A CIRCUIT ARRANGEMENT FOR PRODUCING HIGH VOLTAGES

Abstract: In a circuit arrangement for producing high voltages by means of a voltage multiplier (6) composed of rectifiers (4) and capacitors (5) and fed from an alternating current source (7). a low amplitude voltage lying at the high voltage level is produced by means of the ripple voltage across one or more of the component parts of the voltage multiplier, the average potential of which does not differ substantially from the high voltage. In the case of an X-ray tube the total filament circuit may in this case be approximately on the high cathode voltage and no severe requirements for insula­ tion need to be fulfilled. The ripple voltage may be used to produce other small amplitude voltages lying at a high voltage level.

Description: This invention relates to a circuit arrangement for regulating a voltage or a current in a utilizing circuit operating at a high voltage level such as the filament circuit of an X-ray tube to which high voltage is supplied by a voltage multiplier, e.g. of the Cockcroft-Walton type, that is fed from an alternating current source, said circuit arrangement including means for generating a voltage having a small amplitude with respect to the hight voltage by means of the ripple voltage across at least one of the component parts of the voltage multiplier the average potential of which does not differ substantially from the high voltage, and supplying said small amplitude voltage to the utilizing circuit. It is known per se to provide high-voltages e.g. for discharge tubes, including X-ray tubes, by means of voltage multipliers of the kind concerned. In the case of X-ray tubes it is generally desired to earth the anode because this makes it easier to obtain the necessary dispersion of heat from the anode. In order to avoid extremely high potentials in relation to earth, it is a frequently chosen solution, however, to supply the anode and cathode of the X-ray tube from high-voltage sources of opposite polarities each at half of the full anode-cathode voltage of the tube. In either case, the filament of the tube will attain a high voltage relative to earth and due to the fact that the magnitude of the filament current determining the intensity of the X-radiation shall, moreover, generally be adjustable, certain difficulties arise, in particular as regards fulfilment of the demands for insulation, whereby the means for supplying and controlling the filament current become excessively voluminous and heavy, meaning a considerable increase of the total volume and weight of the X-ray tube. This is particularly inadvantageous in case of mobile X-ray devices for radiological tests of materials. This applies also to other parts of the tube, e.g. a bias grid, the voltage of which deviates by only a small amount from the cathode voltage of the tube. Similar problems occur also in other devices than X-ray tubes, which are supplied from a voltage multiplier of the type in question, and where it is desirable to supply a small amplitude voltage to a utilizing circuit lying at the voltage level of the high voltage. From JP-A-56 129 576 it is known in itself to supply high voltage electric power to a main load circuit by means of a multistage voltage multiplier and supply a small amplitude voltage to an auxiliary load circuit attached to the high voltage terminal of the voltage multiplier and further to compensate for a voltage drop in the load circuit caused by a large capacity of the load by supplying a small amplitude voltage to the load circuit via a leakage transformer. The small amplitude voltage is derived from the ripple voltage over the maximum potential terminals of the voltage multiplier and by means of compensating capacities forming a resonant circuit with the secondary windings of the leakage transformer a series resonance is provided to prevent the voltage drop in the secondary winding and improve the load characteristic. Contrary to this it is the object of the invention to remedy the above-mentioned disadvantage and provide a circuit arrangement which is effective to provide adjustment of a current in the utilizing circuit without any need for excessively voluminious and heavy control means. This is obtained according to the invention in that an earthed adjustment member is included in the low voltage part of the voltage multiplier for generation of an adjustment signal, transfer means being provided for transferring said adjustment signal to the utilizing circuit and an adjustment member being incorporated in the utilizing circuit for controlling said current in response to said adjustment signal. Even though the high voltage produced by the voltage multiplier is principally a D.C. there is, nevertheless, a ripple voltage across the components of the multiplier having a basic frequency corresponding to the frequency of the A.C.-source, and this ripple voltage may be collected, e.g. by a transformer, and supplied to the utilizing circuit, e.g. the filament circuit of an X-ray tube. The entire filament circuit may in this case lie approximately on the high cathode voltage and, therefore, severe demands for insulation do not need to be fulfilled, e.g. between the primary winding and secondary winding of an inserted transformer. The smallest demands for insulation occur if the components of the voltage multiplier, across which the ripple voltage is collected, lie on a high potential level close to the high voltage. This may for instance be the rectifier of the multiplier which attains the highest potential. Economical advantages may, however, also be obtained if the potentials of the components concerned differ not quite immaterially from the high voltage, for example by one third of or perhaps even by half the high voltage relative to earth. In a preferred embodiment of the invention wherein said small amplitude voltage is supplied to the utilizing circuit via a transformer the primary winding of which is supplied with said ripple voltage whereas the secondary winding forms part of the utilizing circuit, the earthed adjustment member can be operated to vary the frequency of said alternating voltage source and said adjustment member in the utilizing circuit comprises a frequency dependent variable impedance. In this case the current in the utilizing circuit may be adjusted by variation of the voltage source frequency. The frequency depending impedance may be constituted by a resonant circuit. If the resonant frequency is close to the frequency variation area of the A.C.-voltage source, a large variation of the current is obtained for a small variation of the frequency. As an alternative the current in the utilizing circuit may also be controlled by optical transmission to the utilizing circuit in that said transfer means comprises optical coupling means for optical transmission of said adjustment signal to said adjustment member in the utilizing circuit. The invention will now be explained in more detail with reference to the drawing illustrating an example of a circuit arrangement according to the invention for supplying and adjusting the filament current of an X-ray tube. In the drawing 1 designates an X-ray tube having an anode 2 and a cathode 3 in the form of a filament. The high-voltage of the X-ray tube is provided by means of a voltage multiplier 6 of the Cockcroft-Walton type and composed of rectifiers 4 and capacitors 5. The voltage multiplier is fed from an A.C.-voltage source 7 via a capacitor 8. A high negative voltage which is approximately a multiple of the peak voltage of the voltage source 7 and which is supplied to the filament 3 of the X-ray tube, the anode 2 of which is earthed, occurs at point 9 of the voltage multiplier. By means of the ripple voltage occurring across the rectifier of the voltage multiplier having the highest potential relative to earth a small voltage is produced and supplied to a utilizing circuit, in the illustrated example to the filament circuit of the X-ray tube. Said ripple voltage is supplied via a capacitor 10 to a primary winding 11 of a transformer, the seconary winding 12 of which forms part of the filament circuit of the X-ray tube, said circuit including besides the secondary winding 12 and the filament 3 a frequency dependent impedance 13. By varying the frequency of the A.C.-voltage source 7 the value of the impedance 13 changes and it is possible in this manner to control the filament current of the X-ray tube. The variable impedance 13 may for instance be constituted by a resonant circuit which may either be a series resonant circuit or a parallel resonant circuit. If the frequency of the A.C.-voltage source is close to the resonant frequency a large variation of the impedance may be obtained and thereby of the filament current for a small variation of the frequency of the A.C.-voltage source. The method of controlling the filament current just described has the advantage of requiring no conductive connections between the filament circuit lying on a high voltage level and the earthed adjusting member located in the low voltage part of the voltage multiplier. An alternative solution to avoid such conductive connections could be the use of optical signal transmission between the earthed adjusting members and an adjustment member such as an adjustable impedance forming part of the filament circuit. In the illustrated embodiment the anode of the X-ray tube is earthed, while the cathode 3 has a high negative voltage. The invention may, however, also be practised in case of using a bipolar high-voltage supply, in which the filament and the anode have opposite polarities relative to earth. The high voltage may in this case be provided by means of two more or less combined voltage multipliers generating oppositely polarized voltages. The voltage multiplier does not need to be of the Cockcroft-Walton type, provided it includes a component having a comparatively high voltage, across which a ripple voltage occurs. The ripple voltage does not need to be collected across the component of the voltage multiplier lying on the highest voltage relative to earth, as illustrated in the drawing, but may be collected across another component having a comparatively high voltage or across a combination of several components. In stead of being used to produce and adjust the filament current of the X-ray tube, as illustrated, the ripple voltage may be used for other purposes. It may e.g. be supplied to a circuit arrangement for producing a voltage for a bias grid controlling the emission current of the X-ray tube. The invention is not limited to X-ray tubes. The circuit arrangement according to the invention is applicable in any case, where a high voltage is produced by means of a voltage multiplier and it is desirable to produce a low amplitude voltage lying at the high potential level of the high voltage with respect to earth and provide adjustability of the current in the utilizing current. The invention is not restricted to the embodiments illustrated in the drawing and explained above, but it may be modified in various ways obvious to an expert within the scope of the following patent claims.

Claims: A circuit arrangement for regulating a voltage or a current in a utilizing circuit operating at a high voltage level such as the filament (3) circuit or the grid bias circuit of an X-ray tube (1) to which high voltage is supplied by a voltage multiplier (6), e.g. of the Cockcroft-Walton type, that is fed from an alternating current source (7), said circuit arrangement including means for generating a voltage having a small amplitude with respect to the high voltage by means of the ripple voltage across at least one of the component parts (4) of the voltage multiplier the average potential of which does not differ substantially from the high voltage, and supplying said small amplitude voltage to the utilizing circuit, characterized in that an earthed adjustment member is included in the low voltage part of the voltage multiplier (6) for generation of an adjustment signal, transfer means being provided for transferring said adjustment signal to the utilizing circuit and an adjustment member being incorporated in the utilizing circuit for controlling said current in response to said adjustment signal. A circuit arrangement as claimed in claim 1, wherein said small amplitude voltage is supplied to the utilizing circuit via a transformer the primary winding (11) of which is supplied with said ripple voltage whereas the secondary winding (12) forms part of the utilizing circuit, characterized in that said earthed adjustment member can be operated to vary the frequency of said alternating voltage source (7) and said adjustment member in the utilizing circuit comprises a frequency dependent variable impedance (13). A circuit arrangement as claimed in claim 1 characterized in that said transfer means comprises optical coupling means for optical transmission of said adjustment signal to said adjustment member in the utilizing circuit.

Inventors: HANSEN, PEER KUHLMANN, MADSEN, JENS ULRIK
Applicants: ANDREX RADIATION PRODUCTS A/S
Application year: 1987
Main classification: H02M7
IPC codes: H05G1, H02M7, H02M3