GB2272617A - Stabilization of EHT voltage and horizontal scan for magnetic deflection cathode ray tube circuits - Google Patents

Abstract

The raster geometry of a cathode ray tube display is stabilized against variations caused by beam current loading. This is done by regulating the B+ voltage supply to the horizontal deflection circuit, which generates EHT, and controlling the regulator according to the magnitude of the EHT. Therefore beam current loading which depresses the EHT will no longer affect the raster geometry. This arrangement is particularly suited for use with 16:9 wide aspect ratio television receivers. <IMAGE>

Description

Stabilization of EHT Voltage and Horizontal Scan for Magnetic Deflection Cathode Ray Tube Circuits Application No. 9214801.4 Filing date 13th July 1992.

Many products such as television receivers, computer monitors etc. use cathode ray tubes with magnetic deflection of the electron beam or beams as display devices.

These cathode ray tubes require a number of electrical supplies for their operation, including deflection currents flowing in scan coils to generate the magnetic fields which deflect the electron beam or beams over the phosphor screen, and also a high d.c. voltage (which may be about 25,000 volts), usually referred to as EHT, to accelerate the same electron beam or beams.

To provide an essentially stable display, it is desirable that the horizontal scan current and EHT voltage are substantially independent of parameters such as the magnitude(s) of the electron beam current(s).

Various circuits have been developed with the aim of achieving this condition. Many of these previous circuits separate the horizontal scan and EHT generating functions.

The present invention minimises the effect of these parameter variations on the display geometry and combines the EHT generation and horizontal scanning circuits.

The invention modifies the conventional horizontal deflection stage with flyback transformer and diode modulator circuits in order to provide controlled EHT and controlled horizontal scan current. The modifications are described by way of example, largely in terms of 16:9 aspect ratio ("widescreen") cathode ray tubes for use in television receivers, but are equally applicable to any application using magnetic deflection cathode ray tube(s).

The essential features of the invention are shown in Fig. 3.

1. The addition of a regulator to the power supply of the Scanning/EHT generation stage. The output of this regulator is referred to as "B+" in the following text.

Feedback is applied from the Scanning/EHT generation stage to this regulator circuit. The regulator may, by way of example, be a switched or linear type.

2. The application of feedback to an existing diode modulator from a circuit which senses the horizontal deflection current.

The principal advantages of the invention are: ThQ gffctiug horizontal deflection current waveform and EHT voltage are both substantially constant, thus minimising picture content and time dependent changes in raster shape previously associated with combined Scanning/EHT generation systems. The performance of the invention in this respect is comparable to that achieved by systems using separate Scanning and EHT generation circuits.

The invention also encloses the diode modulator components within a feedback loop, and thus reduces the parasitic ringing effects which are otherwise associated with standard diode modulator PWM (Pulse Width Modulator) drive circuits.

1. Fig. 1 shows a conventional horizontal output stage incorporating a PWM or linear driven diode modulator circuit for control of width and E-W (East-West) pincushion correction along with the required EHT voltage generator.

Within the normal- horizontal deflection stage, the supply voltage voltage is a function of the P-P (peak to peak) deflection current required to scan the CRT (cathode ray tube) at the EHT voltage obtained from the flyback transformer.

A disadvantage of this conventional circuit is that the peak rectified EHT voltage obtained is not stabilized, and since conduction of the output rectifiers occurs only on the peaks of the waveform, the source impedance is appreciable (typically about 2MQ) resulting in significant variations in EHT voltage with CRT beam current. A typical performance of such a combined Scanning/EHT generating system might be: 25kV at OmA Ibeam and 23.8kV at 1mA 1beam (in the case of this example the source impedance is approximately 1.2MQ). This results in raster width variations with changes in beam current (raster width is approximately proportional to 1/I(EHT), i.e. the raster width is approximately inversely proportional to the square root of the EHT voltage).

Partial compensation for this variation is usually made by adding a current sensing resistor to the low voltage termination of the EHT overwind, and feeding forward the beam current information thus obtained. Since CRT beam current is not linearly related to EHT voltage, it is clear that any height and width correction generated using this technique will only be approximate and the overall picture breathing effects would still be excessive, and these effects would be clearly visible on a large screen 16:9 CRT for example.

2. The normal solution to this problem is to generate the EHT voltage using an EHT transformer and an independent inductance for energy storage instead of the normally used scan coil inductance of the horizontal deflection stage.

By keeping the EHT substantially constant, correction signals are no longer required for height or width circuits and an essentially stable raster is obtained, which is ideally suited for large screen CRT's. A typical system is shown in the block diagram of Fig. 2.

Proposal: The present invention shown in Fig. 3 minimises the above effects by use of a combined EHT-Horizontal Scan system providing substantially stabilized horizontal scan current which eliminates the requirement for separate EHT generating and horizontal deflection circuits. The heart of the circuit is a multiple feedback system used in conjunction with a standard diode modulator circuit and a B+ regulator which by way of example may be either pulse width switching or analogue.

The EHT voltage is stabilized in the same way as with a separate generator. Thus the varying B+ voltage is a function of the CRT beam current since it is used to keep the EHT substantially constant, and this varying B+ voltage is also applied to the deflection coils via the series "S" correction capacitor.

A major benefit of maintaining a substantially constant EHT is that height correction to the vertical deflection stage is no longer required.

However, this varying B+ voltage would normally give rise to unacceptable changes in width with changes in beam current, but by introducing a diode modulator circuit in conjunction with a conventional PWM or linear drive circuit, themselves both prior art, the horizontal deflection current can be controlled by utilising an additional feedback technique which compensates for the above mentioned variations in B+.

In this way, the visible effects of the non-linear relationship between CRT beam current and EHT voltage are minimized and the feedback correction applied to the PWM/linear driver for the diode modulator substantially compensates for the variations in B+ which are a result of regulating the EHT voltage.

By way of example, a means of obtaining information regarding the required amount of applied horizontal scan current is by sampling the peak rectified voltages at both end of the deflection coils (relative to chassis ground), the difference of which is applied to the closed loop feedback system at the virtual earth input of the PWM or linear drive circuit.

Alternative methods of sampling the scan current include the use of a current transformer connected in series with the scan coils. The resulting feedback signal from either system contains information relating to the E-W parabola amplitude, width setting and applied B+.

A further advantage of this system is that the effect known as parasitic ringing due to the transient response of the PWM in conjunction with the diode modulator components is significantly reduced. This is in comparison with a prior art system where the average value feedback is only applied locally around the PWM.

GLOSSARY.

B+: Output of regulator.

CRT: Cathode ray tube.

EHT: High d.c. voltage.

E-W: East-West.

Ibeam: CRT Beam current.

PWM: Pulse width modulator.

P-P: Peak to peak.

Claims (6)

1. What is claimed is: 1. Modification of combined EHT generation and horizontal scan circuits associated with magnetically scanned cathode ray tubes making the raster geometry substantially independent of changes in the cathode ray tube beam current or currents.
2. 2. Modification of the combined EHT generation and horizontal scan circuits associated with magnetically scanned cathode ray tubes accordingly to Claim 1 by the addition of a supply voltage regulator to the said combined EHT generation and horizontal scan circuit.
3. 3. Modification of the combined EHT generation and horizontal scan circuits with supply voltage regulator associated with magnetically scanned cathode ray tubes according to Claims 1 and 2 by the use of a diode modulator in conjunction with either a pulse width modulator or linear drive circuit associated with the said horizontal scan circuit part of the said combined EHT generation and horizontal scan circuit with supply voltage regulator.
4. 4. Minimizing variations in the EHT voltage generated by the combined EHT generation and horizontal scan circuit as described in Claims 1,2 and 3 by a feedback signal applied to the said supply voltage regulator, the said feedback signal being derived from the EHT voltage output.
5. 5. Minimizing variations in the horizontal scan generated by the combined EHT generation, horizontal scan circuit, feedback controlled voltage supply regulator and diode modulator and either pulse width modulator or linear drive circuit as described in Claims 1,2,3 and 4 by the use of a sensing signal derived from the amplitude of the horizontal scan. The said sensing signal being applied to the said diode modulator and pulse width modulator or linear drive circuit associated with the horizontal scan circuit part of the said combined EHT generation and horizontal scan circuit including the said feedback controlled supply voltage regulator.
6. 6. Minimizing variations in the raster geometry of magnetically scanned cathode ray tubes by the use of multiple feedback with a combined EHT generating and horizontal scan circuit including supply voltage regulator, diode modulator and either pulse width modulator or linear drive circuit as described in Claims 1,2,3,4 and 5.