CRT focus tracking arrangement

A video display apparatus includes a cathode ray tube and a high voltage transformer. The high voltage transformer incorporates a high voltage winding having a tap which provides a focus voltage for the electron gun assembly of the cathode ray tube. The transformer also includes a supply winding which provides power to the electron gun assembly drive circuit. The supply winding is wound to be closely coupled to the portion of the high voltage winding not associated with the generation of focus voltage so that increasing beam current produces nonuniform loading of the high voltage winding.

This invention relates to high voltage transformers for video display 
apparatus and, in particular, to high voltage transformers that utilize a 
tertiary winding to generate a focus voltage. 
The electron gun assembly of a color cathode ray tube produces one or more 
electron beams which impinge upon a phosphor display screen in a 
predetermined pattern to form a scanned raster. The electron gun assembly 
is designed to produce a number of spatial regions of different voltage 
potentials through which the electron beam or beams pass. One of these 
voltage potential regions provide focussing of the beams so that the spots 
formed when the beams strike the display screen are of a desirable size 
and sharpness. 
The focus voltage or potential may be generated by providing a tap on the 
high voltage or tertiary winding of the high voltage transformer. The 
electron gun assembly used in the COTY-29 picture tube manufactured by RCA 
Corporation utilizes a high voltage winding tapped to provide a focus 
voltage nominally equal to one-third of the high voltage or ultor 
potential. Changes in electron beam current, due to variations in picture 
brightness, may require that the focus ratio (i.e., the ratio of the focus 
voltage level to the high voltage level) change in order to maintain 
optimum beam focus. This focus tracking in which the focus voltage changes 
in response to variations in beam current, becomes more important for 
picture tubes having large deflection angles (e.g., 110.degree.) or for 
picture tubes utilizing deflection yokes that provide raster distortion 
correction, such as pincushion correction, which may increase the amount 
of deflection defocussing experienced by the electron beams. As the 
electron beam current increases, however, the loading on the high voltage 
supply also increases, which may cause the high voltage level to decrease, 
resulting in an increase in the focus ratio. Some picture tubes, including 
the previously described COTY-29 picture tubes, incorporates electron gun 
assemblies that, in order to produce optimally focussed beams, require the 
focus ratio to decrease as beam current increases. 
In accordance with an aspect of the present invention, a high voltage 
transformer for use in a video display apparatus comprises a high voltage 
winding having first and second terminals located at opposite ends of the 
winding and providing a high voltage potential. An intermediate terminal 
providing a focus potential is located between the first and second 
terminals and defines a focus winding region between the intermediate 
terminal and the first terminal. A supply winding is located adjacent to 
the high voltage winding and is magnetically coupled to the high voltage 
winding. The region occupied by the supply winding and the focus winding 
region have significant portions that do not overlap.

Referring to FIG. 1, there is shown a portion of a video display apparatus 
including a cathode ray tube or picture tube 10 and a high voltage 
transformer 11. Video signals illustratively received via antenna 8 are 
applied to video processing circuitry 9, which demodulates and decodes the 
signal in an appropriate manner for application to video drive circuit 13. 
The output of video drive circuit 13 is applied to picture tube 10, which 
incorporates an electron gun assembly 12. Electron gun assembly 12, when 
energized, may illustratively produce three electron beams. Various 
operating voltage levels may be applied to electron gun assembly 12, 
including a focus voltage level via a terminal 14. The electron beams are 
deflected to form a scanned raster by deflection yoke 15. 
A source of AC voltage 16 is coupled to a rectifying circuit 17 which 
produces an unregulated DC voltage level that is applied to a regulator 
circuit 20. Regulator 20 may illustratively be of various types, such as 
switched-mode or SCR regulators. The output of regulator 20 is a regulated 
DC voltage that is applied to one terminal of a primary winding 21 of high 
voltage transformer 11. The other terminal of primary winding 21 is 
coupled to a horizontal deflection circuit 22 which generates horizontal 
deflection signals that are applied to the horizontal deflection windings 
of deflection yoke 15 via terminal 23. 
High voltage transformer 11 includes a high voltage winding 24, comprising 
winding segments 42, 64 and 65, and rectifying diodes 61, 62 and 63, which 
is energized by primary winding 21 during the horizontal retrace interval 
and produces a high voltage level that is applied to the anode terminal of 
picture tube 10 via conductor 25. Resistor 26 limits the current that can 
be provided by high voltage winding 24 in order to protect various 
electrical components of the video display apparatus. A tap 27 on high 
voltage winding 24 provides a focus voltage that is applied to electron 
gun assembly 12 via terminal 14. Tap 27 is selected so that the focus 
voltage is nominally of the order of one-third the high voltage level. The 
focus voltage generating portion of high voltage winding 24 will therefore 
comprise one-third of the full traverse of high voltage winding 24; i.e., 
one-third of the total number of winding turns of high voltage winding 24. 
The focus voltage is supplied from tap 27 to terminal 14 via an adjustable 
resistor 30. 
High voltage transformer 11 also includes a load circuit supply winding 31 
which, via appropriate rectifying diodes and filtering capacitors, 
produces a voltage level +V.sub.1 at a terminal 32. Voltage level +V.sub.1 
may illustratively be of the order of +230 volts and may be applied to 
video drive circuitry 13. 
As the electron beam current is increased due to viewer adjustment of the 
brightness control or due to changes in the picture scene brightness, the 
focus ratio, that is, the ratio of the focus voltage level to the high 
voltage level, may no longer provide the same quality of beam focus or 
sharpness as that provided at lower beam current levels. The RCA COTY-29 
picture tube, for example, experiences improved beam focus at higher beam 
current levels as a result of a decreasing focus ratio as beam current 
increases beyond, for example, 0.2 milliamperes. In a typical circuit 
application, however, the focus ratio will remain constant or tend to 
increase at higher beam current levels, due to picture tube loading of the 
high voltage supply circuit. 
FIG. 2 illustrates an embodiment of a high voltage transformer 11 in which 
supply winding 31 is wound in a manner that produces the previously 
described desirable decreasing focus ratio at high increasing beam current 
levels. Transformer 11 comprises a primary winding bobbin 33 on which is 
wound the transformer primary winding 21. Primary winding 21 comprises 
upper and lower terminals that are connected to terminal stakes 48 and 49. 
Primary winding 21 is wound to substantially cover the full traverse of 
the high voltage winding 24 to provide uniform loading of the high voltage 
winding 24 during the horizontal retrace interval as is described in U.S. 
patent application, Ser. No. 717,805, filed Mar. 29, 1985 in the name of 
L. W. Nero and entitled "Transformer Winding Arrangement for a Television 
Apparatus", herein incorporated by reference. A tertiary winding bobbin 34 
surrounds the primary winding bobbin 33, and has high voltage winding 24 
wound thereon. The lower terminal 35 of high voltage winding 24 is 
connected via a conductor 36 to a terminal stake 37. The focus-take off 
tap 27 is connected to terminal stake 40 via a conductor 41. In order to 
provide a nominal focus ratio of one-third, tap 27 is located at a 
distance equal to one-third of the total traverse of high voltage winding 
24 from lower terminal 35, thereby forming a focus voltage generating 
winding region 42 as part of high voltage winding 24. The upper terminal 
43 of high voltage winding 24 is connected to the cathode ray tube anode 
terminal conductor 25 via a conductor 44. 
In accordance with an aspect of the present invention, supply winding 31, 
which provides power to the video drive circuitry 13, is wound on bobbin 
33 and overlays primary winding 21. Supply winding 31 is wound to cover or 
overlay only the upper one-half of the traverse of primary winding 21 and 
does not overlap the focus voltage generating portion 42 of high voltage 
winding 24. Supply winding 31 will then be magnetically more tightly 
coupled to the upper portion of the traverse of high voltage winding 24 
and magnetically less tightly coupled to the lower portion of the traverse 
of high voltage winding 24, which includes the focus voltage generating 
winding region 42. The lower terminal 45 and upper terminal 46 of supply 
winding 31 are illustratively connected to terminal stakes 47 and 50 
respectively by conductors (not shown). The previously described bobbins 
and windings are located within a transformer housing 51. Housing 51 is 
filled with an epoxy compound 52 which pots the windings in a conventional 
manner. A magnetically permeable core 53 comprising upper and lower core 
segments 54 and 55, is located within the interior of primary bobbin 33. A 
crushable spacing material 56 separates core segments 54 and 55 to permit 
adjustment of the inductance of primary winding 21. 
Supply winding 31 provides power to video drive circuitry 13 in order to 
drive the electron gun assembly 12 of cathode ray tube 10 and is 
consequently heavily loaded; therefore an increase in electron beam 
current causes an increase in loading of supply winding 31. As previously 
described, supply winding 31 is magnetically coupled to high voltage 
winding 24. This magnetic coupling causes the loading of supply winding 31 
to result in a corresponding loading of high voltage winding 24. 
Substantial loading of high voltage winding 24 by supply winding 31 
occurs, however, only in the region closely coupled to supply winding 31, 
i.e., that portion of high voltage winding 24 not associated with the 
generation of focus voltage. The primary winding generated retrace pulse 
appearing across the portion of high voltage winding 24 that is tightly 
coupled to supply winding 31 becomes flatter and broader because of the 
loading caused by supply winding 31. Rectifying diodes 62 and 63, shown in 
FIG. 1, associated with the loaded portion of winding 24, will conduct for 
a longer period of time than rectifying diode 61 therefore lowering the 
output impedance of the portion of high voltage winding 24 tightly coupled 
to supply winding 31. The overall loading of winding 24 due to increasing 
beam current will therefore cause a greater decrease in focus voltage 
level relative to the decrease in high voltage level, due to the lowered 
output impedance of the high voltage generating portion of high voltage 
winding 24. The focus ratio, i.e., the focus voltage level with respect to 
the high voltage level, will therefore decrease as the beam current 
increases. This results in improved electron beam focus characteristics 
with respect to beam current changes. The previously described winding 
arrangement of the primary winding with respect to the high voltage 
winding results in a constant degree of coupling between the primary and 
high voltage winding. The harmonic tuning of the transformer is not 
affected by changes in beam current or supply winding 31 loading. The 
arrangement of the present invention advantageously relies on the loading 
of the supply winding 31 to control the retrace pulse waveshape in a 
manner that results in desirable focus ratio changes in response to beam 
current variations. 
FIG. 3 graphically illustrates the percent change in focus ratio, with 
respect to a nominal ratio, associated with the use of the inventive 
transformer structure of FIG. 2 as a result of beam current changes. At 
low beam current levels, less than 0.2 milliamperes, for example, an 
increase in beam current results in an increase in the focus ratio. This 
is due to the focus bleeder resistor 30 loading the focus voltage 
generating portion of winding 24 and lowering the output impedance of the 
focus voltage generating circuit so that the high voltage level decreases 
relative to the focus voltage. For low beam current levels, this provides 
optimum focus characteristics. As the beam current increases, however, the 
previously described loading of the upper portion of high voltage winding 
predominates, resulting in a desirable decrease in focus ratio at high 
beam current levels. 
The amount of the traverse of high voltage winding 24 that is overlaid by 
supply winding 31 may be selected to provide the desired change in focus 
ratio with respect to the beam current in order to achieve optimum 
electron beam focus for a given cathode ray tube and video display 
apparatus.