Abstract:
A video display includes a scanner operable at a first frequency and a higher second frequency. A switch mode power supply drives a transformer with three secondaries. First and second rectifiers &amp; filters are associated with the first and second secondaries. A rectifier is coupled to the third secondary and by way of a switch to the first filter. Feedback from the first filter controls the SMPS. In a first operating mode, the scanner is operated at the first frequency, the switch is open, the scanner supply is a first voltage from the first filter, and ancillary equipment is supplied with a third voltage by the second filter. In a second operating mode, the scanner is operated at the second frequency, the switch is closed, the scanner supply is a second voltage, higher than the first, from the first filter, and ancillary equipment is supplied with the same third voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of Provisional application Ser. No. 60/374,281 filed Apr. 19, 2002.  
         [0002]     This invention relates to video displays, and more particularly to power supply arrangements for displays which may operate at different frequencies. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     Multifrequency video displays or monitors are used for both High Definition Television (HDTV) and National Television Standards Committee (NTSC) television viewing and for computer applications. At the current state of the art, charge-coupled device (CCD) displays tend to be costly, or unavailable, especially in large sizes, and in general are not as bright as kinescope or picture-tube types of displays. Plasma displays are not common. Thus, the kinescope display is in common use.  
         [0004]     Kinescope displays are ordinarily scanned by means of inductive or electromagnetic yokes near the neck of the kinescope, to which both vertical and horizontal scan currents are applied, to create magnetic fields which deviate one or more electron beams traversing the tube to the phosphorescent display screen. National Television Standards Committee (NTSC) standards for conventional television specify a horizontal scanning frequency of about 15,734 Hz, and a vertical scan frequency of 60 Hz. A large body of art has grown up around vertical and horizontal deflection circuits. Because of the relatively high horizontal scan frequency and significant power involved in performing horizontal scan, the horizontal deflection scanning circuits commonly operate in a resonant retrace mode, in which currents and the associated power are recirculated back to the power source for re-use during the next following scan cycle.  
         [0005]     The amplitude of current circulating in a resonant horizontal deflection circuit is mainly determined by the value of the B+ voltage. It is also determined, at least in part, by the desired scan frequency. For a given horizontal deflection system with a fixed value of kinescope ultor (High) voltage, a fixed amount of overscan, and a horizontal deflection yoke having a fixed inductance, the product of the scan time multiplied by the deflection circuit B+ tends to be a constant. Thus, the value of energizing voltage or B+ applied to the horizontal deflection circuit multiplied by the scan time is desirably constant. In the past, many video display systems were designed to apply a constant B+ to the horizontal deflection system.  
         [0006]     With the advent of HDTV, many different video formats are available to the consumer. Some of these formats have vertical and horizontal deflection frequencies which differ from those of conventional NTSC broadcast television signals. The higher definition associated with HDTV implies a higher horizontal deflection frequency than for standard-definition television. As the horizontal frequency increases, the scan time must decrease, unless the frequency difference is very small and can be taken up in the retrace time. For most television systems in which the horizontal scanning frequency is varied, the retrace time is held constant, and the scan time is varied. Thus, as the horizontal scanning frequency is increased, the scan time is decreased, and the scan B+ is also increased so that the product of the scan time multiplied by the B+ is a constant.  
         [0007]     Many present-day television display devices, including television receivers, derive their scan B+ voltage from a switch mode power supply (SMPS) including inductive elements and a transformer having a secondary winding. The scan B+ is generated from the scan secondary winding by rectification of SMPS pulsatory signals and subsequent filtration. In many such display devices, in order to avoid the need for plural power supplies, the SMPS must also supply auxiliary or ancillary voltages, for operation of systems or circuits other than the horizontal deflection system. In a television receiver, for example, the ancillary systems may include RF and IF processors, video and sound processors, convergence, and others. These ancillary systems ordinarily require that their energizing voltages remain constant, regardless of the horizontal deflection frequency which happens to be in use. The ancillary energizing voltages may be derived from secondary windings of the SMPS transformer other than the one from which the horizontal scan or deflection B+ is derived. However, simply deriving the ancillary energizing voltages from a separate secondary winding will not guarantee that the ancillary energizing voltage does not change. Since the number of turns per winding in the SMPS transformer is fixed, changing the horizontal scan B+, without more, also changes the ancillary energizing voltage.  
         [0008]     Improved video display arrangements are desired.  
       SUMMARY OF THE INVENTION  
       [0009]     A video display apparatus according to an aspect of the invention comprises a deflection circuit output stage for selectively generating a deflection current in a deflection winding at a first deflection frequency and at a second deflection frequency, and a power supply for producing, via a common power transistor of an output stage, a first supply voltage at a first terminal and a second supply voltage at a second terminal. A first switch is responsive to a control signal indicative of the selected deflection frequency, for selectively coupling the first supply voltage to the deflection circuit output stage when the first deflection frequency is selected, and the second supply voltage, when the second deflection frequency is selected. A power supply regulator is responsive to at least one of the first and second supply voltages for regulating the at least one of the first and second supply voltages via a negative feedback path.  
         [0010]     In a particular version of the video display according to this aspect of the invention, a second switch is responsive to a control signal that is indicative of the selected frequency and coupled in the negative feedback path. The second switch has a first state when the deflection current is at the first deflection frequency, and has a second state when the deflection current is at the second deflection frequency, for providing coarse adjustment. In another avatar of this version of the video display, a source of a fine adjustment signal is coupled to the negative feedback path for providing fine adjustment. In a hypostasis of this avatar, a portion of the negative feedback path contains information derived from at least one of the first and second supply voltages, information derived from the state of the second switch and information derived from the fine adjustment signal.  
         [0011]     In another particular version of the video display according to this aspect of the invention, each of the first and second supply voltages is regulated via the negative feedback path.  
         [0012]     In yet another particular version of the video display according to this aspect of the invention, an output transformer is coupled to the power transistor for producing the first supply voltage from a voltage developed in a first transformer winding and the second supply voltage from a voltage developed in a second transformer winding of the transformer, and the transformer has a third transformer winding for producing a third supply voltage that is coupled to a load circuit, wherein a volts-per-turn ratio in the third transformer winding remains the same at each of the first and second deflection frequencies.  
         [0013]     In yet a further particular version, an output transformer is coupled to the power transistor for producing the first and second supply voltages, and the transformer has a transformer winding for producing a third supply voltage that is coupled to a load circuit, wherein a volts-per-turn ratio in the transformer winding remains the same at each of the first and second deflection frequencies.  
         [0014]     In another particular version, the power supply regulator is responsive to a feedback signal produced at an output terminal of the first switch for regulating each of the first and second supply voltages. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]      FIG. 1  is a simplified schematic diagram of a portion of a television receiver according to an aspect of the invention; and  
         [0016]      FIG. 2  is a more detailed, but still simplified, diagram of a portion of the arrangement of  FIG. 1 . 
     
    
     DESCRIPTION OF THE INVENTION  
       [0017]     In  FIG. 1 , a system designated generally as  10  includes a kinescope  12  associated with a vertical deflection coil  12 V and a horizontal deflection coil  12 H. A block  20  represents ancillary equipment, which may include, for example, any or all of kinescope cathode drivers, radio-frequency receivers and intermediate-frequency amplifiers, demodulators, audio circuits, video processors, and the like, all well known for use in television and video display apparatus. A horizontal deflection or scan circuit, illustrated as a block  100 , is connected to horizontal deflection coil  12 H for applying horizontal scan signals thereto, for causing the desired horizontal deflection, at a selected horizontal scan frequency H 1  or H 2  controlled by a source illustrated as a block  102 . Horizontal deflection circuit  100  receives an energizing voltage designated as B+at an energizing voltage input port  100   i.    
         [0018]     In  FIG. 1 , a switch-mode power supply (SMPS) designated generally as  30  includes a power transformer T 1  including a primary winding T 1 P connected to power transformer T 1  terminals  2  and  4 , a magnetic core designated TIC, a first secondary winding T 1 S 1  including a first portion T 1 S 1   a  connected to transformer terminals  16  and  18 , and a second portion T 1 S 1   b  connected to transformer terminals  16  and  19 , and a regeneration or oscillation inducing feedback winding T 1   f  connected to transformer terminals  1  and  8 . One end of secondary winding portion T 1 S 1   a  is connected to ground by way of terminal  18 . Since both secondary winding portions T 1 S 1   a  and T 1 S 1   b  are connected to transformer terminal  16  and the remote end of portion T 1 S 1   a  is connected to ground, terminal  16  may be considered to be a tap on secondary winding T 1 S 1 . Viewed another way, the end of secondary winding portion T 1 S 1   b  remote from terminal  16  is connected to a terminal  19 , so that the two serially-connected portions of secondary winding T 1 S 1  form a tapped secondary winding with a tap terminal  16 .  
         [0019]     Secondary winding T 1 S 2  of  FIG. 1  has one end coupled to ground by way of a terminal  14 , and the other end connected to a terminal  13 . Secondary winding T 1 S 2  represents any one of a plurality of such windings which may be associated with transformer T 1 , each producing a different or independent output voltage or power for energizing various portions of the apparatus  10 .  
         [0020]     Also in  FIG. 1 , a block  32  represents the electronics and power switching portion of switch mode power supply  30 . Primary winding T 1 P terminals  2  and  4  of transformer T 1  are connected to SMPS electronics and Power switching block terminals  32   2  and  32   4 , respectively, and feedback secondary winding T 1   f  terminals  1  and  8  are connected to SMPS electronics and power switching block terminals  32   1  and  32   8 , respectively. SMPS electronics and power switching portion  32  receives raw B+ as its source of energizing power at a terminal designated RAW B+ relative to an isolated ground, illustrated by a downward-pointing open triangle symbol  32   ig . SMPS electronics and power switching portion  32  switches the power B+ applied to primary winding T 1 P of transformer T 1  so as to periodically store energy in the inductance associated with the winding, and to allow the energy so stored to produce pulsating or pulsatory (alternating) voltages on secondary windings T 1 S 1 , T 1 S 2 , and T 1   f . The pulsatory voltage generated on secondary winding T 1   f  is coupled back to the electronics and power switching portion  32  of switch mode power supply  30  by way of a terminals  32   1  and  32   8  to aid in sustaining oscillation. The pulsatory voltages appearing on secondary windings T 1 S 1  and T 1 S 2  are rectified, as known in the art, to produce pulsatory currents which are smoothed by filtering to produce the desired direct energizing voltages. More particularly, the pulsatory voltage produced at transformer terminal  16  by secondary winding portion T 1 S 1   a  is rectified by a unidirectional current conducting device illustrated as a diode or rectifier CR 107  to produce a voltage at terminal  02 , and applied to a filter designated as F 101  to be smoothed to produce a first “Scan B+” voltage for application to horizontal deflection circuit  100 . Similarly, the pulsatory voltage produced at transformer terminal  13  by secondary winding T 1 S 2  is rectified by a unidirectional current conducting device illustrated as a diode or rectifier CR 108 , and applied to a filter designated as F 102  to be smoothed to produce a direct voltage B 2  for application over a path Pa to the ancillary equipment illustrated as block  20 .  
         [0021]     In  FIG. 1 , the pulsatory voltage produced at terminal  19  of transformer T 1  is greater than the pulsatory voltage produced at tap terminal  16 , because of the additional voltage added by winding T 1 S 1   b  to that voltage appearing at terminal  16 . The voltage at terminal  19  is applied to a unidirectional current conducting device illustrated as a diode or rectifier CR 106 . The pulsatory voltage available at the cathode of device CR 106  is applied to a terminal K 1  of a relay K 101 . Relay K 101  also has a winding KW which, when energized, causes movable contact element K 2  to connect to contact K 1 , but in the illustrated unenergized state of relay K 101 , such contact is not made, and no current flows in unidirectional current conducting device CR 106 .  
         [0022]     In the arrangement of  FIG. 1 , the scan B+voltage at the output of smoothing filter F 101  is applied by way of a path  34  to a power supply controller designated generally as  38 , which includes a voltage divider  22  and an error amplifier U 103 . Voltage divider  22  includes three resistors, namely resistors R 117 , R 118 , and R 119 , having tap points  22   1  and  22   2  between them. The scan B+ voltage is divided in a (reduced by a) fixed ratio by voltage divider  22  and applied from tap  22   1  to a reference input terminal or port U 103   i  of an error amplifier illustrated as U 103 , which in this particular embodiment is a type TL431 integrated circuit, manufactured by Texas Instruments, NEC, Samsung, and others. Error amplifier U 103  has its terminal U 103   g  connected to ground by way of a resistor R 139 . Error amplifier U 103  compares the divided feedback voltage with an internal reference voltage and produces a feedback error signal which is coupled to an error signal input terminal  32   5  of SMPS electronics and switch circuit  32 , for control of the switching power supply  30  in known degenerative fashion.  
         [0023]     As so far described, the switch mode power supply  30  senses the Scan B+ voltage applied to horizontal deflection circuit  100 , and uses feedback to control that sensed voltage. So long as the feedback control of the Scan B+ voltage continues, the horizontal deflection circuit  100  and the ancillary equipments  20  are correctly energized. According to an aspect of the invention, the H drive source  102  is capable of driving the horizontal deflection circuit  100  at different or disparate horizontal frequencies, designated H 1  and H 2 . In one embodiment of the invention, H 1  is about twice the NTSC horizontal frequency of 15,734 Hz., corresponding to about 31,468 Hz., and H 2  is about 2.14 times the NTSC horizontal frequency, corresponding to about 33,750 Hz. The Scan B+ voltage applied to power input terminal  100   i  of horizontal deflection block  100  is required to change when the operating deflection frequency is changed, to maintain the constant product of Scan B+ multiplied by the reduced scan time engendered by the higher operating frequency. In particular, the Scan B+ voltage applied to input port  100   i  of horizontal deflection circuit  100  must theoretically increase by a factor of 2.14/2, or 1.07, in order to maintain constant product for an operating frequency change of 2.14/2, but which may deviate due to different amounts of overscan.  
         [0024]     In  FIG. 1 , regulator B+ switching signal REGB+_SW is applied by way of a port  24  and a signal path  26  to H source  102  for selecting either the 2H or 2.24H horizontal operating frequency. Switching signal REGB+_SW has a logic low level when the horizontal frequency is to be 2H and a high level when the horizontal frequency is to be 2.14H. In order to raise the Scan B+ voltage, it would be a simple matter to adjust the feedback voltage divider  22  to a different division ratio, to thereby increase the power stored by the switch mode power supply in the primary winding T 1 P of transformer T 1  during each operating cycle, thereby increasing the pulsatory voltage produced by secondary winding T 1 S 1   a  and the rectified pulsatory voltage produced by diode CR 107 . This would have the effect, however, of increasing the volts-per-turn of all the secondary windings of transformer T 1 , with the result that the voltage produced by representative secondary winding T 1 S 2 , diode CR 108 , and filter F 102  for application to the ancillary equipment illustrated as block  20 , which in turn would undesirably result in a proportional increase in the ancillary energizing voltage.  
         [0025]     Instead, according to an aspect of the invention, the Scan B+ voltage applied to the horizontal deflection circuit  100  is changed by switching another secondary winding, by means of relay K 101 , into circuit with smoothing filter F 101 . The feedback ratio is also changed to provide the same feedback voltage to the error amplifier so the volts-per-turn does not change as a function of the change in Scan B+. The change of the Scan B+is accomplished by additionally applying switching signal REGB+_SW to the base of a grounded-emitter transistor Q 105 , to turn ON transistor Q 105  when the REGB+_SW voltage has a high level for selecting the higher 2.14H scan frequency. When the higher scan frequency is selected, the scan time decreases, and the Scan B+ voltage must increase. The increase in scan voltage is accomplished by relay K 101 , in which a magnetic winding KW is energized by transistor Q 105  in its ON state. When relay K 101  magnetic winding KW is energized, movable element K 2  is brought into conductive contact with stationary element K 1 , so that the rectified pulsatory voltage at the cathode of diode CR 106  is applied to smoothing filter F 101 . The pulsatory voltage from CR 106 , being greater than the pulsatory voltage from diode CR 107 , keeps diode CR 107  turned OFF (causes CR 107  to cease conduction). In effect, relay K 101  disconnects CR 107  and its associated winding T 1 S 1   a  from filter F 101 , and connects CR 106  and its associated pair of windings T 1 S 1   a , T 1 S 1   b  to the filter. Since the rectified pulsatory voltage produced by CR 106  is greater than that produced by CR 107 , the smoothed B+ is also greater when relay K 101  is energized for 2.14H operation.  
         [0026]     SMPS electronics and power switch  32  of  FIG. 1  responds to degenerative error signals applied to its error signal input port  32   5  from error amplifier U 103 . SMPS electronics and power switch  32  adjusts its switch timing and/or frequency in such a manner as to tend to control the amount of energy stored in transformer primary winding T 1  during each switch cycle in response to the error signal. If the error signal changes as a result of the change in the magnitude of the Scan B+ engendered by the switching of relay K 101 , the ancillary supply voltages produced by representative secondary winding T 1 S 2 , diode CR 108 , and filter F 102  will change, and more particularly the voltage produced by secondary winding T 1 S 2 , diode CR 108 , and filter F 102  will decrease when relay K 101  is energized if the feedback ratio is not adjusted.  
         [0027]     According to an aspect of the invention, the feedback voltage division is changed, preferably in proportion to the change in the nominal Scan B+ which results from the switching of Relay K 101 . More particularly, in  FIG. 1 , the REGB+_SW signal applied by way of terminal  24  and path  26  to H source  102  and to transistor Q 105  is also applied to the base of a further grounded-emitter transistor Q 104 . Transistor Q 104  thus becomes conductive or ON when signal REGB+_SW is logic high. When Q 104  is conductive, a resistor R 120  is introduced into voltage divider  22 , to increase the division ratio. Put another way, the divided feedback signal at tap  22   1  is reduced when the Scan B+ voltage is increased, so that the feedback voltage applied to error amplifier U 103  remains the same, at least in principle. Thus, regardless of whether the Scan B+ voltage is derived from CR 107  and is relatively low, or is derived from CR 106  and is relatively high, the feedback sample at the input port of error amplifier U 103  remains the same. Since the feedback signal remains the same as a result of the switching of the coupling ratio of the feedback path, SMPS electronics and switch block  32  continues to transfer the same amount of energy per cycle to the secondary windings, including secondary winding T 1 S 2 . When secondary winding T 1 S 2  receives the same energy per cycle, the ancillary power supply including CR 108  and filter F 102  produces the same output voltage for operating the ancillary equipment  20 .  
         [0028]     In  FIG. 1 , an additional circuit  39  allows remote control of the magnitude of the Scan B+ voltage. The additional circuit  39  includes a memory (MEM) illustrated as a block  40 , together with a digital-to-analog converter (DAC), for producing a signal B+ALIGN. Signal B+ALIGN is applied to the base of a transistor Q 108  by way of a temperature compensating diode CR 112  and a voltage divider including resistors R 127 , R 128 , and R 129 . Transistor Q 108  has its emitter coupled by a resistor R 130 , so it produces a collector current which is linearly responsive to the magnitude of B+ALIGN, and produces an additional offset of the feedback signal at tap point  22   1  of voltage divider  22 , which can be used to perform fine adjustment of the Scan B+ voltage. The memory  40  is active during all the horizontal frequency modes, but accesses different memory locations under the control of REGB+_SW. The memory locations are preprogrammed with values which compensate for tolerances. An advantage of this arrangement is that the coarse adjustment of the feedback ratio is provided by resistor R 120 , and the combination of memory  40  and DAC  42  can be used over its full dynamic range for trimming the Scan B+voltage.  
         [0029]     The error signal produced by U 103  by comparing the feedback Scan B+signal from tap  22   1  of voltage divider  22  of  FIG. 1  is applied by way of error signal input port  32   5  of  FIG. 2  to input terminal U 101   2  of a photocoupler or optoisolator U 101 . Optoisolator U 101  provides isolation between those portions of the display unit  10  of  FIG. 1  which are at line potential and isolated from ground and the user-accessible or grounded portions of the SMPS electronics and switch block  32 . A resistor R 137  is coupled from terminal U 101   1  of U 101  to terminal U 101   2 , and terminal U 101   1  is connected by a resistor R 114  to a +15 v source.  
         [0030]     In  FIG. 2 , the primary winding T 1 P is illustrated in phantom across terminals  32   2  and  32   4  to aid understanding. Power FET switch Q 101  alternately switches terminal  32   4  of primary winding T 1 P to isolated ground by way of a current sensing resistor R 109 . Since terminal  32   2  of primary winding T 1 P is connected to Raw B+, the switching of Q 101  switches primary winding T 1 P across Raw B+. During those intervals in which Q 101  is conductive, current through T 1 P and through R 109  increases, with a concomitant increase in the energy stored in the inductance of the primary winding. Also during those intervals in which Q 101  is conductive and current is increasing in primary winding T 1 P, a positive-going (+) feedback voltage is generated at terminal  32   1  of feedback secondary winding T 1   f  relative to isolated ground. This positive-going voltage is fed by way of a resistor R 110 , a path  210 , capacitor C 104 , and resistors R 106  and R 107  to the gate of power switch Q 101 , to tend to hold Q 101  in a conductive state. A capacitor C 140 , connected to the gate of Q 101 , co-acts with resistors R 106  and R 107  to limit the rise time of the applied gate voltage, to thereby tend to reduce radio-frequency interference.  
         [0031]     The voltage at terminal  32   1  of  FIG. 2  relative to isolated ground alternates during the cyclical operation of the SMPS of  FIG. 2 . This alternating voltage is used to drive to separate power supplies, one of which is associated with CR 104 , and the other of which is associated with CR 102 . The first power supply has a charge path which includes R 110 , CR 104 , C 120 , and terminal  32   8  of secondary winding T 1   f , and it charges the nongrounded end of capacitor C 120  to a negative voltage, which in a particular embodiment is about −9 or −10 volts.  
         [0032]     The second power supply of  FIG. 2  includes CR 102  and capacitor C 108 , and it produces a positive voltage which tends to be proportional to the magnitude of the raw B+, because increasing B+ increases the voltage across the primary winding T 1 P, which in turn increases the voltage across secondary winding T 1   f . The negative voltage produced by the first power supply across capacitor C 120  tends to be generated during the time that the regulated voltages are generated, and therefore tends to be constant.  
         [0033]     The emitter of Q 103  of  FIG. 2  cannot rise to a voltage above isolated ground greater than +0.7 volts, because of the presence of diode CR 105 . Whenever the optically-controlled transistor  214  connected between terminals U 101   4  and U 101   3  of U 101  conducts, the emitter of Q 103  is taken to a negative voltage relative to isolated ground. CR 103  provides transient protection. Capacitor C 111  is a filter for the control voltage, and is associated with the loop time constant.  
         [0034]     Transistors Q 102  and Q 103  are “SCR-connected” in a regenerative fashion in  FIG. 2 , so that if Q 103  is turned ON, Q 102  is also turned ON, and tends to remain ON. The SCR-connected pair is coupled between the gate of power switch Q 101  and, when transistor  214  of optoisolator U 101  is conductive, the negative voltage at the nongrounded terminal of capacitor C 111 . Thus, when the SCR-connected pair Q 102 , Q 103  is conductive, power switch Q 101  becomes less conductive, which in turn tends to produce a more negative voltage at its gate, so Q 101  turns OFF in a regenerative fashion. The SCR-connected pair Q 102 ,  103  is controlled by the “sawtooth” voltage appearing across current sensing resistor R 109 , in the source circuit of power switch Q 101 . More particularly, as the current increases in primary winding T 1 P as a result of conduction of Q 101 , the increasingly positive voltage on R 109  is coupled by way of a filter, including a capacitor C 107  and a resistor R 108 , to the base of Q 103 . When the base voltage is high enough, Q 103  will turn ON, thereby turning ON Q 102 , and the conduction of the pair discharges the gate of Q 101 , and turns Q 101  OFF. With Q 101  OFF, energy stored in the inductance associated with transformer T 1  is coupled as voltage to the various secondary windings T 1   f , T 1 S 1 , and T 1 S 2 , and is available for use.  
         [0035]     The voltage on feedback secondary winding T 1   f  reverses when Q 101  turns OFF, becoming negative on terminal  32   1 . The negative voltage is coupled to the gate of Q 101  by way of resistor R 110 , path  210 , capacitor C 104 , and resistors R 106  and R 107 , to tend to hold Q 101  in the OFF state, and also turns OFF SCR-connected pair Q 102 ,  103 . Looking at it another way, the current in the SCR-connected pair must be taken low enough to reduce the sum of the alphas of the transistors below unity. Resistors R 103  and  104  are start-up resistors. Once started, the circuit is regenerative. When the energy stored in the primary winding is exhausted into the secondary power supplies, the voltage on the primary winding decreases, which tends to make  32   1  more positive. This positive-going voltage is communicated to the gate of Q 101  to again turn ON Q 101 .  
         [0036]     The magnitude of the positive voltage on C 108  tends to become more positive as the Raw B+increases, and this more positive value is communicated by way of a resistor R 111  to the base of Q 103 , thereby tending to turn ON the SCR-connected pair earlier in the cycle, to compensate for the effects of a larger Raw B+. Resistor R 112  decreases response time to a high load.  
         [0037]     Feedback control of the Scan B+ of  FIG. 1  is accomplished by coupling the error signal from error amplifier U 103  to error input port  32   5  of  FIG. 2 . An increasing value of Scan B+ causes an increasing error current from error amplifier u 103 . An increasing error current from U 103  through the photodiode  212  of optoisolator U 101  causes more photons to be emitted, which is equivalent to increasing base current in transistor  214 . The increasing effective base current, in turn, causes transistor  214  to conduct more heavily, thereby tending to render the emitter of Q 103  of the SCR-connected pair Q 102 ,  103  more negative. With the emitter of Q 103  more negative, it and the SCR-connected pair, will become conductive at a lower value of sawtooth voltage from current sensing resistor R 109 . The turn-ON of the SCR-connected pair is related to the turn-OFF of Q 101 . Thus, a tendency for an increase in the Scan B+ results in a tendency to turn power switch Q 101  OFF at a lower value of current, which results in storage of less energy in the inductance associated with transformer T 1  for that operating cycle. The storage of less energy for the cycle tends to reduce the Scan B+, and the degenerative feedback control is accomplished. In  FIG. 2 , R 113  is a slow-start resistor which slows down the initial turn-on, and provides some fold-back. Resistor R 115  provides a current limit for the transistor  214  in U 101 .  
         [0038]     Other embodiments of the invention will be apparent to those skilled in the art. For example, While serial windings T 1 S 1   a  and T 1 S 1   b  have been described for producing the scan B+, they could alternatively be in separate, mutually parallel windings, with the voltage of winding T 1 S 1   b  being greater than that of winding T 1 S 1   a.    
         [0039]     In the embodiment of  FIG. 1 , the elements have the following characteristics.  
                                                       Scan B+   124/134 volts           F101   220□F, 22□H, 100□F           F102   680□F, 27□H, 10□F           R117   120K ohms           R118   15K ohms           R119   3K ohms           R120   200K ohms           R127   6K2 ohms           R128   1M ohms           R129   10K ohms           R130   62K ohms           R139   10 ohms           T1P   36 turns           T1S1a   23 turns           T1S1b   2 turns           T1S2   3 turns           T1f   2 turns           Q104   Motorola BC847B           Q105   Motorola MPSa06           Q108   Motorola BC847B                      
 
         [0040]     In the embodiment of  FIG. 2 , the elements have the following characteristics.  
                                                       C103   1.1 nF           C104   100 nF           C105   470 pF           C107   1 nF           C108   47 nF           C109   330 pF           C110   330 pF           C111   100 nF           C112   220 pF           C120   2.2 □F           C138   180 pF           C140   470 pF           Q101   Infinion SPP1N60C2           Q102   Motorlay MPS 8599           Q103   Motorola MPSA06           CR103   16 volts           R101   51K ohms           R103   100K ohms           R104   120K ohms           R105   330 ohms           R106   430 ohms           R107   75 ohms           R108   510 ohms           R109   0.22 ohms           R110   68 ohms           R111   22K ohms           R112   10K ohms           R114   1.1K ohms           R115   680 ohms           R137   1K ohms