X-ray or gamma ray systems or devices – Electronic circuit – X-ray source power supply
Reexamination Certificate
2000-02-10
2002-07-09
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Electronic circuit
X-ray source power supply
C378S101000, C363S021030
Reexamination Certificate
active
06418191
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an X-ray apparatus of the type having a d.c.-a.c. converter supplied with voltage that is connected via a line connection to a high-voltage transformer that supplies the high-frequency operating voltage to an X-ray tube.
2. Description of the Prior Art
D.c.-a.c. converter circuits with which an alternating voltage is generated are utilized in X-ray units, this alternating voltage being forwarded via a line connection to the high-voltage transformer that generates the high voltage that is required for the operation of the X-ray tube. The high voltage signal at the secondary side of this transformer is subsequently also rectified, and smoothed. The d.c.-a.c. converter is usually a series resonant circuit converter that is supplied with a d.c. voltage that is usually acquired directly from the mains voltage by rectification and filtering. It contains four semiconductor switches that are conductive in pairs in alternation on the basis of a suitable drive. The line connection is usually required in X-ray embodiments wherein the high-voltage generator, which contains the high-voltage transformer and possibly other components connected following the transformer, is provided in the immediate proximity of the X-ray tube or is contained together therewith in a single tank.
A typical resonant circuit utilized for this purpose is composed of a capacitance C and an inductance L=L
&sgr;
+L
L
, wherein L
&sgr;
is the stray inductance of the high-voltage transformer or transformers and L
L
is the line inductance of the line connection to the high-voltage generator. Particularly at high d.c.-a.c. converter power and operating frequencies of 30 kHz or higher that are currently standard, it is difficult to make the resonant circuit inductance as small as is necessary. The stray inductance L
&sgr;
of the high-voltage transformer cannot be arbitrarily reduced in size without having the high-voltage transformer become too large. The line inductance L
L
makes up a substantial part of the overall inductance L, particularly given single-tank generators that require a longer cable between d.c.-a.c. converter and single tank. An overall inductance which is too high, however, has a disadvantageous effect on the operation of the X-ray apparatus.
Japanese Published Application 6-53 83 discloses an X-ray apparatus of the above-described type.
German Utility Model 85 26 448 discloses a high-voltage X-ray cable with a grid control line. The center of the cable disclosed therein has a grid control line and two electric resistance wires that are stranded with one another. A conductive band that surrounds this configuration is in turn surrounded with the actual high-voltage conductor, which in turn carries an inner conductive layer that is surrounded by an insulation. An outer conductive layer, a shielding as well as an outside insulation are also provided. Either a single high-voltage line, or two high-voltage lines as well as two electrical resistance wires that are stranded with one another are utilized in the inside of the line in the X-ray line disclosed in German Utility Model G 91 07 953. A conductive sheath is provided around the exterior, followed by a high-voltage insulation as well as another outer conductive sheath on which a shielding is disposed that in turn carries an outside cladding. German Patent 39 29 990 discloses a low-impedance, coaxial line that has a number of coaxial cables connected in parallel.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an X-ray means that can also be operated at high operating frequencies and high power.
The above object is achieved in accordance with the principles of the present invention in an X-ray apparatus having a d.c.-a.c. converter circuit supplied with energy that is connected via a line connection to a high-voltage transformer which supplies high-frequency operating voltage to an x-ray tube, wherein the line connection is composed of four concentrically arranged conductors, with a first and second of these conductors carrying the operating voltage and being insulated from each other and being disposed close to each other for achieving a low line inductance, and having a ratio of conductor radius to conductor thickness to produce a low ohmic impedance. A third of these conductors serves for shielding and is insulated from the second conductor for eliminating capacitive shift currents and is connected to a point of constant potential in the d.c.-a.c. converter. A fourth of these conductors serves for shielding and is insulated from the third conductor, and is connected to the housing of the d.c.-a.c. converter circuit, as well as to the housing of the high-voltage transformer or a device component contained within the high-voltage transformer.
In the inventive X-ray apparatus, the line connection has a low line inductance, which is inventively realized by arranging the two conductors that proceed concentrically relative to one another extremely close to one another. Inventively, the line inductance can be ≦0.25 &mgr;H/m, particularly ≦0.15 &mgr;H/m; the spacing between the first and the second conductor should be ≦2 mm, particularly ≦1.5 mm. Even given a long length, which usually amount to approximately 12 m or more, these low inductances contribute only slightly to the overall inductance, so that this is in an acceptable range overall. Moreover, the two current-carrying conductors of the inventive line connection have a low ohmic impedance, which can be achieved by corresponding dimensioning of the conductors as to conductor radius and conductor thickness. i.e., the line connection is also optimized with respect thereto. The ohmic impedance at high frequencies is dependent on the conductor radius and on the effective conductor layer (penetration depth). As is known, the penetration depth is dependent on the conductor material and the operating frequency. The conductor thickness should not be significantly greater than the penetration depth, thereby allowing conductor material to be saved. Given copper and a frequency of 30 kHz, for example, the penetration depth amounts to barely 0.4 mm. It is therefore inventively provided that the thickness of the conductors is ≦1 mm, particularly ≦0.5 mm. The ohmic impedance should be ≦20 m&OHgr;/m, particularly ≦15 m&OHgr;/m in accordance with the invention. The low ohmic impedance leads to low losses.
Since, due to the skin effect, current conduction occurs only in the outer layer of conductors, the first, most interiorly disposed conductor in the invention can be a waveguide, which is advantageous in view of the lower use of material as a result thereof as well as in view of the flexibility and pliability of the line connection.
As described, a concentrically arranged, third conductor serving as shielding and insulated from the second conductor is inventively provided for carrying away capacitive shift currents. Such shift currents, otherwise would flow (drain) via the capacitance between the second conductor and the grounded metal parts located in the proximity without employing this shielding conductor, and would cause significant disturbances in the system as stray currents. These shift currents, are eliminated via this third conductor, which preferably at constant potential in the d.c.-a.c. converter for this purpose, i.e. the capacitive shift currents are returned into the d.c.-a.c. converter in this case and do not flow via the housing of the d.c.-a.c. converter or some other metallic article.
A concentrically arranged fourth conductor serving for shielding and insulated from the third conductor, is provided, with which the—albeit slight—magnetic field generated by the current through the third conductor is shielded, so that no shift currents having a disadvantageous influence are generated in this case, either. Inventively, the fourth conductor is connected to the housing of the d.c.-a.c. converter circuit and to the housing of the high-voltage transf
Fehre Jens
Kuehnel Werner
Bruce David V.
Hobden Pamela R.
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