CA1309170C - Regulating power supply for video display apparatus - Google Patents

Abstract

RCA 81,568A

ABSTRACT OF THE DISCLOSURE
A regulating power supply for a video display apparatus includes a switching transistor that periodically energizes a transformer winding. Input switching pulses for the transistor are varied in width and frequency to optimize power transfer via the transformer. The charge and discharge rates of a capacitor determine the pulse width and frequency. The capacitor charging current is selectively switched to be connected to the capacitor while the capacitor discharge current is controlled in response to a feedback signal derived from the regulated voltage level.

Description

RCA 81,568A

REGULATING POWER SUPPLY FOR VIDEO DISPLAY APPARATUS
This application is a division of Canadian Application Serial No. 508,069, filed May 1, 1986.
This invention relates to switching-type regulating power supplies for video display apparatus and, in particular, to switching-type supplies that operate in response to variable width and frequency pulses.
Video display apparatus, such as television receivers or computer monitors, incorporate power supplies that provide sne or more regulated voltage levels that are used to pbwer various load circuits of the video display apparatus. one such power supply utilizes a switching device, such as a transistor, that periodically energizes a primary winding of a transformer from an unregulated voltage source in response to trigger pulses. The trigger pulse width is controlled so that the transformer secondary winding develops a voltage that remains constant independent of load changes and variations in the unregulated voltage level.
Wide variations in load circuit power requirements, such as may occur with circuitry incorporated in video display apparatu~, may cause the pulse width variations of the previously described pulse width modulation power supply to be quite large. Large pulse width variations may consequently result in large flux density variations in the transformer windings, thereby increasing the complexity of the circuit and transformer design. A power supply having a pulse frequency modulation regulator arrangement with a fixed pulse width simplifies transformer design and construction but may introduce other difficulties. For example, at very low load conditions, the pulse frequency may decrease into the audible range.
At high load conditions, insufficient power may be transferred to maintain voltage level regulation.
In accordance with an aspect of the present invention, a power supply for a video display apparatus I 309 1 70 -2- RCA 81,568A

comprises a source of unregulated voltage, a transformer winding and a switch for coupling the source of voltage to the winding in response to inpu~ pulses. A pulse generator comprises a source of current that produces a predetermined current level. ~ first current path that includes a capacitor is coupled to a second current path that includes a control circuit. Circuitry responsive to the voltage level across the capacitor couples the current source to -~he first and second current paths when the capacitor is discharged below a first voltage level and decouples the current source from the first and second current paths when the capacitor is charged above a second voltage level. Circuitry is coupled to the second cuxrent path control circuit and is responsive to a reference voltage for controlling the current flow through the second current path. The second current path in a first mode diverts current from the current source away from the capacitor to control the charging time of the capacitor a~d in a second mode discharges the capacitor. The control circuit controls the discharge time of the capacitor.
In the accompanying drawing:
FIGURE 1 is a schematic and block diagram of a portion of a video display apparatus incorporating a power supply in accordance with an aspect of the present invention;
FIGURE 2 illustrates waveforms useful in understanding the circuit of FIGURE 1; and FIGURE 3 is a schematic diagram of an embodiment of a pulse shaping circuit shown in FIGURE 1.
Referring the FIGURE 1, there is shown a portion of a video display apparatus, such as a television receiver or a computer monitor, in which an AC voltage from an AC line supply 10 is applied to a rectifying circuit 11 and a filter capacitor 12 to provide an unregulated DC voltage. The unregulated DC voltage is applied to one terminal of a primary winding 13 of a 1 30~ 1 70 3 RCA 81,568A
transformer 14. Transformer 14 may illustratively provide electrical isolation between the AC supply 10 and customer access te~minals (not shown) such as audio and video input and output terminals. This isolation is provided by electrically isolating the video and audio load circuits, for example, from the supply 10 via transfoxmer 14. The other termlnal of primary winding 13 is coupled to ~he drain electrode of a ~OSFET 15 which, in accordance with an aspect of the present invention, recei~es gate input pulses of ~arying width and frequency generated in a manner that will be described later. The gate input pulses cause MOSFET 15 to switch conduction states in a manner that generates a regulated voltage across secondary winding 16 of transformer 14. Additional secondary windings may be provided in order to generate other regulated voltage levels that may be used to power various load circuits of the video display apparatus. The voltage generated across a sense winding 34 is rectified and filtered to provide a voltage level at a terminal 35 that is indicative of the re~ulated voltage level produced by winding 16. The voltag~ at terminal 35 provides eedback to the pulse generator circuitry that controls the switching of MOSFET 15.
The voltage developed across winding 16 is rectified and filtered by diode 17 and capacitor 20, respectively, and is applied to one terminal of a primary winding 21 of a high voltage transformer 22. The other terminal of primary winding 21 is coupled to a conventional horizontal deflection circuit 23 which comprises a horizontal driver circuit 24 that supplies switching pulses to a horizontal output transistor 25.
Horizontal deflection circuit 23 also comprises d~mper diode 26, retrace capacitor 27, horizontal deflection winding 30 and S-shaping capacitor 31. Horizontal deflection circuit 23 generates a deflection current in deflection winding 30 which is located about the neck of a cathode ray tube (not shown) in order to provide horizontal deflection of the cathode ray tube electron 1 30q 1 70 -4- RCA 81,568A

beams. Switching of horizontal out~ut transistor 25 generates horizontal retrace pulses across winding 21 which are stepped up and rectified by high voltage winding 32 to produce a high voltage level of the order of 30,000 volts at an ultor terminal 33.- The high voltage is applied to the ultor terminal (not shown) of the cathode ray tube. High voltage transformer 22 may also comprise additional windings, such as winding 48, which may develop a voltage that is rectified and filtered to provide power to one or more of the video display apparatus via a terminal 39.
The generation of input pulses for MOSFET 15 occur in the following manner. The pulses generated are caused to vary in both pulse width and frequency. The allowable variation in pulse width is less than the corresponding variation in a fixed frequency pulse width modulated power supply since the pulse freguency can also vary. In the illustrative circuit of FIGURE 1, during operation, the pulse width is constrained to vary only within a range of the order of 4 ~S to 8 ~S while the pulse fre~uency may vary in a range of the order of S K~Z
to 80 ~C~z. During normal operation the pulse frequency tends to vary from 40-50 KH2. The charge and discharge rates of a capacitor 36, shown by the solid waveform in FIGURE 2A, dete.rmine the pulse width and frequency. The charging time of capacitor 36 bet.weèn two threshold values THl and TH2 determines the pulse width while the discharge time of capaci~or 36 between the threshold values TE2 and TU1 determines the time between pulses, which is equal to pulse frequency. The threshold values TH1 and TH2 are set by flip-flop circuit 37, which may be of a R-S type designed ~o switch at threshold values which are at 1/3 and 2/3 of the supply voltage. The voltage developed across capacitor 36, present at terminal 40, is applied to the S and R inputs of flip-flop 37. The Q and output terminal of flip-flop 37, under control of the voltage levels at terminal ~0, swi~ches in response to capacitor 36 attaining respective threshold levels during charging 1 309 1 70 _5~ RCA 81,568A

or discharging. The Q output of flip-flop 37, which produces a signal such as that shown by the solid waveform in FIGURE 2B, is applied to a pulse shaping circuit 131.
The output of pulse shaping circuit 81, shown by the solid waveform in FIGURE 2C, is app~ed to the gate of MOSFET
15.
Capacitor 36 is charged from the +Vl supply via a charging path comprising resistor 41, transistor 42 and transistor 43 of differential amplifier 4~. The input to the gate or base terminal of transistor 43 is connected to a voltage xefeLence source VREF6 which is equal to one-half the logic swing of flip-flop 37, i.e., one-half the voltage swing of the Q output. The gate or base electrode of transistor 45 of differential amplifier 44 is coupled to the Q output of flip-flop 37. The discharge path of capacitor 36 comprises transistor 46 and resistor 47. The charging current through resistor 41 and transistor 42 represents a known current level determined in a manner that will he explained later. This current is conducted by either transistor 43 or transis~or 45, determined by the level state of the input terminal 4~ of transistor 45, which is the Q output of flip-flop 37. The deyre~ of conduction of transistor 46 will determine the charge and discharge rates of capacitor 36. ~lile transistor 43 is conducting, transistor ~6 controls the diversion of charging current from capacitor 36, which increases the charging time. The maximum current diverted by transistor 46 is established, in a manner to be described, to be one-half the charging current, such that capacitor 36 will continue to charge as long as transistor 43 is conducting. When transistor 43 is not conducting, conduction of tra~sistor 46 will cause capacitor 36 to discharge. Therefore, the degree of conduction (i.e., current flow) of transistor 46 will determine hoth the charging and discharging rates of capaci~or 36, and hence bo~h the width and frequency of pulses for MOSFET 15. The voltage level at the base of transistor 46, which controls the degree of conduction of transistor 46, is determined 1 309 1 70 -6- RCA 81,568A

by a differential amplifier 50, essentially comprising transistors 51, 52, 53 and 54 and a current source transistor 66. The input of transistor 54, one input of differential amplifier 50, is derived from a carefully controlled refPrence voltage VREFl. The other input of differential amplifier 50, transistor 51, receives a regulator feedback signal from terminal 35 that is derived from the voltage developed across sense winding 34, which, in turn, is derived from the regulated ~oltage produced by transformer 14. ~5 the feedback voltage decreases with respect to VR~Fl, indicating a drop in the regulated voltage level, transistor 52 becomes more conductive which, via diode-connected transistor 58, raises the base voltage of transistor 46, which causes transist~r 46 to also become more conductive. This results in capacitor 36 charging at a lower rate, as shown by the dashed waveform in FIGURE 2A, thereby increasing the MOSFET input pulse width, shown by the dashed waveform in FIGURE 2C, when the charging current is switched on via ~ransistor ~3. When the charging current is switched off, and the current flows via transistor 45, increased conduction of txansistor 46 will cause capacitor 36 to discharge at a faster rate, thereby increasing the MOSFET input pulse frequency, as can also be seen in FIGURE 2C. MOSFET 15 will thereore conduct longer and more frequently, as shown by the dashed wave~orm in FIGURE 2D, causing the regulated voltage level to rise. Conversely, a rise in regulated voltage level will cause an increase in the voltage applied to transistor 51, causiny it to conduct less, which causes transistors 52 and 46 to conduct less, thereby decreasing the pulse width and frequency so that the regulated voltage decreases. The circuit operates in this manner by simultaneously varying the pulse width and re~uency to maintain the regulated voltage at a constant level.
As previously described, the charging current supplied ~ia transistor 42 and transistor 43 is a known value, with the maximum discharge current flow via - 1 30q 1 70 -7- RCA 81,568A

transistor 46 referenced to the charging current in the following manner. Differential amplifier 60, comprising transistors 61, 62 and 63, has one input, the base of transistor 62, coupled to a reference voltage VREF2.
Conduction of transistor 62 c~-uses its collector voltage to drop, thereby making transistor 64 more conductive, since transistor 64 has its base connected to the collectox of transistor 62. Increased conduction of transistor 64 increases the curxent flow through resistor 65, which increases the voltage drop across resistor 65, thereby raising the base voltage of transistor 61, making it more conductive. Increased conduction of transistor 61 decreases the conduction of transistor 62 so that conduction of transistor 64 and consequently transistor 61 is decreased. In this feedback manner the voltage at the base of transistor 61 is maintained substantially equal to the voltage at the base of transistor 62, which is e~ual to VRE~. Since the collector voltage of transistor 64 is known, selecting the desired value of resistor 65 deter~ines the current flow via transistor 64. The base of transistor 64 is also coupled to the bases of transistors 42 and 66, so that their current flow is also determined. Transistor 42 conducts the charging current via resistor 41 while transistor 66 controls the discharge current of transistor ~6 via a resistor 67. Therefore, the ratio of values of resistors 41 and 67 will determine the maximum ratio of charge current to discharge current.
Illustratively, the charge current is selected to be twice the discharge current at maximum conduction of transistor 52. Under those conditions, this results in equal currents charging and discharging capacitor 36, thereby providing MOSFET input pulses having a 50% duty cycle, which is desirable or optimum power transfer.
During startup operation of the power supply when the video display apparatus is initially energized, for example, it is desirable to limit the generated pulse width and freguency to allow the circuit supply voltages to increase to their noxmal levels without undue loading.

1 309 1 70 8 RCA 81,568A
A slow start circult is provided which allows the pulse width and frequency to increase slo~ly. When the video display apparatus is turned off, logic circuitry 70 causes transistor 71 to momentarily conduct, thereby discharging capacitor 72. When the video ~isplay apparatus is energized1 capacitor 72 will be discharged but will begin charging from the slowly increasing supply +V1 via resistor 73, which forms a voltage divider with resistor 74. With capacitor 72 discharged, the voltage level at the base of transistor 75 will cause it to turn on , thereby turning on transistor 76 via diode-connected transistor 77. Transistor 76 will divert or conduct the discharge control current from transistor 52, so that the generated MOSFET input pulses have narrow widths and low frequency. As the operating supply +Vl increases, the voltage developed across capacitor 72 will cause a decrease in conductlon of transistor 75, and hence a decrease in conduction of transistor 76. The discharge control current will then begin to control the base voltage of transistor 46, so that the generated pulses become wider and more frequent. When the operating supply +V~ substantially reaches its normal level, the voltage across capacitor 72 will cause transistor~ 75 and 76 to turn off, thereby ending t~e slow start i.nterval and permitting the pulse generator to operate in its normal manner.
Logic circuitry 70 may also be utilized to disable the power supply, and hence the video display apparatus, under certain fault conditions. For example, conductor 80 provides an overcurxent sensing signal to logic circuitry 70 from MOSFET 15. In the event of an overcurrent condition, the increased voltage drop across resistor 79 is coupled by conductor 80 to logic circuitry 70 to cause transistor 71 to conduct, which discharges capacitor 72. This causes conduGtion of transistor 76 which effectively disables thP power supply. Other fault conditions may also cause logic circuit 70 to operate in a 1 30q 1 70 9 RCA 81,568A

similar manner such that the video display apparatus is prevented from operating during fault conditions.
The output of flip~flop 37, shown in FIGURE 2B, i5 a square wave pulse corresponding to the desired switching si~nal for MOSFET 15. In order to reduce possible radio fre~uency interference (rfi) problems that may be caused by providing a sharp-edged pulse to MOSFET
15, pulse shaping circuit 81 is provided which processes the output signal of flip-flop 37 to generate a signal comprising pulses having controlled rise and fall times, as shown in FIGURE 2C. One embodiment of pulse shaping circuit 81 is shown in FI~URE 3. The output signal, from the Q output of flip-flop 37, is applied to terminal 82, which is the base of transistor 83. The positive-going pulses from flip~flop 37 turn transistor 83 on, which, via current flow through diode 84 and resistor ~5, raises the base voltage of transistor 86, turning it on. Conduction of transistor ~6 discharges capacitor 87 which is ordinarily charged from a constant current source comprising transistor 90 and resistor 91. Diodes 92 and 93 clamp the level to which capacitor 87 can charge. The discharge rate of capacitor 87 is determined by the values of resistors 85, 94 and 9S. Discharge of capacitor 87 causes the base voltage of transistor 96 to decrease, thereby decreasing its conduction and causlng the current through resistors 97 and 100 to decrease. As the voltage across resistor 100 decreases, conduction o~ transistor 101 decreases resulting in a linear rise in the voltage across resistors 102 and 106. The rise in voltage is lineax because a relatively small portion of the RC
charging vol~age characteristic of capacitor 87 is amplified by transistor 96 and 101. The emitter voltage of transistor 103 also rises linearly to a level equal to the level of the +Vl supply less the base emit~er drops of transistors 104 and 105. The emitter o~ transistor 103 is coupled to the base of MOSFET 15 and provides the actual switching pulses for MOSFET 15.

-- 1 30q 1 70 ~lo- RCA 81,568A

The negatlve going edges of the pulses from flip-flop 37 at terminal 82 turn transistors 83 and 86 off. Capacitor 87 then charges from the +Vl supply via resistor 91 and transistor 90. This causes conduction of S transistors 96 and 101 to inc~ease, resulting in a linear decrease in voltage across resistor 102. Conduction of transistor 103 increases, so that the emitter voltage, and he~ce the MOSFET input pulse, linearly falls. The selection of values for resistors 85 and 9S determines the pulse rise time, while the selection of resistor 91 determines the pulse fall time.
Logic circuitry 70 illustratively provides a signal to terminal 110 of pulse shaping circuit 81 during an overcurrent condition such that MOSFET 15 remains turned off, thereby providing further disabling of the power supply. As shown in FIGU~E 3, ~erminal 110 is connected to the collector of transistor 101.

.

Claims

RCA 81,568A

The embodiments of the invention in which a exclusive property or privilege is claimed are defined as follows:
1. A power supply for a video apparatus comprising:
a source of input voltage, an inductance coupled to a load circuit, a field effect transistor coupled to said source and to said inductance, a source of input pulses for switching said field effect transistor to transfer energy from said source to said load circuit via said inductance, - a pulse shaping circuit for coupling said source of input pulses to a control electrode of said field effect transistor, said circuit comprising a capacitance which is charged in response to transitions of said input pulses of a first polarity to generate first transition edges of shaped pulses having a shape determined in accordance with the charging of said capacitance, and discharged in response to transitions of said input pulses of an opposite polarity to generate second transition edges of said shaped pulses having a slope determined in accordance with the discharging of said capacitance, and means for coupling said shaped pulses to said control electrode of said field effect transistor in a manner that preserves the slopes of said transition edges.
2. A power supply as defined by Claim 1, in which said capacitance is charged from a source of constant current.
3. A power supply as defined by Claim 1, in which said means coupling said shaped pulses amplifies only a portion of the voltage charge characteristic of said capacitance so as to provide a linear change in said shaped pulse.