64  transitions from the level of the power supply voltage to the ground, or reference, potential. As indicated by the waveforms shown in FIGS. 3 c  and  3   d , when this transition occurs is also a function of the condition of the load. This transition causes the transistor TR 3  to turn on. The voltage at the junction of the cathode of the diode D 2  and the anode of the diode D 3  goes to a negative voltage. The diode D 3  thus turns off and the diode D 2  turns on, thereby turning off the switch element TR 1 . As a result, the switched retrace capacitor CR 2  is no longer in parallel with the fixed retrace capacitor CR 1 , and the effective retrace capacitance is thus reduced and is equal to the fixed retrace capacitance CR 1 . The peak-to-peak magnitude of the retrace voltage VR at the junction J 1  increases due to the decrease in the effective retrace capacitance. Consequently, the anode voltage rises toward the predetermined level as the retrace voltage VR, with its increased peak-to-peak magnitude, is coupled through the primary winding  30  of the transformer T 1  to the secondary winding  40 , and then rectified by the diode D 1  to provide the anode voltage EHT. 
     When the switch element TR 1  turns off, the flow of current I 1  is interrupted, and the switched retrace capacitance CR 2  maintains a constant charge and thus a fixed voltage drop. The voltage VR′ at the collector electrode of the switch element TR 1  tracks the retrace voltage VR during the retrace period by virtue of the fixed voltage drop across the switched retrace capacitance CR 2 , as indicated by the waveform shown in FIG. 3 b . As indicated in FIG. 3 b , the magnitude of the voltage VR′ is also a function of the load condition. In the exemplary embodiment shown in FIG. 2, the voltage VR′ may vary from approximately 100 V for 0 mA of beam current to approximately 500 V for 1.6 mA of beam current. The theoretical range for the voltage VR′ is from approximately 0 V to approximately 1200 V, where approximately 1200 V is the peak value of the retrace voltage VR during the retrace period. The diode D 4  clamps the collector electrode of the switch element TR 1  to approximately the ground, or reference, potential if the voltage VR′ attempts to go below the ground, or reference, potential. This allows switch element TR 1  to switch at a zero voltage, further minimizing the power dissipated by the switch element TR 1 . 
     The inventive high voltage regulation circuit of FIG. 2 thus advantageously provides for precise control of the point during the retrace period at which the switch element TR 1  will turn off to reduce the effective retrace capacitance of the horizontal deflection circuit  10 . The point at which the switch element TR 1  is turned off is made to be as close as possible to the beginning of the retrace interval in order to minimize power dissipation in the switch element TR 1 . 
     Additionally, the high voltage regulation circuit  100  also delivers a signal for more precise dynamic beam current compensation than is currently provided by conventional solutions. For instance, fast changes in the anode voltage EHT may not be well-regulated because of the particular the cut-off frequency chosen for the loop filter of the high voltage regulation circuit  100 . Recall that the cut-off frequency of combination of the resistor R 3  and the capacitor C 3  in the high voltage regulation circuit  100  of FIG. 2 is equal to approximately 20 Hz. As a result, raster width compensation is still necessary, but only with respect to an AC component of such compensation. 
     The high voltage regulation circuit shown in FIG. 2 advantageously provides for such correction. In a feature of the present invention, the output  53  of the error amplifier  50  is coupled to a particular East-West pincushion correction circuit by a resistor R 10  and a DC blocking capacitor C 5  to provide a raster width compensation voltage V EW . The pincushion correction circuit need not be limited to a particular configuration. For example, referring to FIGS. 4 a  and  4   b , the high voltage regulation circuit  100  of FIGS. 1 and 2 can be used in conjunction with a forward regulated output stage  91 , as shown in FIG. 4 a , or in conjunction with a diode modulator configuration  92 , as shown in FIG. 4 b . The E-W (East-West) drive  90  shown in FIG. 4 a  may be a conventional type that is well-known to those having ordinary skill in the art and will not be described further. 
     A conventional manner of providing information about the raster width to an East-West pincushion correction circuit is to use the voltage at the footpoint of the high voltage secondary winding of the transformer. For example, in the high voltage regulation circuit  100  shown in FIG. 2, the conventional approach would couple the point labeled BCL on the secondary winding  40  of the transformer T 1  to the East-West pincushion correction circuit. Coupling the output  53  of the error amplifier  50  of the high voltage regulation circuit  100 , rather than the voltage at the BCL point of the secondary winding  40 , to the East-West pincushion correction circuit is a preferable approach because the error voltage Verr more precisely follows the anode voltage EHT than does the voltage at the BCL point of the secondary winding  40 .