Abstract:
A power supply for a video display tube includes a deflection transformer with a primary winding powered with a scan voltage which has a first value when displaying signal according to a first video standard and a second voltage when displaying video according to a second standard. A secondary winding of the transformer produces filament voltage for the display tube. A variable coupler couples the filament voltage to the filament of the picture tube in an amount established by a control signal. A memory produces a first control signal in response to the first scan voltage and a second control signal in response to the second scan voltage. The first and second control signals are selected so that the voltage coupled from the transformer to the filament is the same at both scan voltages.

Description:
[0001]    This application claims the priority of Provisional application No. 60/373,875 filed Apr. 19, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to filament power supplies for display tubes, and more particularly to such supplies for powering filaments in the context of multiple scan frequencies.  
         BACKGROUND OF THE INVENTION  
         [0003]    Video display tubes such as television picture tube are vacuum tubes which ordinarily use a filament to heat the cathode in order to free electrons from the cathode. The electrons from the cathode are accelerated toward the ultor or phosphor faceplate to generate a luminescent dot. The dot is scanned over the face of the picture tube, which creates a bright display. The intensity of the electron beam is modulated in consonance with the scanning to create a picture.  
           [0004]    In the past, television or video picture tubes have been supplied with voltage from a horizontal deflection transformer driven from a regulated scan voltage at the horizontal scan frequency, which pursuant to National Television Standards Committee (NTSC) standards corresponds to about 15,734 Hz. The high voltage for the picture tube ultor is commonly generated by rectifying the voltage from a high voltage secondary winding of a deflection transformer, and the filament of the picture tube was driven by alternating voltage from a filament secondary winding of the transformer. The regulation of the scan voltage was believed to be sufficient to maintain the filament voltage within the nominal filament supply voltage range of the picture tube. If the filament voltage deviates from the nominal value, picture tube life can be adversely affected.  
           [0005]    The advent of multi-function high definition television (HDTV) and NTSC display systems requires that the display system be operable at both the NTSC standard deflection frequency and another, higher, HDTV deflection frequency When a picture tube is to be used with both NTSC and with HDTV, the deflection frequency used for NTSC is often 2H, where H represents the NTSC deflection frequency. The HDTV deflection frequency used for this purpose may be, for example, 2.14H. Because of the differing horizontal scan frequencies of HDTV and NTSC systems, a picture tube scanned at those disparate frequencies would, in the absence of compensation for the effects of the deflection frequencies, scan different portions of the phosphor screen, depending upon the scan frequency. In order to cause the scanned portion of the phosphor screen to be the same when displaying NTSC or HDTV, the scan voltage may be increased in proportion to the scan frequency. It should be noted that, by contrast with HDTV, video pursuant to NTSC standards is often referred to as having “standard definition.” 
           [0006]    When two different scan voltages are applied to the deflection transformer, depending upon whether the video is standard definition or HDTV, the voltage produced by the filament winding of the deflection transformer varies.  
           [0007]    One possible way to regulate the filament voltage of a television display tube is to produce alternating filament voltage, rectify the alternating voltage to produce pulsating voltage or current, and to filter the pulsating current by the use of capacitors, to thereby produce direct voltage. In this context, the term “pulsating” differs from “alternating” in that pulsating voltage or current is principally unidirectional, while alternating generally suggests voltages taking opposite polarities relative to zero and currents flowing in generally equal amounts in both directions. The direct voltage produced by rectification and filtration can then be regulated in a conventional manner to produce the desired value of filament voltage. This approach is effective, but the cost may be greater than desired, because filament currents in video display devices may be on the order of ¾ ampere, and the capacitors required for filtering may be large, expensive, and subject to aging. The active devices required for regulating voltage at the required currents tend to be expensive because of the relatively high power being controlled, and their mountings also tend to be expensive because of the heat to be removed.  
           [0008]    According to an aspect of the invention, a controllable coupling device is coupled between the filament voltage winding of a deflection transformer and the filament(s) of a display tube, to control the magnitude of the coupling under the control of a control signal, and the control signal is selected to provide the same filament voltage regardless of the applied scan voltage. The controllable coupling device may be viewed as a variable voltage divider, a variable load, a variable current shunt, or a level shifter.  
         SUMMARY OF THE INVENTION  
         [0009]    A video display power supply for energizing a filament load of a cathode ray tube according to an aspect of the invention comprises a source of an input supply voltage, and also comprises a switching semiconductor coupled to the input supply voltage source for generating an alternate current, unregulated supply output at a frequency related to a deflection frequency. The unregulated supply output is applied to the filament load unfiltered, with respect to the frequency of the unregulated supply output, to develop in the filament load an unfiltered, second supply output. The video display power supply also includes a regulator for regulating the second supply output, and a source of regulation control signal. The regulation control signal has a value selected from a range of values, and is coupled to an input of the regulator for regulating the second supply output in accordance with the selected value of the control signal.  
           [0010]    According to another aspect of the invention, a video display power supply energizes a filament load of a cathode ray tube. The video display power supply according to this other aspect of the invention comprises a power supply power stage for generating an unregulated supply output voltage at a frequency related to a deflection frequency, where the deflection frequency being selected from a range of frequencies. A regulator applies the unregulated supply output voltage to the filament load to generate in the filament load a regulated supply output voltage. A source of regulation control signal is coupled to a control input of the regulator and has a selectable first value when a first deflection frequency is selected and a selectable second value when a second deflection frequency is selected for regulating the unregulated supply output voltage in accordance with the selected deflection frequency. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    [0011]FIG. 1 a  is a simplified diagram in block and schematic form of a first embodiment of a display arrangement according to an aspect of the invention, and FIG. 1 b  is an amplitude-time plot of voltages which may occur in the arrangement of FIG. 1 a  during operation; and  
         [0012]    [0012]FIG. 2 is a simplified diagram in block and schematic form of another embodiment according to an aspect of the invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0013]    In FIG. 1 a , a video display apparatus  10  includes a picture tube or cathode-ray tube (CRT)  12  having an ultor or high voltage terminal  12   u  and a cathode (K) filament  12   f . A horizontal deflection winding  14  having terminals  14 A and  14 B is illustrated as being associated with picture tube  12 .  
         [0014]    Also in FIG. 1 a , a horizontal deflection arrangement designated generally as  20  includes a transformer  22  and a horizontal deflection circuit  24 . Transformer  22  includes a primary (PRI) winding  22 P, a high voltage winding  22 HV with terminals  22 HV 1  and  22 HV 2 , and a filament winding  22 FIL. A terminal  22 P 1  of primary winding  22 P is connected to the output terminal  26   o  of a controllable scan voltage source  26 . Scan voltage source  26  produces a first scan voltage under the control of the first state of a signal applied to its input terminal  26   i , and produces a second voltage, greater than the first, in response to the second state of the control signal. Another terminal  22 P 2  of primary winding  22 P of transformer  22  is connected to the collector of a high voltage switching NPN transistor  28 . High voltage switching transistor  28  has its emitter coupled to ground. A diode  30  has its anode connected to the collector of transistor  28 . A horizontal-frequency (H) source  32  produces base drive for transistor  28  at disparate drive frequencies f 1  or f 2 , which in one embodiment of the invention corresponds to either 2 h or 2.14H, under the control of a control signal applied to its input terminal  32   i.    
         [0015]    Terminal  22 HV 1  of high voltage winding  22 HV of transformer  22  of FIG. 1 a  is connected to ultor  12 U of picture tube  12  by way of a rectifying arrangement, illustrated in simplified form as a single rectifier  34 . Rectifier  34  rectifies the high voltage pulses produced by winding  22 HV to generate the ultor voltage for ultor terminal  12 U. Terminal  22 HV 2  of winding  22 HV is connected across a capacitor C 38  and resistor R 38 , and by way of a series resistor RA to a video integrated circuit  36 . Integrated circuit  36  is coupled to the cathode K of picture tube  12 , as suggested by path  36   p . A resistor RV is connected to a direct supply voltage source V 1  and video IC  36  for energizing the IC. During normal operation, capacitor  38  is normally at a low positive voltage, and little current flows through resistor RA. When a very bright scene occurs, current in the high voltage winding  22 HV tends to discharge capacitor C 38 , which tends to draw current through resistor RA, which in turn tends to reduce the available energizing voltage for video integrated circuit  36 . The reduction in the energizing voltage for integrated circuit  36 , in turn, tends to reduce the anode current, all in known manner.  
         [0016]    Horizontal deflection winding  14  of FIG. 1 a  is coupled to the collector of switching transistor  28 . The conventional operation of switching transistor  28  at the frequency established by a horizontal (H) source, in conjunction with a conventional damper diode  30  and retrace capacitor Cr, generates deflection current through horizontal deflection winding  14  of FIG. 1 a , which in turn causes the electron beam of the picture tube  12  to deflect in a horizontal direction.  
         [0017]    Filament winding  22 FIL of FIG. 1 a  produces pulsatory voltages, which are illustrated generally as  200  in FIG. 1 b . In FIG. 1 b , the average voltage  200  produced by winding  22 FIL is zero volts, and the positive-going pulses  210  have a peak value of about 23 volts and a duration of about 4.5:S. The negative value  212  lying between the pulses  210  is in the range of about 2 volts. The pulsatory voltages produced by filament winding  22 FIL are applied through a rectifier  70  and a resistor  71  to the filaments  12   f  of picture tube  12 . A control circuit designated generally as  40  is connected between ground and the junction  39  of resistor  71  with filament  12   f . In effect, control circuit  40  is coupled across filament  12   f , where the term “across” is used in its electrical, rather than physical, sense. With this connection, control circuit  40  is effectively in parallel (shunt) with filament  12   f . Thus, increasing conduction in control circuit  40  increasingly bypasses or shunts current away from filament  12   f , thereby decreasing the current available to filament  12   f , and effectively reducing the voltage at the filament. Correspondingly, reduced conduction of control circuit  40  bypasses or shunts less current away from filament  12   f , thereby leaving more current available for the filament, thereby increasing the filament voltage. Thus, varying conduction of control circuit  40  adjusts the voltage across, and the current through, filament  12   f.    
         [0018]    Control circuit  40  of FIG. 1 a  includes an NPN transistor  42  having its emitter coupled to ground by way of a resistor  46 , and its collector coupled by way of a resistor  44  to junction  39 . The conduction of transistor  42  is controlled by base current or voltage applied to the base of the transistor by way of a resistive voltage divider  47  including resistors  48  and  50 . Looking at the operation of the control circuit  40  as being a level shifter, the collector current of transistor  42  may be considered to be a constant current during those intervals in which its collector-to-base voltage reverse-biases the junction. This constant current flows to ground from junction  39 , and reduces the current available for filament  12   f . Alternatively, the constant current through the collector of transistor  42  may be viewed as increasing the voltage drop across series resistor  70 , which in turn reduces the voltage available for filament  12   f.    
         [0019]    The action of the control circuit  40  may be viewed in another way. More particularly, the serial combination of resistors  44  and  46 , in conjunction with transistor  42 , may be viewed as being one leg of a voltage divider including series resistor  71  and the shunt (as to filament  12 F) control circuit  40 . As the conduction of the shunt leg (the leg in parallel with the filament  12 F) of the voltage divider increases, more of the pulsatory voltage produced by filament winding  22 FIL is developed across resistor  41 , and less across the parallel combination of the shunt leg  40  and filament  12   f . Similarly, as the conduction of the shunt leg  40  of the voltage divider decreases, more of the pulsatory voltage produced by filament winding  22 FIL is produced across the parallel combination of shunt leg and the filament  12   f , so more voltage is available across filament  12   f.    
         [0020]    A memory  60  of FIG. 1 a  addresses an internal memory location under the control of a control signal applied to its input port  60   i  by way of a coupling path  64   c . In a first state of the control signal applied to port  64 , the memory addresses a first memory location, and in a second state of the control signal, the memory addresses a second memory location. The signal from the addressed memory location is applied to a digital-to-analog converter (DAC)  62 , which converts the digital signal from the memory into a corresponding analog signal. The analog signal is applied from DAC  62 , by way of a resistor  54  and an emitter follower  52 , to the voltage divider  47  and the base of transistor  42 , for controlling the conduction of transistor  42 .  
         [0021]    In operation of the arrangement of FIG. 1 b , an HDTV/NTSC control signal is applied by way of terminal  64  to input port  26   i  of scan voltage source  26 , input port  32   i  of H source  32 , and to input port  60   i  of memory  60 . In a first state of the control signal, which may be assumed to be for selecting NTSC operation, scan voltage source  26  produces a first, relatively low voltage, H source  32  produces signals at frequency 2H for driving horizontal deflection circuit  24 , and memory  60  addresses a first memory location, preprogrammed with a digital word representing a voltage which, when applied by way of resistor  54 , emitter follower  52 , and voltage divider  47  to the base of transistor  42 , causes conduction of control circuit  40  sufficient to maintain the filament voltage of filament  12   f  within its nominal value. When HDTV operation is commanded by altering the state of the HDTV/NTSC signal, the voltage produced by scan voltage source  26  increases to maintain constant product of scan time and supply voltage This increase in scan voltage would ordinarily be expected to increase the value of the pulsatory filament voltage produced by filament winding  22 FIL. When HDTV operation is commanded by the HDTV/NTSC signal, H source  32  produces frequency 2.14H, and memory  60  addresses another memory location, which is preprogrammed with a digital word representing a voltage which, when applied by way of resistor  54 , emitter follower  52 , and voltage divider  47  to the base of transistor  42 , causes conduction of control circuit  40  sufficient to maintain the filament voltage of filament  12   f  within its nominal value. Thus, the filament voltage is held near its nominal value regardless of the scan voltage.  
         [0022]    [0022]FIG. 2 illustrates an alternative embodiment of this aspect of the invention. FIG. 2 is very similar to FIG. 1 a , with the only difference lying in the location of rectifier  70 . In FIG. 2, rectifier  70  no longer lies in the path extending from filament winding  22 FIL and the filament  12   f , so a part of the alternating voltage  200  of FIG. 1 b  is applied to the filament  12   f  Rectifier  70  is instead located in series with resistor  44  of control circuit  40 , so that the collector-to-base junction of transistor  42  does not become forward biased by application of a negative voltage ( 212  of FIG. 1 b ) thereto.  
         [0023]    A salient advantage of the arrangement according to the invention is that a series regulator with a associated filter capacitor need not be used to control the voltage applied to filament  12 F.  
         [0024]    Other embodiments of the invention will be apparent to those skilled in the art For example, while FIG. 1 a  shows the series resistor  71  connected between filament winding  22 FIL and the filament  12   f  as being a single resistor, it may be advantageous from a cost point of view to use a plurality of resistors, so that inexpensive low power resistors may be used instead of more expensive higher-power resistors.