Electric lamp and discharge devices: systems – Cathode ray tube circuits – Power supply from deflection circuit source
Reexamination Certificate
2000-01-05
2002-04-16
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Power supply from deflection circuit source
C336S170000
Reexamination Certificate
active
06373203
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diode-split high-voltage transformer having a core, a primary winding and a high-voltage winding, which is arranged in chambers of a coil former.
2. Description of the Related Art
A diode-split high-voltage transformer of this type is disclosed in EP 0 529 418 B1, for example. This transformer contains a first coil former, which accommodates the primary winding and further auxiliary windings, and a second coil former, in which is arranged the high-voltage winding in the form of a chamber winding. The two coil formers are usually produced and wound separately. During final assembly, the coil former with the high-voltage winding, which has a correspondingly larger inner diameter, is pushed over the coil former with the primary winding. The coil formers are subsequently surrounded with a plastic housing and additionally potted with a synthetic resin composition for the purpose of suppressing corona effects and high-voltage flashovers. Embodiments of this type are used in television sets, for example, and supply high voltages in continuous operation of 24 kV up to above 30 kV.
DE 38 22 284 A1 discloses a high-voltage transformer having small dimensions for about 7 kV for copiers and the like. This transformer likewise has two coil formers, the coil former with the primary winding being pushed over the coil former with the high-voltage winding and latching into place therein. It is not designed as a diode-split high-voltage transformer and it cannot achieve high voltages of above 20 kV as required for television sets. It contains no rectifier diodes—these are arranged separately in the associated circuit. The particular intention, by using a chamber-type coil former, was to solve the high-voltage problems which arise here due to the small distance between the high-voltage winding and the core. Despite the considerably lower voltage of 7 kV, however, this design has not demonstrated satisfactory high-voltage strength in sustained operation, even with complete potting, and has therefore not been put into production.
SUMMARY OF THE INVENTION
The object of the present invention is to specify a diode-split high-voltage transformer of the type mentioned in the introduction which is constructed very compactly and cost-effectively and, in particular, has good high-voltage strength in continuous operation at voltages of above 20 kV.
This object is achieved by means of the invention specified herein. Advantageous developments of the invention are specified in the foregoing description.
In the case of the diode-split high-voltage transformer of the invention, the primary winding lies above the high-voltage winding and the high-voltage transformer contains means by which the electric field between the coil former and the core is reduced in order to avoid corona effects. For example, the surface of the inner cavity of the coil former is provided with a conductive coating, which, during operation, is at earth as a result of contact with the core, or at the same potential as the core. As a result, the electric field can be screened in the inherently unavoidable air gap between core and coil former, thereby effectively suppressing corona effects and voltage flashovers. Corona effects are produced in particular by ozone produced in the air by a high electric field. The conductive coating concentrates the electric field in the material between the high voltage winding and the conductive coating of the coil former, which ensures long-term high-voltage strength with an appropriate material and dimensioning.
The conductive coating used must be a high-impedance layer, for example colloidal graphite, which can be applied in a simple manner by means of a nozzle which sprays in the radial direction. A low-impedance, for example metallic, layer would constitute a short-circuited turn and lead to losses.
As an alternative, instead of the conductive coating, the remaining cavity between core and coil former may be filled with a material, so that corona effects are also avoided by this means. The material preferably has the highest possible relative permittivity ∈
r
, for example 2-3 or 4, and may be, for example, a viscous paste, possibly also the potting material of the high-voltage transformer itself. The material may also have a low conductivity. Air inclusions must not occur in the course of filling since, on account of the low relative permittivity ∈
r
=1, a high electric voltage builds up in the said inclusions and air can easily be ionized under the voltage conditions prevailing here.
Since the primary winding bears together with an insulating layer directly on the high-voltage winding, the entire arrangement becomes very compact. The chambers of the coil former also provide, with a multiple sheet winding, a sufficiently smooth surface onto which the primary winding can be wound uniformly and tightly with a wire thickness of, for example, 0.3 to 0.8 mm.
The wall thickness under the chambers of the high-voltage winding in the direction of the core are advantageously chosen such that they increase as the high voltage rises at the bottom of the chamber.
The high-voltage diodes may be arranged laterally with respect to the high-voltage chambers on the coil former, or alternatively they may be integrated between the high-voltage winding and the primary winding. In order to obtain a very inexpensive embodiment, the high-voltage winding is subdivided into four windings, a diode being respectively connected between the first and the second and the third and the fourth winding and a tap being routed out between the second and third winding for the focus voltage of a picture tube.
The compact structure of the coil former enables not only the housing of the high-voltage transformer but also its core to be considerably reduced in size. As a result of this, the potting compound can also be considerably reduced since there are no longer any high-voltage potentials on the outside of the high-voltage transformer. This not only leads to a considerable cost reduction but also affords space and weight advantages. Thus, it is possible to achieve a weight reduction of 25%, given the same electrical properties, with a diode-split high-voltage transformer (DST) having two diodes compared with a diode-split high-voltage transformer having three diodes. In addition, RLC circuits for attenuating the interference radiation are obviated.
In a further exemplary embodiment, the diode-split high-voltage transformer contains only one coil former, in which the high-voltage winding is arranged in chambers, the primary winding lying above the high-voltage winding and being wound onto an interposed sleeve or sheet winding. As an alternative, it is also possible to use a simple coil former for the primary winding, which coil former is pushed over the coil former with the high-voltage winding. If a sleeve is used, it may also be composed of two or more parts.
In an advantageous manner, the primary winding is somewhat wider than the high-voltage winding and covers the latter as far as possible completely. The higher-frequency interference radiation produced in the high-voltage winding is virtually completely screened by this means since the core (usually at earth potential) of the high-voltage transformer is situated on the inside of the high-voltage transformer and the covering, tightly wound primary winding is situated on the outside, and the outer chambers of the high-voltage winding carry either no or only a very small pulse voltage, depending on the design, since they are connected either to the reference potential or to the high-voltage connection directly or via a further chamber. These interference voltages are produced as a result of oscillations between the inductances and stray capacitances of the high-voltage transformer when the diodes change over from the conducting phase to the blocking phase. These facts have already been explained comprehensively in the literature, for example in EP 0 735 552 A1, and are not, therefore
Goseberg Walter
Heidrich Rolf
Renner Hans
Fried Harvey D.
Kolodka Joseph J.
Thomson Licensing SA
Tran Thuy Vinh
Tripoli Joseph S.
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