EP0416708B1 - X-ray generator for operating an X-ray tube with earthed radiological components - Google Patents

Description

X-ray generator with an X-ray tube with tube parts connected to ground

The invention relates to an X-ray generator according to the preamble of claim 1. Such an X-ray generator is known from FR-A-2 527 035.

In X-ray tubes with a tube part connected to ground, for example a metal piston, which can optionally also be connected to a metal part located between the anode and cathode, the current generated at the cathode does not flow completely to the anode; part of this current flows to ground via the relevant tube part. As a result, the cathode-side high-voltage source is subjected to greater stress than the anode-side high-voltage source, which in the case of high-resistance, symmetrically designed high-voltage sources leads to an asymmetry between the high voltage at the anode or at the cathode (i.e. the high voltage between the anode and ground is greater than the high voltage between the cathode and Dimensions).

• a) Bei großen Röhrenspannungen erreicht die Spannung zwischen Anode und Masse bereits einen Wert von mehr als der Hälfte der maximal zulässigen Röhrenspannung, bevor die Spannung zwischen Anode und Kathode ihren maximal zulässigen Wert erreicht. Um eine hochspannungsmäßige Überlastung der Röntgenröhre zu vermeiden, darf die Röntgenröhre in einem solchen Fall nicht mit der vollen Spannung betrieben werden, für die sie ausgelegt ist.
• b) Bei kleinen Röhrenspannungen kann die Kathodenspannung so niedrig werden, daß der an der Kathode emittierte Strom durch Raumladungseffekte begrenzt wird. Um einen bestimmten Röhrenstrom zu erreichen, muß in diesem Fall der Heizstrom für die Kathode unnötig groß gemacht werden, was zu einer Verkürzung der Lebensdauer der Röhre führen kann.

This asymmetry has negative effects that depend on the level of voltage between the anode and cathode:

• a) With large tube voltages, the voltage between the anode and ground already reaches a value of more than half of the maximum permissible tube voltage before the voltage between the anode and cathode reaches its maximum permissible value. In this case, in order to avoid a high voltage overload of the X-ray tube, the X-ray tube must not be operated at the full voltage for which it is designed.
• b) With small tube voltages, the cathode voltage can become so low that the current emitted at the cathode is limited by space charge effects. At a certain tube current To achieve this, the heating current for the cathode must be made unnecessarily large in this case, which can lead to a shortening of the life of the tube.

In the known X-ray generator, the voltage asymmetry and the resulting negative effects are eliminated in that a special inverter is provided which can be operated in such a way that the electrical power supplied via the primary and secondary windings for the cathode voltage becomes greater than that via the primary - And secondary windings for the anode voltage supplied electrical power.

It is an object of the present invention to eliminate the unwanted asymmetry without a specially designed inverter.

This object is achieved according to the invention by the characterizing part of claim 1.

Generation of the anode voltage an inductance can be switched on by means of a switching device.

According to the invention, each secondary winding is also assigned a primary winding, so that the voltages on the secondary windings can be predetermined independently of one another, at least in a certain range. If one assumes that the primary windings correspond to one another and also the secondary windings, then the X-ray generator according to the invention delivers to a "normal" X-ray tube, i.e. an X-ray tube without tube parts connected to ground, e.g. an X-ray tube with a glass bulb, a symmetrical voltage distribution, i.e. the amount of voltage between the anode and ground is the same as the voltage between cathode and ground. On the other hand, when an X-ray tube is connected, the anode current of which deviates from the cathode current, the inductance is connected in series with the primary winding for generating the high voltage on the anode side by means of the switching device. As a result, the voltage on the anode-side primary winding is reduced in comparison to the voltage on the cathode-side primary voltage, as a result of which, when the inductance is suitably dimensioned, the anode voltage is reduced at least approximately by the same amount as the cathode voltage as a result of the higher cathode current.

However, it is also possible to measure the inductance such that the anode voltage drops more than the cathode voltage, so that the cathode voltage is greater than half the tube voltage - as long as the cathode voltage does not exceed half the maximum tube voltage. In this case, the space charge in the region of the cathode can be eliminated, so that the current through the X-ray tube increases or at a given cathode temperature given tube current, the cathode temperature is lowered and the life of the cathode can be increased.

In principle, it would be possible to provide a separate transformer for generating the cathode voltage and the anode voltage and to arrange the additional inductance in series with the primary winding for the transformer for the anode voltage. However, the effort for two separate transformers is still relatively large, as is the space required for the two high-voltage transformers. A preferred embodiment therefore provides that the primary windings and the secondary windings are wound on a common core and are arranged in such a way that the leakage inductance between the mutually assigned primary and secondary windings is substantially smaller than the leakage inductance between the windings which are not assigned to one another.

If several windings are wound on a common core so that they are practically penetrated by the same induction flux, the voltages in these windings are given per se, so that separate control of the primary winding and secondary windings for the anode or the cathode is initially not possible appears. In a high-voltage transformer for an X-ray tube, however, the primary windings and the secondary windings carrying the high voltage must be isolated from one another, which results in a certain leakage flux or a certain leakage inductance between the mutually associated primary and secondary windings. It is now ensured by a suitable arrangement of the windings that the leakage flux or the leakage inductance between windings which are not assigned to one another (for example between the primary winding for the anode voltage and the secondary winding for the cathode voltage) is still substantially greater than the leakage flux or the leakage inductance between mutually assigned windings, then the winding pairs behave within certain limits like two separate high-voltage transformers.

Another embodiment of the invention provides that the inductor consists of several partial inductors lying in series and that a switchover device can be coupled to the partial inductors in such a way that the inductor can be switched completely or only partially into the feed line to the primary winding. This configuration allows the (partial) inductance connected in series with the anode-side primary winding to be adapted to the respective requirements in stages: at high tube voltages, a relatively small partial inductance is switched on by means of the switching device, which is dimensioned such that the anode and cathode voltage are at least approximately are equal in amount. With small tube voltages, on the other hand, a larger inductance is made effective, so that the cathode voltage becomes greater than the anode voltage, which makes it possible to lower the cathode temperature for a given tube current.

A further development of the invention provides that the inductance is designed as an air throttle. In principle, the inductance could also be formed by a coil with a ferromagnetic core. However, since the inductances required are relatively small, such a coil would have only one or a few turns, so that exact dimensioning would be difficult. In addition, saturation phenomena would occur with such a coil because of the high currents which can flow through the primary winding in an X-ray image (a few hundred A). In contrast, an air choke, ie a coil without a ferromagnetic core, can have a sufficient number of turns and shows no saturation effects.

In a further embodiment of the invention it is provided that the inductor is wound on a toroid. An air throttle could in itself be wound particularly easily on a cylinder core. An air choke with turns evenly distributed around the circumference of the (non-ferromagnetic) ring core is more difficult to wind, but it creates a smaller magnetic stray field in its environment.

• Fig. 1 ein Prinzipschaltbild eines Teils eines erfindungsgemäßen Röntgengenerators und
• Fig. 2 einen Querschnitt durch einen dafür geeigneten Hochspannungstransformator.

The invention is explained below with reference to the drawing. Show it:

• Fig. 1 is a schematic diagram of part of an X-ray generator according to the invention and
• Fig. 2 shows a cross section through a suitable high-voltage transformer.

In Fig. 1, two X-ray tubes 1 and 2 optionally connectable to the X-ray generator are provided. While the x-ray tube 2 has the same cathode current as the anode current because this tube has a glass bulb, for example, this is not the case with the x-ray tube 1. As indicated schematically, this X-ray tube has a grounded metal piston and a central part connected to it in an electrically conductive manner and arranged between the anode and cathode. In such an x-ray tube known per se (see also EP-OS 74 141), part of the cathode current can flow to earth via the central part and the metal bulb, so that the cathode current is greater than the anode current.

One of the two X-ray tubes 1 or 2, each located at different workplaces (in clinical practice, even more X-ray tubes can be provided) can be connected to the high voltage generated in the X-ray generator by means of a high-voltage switchover device 3 - which can be coupled with the workstation selector (not shown). The high voltage for the rectifiers 11 and 12 is supplied by the secondary windings 21 and 22, to which a primary winding 31 and 32 is assigned. The four windings mentioned are wound on a common transformer core 4. Instead of the secondary windings 21 or 31, a secondary winding arrangement consisting of several individual windings can also be used.

The output voltage of the rectifiers 11 and 12 is smoothed by capacitors 41 and 42 and fed to the changeover switch 3 via a damping resistor 51 and 52, respectively. The positive or negative high voltage, to which one of the X-ray tubes 1 or 2 is connected in the operating state, is detected by a voltage divider 61 or 62 for measurement and control purposes.

FIG. 2 shows a cross section through the high-voltage transformer with the core 4, the two primary windings 31 and 32 and the two secondary windings 21 and 22. The core 4, a cutting tape core, has the shape of a rectangular ring core. Such a core is expediently composed of two identical cores with a U-shape, so that the windings can be produced before they are each applied to one core and before the two cores are assembled. The secondary windings 21 and 22 each enclose the primary winding 31 and 32 assigned to them, and the primary windings 31 and 32 enclose the same leg of the core 4. Since the primary windings have the same number of turns - and likewise the secondary windings - this results in a transformer structure that is symmetrical with respect to the center line 40.

With this structure, the magnetic coupling between mutually unassigned windings - for example the primary winding 32 and the secondary winding 21 is considerably weaker - and accordingly the leakage inductance or the leakage induction flux is considerably greater - than between the mutually associated windings, for example the primary winding 31 and Secondary winding 21. A ratio of the mentioned leakage inductances of 6: 1 has already proven to be sufficient to enable the windings to be fed asymmetrically without inadmissibly high equalizing currents flowing.

As can be seen from Fig. 1, the two primary windings 31 and 32 are fed by a controllable AC voltage source, for example with a medium-frequency series resonance inverter with an operating frequency of e.g. 3-12 kHz. However, while the primary winding 32 for generating the cathode voltage is connected directly to the output of the AC voltage source 5, an inductor is provided in one of the connecting lines between the primary winding 31 and the AC voltage generator, which consists of the partial inductors 7, 8 and 9 connected in series. which each have a switch 70, 80 and 90 connected in parallel. While the primary and secondary windings must be arranged in a boiler filled with transformer oil, for example, the partial inductors 7, 8 and 9 and the switches 70, 80 and 90 can be arranged outside of this boiler.

The x-ray generator is operated as follows: when the x-ray tube 2 is connected (in the extended position of the high-voltage switch 3), all the switches 70, 80 and 90 closed so that the inductance 7, 8, 9 is short-circuited. The primary windings 31 and 32 are fed with alternating voltages of the same size, so that there is a symmetrical voltage distribution on the X-ray tube 2, ie the amount of the anode is the same as the cathode voltage (in each case with respect to ground or ground).

To connect the X-ray tube 1, the high-voltage switch 3 is switched to the position not shown in FIG. 1. At high tube voltages, only one switch is opened in this case, for example switch 70, so that only partial inductance 7 in series with primary winding 31 is effective. As a result, the voltage across the primary winding 31 is less than that across the primary winding 32 and, accordingly, the open circuit voltage (ie, the voltage under no load from the X-ray tube 1) is lower than the open circuit voltage at the output of the rectifier 12. Due to the difference between cathode and anode current however, the operating voltage at the cathode drops more than at the anode, so that, with suitable dimensioning of the partial inductance 7, a symmetrical voltage distribution at the x-ray tube 1 arises at least approximately.

At lower tube voltages, two of the three switches or all three can be open. The voltage across the primary winding 31 then decreases to such an extent that the anode voltage is always lower than the cathode voltage. The advantage of this asymmetrical operating mode is that the maximum possible emission current is increased for a given voltage between the anode and cathode, or that the cathode temperature can be reduced for a given tube current, so that its service life is extended.

In this case, the switches 70, 80, 90 must be controlled as a function of the voltage on the X-ray tube. If, on the other hand, there is only a single switch and only a single inductance, the switch is controlled as a function of the workplace selector (not shown in more detail) which also operates the high-voltage switch 3.

It has been shown that even relatively small inductances are sufficient to achieve a symmetrical voltage distribution in an X-ray tube of the tube 1 type; a maximum operating voltage asymmetry (this is the difference between anode voltage and cathode voltage - without the inductance) of 14 kV could be almost completely compensated by means of an inductance of around 13 »H. If one were to provide a ferromagnetic core for the production of such a coil, then this coil would have only one or a few turns, so that an exact production would be difficult. In addition, saturation effects could occur in the core due to the very large currents that flow over the primary windings in an X-ray (some 100 A). The inductance is therefore designed as an air choke. The windings of this air choke are preferably wound on a non-ferromagnetic toroid - evenly distributed - so that only small magnetic stray fields result in the vicinity of the air choke.

Claims (5)

1. An X-ray generator, comprising an X-ray tube (1) having parts connected to ground, and a high-voltage transformer arrangement comprising a primary winding arrangement (31) and a secondary winding arrangement (21) for generating the anode voltage for the X-ray tube, and a primary winding arrangement (32) and a secondary winding arrangement (22) for generating the cathode voltage for the X-ray tube, characterized in that an inductance (7, 8, 9) can be switched in series with the primary winding (31) for generating the anode voltage by means of a switching device (70, 80, 90).
2. An X-ray generator as claimed in Claim 1, characterized in that the primary windings (31, 32) and the secondary winding arrangement (21, 22) are wound onto a common core (4) and are arranged so that the leakage inductance between the associated primary and secondary windings (for example, 31, 21) is essentially smaller than the leakage inductance between the non-associated windings (for example, 31, 22).
3. An X-ray generator as claimed in any one of the preceding Claims, characterized in that the inductance consists of several series-arranged sub-inductances (7, 8, 9) and that a switching device (70, 80, 90) can be coupled to the sub-inductances in such a manner that the inductance can be switched entirely or only partly into the lead to the primary winding (31).
4. An X-ray generator as claimed in any one of the preceding Claims, characterized in that the inductance (7, 8, 9) is formed as an air-core coil.
5. An X-ray generator as claimed in Claim 4, characterized in that the inductance (7, 8, 9) is wound on an annular core.

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