Power supply protection circuit

Abstract: A horizontal deflection circuit (100) includes a flyback transformer (T1). A secondary winding (W2) of the flyback transformer (T1) is coupled in series with a diode switch (70) to produce a trace rectified supply voltage (V) that energizes a vertical deflection amplifier (40b). The diode switch (70) also provides switching operation in an East-west diode modulator (30). During, for example, service mode operation, when the vertical raster is collapsed, the current in the secondary winding (W2) during trace is substantially reduced. The reduced current during trace is sensed (by R15) for producing a control signal that causes the peak current supplied to the diode switch (70) by the diode modulator (30) to be smaller than the current in the secondary winding (W2). This ensures that the diode switch (70) remains conductive throughout horizontal trace, thereby preventing an undesirable increase in the supply voltage (V) from occuring.

Description: The invention relates to an over-voltage protection circuit, e.g. for a television apparatus power supply. The horizontal deflection circuit in some conventional television receivers encompasses an output stage that includes a horizontal deflection winding and a trace capacitance that supplies deflection current to the deflection winding during each trace interval. A retrace capacitance is coupled across the deflection winding during the retrace interval. Energy is replenished during retrace via a flyback transformer. A deflection transistor switch, that is conductive during trace, causes a DC voltage B+ to be developed across a primary winding of the flyback transformer during trace. Aspects of the invention are specified in the claims. In a circuit embodying the invention, a secondary winding of the flyback transformer is series coupled through a rectifier arrangement to a common conductor, referred to as ground. The rectifier arrangement includes a diode switch . The anode of the diode is at, for example, ground potential. The cathode of the diode switch is coupled to one end terminal of the secondary winding. A filter capacitor is coupled between ground and a second end terminal of the secondary winding that is remote from the cathode of the diode switch. A load circuit is coupled across the filter capacitor. During trace, a trace voltage developed in the secondary winding produces a trace current in the secondary winding that is coupled to the filter capacitor and to the load circuit. The secondary winding current develops a rectified DC output supply voltage in the filter capacitor. The secondary winding current flows in a current path that includes the diode switch. The secondary winding current produces a forward current in the diode switch that causes the diode switch to be conductive during trace. When the diode switch is conductive, the secondary winding forms a voltage source having a relatively low output impedance. As long as the diode switch is conductive, the forward voltage developed at the cathode of the diode switch is negligible. Therefore, the output supply voltage is determined mainly by the primary-to-secondary turn ratio of the flyback transformer and by voltage B+, and is not affected by the voltage at the cathode of diode switch. During retrace, a retrace pulse voltage is developed in the secondary winding of the transformer in the opposite polarity that causes the diode switch to be nonconductive. A retrace voltage is developed at the cathode of the diode switch. Thus, in normal operation, the diode switch is conductive throughout trace and nonconductive during retrace, so as to provide the DC output supply voltage that is rectified by the diode switch. In a well known side pincushion distortion correction circuit of the diode modulator type, the same diode switch that was discussed before also provides switching operation in the diode modulator. To that end, a circuit branch of the diode modulator has an end terminal that is coupled between the secondary winding and the cathode of the diode switch, such that the cathode forms a three way junction node where three branches intersect: the circuit branch of the diode modulator, a circuit branch that includes the diode switch and a circuit branch that includes the secondary winding. A current flowing through the above mentioned circuit branch of the diode modulator, during, for example, the first half of trace, may supply a portion of the current that flows in the secondary winding and, hence reduce the forward current in the diode switch. Thus, the secondary winding current may be equal to the sum of the absolute value of the forward current and the current in the circuit branch of the diode modulator. The current in the secondary winding is determined by the load current. Therefore, a decrease in the load current causes the forward current in the diode switch to decrease. The load circuit that is coupled across the filter capacitor associated with the diode switch may include a vertical amplifier integrated circuit of the television receiver that is energized by the output supply voltage. During degaussing, and during operation of the service switch, when the vertical raster is collapsed, or in the event of a fault condition, for example, the vertical amplifier output current, hence the load current, may be significantly smaller than during normal operation. Should the load current decrease, the secondary winding current would also decrease. Should the secondary winding current decrease to a level that is, for example, equal to that of the current in the circuit branch of the diode modulator, the current in the circuit branch of the diode modulator will cause the forward current of the diode switch to become zero. A further decrease in the secondary winding current will cause the cathode voltage of the diode switch that is then nonconductive to increase substantially above ground potential. The trace voltage that is developed across the secondary winding is coupled in series with the voltage at the cathode of the diode switch. An increase in the cathode voltage during trace, disadvantageously, may cause the output supply voltage to exceed its normal operation level. An increase in the output supply voltage may damage other integrated circuits such as, for example, audio processing integrated circuits that are also energized by the output supply voltage. A television apparatus power supply embodying the invention includes a controllable switch coupled to a horizontal deflection winding for producing a horizontal deflection current in the horizontal deflection winding, and an output supply voltage. A raster distortion correction circuit is coupled to the horizontal deflection winding for producing a modulation of the horizontal deflection current. The raster distortion correction circuit produces a second current. A vertical deflection circuit is coupled to the output supply voltage that produces a load current in the vertical deflection circuit for energizing the vertical deflection circuit. The load current has a magnitude that is within a predetermined range of values during normal operation and outside the range of values when the vertical deflection circuit is disabled or when a fault condition occurs. A first control signal is generated that is indicative when the magnitude of the load current is outside the range of values. The control signal is coupled to the raster distortion correction circuit for disabling the operation of the raster distortion correction circuit when the magnitude of the load current is outside the range of values. In accordance with an embodiment of the invention, the control signal prevents the second current from causing an increase in the output supply voltage. In the Drawing: The sole FIGURE illustrates an East-west raster distortion correction circuit of a horizontal deflection circuit that includes a protection circuit, embodying an aspect of the invention. The sole FIGURE illustrates a horizontal deflection circuit output stage 100 that operates also as a power supply for generating a supply voltage V. Voltage V of, for example, +26 volts, is developed across a filter capacitor C5. Voltage V energizes a load circuit 40a and a vertical deflection amplifier 40b. Load circuit 40a may be, for example, an audio processing stage of a television receiver, not shown. Output stage 100 includes a deflection switching transistor Q1 that is antiparallel coupled with a damper diode D1. A retrace capacitor C2 is coupled across diode D1 and between the collector and emitter electrodes of transistor Q1. A primary winding W1 of a flyback transformer T1 is coupled between an input supply voltage B+ and the collector electrode of transistor Q1. A horizontal deflection winding L y , a linearity inductor L LIN , a primary winding T2a of a transformer T2 of an East-west pincushion distortion correction circuit 30 and a trace capacitor C1 form a series arrangement that is coupled between the collector of transistor Q1 and hot ground. Transistor Q1 is switched at a horizontal rate in accordance with a drive signal H r that is developed at its base electrode. Signal H r is produced in a horizontal oscillator and driver stage 150 in a well known manner. The switching operation of transistor Q1 produces a horizontal deflection current i y in a well known manner. A secondary winding W2 of flyback transformer T1 has a first end terminal W2b that is coupled to filter capacitor C5 where voltage V is developed. A second end terminal W2a of winding W2 is coupled via a rectifier arrangement 70 to chassis ground. Rectifier arrangement 70 includes diodes CR01 and CR02 that are coupled in series with winding W2. The anode of diode CR02, operating as a diode switch, is at chassis ground potential. The cathode of diode CR02 is coupled to the anode of diode CR01 via a terminal 32. Diode CR01 is coupled between terminal 32 and terminal W2a of winding W2. During trace, when transistor Q1 (or diode D1) is conductive, a trace voltage that is approximately equal to voltage B+ is developed in winding W1 of flyback transformer T1. During trace, a voltage V W2 , forming a trace portion of an AC voltage, is developed by transformer action across winding W2 in the polarity shown. As a result of rectifier arrangement 70, voltage V W2 produces a half wave rectified current i W2 in winding W2 that is coupled to filter capacitor C5 and to circuits 40a and 40b. In normal operation, during trace, current i W2 flows in a current path that includes forward biased diodes CR01 and CR02. Consequently, voltage V, that is developed at terminal W2b, is equal to the value of voltage B+ multiplied by the turns ratio between windings W2 and W1. During retrace, when transistor Q1 is nonconductive, a relatively high retrace voltage of the opposite polarity is developed across winding W1. A retrace pulse voltage is developed in winding W2 of the opposite polarity to the polarity during trace, that causes diodes CR01 and CR02 to operate as a nonconductive switch. Thus, in normal operation, diodes CR01 and CR02 are conductive throughout trace and nonconductive during retrace. A secondary winding T2b of transformer T2, of pincushion distortion correction circuit 30, has a first end terminal 31 that is coupled to a modulation voltage developing capacitor C3. Terminal 32, that is coupled between diodes CR02 and CR01, forms a second end terminal of winding T2b. During horizontal trace, a trace voltage developed across trace capacitor C1 produces, by transformer action of transformer T2, a trace voltage V T2b across winding T2b in the polarity shown. Voltage V T2b is applied in series with capacitor C3 for producing a voltage V m developed in capacitor C3 in the polarity shown by the arrow, and for producing an oscillatory current i T2b . During horizontal retrace, diodes CR01 and CR02 become nonconductive, as explained before. Consequently, a retrace voltage that is developed in winding T2b causes current i T2b in winding T2b to reverse its direction and to flow in the direction shown by the arrow, at the end of retrace and during the first half of trace. Also, a retrace voltage is developed at terminal 32 that is coupled between diodes CR01 and CR02. During the second half of trace, trace current i T2b flows in the opposite direction to that of the arrow. A control circuit 30a of pincushion distortion correction circuit 30 is coupled to end terminal 31 of capacitor C3. Control circuit 30a generates, in a manner that is later described, a modulation sink current i m having a controllable average value that controls voltage V m in capacitor C3 in accordance with the average DC component of current i m . When the average value of sink current i m is, for example, zero, voltage V m is at a maximum. On the other hand, when the average value of current i m is at a maximum, voltage V m is at a minimum. Modulation voltage V m , as controlled by control circuit 30a, establishes by the transformer action of transformer T2, the level of voltage V T2a across primary winding T2a of transformer T2. Thus, voltage V T2a is modulated in accordance with voltage V m or current i m . Voltage V T2a is applied in a direction that causes the trace voltage across deflection winding L y to be smaller than voltage V C1 , developed across trace capacitor C1. When the average value of current i m is at a minimum, voltage V m is at a maximum, and voltage V T2a is at a maximum. Voltage V T2a at the maximum causes the amplitude of deflection current i y to be at a minimum. On the other hand, when the average value of current i m is at a maximum, voltage V m is at a minimum and the amplitude of deflection current i y is at a maximum. Control circuit 30a, that produces sink current i m , includes transistors Q2, Q3, Q4, Q5 and Q6 that form a switched amplifier. Transistors Q2 and Q3 form an input, differential stage of the switched amplifier. The base of transistor Q3 forms a noninverting input terminal of the switched amplifier. A reference level V ref developed at the base of transistor Q3 is produced in a zener diode Z. The base of transistor Q2 forms an inverting input terminal of the switched amplifier. The collector of transistor Q3 is coupled to a load resistor R10 and to the base of transistor Q4. The emitter of transistor Q4 is coupled to the base of switching transistor Q6 in a Darlington configuration. The emitter of transistor Q6 is coupled to chassis ground. The collector of transistor Q6 is coupled via an inductor L m to capacitor C3. The collector of transistor Q6 forms a switching output terminal of the switched amplifier for producing sink current i m in inductor L m that provides East-West pincushion distortion correction. A horizontal retrace pulse signal is coupled from a winding W3 of flyback transformer T1 via a series arrangement of a capacitor C7 and a resistor R13 to the base of transistor Q2. During horizontal retrace, the retrace pulse signal in winding W3 produces an upramping portion UR of a sawtooth signal V INV developed at the base of transistor Q2. During horizontal trace, a downramping portion DR of signal V INV changes at a smaller rate than umpramping portion UR. During an interval having a controllable length that includes a portion of upramping portion UR and an immediately following portion of down ramping portion DR, signal V INV is more positive than voltage V ref developed at the base of transistor Q3. Assuming a zero offset voltage, as long as the base voltage of transistor Q2 is more positive than that of transistor Q3, transistor Q6 operates as a switch that is turned on. Conductive transistor Q6 couples inductor L m across capacitor C3 to produce an upramping portion of sink current i m in inductor L m . After transistor Q6 is turned off during trace, sink current i m continues flowing, as a result of the magnetic energy stored in inductor L m , in a current path that includes a diode D2. Diode D2 is coupled between the collector of transistor Q6 and voltage V. The length of the interval when transistor Q6 is conductive is modulated by, for example, a parabola voltage V p that varies in a vertical rate parabolic manner. Voltage V p is coupled to the base of transistor Q2 via an R-C network 50. Network 50 includes a potentiometer 50a that is adjustable for controlling the amplitude of the vertical parabola voltage developed at the base of transistor Q2. Network 50 includes also an integrating capacitor 50b that determines the slope of the corresponding portions of sawtooth signal V INV . Voltage V p causes the average DC value of signal V INV at the base of transistor Q2 to vary in a vertical rate parabolic manner. The result is that the length of the interval when transistor Q6 is conductive, hence each of voltage V m and the average value of current i m , varies in a vertical rate parabolic manner. A resistor R141 is coupled between the collector of transistor Q6 and the base of transistor Q2. Resistor R141 produces a negative feedback current having, during a given horizontal period H, a DC or average value that is proportional to voltage V m . The current in resistor R141 is coupled to integrating capacitor 50b. As a result of the negative feedback, voltage V m , that is controlled in a closed loop, varies in a parabolic manner. Voltage V p is produced in a vertical deflection circuit 60 that includes vertical amplifier 40b of the type LA7831 produced by Sanyo, for example. A series arrangement that includes a vertical deflection winding L v , a sampling resistor R v and a DC blocking capacitor C8 is coupled between an output terminal of amplifier 40a and chassis ground. Parabola voltage V p is developed across capacitor C8 in a well known manner. Vertical deflection circuit 60 includes a vertical control circuit 61 that produces an input signal 61a having a sawtooth waveform that is coupled to an input terminal 40b2 of amplifier 40b. Vertical control circuit 61 is responsive to a control signal SERVICE/DEGAUSSING that is used for disabling the generation of a vertical deflection current i v in winding L V during, for example, operation of a service switch and degaussing. An example of a vertical deflection circuit that includes an amplifier of the type LA7831 and a service mode operation feature is described in U.S. Patent 4,694,226 in the name of James A. Wilber, entitled VERTICAL DEFLECTION CIRCUIT WITH SERVICE MODE OPERATION. As a result of parabola voltage V p , the instant within horizontal retrace when transistor Q6 becomes conductive occurs earlier at the center of vertical trace than at the top and bottom. Hence, each of the average value of current i m that sinks charge from capacitor C3 in each horizontal period H, and the level of voltage V m , is at a maximum at the center of vertical trace and at a minimum at the top and bottom of vertical trace. Consequently, the amplitude of deflection current i y is at a maximum at the center of vertical trace and at a minimum at the top and bottom, in a manner that provides East-West pincushion distortion correction. A potentiometer R14, that is coupled to the base of transistor Q2, provides width adjustment. When, during horizontal trace, transistor Q2 becomes nonconductive, the collector voltage of transistor Q2 increases. Consequently, transistor Q5, having a base electrode that is coupled to the collector of transistor Q2, becomes conductive. When transistor Q5 becomes conductive, it forms a low impedance between the base and emitter of transistor Q6 that, advantageously, speeds up the turn-off of transistor Q6. Transformers T1 and T2 electrically isolate, with respect to electrical shock hazard, voltage V and control circuit 30a from voltage B+ and from AC mains supply voltage V AC . Voltage B+ is electrically nonisolated with respect to electrical hazard, from AC mains supply voltage V AC that is used for producing voltage B+. Thus transformers T1 and T2 provide an isolation barrier. Supply voltage V is coupled via a resistor R15 to a supply terminal 40b1 of vertical amplifier 40b. During, for example, degaussing, it may be desirable to disable the vertical deflection current in order to prevent introducing purity error in a cathode ray tube of the receiver, not shown. Thus, during degaussing and also during operation of a service switch, signal SERVICE/DEGAUSSING is generated in a well known manner, not shown, for disabling the generation of the vertical deflection current i v . When signal SERVICE/DEGAUSSING is generated, or during the occurrence of a fault condition, a supply current i 40b flowing in vertical deflection amplifier 40b and through resistor R15 may be at a substantially smaller magnitude than during normal operation. The result is that trace current i W2 in winding W2 will also decrease. Assume that throughout the first half of normal horizontal trace operation, current i W2 is larger than current i T2b . Therefore, diode CR02 that conducts the difference between currents i W2 and i T2b remains conductive throughout horizontal trace. It follows that, during normal operation, diode CR02 prevents trace current i T2b from affecting voltage V. Should current i W2 decrease significantly relative to its normal operation level, as a result of the aforementioned decrease in load current i 40b , positive trace current i T2b in winding T2b of transformer T2 might become equal to current i W2 , during a significant portion of the first half of the horizontal trace. Therefore, diode CR02 might become nonconductive during such significant portion of the first half of horizontal trace. The result is that current i T2b might produce a wide pulse of voltage V₃₂ at the cathode of reverse-biased diode CR02 having a relatively large positive peak level. The sum of voltages V W2 and V₃₂ forms voltage V. Therefore, should the peak level of the pulse of voltage V₃₂ become large and its width significant, during the aforementioned vertical raster collapse, voltage V₃₂ would cause voltage V to be larger than during normal operation. Voltage V might harm some integrated circuits depicted as load 40a that are energized by supply voltage V. In an illustrative implementation of the invention, a transistor Q7 has an emitter electrode and a base electrode that are coupled to end terminals of resistor R15, respectively, for sensing load current i 40b in vertical amplifier 40b. The collector of transistor Q7 is coupled via a parallel arrangement of a filter capacitor C9 and a bleeder resistor R16 to ground. A switch diode D3 is coupled between the base of transistor Q2 and a junction terminal that is coupled between the collector of transistor Q7 and the parallel arrangement of filter capacitor C9 and resistor R16. During normal operation, supply or load current i 40b in vertical amplifier 40b that flows also in resistor R15 produces a voltage drop across resistor R15 that is sufficiently large to cause transistor Q7 to be conductive during at least a portion of each vertical deflection cycle. Consequently, a DC positive filtered voltage that is developed across capacitor C9 is maintained sufficiently high to maintain diode D3 back-biased throughout each vertical deflection cycle. Should the maximum value of load current i 40b in a given vertical deflection cycle become smaller than a predetermined level, as a result of, for example, vertical deflection circuit 60 being disabled, transistor Q7 would remain turned-off. Therefore diode D3 would become conductive throughout each vertical deflection cycle. Consequently, the base voltage of transistor Q2 would become smaller than that of transistor Q3. The result is that transistor Q6 would be maintained nonconductive indefinitely, causing the average value current i m to be zero. When the average value of current i m is at a maximum, voltage V m is at a minimum and the positive peak amplitude of current i T2b in winding T2b, at the beginning of trace, is at a maximum. Conversely, when the average value of current i m is at a minimum, the peak positive amplitude of current i T2b is at a minimum. When, for example, the vertical raster is collapsed, current i m is zero because of the operation of diode D3 that was explained above. The result is that current i T2b is at a minimum positive peak amplitude that is smaller than current i W2 flowing in winding W2. Since the difference between current i W2 and current i T2b flows as the forward current of diode CR02, diode CR02 remains conductive throughout the first half of horizontal trace. It follows that voltage V₃₂ is maintained at the forward voltage level of diode CR02 throughout horizontal trace. When diode CR02 is conductive, diode CR02 advantageously prevents current i T2b from affecting current i W2 . Thus, diode CR02 decouples a current path that includes winding T2b from a current path that includes winding W2. In this way, advantageously, voltage V is prevented from exceeding its normal operation level when, for example, the vertical raster is collapsed. When vertical amplifier 40b is disabled during, for example, operation of a service switch, voltage V will continue to be generated for enabling horizontal scanning operation. Such normal horizontal scanning operation may be required during operation of the service switch. Thus, voltage V is protected by the operation of transistor Q7 in a way that, advantageously, does not necessitate a complete shutdown of horizontal deflection circuit output stage 100.

Claims: A power supply apparatus for a television display, comprising: a vertical deflection circuit (40b); means (R15) for generating a control signal (61a) indicative of disablement of normal operation of said vertical deflection circuit; a horizontal deflection circuit (100) for producing a horizontal deflection current (i Y ) in a horizontal deflection winding (L Y ); a side pincushion distortion correction circuit (30) coupled to said horizontal deflection circuit (100) for producing a modulation of said horizontal deflection current (i Y ); characterized by means (Q7,D3,Q2) responsive to said control signal (61a) and coupled to said pincushion distortion correction circuit (30) for disabling said pincushion distortion correction circuit (30) when normal operation of said vertical deflection circuit (40b) is disabled. A power supply apparatus according to Claim 1 wherein said horizontal deflection circuit (100) comprises a horizontal flyback transformer (T1) having first (W1) and second (W2) windings and a horizontal deflection switch (Q1) operating at a horizontal deflection frequency and coupled to said first winding (W1) of said transformer (T1) for generating an AC supply voltage (V W2 ) in said second winding (W2) at a horizontal deflection frequency, characterized by a diode switch (70) coupled to said second winding (W2) for rectifying said AC supply voltage (W2) to produce an output supply current (i W2 ) that is coupled to said vertical deflection circuit (40b) for energizing said vertical deflection circuit, said supply current (i W2 ) being coupled via said second winding (W2) to said diode switch (70) to produce a forward current in said diode switch that causes said diode switch to be conductive during a horizontal trace interval and nonconductive during a horizontal retrace interval such that when said vertical deflection circuit (40b) is disabled, a magnitude of said supply current (i W2 ) becomes substantially smaller than during normal operation, wherein said pincushion distortion correction circuit (30) comprises a second transformer (T2) having a first winding (T2a) that is coupled to said deflection winding (L Y ) and having a second winding (T2b) that is coupled to said diode switch (70) such that the switching operation of said diode switch also produces an oscillatory second current (i T2b ) in said second winding (T2b) of said second transformer (T2), said second current (i T2b ) being combined in said diode switch (70) with said supply current (i W2 ) that flows in said second winding (W2) of said flyback transformer (T1) in a manner that reduces said forward current in said diode switch during said trace interval and wherein said pincushion distortion correction circuit disabling means (Q7,D3,Q2) decreases the magnitude of said second current (i T2b ) in said second winding (T2b) of said second transformer (T2) when the magnitude of said output supply current (i W2 ) is substantially smaller than during normal operation to prevent said second current (i T2b ) from back-biasing said diode switch (70) at least substantially throughout said horizontal trace interval.

Inventors: STEPHENS JOSEPH CURTIS, WILBER JAMES ALBERT, STEPHENS, JOSEPH CURTIS, WILBER, JAMES ALBERT
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS, INC.
Application year: 1989
Main classification: H04N3
IPC codes: H04N3