HIGH VOLTAGE REGULATION BY SWITCHING RETRACE CAPACITORS

Abstract: A circuit for regulating an anode voltage (EHT) in an information display apparatus. A horizontal deflection circuit (10) has trace and retrace modes of operation. The horizontal deflection circuit further has a fixed retrace capacitance (CR1); and a series combination of a switched retrace capacitance (CR2) and a switch element (TR1), 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 (R1, R2) samples the anode voltage. A control circuit (20) is responsive to the sample voltage (V1) 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: HIGH VOLTAGE REGULATION BY SWITCHING RETRACE CAPACITORS 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'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 typ e 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. 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: FIGURE 1 is a block diagram of a high voltage regulation circuit that embodies the present invention; FIGURE 2 is a schematic diagram of the high voltage regulation circuit of FIGURE 1 ; FIGURES 3a-3d show voltage and current waveforms useful for explaining the operation of the high voltage regulation circuit of FIGURES 1 and 2; and FIGURES 4a and 4b show, in block and schematic form, th e high voltage regulation circuit in combination with different raster correction circuits of the information display apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGURE 1 illustrates the principle of operation of th e 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 th e high-voltage anode voltage EHT required by the cathode ray tube . The retrace voltage VR is shown in FIGURE 3a; the retrace voltage VR typically has a substantially sinusoidal shape at a low load, o r 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 FIGURES 3a-3d 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 SW2 should be placed in the open position. For example, the control circuit 20 opens the switch element SW2 sooner if th e anode voltage EHT drops below a predetermined level; conversely, the control circuit 20 opens the switch element SW2 later if th e anode voltage EHT exceeds the predetermined level. In this manner the anode voltage EHT can be precisely regulated. FIGURE 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 SW1, the S-shaping capacitance CS, th e horizontal deflection yoke LH, the fixed retrace capacitance CRl, and the switched retrace capacitance CR2. For purposes of th e present description the switch element TR1 is initially closed so that the retrace capacitances CRl and CR2 are in parallel; thus, th e 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 th e switch element SW1 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 SW1 typically consists of an np n - 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 th e transistor and the anode of the diode are coupled to the ground, o r reference, potential. During the period of time that switch SW1 is in a closed position, also referred to as the "trace" period, th e electron beam is deflected from the left edge of the screen to th e 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 Jl of the S-shaping capacitor CS and the retrace capacitances CRl and CR2 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 SW1 is i n 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 th e resonant interaction of the horizontal deflection yoke LH and th e retrace capacitances CRl and CR2. During this retrace period th e retrace voltage VR at the junction has a substantially sinusoidal shape, as shown in FIGURE 3a, and a peak magnitude that is equal to approximately 1200 V for the exemplary embodiment of FIGURE 2. The retrace voltage VR is coupled through a primary winding 30 of transformer Tl to a secondary winding 40, where it is rectified by the diode Dl 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 FIGURE 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 VI . In FIGURE 2, the sampling network is implemented by a voltage divider network. The anode voltage EHT is divided by the voltage divider formed by the resistors Rl and R2 to provide the sample voltage VI, which is applied to th e non-inverting input 51 of the error amplifier 50. The sample voltage VI 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 VI is amplified and provided at th e output 53 of the error amplifier 50. The gain of the error amplifier 50 is determined by the ratio of the resistor R4 to th e resistor R5. This ratio also affects the output impedance of th e high voltage regulation circuit, which, for the exemplary embodiment of FIGURE 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 R3 and the capacitor C3 to produce a control voltage Vctrl, which is applied to the non-inverting input 61 of a comparator 60. I n the exemplary embodiment shown in FIGURE 2, it has bee n experimentally determined that the high voltage regulation circuit 100 performs best when the low-pass filter formed by the resistor R3 and the capacitor C3 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 i n each embodiment of the particular high voltage regulation circuit 100 and involves a trade-off between two competing demand s placed on the filter. Typically, a higher cut-off frequency for thi s filter, and thus a faster response for the control loop of the high voltage regulation circuit, is undesirable. Because of coupling between the transformer Tl, 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. A n 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 b y charging capacitor Cl through resistor R3. Capacitor Cl is discharged to ground through transistor TR2 by using a horizontal-rate waveform 70 to turn transistor TR2 on at a horizontal-rate frequency. The horizontal-rate waveform 70 m a y be provided by a secondary winding (not shown) of th e transformer Tl and may thus have the same general shape, although inverted, as the retrace voltage VR at junction Jl . In th e exemplary embodiment of the high voltage regulation circuit 1 00 shown in FIGURE 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 TR2 by a differentiator formed by a capacitor C4 and resistors R8 an d R9. The values for the components of the differentiator are selected such that transistor TR2 turns on and discharges th e capacitor Cl 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 th e control voltage Vctrl; at that time, the switch drive signal 64 goes to a low level, as shown in FIGURE 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 th e reference voltage Vref. The error voltage Verr, and thus th e 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 th e 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 TR1. 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, a s indicated by the waveform shown in FIGURES 2 and 3c. 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 TR3, the base electrode of which is coupled to th e comparator 60 and controlled by the switch drive signal 64, is thus off, and the capacitor C2 charges to the supply voltage potential through the path defined by the resistors R5 and R6, th e capacitor C2, and the diode D3. The diode D2 is off, and the switch element TRl is on by virtue of being coupled at its base electrode to the supply voltage potential through the resistor R7. The switched retrace capacitance CR2 is thus coupled in parallel to th e fixed retrace capacitance CRl, so that the effective retrace capacitance is the sum of the fixed CRl and switched CR2 retrace capacitances. Note that minimal power is dissipated by the switch element TRl because the retrace voltage VR at junction Jl and a current II through the switch element TRl are both equal to approximately zero during the trace period. The switch element TRl may be a transistor having an industry part number BU506DF, which has an integrated body diode that is labeled D4 i n FIGURE 2. The switch element TRl must have a voltage rating that is sufficiently high to enable switch element TRl to withstand the peak magnitude of the retrace voltage VR. As the retrace period begins, the switch element TRl is still on; the current II flows to ground through the switch element TRl, as indicated by the waveform shown in FIGURE 3d, thereby charging the switched retrace capacitor CR2; and the ramp voltage Vramp approaches the control voltage Vctrl. The peak magnitude of the current II flowing through the switch element TRl 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 th e ground, or reference, potential. As indicated by the waveforms shown in FIGURES 3c and 3d, when this transition occurs is also a function of the condition of the load. This transition causes th e transistor TR3 to turn on. The voltage at the junction of th e cathode of the diode D2 and the anode of the diode D3 goes to a negative voltage. The diode D3 thus turns off and the diode D2 turns on, thereby turning off the switch element TRl . As a result, the switched retrace capacitor CR2 is no longer in parallel with th e fixed retrace capacitor CRl , and the effective retrace capacitance is thus reduced and is equal to the fixed retrace capacitance CRl . The peak-to-peak magnitude of the retrace voltage VR at the junction Jl increases due to the decrease in the effective retrace capacitance. Consequently, the anode voltage rises toward th e predetermined level as the retrace voltage VR, with its increased peak-to-peak magnitude, is coupled through the primary winding 30 of the transformer Tl to the secondary winding 40, and then rectified by the diode Dl to provide the anode voltage EHT. When the switch element TRl turns off, the flow of current II is interrupted, and the switched retrace capacitance CR2 maintains a constant charge and thus a fixed voltage drop. The voltage VR' at the collector electrode of the switch element TRl tracks the retrace voltage VR during the retrace period by virtue of the fixed voltage drop across the switched retrace capacitance CR2, as indicated by the waveform shown in FIGURE 3b. As indicated in FIGURE 3b, the magnitude of the voltage VR' is also a function of the load condition. In the exemplary embodiment shown in FIGURE 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 D4 clamps the collector electrode of the switch element TRl to approximately the ground, or reference, potential if the voltage VR' attempts to go below th e ground, or reference, potential. This allows switch element TRl to switch at a zero voltage, further minimizing the power dissipated by the switch element TRl . The inventive high voltage regulation circuit of FIGURE 2 thus advantageously provides for precise control of the point during the retrace period at which the switch element TRl will turn off to reduce the effective retrace capacitance of th e horizontal deflection circuit 10. The point at which the switch element TRl is turned off is made to be as close as possible to th e beginning of the retrace interval in order to minimize po wer dissipation in the switch element TRl . 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 m a y 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 R3 and the capacitor C3 in the high voltage regulation circuit 100 of FIGURE 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 FIGURE 2 advantageously provides for such correction. In a feature of th e present invention, the output 53 of the error amplifier 50 is coupled to a particular East- West pincushion correction circuit b y a resistor R10 and a De blocking capacitor C5 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 FIGURES 4a and 4b, the high voltage regulation circuit 100 of FIGURES 1 and 2 can be used in conjunction with a forward regulated output stage 91, as shown in FIGURE 4a, or i n conjunction with a diode modulator configuration 92, as shown i n FIGURE 4b. The E-W (East-West) drive 90 shown in FIGURE 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 th e raster width to an East-West pincushion correction circuit is to u s e 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 FIGURE 2, the conventional approach would couple the point labeled BCL on the secondary winding 40 of th e transformer Tl 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. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to b e understood that the invention is not limited to those precise embodiments, and that various changes and modifications may b e effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.

Claims: CLAIMS: 1. A circuit for regulating an anode voltage (EHT) in a n information display apparatus, said circuit comprising: (a) a horizontal deflection circuit (10) having trace and retrace modes of operation; said horizontal deflection circuit (10) comprising: a switched retrace capacitance (CR2); a switch element (TRl) coupled in series with said switched retrace capacitance (CR2), said switch element being in a closed position as the horizontal deflection circuit enters said retrace mode of operation; and a fixed retrace capacitance (CRl) coupled in parallel with said series combination of said switched retrace capacitance an d said switch element; (b) a sampling network (Rl, R2) for providing a sample voltage (VI) indicative of a magnitude of said anode voltage (EHT); and (c) a control circuit (20) responsive to said sample voltage (VI) for controlling said switch element (TRl), such that said control circuit effects regulation of said anode voltage (EHT) b y selecting an instant during said retrace period at which to place said switch element in an open position. 2. The circuit of claim 1, wherein said anode voltage (EHT) is increased when said switch element (TRl) is placed in an open position early in said retrace period. 3. The circuit of claim 2, wherein said anode voltage (EHT) is decreased when said switch element (TRl) is placed in an open position relatively later in said retrace period. 4. The circuit of claim 1, wherein said switch element comprises a transistor (TRl). 5. The circuit of claim 4, wherein said switch element further comprises a diode (D4) coupled in parallel with said transistor (TRl), such that a direction of conduction of said diode is opposite that of said transistor. 6. The circuit of claim 4, wherein said transistor (TRl ) comprises a bipolar junction transistor. 7. The circuit of claim 1, wherein said sampling netw ork comprises a voltage divider network (Rl , R2). 8. The circuit of claim 1, wherein said control circuit (20) comprises an error amplifier (50) for comparing said sample voltage (VI) with a reference voltage (Vref) to provide an error voltage (Verr) indicative of a deviation of said anode voltage (EHT) from a desired level. 9. The circuit of claim 8, wherein said control circuit (20) further comprises a low-pass filter (R3, C3) coupled to an output (53) of said error amplifier. 10. The circuit of claim 9, wherein said control circuit ( 20) further comprises a pulse width modulator circuit having an input (61) coupled to said low-pass filter (R3, C3). 11. The circuit of claim 10, wherein said pulse width modulator circuit (60) provides a switch drive signal (64) for said switch element (TRl) at an output (63). 12. A circuit for regulating an anode voltage (EHT) in a n information display apparatus, said circuit comprising: a horizontal deflection circuit (10) having trace and retrace modes of operation; said horizontal deflection circuit (10) having a variable retrace capacitance (CRl, CR2, TRl); a sampling network (Rl , R2) for providing a sample voltage (V I ) indicative of a magnitude of said anode voltage (EHT); a control circuit (20) for varying said variable retrace capacitance (CRl, CR2, TRl) during said retrace period responsive to said sample voltage (VI ); and a raster width compensation circuit for controlling the width of a raster formed by said horizontal deflection circuit (10), said raster width compensation circuit coupled to said control circuit (20) . 13. The circuit of claim 12, wherein control circuit (20) comprises an error amplifier (50) for comparing said sample voltage (VI ) with a reference voltage (Vref) to provide an error voltage (Verr) indicative of a deviation of said anode voltage (EHT) from a desired level. 14. The circuit of claim 13, wherein said raster width compensation circuit comprises an east-west pincushion correction circuit. 15. The circuit of claim 14, wherein said east-west pincushion correction circuit is coupled to an output (53) of said error amplifier (50). 16. The circuit of claim 15, wherein said error voltage (Verr) of said error amplifier (50) provides a raster width compensation voltage (V EW ) for said east-west pincushion correction circuit.

Inventors: TRIPOD LUC, TRIPOD, LUC
Applicants: RCA THOMSON LICENSING CORP, TRIPOD LUC, RCA THOMSON LICENSING CORPORATION, TRIPOD, LUC
Application year: 1997
Main classification: H04N3
IPC codes: G09G1, H04N3