The invention relates to a display device comprising a cathode ray tube having an electron gun and a deflection unit with line and frame deflection coils and a yoke.

The invention also relates to a deflection unit for a cathode ray tube having a yoke ring.

Display devices having cathode ray tubes with a deflection unit with a yoke ring are commonly known.

In such known devices in an evacuated envelope one or more electron beams are generated by a means for generating (an) electron beam (s), e. g. an electron gun. The electrons are accelerated and impinge on a phosphor screen. For the acceleration a high voltage source is provided. In between the display screen and the electron gun a colour selection electrode or shadow mask may be present. To scan the display screen the electron beams are deflected by a deflection unit positioned around the evacuated envelope in between the means for generating the electron beam (s) and the phosphor screen. The deflection unit comprises line and frame deflection coils, the line deflection coils deflect the electron beam (s) at the line (high) frequency in a line deflection direction and the frame deflection coils deflect the electron beams in a direction perpendicular to the line deflection direction and at a lower frequency. The deflection unit furthermore comprises a yoke. The yoke is comprised of material with a high magnetic permeability. The yoke ring surrounds at least some of the coils and enhances the field generated by the coils.

The design of the display device is rather complex and thus costly, in particular the high voltage generator is costly.

It is an object of the invention to provide a display device of the type mentioned in the opening paragraph, the design of which is less costly and less complex, and a deflection unit for such a display device.

To this end the display device in accordance with a first aspect of the invention is characterized in that the yoke is provided, in a symmetry plane of the line coils parallel to a line deflection direction, with at least one pick-up coil surrounding the yoke and a means for deriving a high voltage from said at least one pick-up coil. The inventors have realised that in conventional display tubes, the extreme high tension (EHT) voltage for acceleration of the electron beams is generated by a so called line output transformer (LOT) which is also part of the line deflection circuitry. Besides the display tube itself, the LOT is the most expensive part of a television/monitor set, occupying a substantial part of the accompanying printed circuit boards. The invention provides a new design and method for generating the EHT voltage, namely by means of pick-up coils added to the deflection coil in such a way that a transformer is created. This concept will hereinafter also be referred to as DULOT, indicating the combination of Deflection Unit (DU) and Line Output Transformer (LOT). The invention allows a considerable reduction of the complexity and cost of the display device.

It is remarked that from GB 463,972 deflection means are known in which deflection coils are so disposed on a core that they produce a deflecting magnetic field, but the magnetic fields produced by the mean anode and the mean grid current cancel each other.

Although GB 463,972 describes a combination of deflection coils and coils for generating high voltages, no indication is given of the use of pick-up coils separate from the line coils, or of the position of the pick-up coils.

The position of the pick-up coils around the yoke, in a symmetry plane of the line coils, allows for a reasonable energy transfer to the pick-up coils, while having only a moderate influence on the deflection per se.

In preferred embodiments the pick-up coil (s) and the means for deriving a high voltage from the pick-up coils are arranged such that energy is picked up substantially only during fly-back. Line deflection is usually performed in lines, wherein lines are written (usually from left to right) having the image information, and when the electron beam has reached the end of the line, the electron beam is instantly (or as fast as the electronics and the device allows) brought back to the start of the line, to scan the next line. Bringing back the electron beam (s) from the end of the line to the beginning of the next line is called the"fly- back". No useful information is supplied to the beams during fly-back.

During normal deflection the pick-up coils are substantially inactive, whereas during fly-back energy for the high voltage is picked up. The influence of the pick-up coils on the deflection per se is then small or non-existent, thus they do not interfere with the

image. During fly-back the pick-up coils pick up energy. During fly-back the energy present in the magnetic field has to be dumped anyway, no image is generated on the screen, so the energy pick-up by the pick-up coils does not interfere with the functioning of the device (in fact it may be beneficial), nor is the image detrimentally influenced. Furthermore during fly- back the change in flux is the highest, therefore the highest voltages may be obtained in the pick-up coil (s).

Preferably the device comprises two oppositely arranged pick-up coils.

Although one pick-up coil might suffice, the use of two pick-up coils increases the efficiency.

The two pick-up coils may be arranged in series or in parallel.

Preferably one of the pick-up coils has a short-circuit. Although short- circuiting one of the pick-up coils may seem odd, the inventors have realized that the net result is that the whole voltage difference is now present on the coil that is not short- circuited. The disadvantage of connecting the pick-up coils electrically in series is that high voltage leads will have to be arranged going from one coil to another. This may lead to high voltage problems. Having all of the high voltage over one coil reduces such problems. In a preferred embodiment the short circuit comprises a diode. A diode short-circuits the relevant coil for one current direction, which in preferred embodiments is such that the coil is short- circuited during flyback.

Preferably the means for deriving a high voltage from the pick-up coils are arranged to multiply the voltages over the coils per se. Voltage doublers and triplers exist.

Such circuits increase the costs, but they have the advantage of keeping the voltage differences near the deflection unit in bounds, e. g. only 10 or 15 kVolts instead of 30 kVolts, thus reducing greatly the risk of flashovers.

In a second aspect of the invention the deflection unit is provided with a system of pick-up coils, said system comprising at least one coil wound around or co-wound with a line coil, said at least one coil forming a closed electrical circuit with a coil wound around the yoke, the deflection unit further comprising a second coil wound around the yoke, and means for deriving a high voltage from said second coil.

The coil wound around, or preferably co-wound with, the line coil is magnetically tightly coupled with the line coil. The coil wound around the line coil or co- wound with the line coil forms a transformer. If the high voltage were directly derived from the coil wound around or co-wound with the line coil, a high voltage would be present over this coil. This however, also means that the distance between the co-wound coil and the line coil is small. If the voltage over this coil would be large, there is a considerably risk of high-

voltage breakdown between the co-wound coil and the line coil. To prevent this the co- wound coil (or coil (s) ) forms a closed electrical circuit with a further coil which is wound around the yoke, thereby preventing high voltages. The flux generated in this coil induces a voltage in the second coil, which need not be in close physical contact with the line coil, thus reducing the risk of electrical breakdown.

In this embodiment there are two preferred positions for the first and second coils wound around the yoke. One of these positions is in a plane of symmetry of the line coils perpendicular to the line deflection direction. In this case the first and second coil are not magnetically coupled with the line coil and consequently have less influence on the line deflection field. For normal scanning this means at or near the North and South positions (in a vertical plane) A second preferred position is in a plane of symmetry of the line deflection coils parallel to the line deflection direction. For normal scanning this means at or near the East and West positions (in a horizontal plane). Although this has the disadvantage that there is some influence on the line deflection, the advantage is that a much higher efficiency is achievable. In the latter embodiment the resulting deflection unit combines in effect both aspects of the invention.

These and other objects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings: Fig. 1 is a sectional view of a display device.

Fig. 2 illustrates in perspective view a conventional deflection unit.

Fig. 3 illustrates the principal aspect of the invention Fig. 4 illustrates magnetic field lines in a deflection unit Fig. 5 illustrates the position of the pick-up coils in a deflection unit in accordance with the invention.

Fig. 6 schematically further illustrates the position of the pick-up coils in a deflection unit in accordance with the invention Fig. 7 illustrates an embodiment of the invention using two electrically interconnected pick-up coils Fig. 8 illustrates an embodiment of the invention in which one of the pick-up coils is short-circuited

Fig. 9 illustrates in a graphical form the result of short-circuiting one of the pick-up coils Fig. 10 illustrates pick-up coils for DC generation.

Fig. 11 illustrates a further example of a pick-up coil system operating mainly during fly-back.

Fig. 12 illustrates schematically a pick-up coil system strongly coupled to the line coils according to the second aspect of the invention.

Fig. 13 illustrates further a pick-up coil system strongly coupled to the line coils Fig. 14 illustrates a Litz wire Fig. 15 illustrates an embodiment in which both aspects of the invention are combined.

Figs. 16 and 17 illustrate embodiments with changes in the form of the yoke Fig. 18 illustrates an embodiment using a voltage tripler.

Fig. 19 illustrates a detail of a further embodiment.

The Figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the Figures.

Fig. 20 illustrates a further embodiment in which the yoke ring is extended a separate high voltage isolated coil.

Fig. 21 illustrates a further embodiment in which the yoke ring comprised an air gap Figs. 22 and 23 illustrate further embodiments in which the yoke ring is provided with an additional coil mimicking a physical air gap.

The Figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the Figures.

Fig. 1 is a schematic, sectional view of a known design of a display device comprising a cathode ray tube 1 comprising a deflection unit 11. In the Figure, a tube axis is indicated by means of reference numeral 15. Said tube axis 15 substantially coincides with the axis of symmetry of the deflection unit 11. This axis is in the following Figures also referred to as z-axis. Said deflection unit includes a coil holder 18 of an electrically insulating material (often a synthetic resin) having a front end portion 19 and a rear end portion 20. A line deflection coil system 21 is situated on the inside, between these end portions, which line

deflection coil system serves to generate a (line) deflection field for deflecting electron beams generated by an electron gun 6 in, usually, a horizontal (line) direction; and on the outside of the coil holder, there is a frame deflection coil system 22 for generating a (frame) deflection field for deflection in the vertical direction. Within the framework of the invention, the line deflection is the fast (high frequency) deflection, the frame deflection is the comparatively low frequency deflection. Each coil system 21,22 generally includes two sub- coils. The deflection unit further comprises a yoke ring 23. The yoke ring has an axially extending external surface tapering outwardly toward the display screen. The tube is provided with an anode button to which a HV (High Voltage) of the order of 30 kVolts is provided. Via a conductive layer at the inner side of the envelope and springs which are on the one hand attached to the electron gun and on the other hand touch the conductive layer, the high voltage is supplied to an anode (usually to the anode cup) of the electron gun 6. The high voltage is commonly supplied by a LOT (line output transformer) which also provides the line deflection current. Besides the display tube itself, the LOT is the most expensive part of a television/monitor set, occupying a substantial part of the accompanying printed circuit boards.

Both coil systems 21,22 are attached to the coil holder 18.

Fig. 2 illustrates in perspective view a conventional deflection unit. The line deflection coils 21 are arranged at the inside of the holder 18, the frame or field deflection coils are arranged at the outer side of the holder 18 and the yoke ring 23 is arranged around the field deflection coils 22. It is remarked that the form of the yoke ring may be substantially circular, as in Fig. 2, but may alternatively be more or less rectangular. The yoke ring 23 is made of a material having a high magnetic permeability e. g. a ceramic type material.

Fig. 3 illustrates the basic idea of the invention in comparison to prior designs.

The top part of Fig. 3 illustrates the simplified version of the widely used system set-up of a high voltage generator and a deflection unit. The high voltage is generated by means of the line output transformer (LOT) and electron beams are deflected by means of the line coil, which is part of the deflection yoke. High voltage (EHT) rectification and stabilization is left out of consideration.

One switching device SW1 serves both the line output transformer and the deflection coil. The AC parts of the voltages across the line output transformer and the deflection coil are more or less the same. In conventional display tubes, the extreme high tension (EHT) voltage for acceleration of the electron beams is generated by a so called line output transformer (LOT). Besides the display tube itself, the LOT is the most expensive part

of a television/monitor set, occupying a substantial part of the accompanying printed circuit boards. The present invention aims at providing a different, simplified design.

The proposed high voltage generator is based on a conventional deflection yoke and designed conform the standard transformer concept, i. e. consisting of a primary and secondary coil. The horizontal deflection coil (line coil) 21 acts as the primary coil of the transformer, the yoke 23 acts as the core of the transformer, and the secondary coil of the transformer is formed by a pick-up coil 24 which is added to the deflection yoke at a location (or locations) that will be more clearly shown in further Figures. The general system set-up is depicted in the lower part of Fig. 3.

Fig. 4 illustrates the magnetic field lines and the position of the deflection coils 21 (line) and 22 (frame). The field lines are those of the line deflection field, 40 depicts the centre of the deflection field, 42 indicates the parts of the yoke ring 23 in which the magnetic field is the lowest, 43 indicates the parts of the yoke ring in which the magnetic field is the highest. Maximum magnetic coupling can be obtained by maximum enclosure of flux. From Fig. 4 we can see that three locations are suitable for enclosing the maximal amount of flux, namely: 1. in the centre of the deflection field, on top of or beneath the line coil 2. on the left side of the yoke ring, -90 degrees rotated with respect to the physical centre line of the line coil 3. on the right side of the yoke ring, +90 degrees rotated with respect to the physical centre line of the line coil.

In a deflection unit in accordance with the first aspect of the invention, a pick- up coil is positioned at either one, and preferably at both, of the positions 43, i. e. in the x-z symmetry plane of the line coils. "In the plane of symmetry"means that the windings of the coil (s), or at least the bulk of the windings of the coil (s), are at or about + 90 or-90 degrees rotated with respect to the centre of the line coil, preferably within 15 degrees, plus or minus, of the 90 degree mark, more preferably within 10 degrees even more preferably within 5 degrees, plus or minus, of the 90 degree mark. These positions are situated in a plane parallel to the line deflection direction (which in this Figure is the horizontal direction) and symmetrical in respect of the line coils 21 (i. e. one line coil 21 is situated above the horizontal symmetry plane, the other symmetrically below this plane). In this Figure this symmetry plane is the x-z plane. In case the deflection unit is a transposed scan deflection unit the symmetry plane is the vertical (y-z) plane. In transposed deflection systems, the line deflection direction is the vertical direction. In standard deflection systems, the line

deflection direction is the horizontal direction. It is remarked that the energy pick-up by the pick-up coils is a function of the change in flux ; the line field has the highest change of flux (in comparison to the frame field).

Fig. 5 illustrates the positions of the pick-up coils 24,25 in a perspective view.

Fig. 6 schematically further illustrates that coils 24,25 are wound around the yoke 23 at the indicated position in the x-z symmetry plane of the line deflection coils 21. As the total amount of generated flux lines in the centre of the deflection field returns via two paths through the yoke ring, one coil 24 encloses (maximally) only half of the generated flux.

In order to increase the magnetic coupling a second coil 25,180 degrees rotated with respect to the yoke ring, is preferably added. Arranging these two coils in series (schematically illustrated in Fig. 7, or in parallel) will increase the coupling factor. The winding sense has to be in accordance with the series or parallel coupling of the coils.

In Fig. 7, the coils are arranged electrically in series. Although this is possible, the inventors have realized that the problem arises that, if the maximum voltage reaches 30kV, a line carrying 1 5kVolts has to span from one side of the deflection unit to the other.

This poses a potential safety hazard.

Fig. 8 illustrates a preferred embodiment of a deflection unit in accordance with the invention in which this problem is reduced. One of the pick-up coils (in this case coil 24) is short-circuited by a short circuit 81.

Short circuiting one of the coils (as in Fig. 8) also provides a higher output voltage, not by putting two coils in series (as in Fig. 7), but by increasing the flux enclosed by one coil. This is done without increasing the flux as generated by the primary (line) coil (21) and so the efficiency of this newly proposed transformer will be comparable to that of the embodiment shown in Fig. 7.

A coil enclosing flux lines will generate a certain voltage according to equation (1).

V=-N#d#/dt, (1) in which V is the voltage generated in a coil with N turns (no load impedance connected), enclosing a certain amount of flux 0.

When constructing a transformer from which the secondary coil is terminated with a load resistance R, a current Iso"d, y will flow through this secondary coil, according to equation (2).

Isecondary = -Nsecondary/R#d#'/dt, (2) in which in which Equation (3) shows that the total amount of flux 0'is the sum of the flux generated by the primary coil p, ;", Q, y and the flux generated, due to the load current, by the secondary coil.

Now we can write CdY'primary secolzdary implying that when R=0# # #primary+#secondary = 0 or #primary = -#secondary. (5) From equation (5) it can be seen that we can create a counter flux field (- secaldary), schematically indicated in Fig. 8 by line 82, with the same amplitude as the original flux field) by terminating the secondary coil with a low (~ 0 Q) load impedance. The number of turns of the short circuited coil does not substantially influence this mechanism. When this coil is constructed on core material, flux lines will be guided according to the shape of the core material. This flux line trajectory is not necessarily the same as the trajectory of the original (primary) flux lines. Flux lines as created by the line deflection coil are running according to the lines 41. Half of the total amount of flux lines is enclosed by coil 24, the other half by coil 25. Because coil 24 is terminated by a 0# impedance, coil 24 will generate a flux field as large as, but opposite to, the original flux field. Because the flux is generated by a coil constructed on a core material with high permeability, flux lines will follow the shape of the core material, resulting in the circular path as indicated by line 82. As shown in Fig 8, coil 25 encloses both flux lines from the line deflection field (41), as well as flux lines (82) generated by coil 24. Because the direction/sign of the flux lines 41 and 82 is the same, adding them up is justified. Assuming zero losses, the flux field as generated by coil 24 equals half the flux field as originating from the line deflection coil. This results in the fact that coil 25 now encloses double the amount of flux lines as it did before coil 24 was terminated with a short circuit.

With respect to the design as shown in Fig. 7, this embodiment effectively consists of only one coil of, as a reference, 500 turns, i. e. coil 25. For coil 24 there is no need to use a large number of turns, only one (or a few) turn (s) will suffice, which is an advantage; another advantage is that there is no need to have a high voltage line spanning across the line deflection coil, as in the embodiment shown in Fig. 7.

The advantage is that the output voltage is applied ( ?) over one of the coils (coil 25) only; this reduces high voltage problems.

Fig. 9 illustrates the effect of short-circuiting one of the pick-up coils.

The horizontal axis denotes the load resistance, whereas the vertical axis denotes the output voltage. The steeper the curve the better the magnetic coupling, and the more efficient the coils operate. Line 91 illustrates an embodiment having a single coil (24 or 25), line 92 illustrates an embodiment with two coils 24 and 25 in accordance with the embodiment of Fig. 8, in which short-circuited coil 24 has 1 winding (line 93) and 76 windings (line 92). Comparing line 91 representing a single-coil embodiment with lines 92 and 93 representing embodiments having two coils, one of which is short-circuited, shows that the coupling is significantly improved. Line 94 depicts a calculated curve with a coupling factor of 0.35.

The pick-up coils 24 and 25 pick-up energy from the field lines. This energy has to come from the magnetic field generated by the line coils 21. Thus, there is an influence of these pick-up coils on the magnetic field. Increasing the coupling increases the efficiency with which the pick-up coils pick-up energy, which could, as a negative effect, also increase the influence of the pick-up coils on the magnetic deflection field. One way of counteracting such effects is to slightly change the current through the line deflection coils to compensate for the above mentioned energy draining effect of the pick-up coils. Although this is possible, it is preferred that the influence of the pick-up coils on the deflection as such is minimized. In preferred embodiments this is accomplished by arranging the means for deriving a high voltage such that substantially only during flyback energy is picked up by the pick-up coils.

Figs. 10A and 10B illustrate this. The EHT voltage (HV) is preferably a DC voltage with only little AC ripple.

When using, in a simple embodiment (Fig. 10A), a single diode and a single capacitor on the output terminals of the secondary coil, a DC voltage will arise. As for the secondary coil (coil 25), current will flow only during one period, chosen to be the flyback period to prevent disturbances in the deflection field during the scan period. This embodiment is illustrated in Fig. 10A. It is also possible to choose a different kind of

rectifying, namely by using a bridge rectifier. In that case the voltage across the output capacitor can be higher with respect to the single diode version, because the voltage created across the secondary coil is an AC voltage. However, this implies that a current can flow through the secondary coil during the scan period as well, resulting in a possible disturbed deflection field. Therefore, the bridge rectifier 102 as shown in Fig. 10B is less preferred than the arrangement 101 shown in Fig. 10A. The major difference between the arrangements of Fig. 10A and 1 OB is that, whereas in the design of Fig. 1 OB the coil 25 is in effect operative during line deflection and flyback, in the design in accordance with Fig. 10A the effectiveness of the coil 25 is much larger during flyback (when the diode conducts current) than during line deflection (when the diode stops the current).

Even when using an arrangement as schematically illustrated in Fig. 10A, the pick-up coils, including the short circuited secondary coil (coil 24), will influence the deflection field continuously (even when there is no load current). This is due to the fact that the short circuited coil 24 around the yoke ring 23 will always generate a counter flux with respect to the flux as generated by the line deflection coil. This effect can be eliminated by using a simple diode for short circuiting this coil (coil 24). By choosing the forward direction of the diode in such a way that it will only conduct during the flyback period, disturbances during the scan period will be eliminated. Fig. 11 illustrates the location of the additional diode 83. Thus, short circuiting of one of the coils preferably takes place during fly-back (e. g. by means of the diode 83).

When constructing a transformer, the coupling factor k, describing the efficiency of the power conversion from the primary side of the transformer (i. e. the line coil (s) ) towards the secondary side of the transformer, (i. e. the pick-up coil (s) ) is of great importance. Besides efficiency, the coupling factor influences the maximum reachable output voltage and the required number of turns of the coil (s) significantly. DULOT implementations shown in the above mentioned Figures show a maximally achieved coupling factor of approximately k=0.42. Although simulations indicate that a coupling factor of k=0.42 should be sufficient for creating a 30 kV high voltage generator, it is preferred that the coupling factor is higher.

For a DULOT constructed by means of the line coil and one coil on the yoke ring, the coupling factor k is determined by two different"coupling-paths", namely a coupling from the primary side to the secondary side kps and a coupling from the secondary side to the primary side ksp, which is illustrated by Fig. 12, upper part. A high efficiency transformer can only be constructed if both"coupling-paths"kps and ksp are efficient.

Due to the construction of the deflection unit, the coupling factor of transformer concepts shown so far is dominantly determined by the coupling from the secondary side towards the primary side (kp). This can be attributed to the fact that due to the high permeability of the yoke ring material, the majority of the flux lines generated in the yoke ring will stay inside the yoke ring.

In a second aspect of the invention the coupling is increased as follows: Increasing the coupling factor of the DULOT is done by means of extra coils located in the centre of the deflection field as it is generated by the deflection unit. These coils, when constructed and located properly, do have a large magnetic coupling with the line coil (s) and vice versa. Fig. 12, lower part, illustrates the additional coils 122 and 121 on top of the line coils. The coupling factors kl «, between coils 21 and coils 122 and 121 respectively are large. To ensure a good coupling the additional coils are wound around or co-wound with the line coils. Wound around means that the coils 121,122 enclose at least partly the line coil, enclosing a large part of the flux generated by the line coil and a large coupling factor.

Co-wound, which is preferred, means that coils 121,122 and the line coil are simultaneously wound in a winding machine. The line coil and the additional coils then form a transformer with a very large coupling factor due to the fact that the line coils and the additional coils are co-wound.

The voltage sources which are created by the additional coils 121 and 122 that are constructed on top of the line coil, can be used in various configurations, e. g. as input for a separate transformer or put (electrically or magnetically) in series with the voltage source (s) generated by the coil (s) mounted on the yoke ring. A problem arises in that large voltages may be generated in the additional coils, creating a risk of high-voltage breakdown. Using a separate transformer is possible in theory, however, due to cost and volume issues, in the second aspect of the invention the second transformer is constructed on the yoke ring by coils 124 and 125. The voltage differences in the electrically closed ring coils 121-122-coil 124 are small. The voltage difference VA-Va is generated over the coil 125. This coil is not in close contact with the line coil 21, thus the risk of high voltage breakdown is strongly reduced. In Fig. 12, the coils 124,125 are positioned at the North and South position, i. e. above and below the line deflection coils (corresponding to positions at or near positions 42 in Fig. 4).

Fig. 13 illustrates the embodiment of Fig. 12 in further detail. In this example coils 121 and 122 are wound around the line coil.

Using coils 121 and 122 that are wound close to the line deflection coil 21 a much larger overall coupling factor and thus a larger efficiency (of up to k=0.6 to 0.7, i. e.

some 50 to 80% increase with respect to the designs shown in earlier Figures) was achieved in experiments.

Yet, although a larger coupling factor is achieved with this provision of coils 121 and 122, there is still room for improvement. The coupling factors between coils 121, 122 and coil 21 can be improved.

In a preferred embodiment the line coil (s) and the additional coils 121,122 are co-wound in a bundle of wires. The bundle of wires are then wound together in a coil in a winding machine. "Co-wound"thus means that the bundle of wires are wound in winding machine as if it were one wire. A special type of co-wound wires is a so-called Litz wire, as shown in Fig. 14. The inventors have realized that even higher coupling factors may be obtained making use of co-wound wires, for instance Litz wires. A Litz wire 140 is a composite wire with an outer electrically isolating layer 141, within which there are a number of strands 142A and 143A, each also covered with an electrically isolating outer layer 143.

These strands are wound into a bundle within the outer layer 141. Usually the Litz wire comprises a central strand, surrounded by a number (usually 6 to 8) of outer strands. The inventors have realized that by winding Litz wire to construct the line coil 21, and then splitting the Litz wire at the ends, and making one (preferably the centre strand) or two or more (preferably situated at opposite sides with respect to the central strand) of the strands into the additional coils 121,122, a strongly increased coupling factor is obtained. When use is made of a simple bundle of co-wound wires the outer layer 141 is not present.

A prototype of the preferred embodiment with a bundle of co-wound wires without outer layer 141 resulted in a transformer with a coupling factor of k=0. 9 and higher.

This embodiment had two modifications with respect to the embodiment mentioned in Fig.

13. The most important enhancement was the improvement of the magnetic coupling between the line coil 21 and the additional coil 121,122 in the centre of the deflection unit.

This was achieved by using one of the wires of the bundle of co-wound wires (5 wires in parallel), which was used for constructing the line coil. This resulted in a kl=0. 99 and can be relatively easily implemented in a manufacturing process. This improvement leads to an overall coupling factor of k=0. 89. Finally, it was found that putting the coils on the yoke ring on the east and west side, leads to an extra improvement, leading to k=0.94.

The advantage of positioning the coils 124 and 125 at said North-South positions is that the coils 124,125 do not, or hardly, influence the line deflection field.

Although this may be preferred in some circumstances, in others a further increase in efficiency may be desired. Such an embodiment is shown in Fig. 15. The difference between

the design as shown in Figs. 12 and 13 and the design shown in Fig. 15 is that the coils 154, 155 are not positioned at the North and South positions, but at the East and West positions.

Fig. 15 shows the preferred embodiment, wherein the'dashed-coils'on top of the line coil, illustrate the very well magnetically coupled co-wound wires 121 and 122 and the line coil 21 and the additional coils 154 and 155. The embodiment of Fig. 15 combines the two aspects of the invention. On the one hand, coils 154 and 54 are positioned in the line deflection plane (the x-z plane, wherein one of these coils is short-circuited via coils 121, 122), and thus this embodiment is a preferred variation of the first aspect. The coils 121,122 are co-wound with (they could also be"wound around") the line coils 21, and form a closed electrical circuit with coil 154 wound on the yoke; coil 155 is the second coil wound on the yoke and the high voltage is generated over said coil.

It will be clear that within the framework of the invention many variations are possible.

Figs. 16 and 17, for example, illustrate an embodiment in which the form of the yoke 23 is changed at the position where the pick-up coils 24,25 are wound around the yoke. In this preferred embodiment the yoke 23 is ring-shaped and the yoke ring is provided with protuberances, whereby the inside diameter of the yoke ring is increased at the positions where the pick-up coils are wound around the yoke ring.

Usually the yoke ring is as close as possible to the line and frame deflection coils, for minimization of the line and frame deflection field energy and the energy dissipation of the deflection system. With the provision of the pick-up coils, the distance between the yoke ring and the line and frame deflection coils needs to be larger to prevent high tension breakdown. The energy of the line and frame deflection field increases roughly with the enclosed volume within the yoke ring. Therefore, enlarging the diameter of the yoke ring increases the energy and energy dissipation of the deflection unit. By increasing the effective radius only at the positions of the pick-up coils, i. e. near the x-z plane, the volume enclosed by the yoke ring increases only slightly, thus the effect on the energy dissipation is relatively moderate. Fig. 16 illustrates a yoke ring of substantially circular design, apart from the provision of the protuberances or"ears"161 and 162 for obtaining spaces 163 and 164 to accommodate the pick-up coils 24 and 25. Fig. 17 is comparable to Fig. 16, except for the fact that the yoke ring 23 has a different basic form ; instead of having a circle as the basic form, the basic form of the yoke of Fig. 17 is a rectangle with rounded corners.

Fig. 18 illustrates a preferred embodiment in which the output of the LOT formed by line coil 21 and pick-up coils 24,25 is tripled by means of tripler T to an output

voltage Vout of three times the voltage over the pick-up coils 24 and/or 25. The risk of high- tension breakdown, which is greatest near the yoke, is thereby reduced. A voltage doubler can be made by removing one of the repetitive elements in the tripler T.

According to one preferred embodiment, the co-wound wires (for instance a Litz-wire) are used in such a way that all wires or strands of the Litz wire are part of the line coil 21 during scanning, while during flyback the bundle of wires or the Litz-wire is split-up into two parts. During scanning, all wires are switched in parallel, ensuring minimal resistive losses, during flyback one part, for example three wires, still serves as line coil 21, wires 142A, and the remaining part (two 142B wires in case the bundle of wires or the Litz-wire is constructed by five parallel wires) are used for short circuiting the coil 24 which is constructed on the yoke 23. In this way, a standard DU can be used without the necessity of adding extra wires to the line coil, while at the same time the high coupling factor facility required for constructing a highly efficient transformer is possible as well. Fig. 19 illustrates a switching part 191 for the proposed embodiment.

The invention resides in each and every novel feature and each and every combination of the features described hereinabove. So, for instance, in both aspects of the invention the pick-up coils or pick-up coil system may be arranged to operate mainly during fly-back. Figs. 17 and 18 show"ears"to accommodate the coils 24,25 at the East-West positions. When coils are used at the North-South positions (see Figs. 12-13) "ears"may be provided at the North-South positions.

With respect to embodiments in accordance with the first aspect of the invention in which coils 24 and 25 are electrically coupled in series (see Fig. 7), the best performance is obtained when the number of coil windings in coils 24 and 25 is approximately equal, i. e. the ratio of the number of coil windings is preferably approximately 1 (between 0.8 and 1.2, preferably between 0.9 and 1. 1, or closer to 1).

With respect to embodiments in accordance with the first aspect of the invention in which the coils are magnetically coupled (see Figs. 8,10A, 10B, 11, 15), it holds that if the resistance of the short-circuited coil is zero, the ratio is not important. Fig. 9 illustrates this to some extent since lines 92 (using a coil 24 with 1 winding) and line 93 (using a coil with 76 windings) do not show a large difference in behaviour.

When the resistance is not zero, it is best to minimize the loss (I2R) in the short-circuited coil.

With respect to the best choice for the embodiment as shown in Fig. 12, the following is the best choice:

LC/Ly~ ((l a2)/2ll2 In this formula Lc is the self-inductance of the coil system (121,122) wound around, or co- wound with, the line coil 21 Ly is the self-inductance of the coil (124) on the yoke 23 X is the coupling factor between coils 121,122 and coil 21 (thus basically the coupling factor of the first transformer formed by the system line coil 21-coils 121,122) il is the coupling factor between coil 124 and 125 (i. e. the coupling factor of the second transformed formed by the two coils wound around the yoke).

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope.

Use of the verb"to comprise"and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article"a"or"an"preceding an element does not exclude the presence of a plurality of such elements.

The present invention has been described in terms of specific embodiments, which are illustrative of the invention and not to be construed as limiting. The invention may be implemented in hardware, firmware or software, or in a combination of them. Other embodiments are within the scope of the following claims.

Other embodiments are e. g. schematically shown in Fgures 20 to 23.

Fig. 20 shows schematically an embodiment in which the yoke ring 23, provided with a coil 224 is extended with a separate high voltage isolated coil 225. This separate coil 225 is constructed on an additional core 23A, which are both magnetically connected with the yoke ring. The high voltage coil 225 is further away from the yoke ring 23 itself and from the line coils 21. This reduces problems that could occur when a high voltage coil is brought in close vicinity to the line coils and the yoke ring 23.

The inventors have realized that in the embodiment shown in Fig. 20 the magnetic coupling between coil 224 and coil 225 leaves to be desired as a consequence of the fact that flux lines running through the yoke ring 23 will not flow through the additional core

23A automatically due to air gaps between the yoke ring 23 and the additional core 23A which results in a flux path with qualitatively less magnetic properties than the original flux path via the yoke ring 23. In order to increase the magnetic coupling between coil 224 and coil 225, in the embodiment shown in Fig. 21 the magnetic path via the yoke ring 23 is deteriorated at the location where the additional core 23A is mounted on the yoke ring 23A by adding, at the location of the additional core 23A, a physical air gap in the yoke ring 23.

Although a physical air gap in the yoke ring does result in the desired functionality, the yoke ring 23 needs to be adapted and is no longer a standard component. However, the functionality of the embodiment which is illustrated in Fig. 21 can also be obtained in a different way, namely by replacing the physical air gap by a short-circuited coil 226 which is mounted on the yoke ring 23 at the location where the physical air gap was situated. This alternative embodiment is illustrated in Fig. 22.

In Fig. 22, the short-circuited coil 226 on the yoke ring 23 mimics the physical air gap in the yoke ring of Fig. 21. Both embodiments deteriorate the magnetic properties of the yoke ring at the location where the additional core 23A is added, resulting in the fact that flux lines running through the yoke ring 23 will run through the additional core 23A, enabling coil 225 to enclose all present flux. The air gaps between the yoke ring and the additional core are preferably as small as possible, as a result of which the magnetic path length of the additional core is less important. Because the magnetic path length is of minor importance, this construction provides a large degree of freedom for constructing a rather large high-voltage isolated coil 226 When constructing a device that combines deflection and high-voltage generation functionality, it is desired that the two separate functions do not influence each other too much. The embodiments as depicted in Fig. 21 and Fig. 22 do influence to some extent the flux path of the line deflection field through the yoke ring in an asymmetrical way. Adding a diode 227 in the embodiment of Fig. 22 prevents an asymmetrical flux path during the time a picture is written on the display screen. This embodiment is illustrated in Fig. 23.