Horizontal deflection stage linearity control device

Abstract: A device (10) for controlling the linearity of a horizontal deflection stage (1) comprising a deflection yoke (6) supplied with a yoke current (ig) controlled by a switch (2, 4), and series connected to a capacitor (7) for varying the yoke voltage according to the phase of the yoke current (ig); the device (10) comprising a circuit (11, 12) for detecting and generating a real signal proportional to the yoke current (ig); a ramp generator (13-15) for generating an ideal ramp signal having rising portions defining a given S-shaped waveform; and an error amplifier (16) for receiving the real and ideal ramp signals and generating an error signal for driving a power transistor (17) having its emitter and collector terminals connected parallel to the capacitor (7), for modifying the voltage in the capacitor so that the yoke current assumes the required waveform.

Description: The present invention relates to a device for controlling the linearity of horizontal deflection stages of monitors and television screens according to the preamble of claim 1. The horizontal deflection stages of monitors and television screens provide for generating a substantially sawtooth current in a deflection yoke, which in turn produces the magnetic field required for sweeping the electron beam of the cathode-ray tube. For this purpose, unlike vertical deflection stages, currently used horizontal deflection stages employ switch circuits, due to the values of the physical quantities involved. Number 1 in Fig.1 indicates a typical known deflection stage comprising an NPN transistor 2 defining the switching element, and having the emitter grounded, the collector connected to one terminal of primary winding 3 of a line transformer (shown only partially), and the base defining the control terminal to which is supplied a rectangular control signal V 1 . The other terminal of winding 3 is connected to the positive supply, while the collector of transistor 2 is connected to the cathode of a recirculating or flyback diode 4 (the anode of which is grounded), to one terminal of a so-called "flyback" capacitor 5 (the other terminal of which is grounded), and to one terminal of a deflection yoke 6 represented in Fig.1 by a coil. The other terminal of yoke 6 is connected to one terminal of a control capacitor 7, the other terminal of which is grounded. Consequently, assuming the components are ideal, when transistor 2 is turned off by control signal V 1 switching to low (instant t 0 , see Fig.2a), capacitor 5 and coil 6 (as well as capacitor 7, the high value of which, however, makes it negligible as compared with capacitor 5) define a freely oscillating resonant circuit, so that the voltage in capacitor 5 (equal to collector-emitter voltage drop V cet ) assumes a sinusoidal waveform (Fig.2b), and the yoke current i g (which was maximum at instant t 0 ) decreases sinusoidally to minimum, so that the electron beam returns to the starting point. At instant t 1 , the capacitor voltage returns to zero, thus turning on diode 4, which maintains a zero voltage, thus preventing further oscillation of circuit 5, 6. In this phase, current i g in coil 6 is recirculated via diode 4, and presents a waveform (Fig.2c) depending on the voltage at the coil terminals and, therefore, on the voltage at the terminals of capacitor 7, which in this phase is parallel to coil 6. More specifically, if the capacitor voltage were constant, the waveform of the current would be linear, increasing as of instant t 1 so as to produce an ideal sawtooth waveform (at least over the useful, i.e. rising, deflection portion, as shown by the dotted line in Fig.2c). In actual practice, however, by virtue of the cathode-ray tube screen being roughly flat, as opposed to semispherical (with a constant radius), the rising portion of the deflection current waveform necessarily assumes the form of an S, as shown by the continuous line in Fig.2c. This is due to capacitor 7, which is charged during conduction of diode 4, and discharged in the following interval, thus resulting in a parabolic voltage at the yoke terminals and the S-shaped current waveform shown in Fig.2c. Theoretically, the above situation should continue up to instant t 3 , at which point, the current in the yoke is zeroed, thus turning off diode 4 and turning on transistor 2 for increasing the yoke current over the positive portion. In actual fact, however, due to the time lapse involved in turning off diode 4 and turning on transistor 2, the latter is turned on in advance so that a further control pulse is produced at instant t 2 . Consequently, the transistor is turned on and saturated, as shown, thus maintaining a substantially zero V cet voltage, so as to discharge capacitor 7 and increase the yoke current as shown by the S-shaped curve. The above operating mode is based on the assumption (false) that, when conducting, diode 4 and transistor 2 are capable of maintaining a zero V ce voltage (zero direct current voltage of diode 4 and saturation voltage of transistor 2), in which case, said voltage would present the theoretical V cet curve shown in Fig.2b and described so far. In actual fact, however, voltage V ce presents the curve shown in Fig.2d, which, in interval t 1 -t 2 , presents a negative portion due to the direct current voltage of diode 4; and, in the interval t 3 -t 0 , a rising portion of other than zero due to the saturation voltage of transistor 2. As such, the actual curve of yoke current i g differs from that shown in Fig.2c, thus resulting in serious, and highly visible, distortion of the picture, particularly in the first part of the screen. Existing solutions devised to overcome the above drawback provide for an additional coil series connected to deflection coil 6, and the inductance of which varies according to the direction of the current, so as to compensate, in particular, the direct current voltage drop in diode 4 when conducting. Though relatively cheap, the above solution provides for no more than a partial solution to the problem, in that it fails to compensate for distortion caused by the transistor, and even that of the diode is compensated by a fixed amount, thus resulting in only approximate correction. A further drawback of the above known solution is that the inductance of the additional coil is highly dependent on the characteristics of the transistor and the diode, so that the cost advantage is offset by requiring painstaking adjustment for achieving a high degree of performance. A device for controlling the linearity of a horizontal deflection stage of the type defined in the preamble of claim 1 is disclosed in US-A-4 423 358, wherein, to overcome the effects of losses incurred during a deflection cycle, a DC current is injected in the node formed by the deflection yoke and the control capacitor. Such correction is however not sufficient to solve the problems caused by non-ideal components. US-A-3 996 611 discloses a cathode ray tube deflection circuit with a closed-loop circuit for correcting the shape of the deflection current, including a linear amplifier arranged in series to the deflection yoke, so that the linear amplifier is flown by the entire deflection current. No control capacitor is provided; the output of the linear amplifier is maintained at virtual ground and has the function to set the deflection current; thus the linear amplifier has not the function to and cannot modify the voltage of any control element of the circuit. GB-A-2 072 447 describes a linearization circuit for cathode ray tubes of videocameras, having monitors of much smaller dimension than cathode tubes for computer monitors and television screens and not requiring the use of a S-shaped deflection current, but using a linear deflection current. Thus the structure of the deflection stage is different from the considered case. Furthermore, in this known circuit, a closed loop is provided, comprising and emitter follower generating an error voltage summed to a biasing voltage and fed directly to the yoke to set its operating voltage. It is an object of the present invention to provide a device enabling improved correction of the yoke current for achieving the required S-shaped waveform, and which entails no special adjustment due to the spread of the deflection stage components. According to the present invention, there is provided a device for controlling the linearity of horizontal deflection stages, as claimed in Claim 1. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows an electric diagram of a known deflection stage to which the device according to the present invention is applied; Fig.s 2a-2d show waveforms relative to the Fig.1 diagram; Fig.3 shows a block diagram of one embodiment of the device according to the present invention; Fig.4 shows a more detailed electric diagram of part of the Fig.3 device; Fig.s 5a-5e show waveforms relative to the Fig.4 diagram. Fig.3 shows the device 10 according to the present invention as applied to horizontal deflection stage 1 described with reference to Fig.1. According to the present invention, device 10 comprises means for detecting the real current in yoke 6; means for generating an ideal yoke current waveform; means for comparing the real current with the ideal waveform, and generating an error signal; and means for controlling the voltage at the terminals of capacitor 7 on the basis of the error signal, so that the real yoke current presents the set ideal waveform. In the embodiment shown, the means for detecting the real current i g in yoke 6 comprise a sensing resistor 11 of negligible value (about 0.1 Ω) series connected to yoke 6 and capacitor 7; and an operational amplifier 12 input-connected to both terminals of resistor 11, for measuring its voltage and generating an output signal proportional to the real yoke current. The ideal waveform generating means comprise a ramp generator 13 supplied (terminal 20) with the flyback pulses (v f voltage pulses at the terminals of primary winding 3 during flyback, and obtained via secondary winding 18 of line transformer 19); a parabola generator 14 receiving the sawtooth signal supplied by ramp generator 13; and an ideal ramp generator 15 receiving both the output signals from generators 13 and 14, for generating the required S-shaped waveform of the yoke current. The means for comparing the real current with the ideal waveform and generating the error signal consist, in this case, of an error amplifier 16 having one input A connected to the output of ideal ramp source 15, and another input B connected to the output of operational amplifier 12. The means for controlling the voltage at the terminals of capacitor 7 consist of a power transistor 17, in this case an NPN transistor, having the base terminal connected to the output of error amplifier 16; the emitter terminal grounded; and the collector terminal connected to the common point between capacitor 7 and sensing resistor 11. The Fig.3 device therefore detects the real current i g in yoke 6; compares it with the ideal waveform generated by generator 15; and, via transistor 17, accordingly adjusts the voltage at the terminals of yoke 6 so that current i g assumes the set ideal waveform. Obviously, the greater the loop gain of the loop system defined by device 10 and capacitor 7, the closer the resemblance will be between the real and set ideal waveforms. A simplified embodiment of generators 13-15 is shown in Fig.4 wherein terminal 20 is defined by the base terminal of an NPN transistor 21 having the emitter terminal grounded, and the collector terminal (defining point C) connected to one terminal of a capacitor 22, the other terminal of which is grounded. Point C is also connected to the input of an amplifier 23 defining a decoupling buffer, the output of which is connected to point A and to the input of a full-wave rectifying circuit 24. Circuit 24 is cascade connected to an x 2 parabola generating block 25 in turn cascade connected to a variable-gain amplifier 26 and a series RC filter 27 defined by a capacitor 28 and a resistor 20. The output of filter 27 is connected at point D to one terminal of resistor 33, the other terminal of which is grounded. The same point D is connected to the inverting input of an operational amplifier 34, the non-inverting input of which is connected to a temperature-stabilized direct-current voltage V REF , and the output of which is connected to the base terminal of an NPN transistor 35 having the emitter terminal connected to point D, and the collector terminal connected to a current mirror circuit 36 output-connected to point C. Operation of the Fig.4 circuit will be described with reference to Fig.s 5a-5e. Under normal operating conditions, the voltage at the inverting input of operational amplifier 34, and consequently in resistor 33, is maintained equal to the stabilized voltage at the non-inverting input, so that resistor 33 presents a constant current of I = V REF /R, where R is the resistance of resistor 33. Disregarding for the time being the current contributed by the feedback loop comprising components 24-29, current I is supplied by current mirror circuit 36 via transistor 35, and is equal (or at any rate in a fixed relationship) to that supplied by mirror circuit 36 to capacitor 21, which is thus charged linearly as shown in interval t 1 -t 0 in Fig.5b. When a flyback pulse v f is supplied to terminal 20, transistor 21 is turned on and saturated, so as to ground capacitor 22, which thus discharges, as shown in interval t 0 -t 1 in Fig.5b, to produce a sawtooth waveform. This is d.c.-decoupled by buffer 23 to produce signal S 1 in Fig.5b, which is rectified by rectifier 24 to produce output signal s 2 in Fig.5c, which presents a first portion decreasing between t 1 and t 5 , and a second portion increasing between t 5 and t 0 . Signal s 2 is processed by block 25 for generating a parabolic waveform which, amplified by amplifier 26 and filtered by filter 27 to remove the direct-current offset, produces current signal s 3 as shown in Fig.5d. Said current reduces (when positive) or increases (when negative) the current to be supplied by mirror circuit 36 via transistor 35, so that, at t 1 and t 0 , at which the current supplied by filter 27 is maximum, the current supplied by mirror circuit 36 is minimum; the capacitor charges slowly; and the voltage at its terminals presents a minimum slope. As the value of signal s 3 decreases, the current supplied to the capacitor increases, thus increasing the slope of the voltage at its terminals, which reaches maximum at t 5 when current s 3 is minimum. Subsequently, as current s 3 increases, the voltage increase in capacitor 22 slows down until it eventually returns to minimum at t 0 , so that point A presents signal s 4 , the waveform of which presents a minimum and maximum slope, as described, and thus the desired S-shape pattern. In other words, components 33 and 34 define a constant current I source; capacitor 22 defines a ramp generator; transistor 21 defines a switch for periodically discharging the capacitor; components 23-25 define a multiplying element for producing a first parabolic quadratic signal; filter 27 defines a high-pass filter for producing a second quadratic signal minus the direct-current offset; and point D defines an adding element. In the Fig.4 circuit, by varying the gain of variable amplifier 26 (which, for the purpose, features a control input (not shown) connected to an external pin on the device), it is possible to modify the amplitude of signal s 3 and, therefore, the difference between the ideal ramp and sawtooth waveform with linearly-increasing portions, for adapting the ideal waveform easily to the design of the screen. The advantages of the device according to the present invention will be clear from the foregoing description. In particular, the loop solution for tracking the ideal waveform (by compensating the yoke current according to the amount of error involved) provides for a considerable improvement in performance of the deflection stage, the yoke current of which assumes the exact desired S-shaped waveform for any cathode-ray tube, regardless of the characteristics of drive transistor 2 or diode 4. The solution described therefore requires no setting adjustments other than the gain of amplifier 26, which provides for adapting the device easily to any cathode-ray tube or screen, even semispherical (constant-radius) types. The ideal signal is already timed in relation to the yoke current curve, by virtue of employing the flyback signal for synchronizing the Fig.4 circuit. Even in the event of changes to the yoke current waveform, the general solution shown in Fig.3 is nevertheless adaptable by simply providing for appropriate ideal waveform generators. Finally, the device according to the present invention is readily integratable, with the exception of a few components, such as capacitor 22, for reasons of size, and filter 27 and resistor 33 for reasons of precision. To those skilled in the art it will be clear that changes may be made to the device as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the sawtooth, parabola and ideal ramp generators may differ from those described herein, e.g. block 25 may be replaced by an integrator. The same also applies to the other components of the device, which may be replaced by equivalents, For example, the yoke current sensing resistor may be replaced by a transformer having the primary winding series connected to yoke 6.

Claims: A device (10) for controlling the linearity of a horizontal deflection stage (1), said stage (1) comprising a deflection yoke (6) connected in series between a flyback section (2, 4, 5) and a control capacitor (7), said flyback section including an input switch (2), a flyback diode (4) and a flyback capacitor (5) connected in parallel to each other, said input switch (2) receiving a control signal determining a first phase, wherein said switch (2) is open, said diode (4) is off and said deflection yoke (6) forms an oscillating circuit with said flyback and control capacitors (4, 7), a second phase, wherein said switch is off, said diode is on, said deflection yoke (6) and said control capacitor (7) form an oscillating circuit, with said control capacitor charging, and a third phase, wherein said input switch is on, and said control capacitor discharges, thus varying the voltage in said yoke (6) so that, in said second and third phases, said yoke is flown by a real yoke current (i g ), characterized in that said linearity control device (10) comprises means (11, 12) for detecting said yoke current (i g ); generator means (13-15) for generating an ideal signal (s 4 ) having an ideal S-shaped waveform; means (16) for comparing said real yoke current (i g ) with said ideal signal (s 4 ) and generating an error signal; and control means (17) arranged in parallel to said control capacitor, receiving said error signal and generating an output signal modifying the voltage in said control capacitor, so that said real yoke current (i g ) assumes said S-shaped ideal waveform. A device as claimed in Claim 1, characterized by the fact that said detecting means comprise a resistor (11) series connected to said yoke (6) and said control capacitor (7); said resistor (11) presenting two terminals connected to respective inputs of an operational amplifier (12). A device as claimed in Claim 1 or 2, characterized by the fact that said comparing means comprise an error amplifier (16) having two inputs (A, B) connected respectively to said generator means (13-15) and said detecting means (11, 12). A device as claimed in one of the foregoing Claims from 1 to 3, characterized by the fact that said control means comprise a bipolar power transistor (17) having emitter, collector and base terminals; said transistor (17) presenting said emitter and collector terminals connected parallel to said control capacitor (7), and said base terminal connected to said comparing means (16). A device as claimed in one of the foregoing Claims from 1 to 4, characterized by the fact that said generator means comprise a sawtooth generator (13) for generating a sawtooth signal (s 1 ); a parabola generator (14) for receiving said sawtooth signal (s 1 ) and generating a substantially parabolic signal (s 3 ); and an ideal ramp generator (15) for receiving said sawtooth signal (s 1 ) and said substantially parabolic signal (s 3 ), and generating a ramp signal (s 4 ) having rising portions defining a substantially S-shaped waveform. A device as claimed in any one of the foregoing Claims from 1 to 5, characterized by the fact that said generator means (13-15) comprise a current source (33, 34) for supplying a constant current (I); a charge capacitor (22) supplied by said current source and generating a ramp signal (s 1 ); switch means (21) for periodically discharging said charge capacitor (22); a multiplying element (23-27) for receiving said ramp signal and generating a quadratic signal (s 3 ); and an adding element (D) for adding said constant current to said quadratic signal. A device as claimed in Claim 6, characterized by the fact that said current source (33, 34) comprises a resistor (33) having a terminal (D) connected to a constant voltage source (34) and to the output of said multiplying element (23-27); and by the fact that said multiplying element comprises a buffer (23) connected to said charge capacitor (22) for eliminating the direct-current offset of said ramp signal (s 1 ); a rectifier (24) connected to the output of said buffer, for generating a rectified signal (s 2 ); a quadratic circuit (25) connected to the output of said rectifier, for generating a signal proportional to the square of said rectified signal; a variable-gain amplifier (26) connected to the output of said quadratic circuit; and a high-pass filter (27) connected between said amplifier (26) and said terminal (D) of said resistor (33).

Inventors: GORNATI SILVANO, MERLO MAURO, GORNATI, SILVANO, MERLO, MAURO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H04N3
IPC codes: H04N3