Operating an X-ray tube

Abstract

Method for operating an X-ray tube (12) having at least one grid electrode (18) arranged between an anode electrode (14) and a cathode electrode (16), wherein - by means of a focusing unit (24) an electron current (26) is focused from the cathode electrode (16) to the anode electrode (14), in that the focusing unit (24) applies a first electrical grid potential to the grid electrode (18) at least in a focusing operation in order to focus the electron current (26), - the focusing unit (24) is supplied with electrical energy in a galvanically isolated manner by means of an energy converter (38), - the first electrical grid potential is provided by means of an adjustable voltage divider (36) of the focusing unit (24), and - the adjustable voltage divider (36) is adjusted by means of a control circuit (40) of the focusing unit (24) by applying to the control circuit (40) at least one galvanically isolated control signal (42, 44) of a control unit (74) galvanically isolated from the X-ray tube (12), wherein the control signal (42, 44) depends on a predetermined value for the first electrical grid potential, characterized in that - an electrical power of the energy converter (38) is set depending on the predetermined value for the first electrical grid potential.

Description

The invention relates to a method for operating an X-ray tube which has at least one grid electrode arranged between an anode electrode and a cathode electrode, wherein an electron current is focused from the cathode electrode to the anode electrode by means of a focusing unit, in that the focusing unit applies a first electrical grid potential to the grid electrode at least in a focusing operation in order to focus the electron current, and the focusing unit is supplied with electrical energy galvanically separately by means of an energy converter. Furthermore, the invention relates to a circuit arrangement for operating an X-ray tube, which has at least one grid electrode arranged between an anode electrode and a cathode electrode, having a focusing unit for focusing an electron current from the cathode electrode to the anode electrode, wherein the focusing unit is designed to subject the grid electrode, at least in a focusing operation, with a first electrical grid potential in order to focus the electron current, an energy converter for supplying the focusing unit with electrical energy in a galvanically isolated manner, and a control unit galvanically isolated from the X-ray tube for adjusting an electrical power of the energy converter. Finally, the invention also relates to an X-ray apparatus having an X-ray tube which has at least one grid electrode arranged between an anode electrode and a cathode electrode, and a circuit arrangement for operating the X-ray tube which is connected to the X-ray tube by means of a connecting line.X-ray tubes, methods for their operation and also circuit arrangements therefor are widely known in the prior art; see, for example, US 2004 / 0 114 722 A1, US 2002 / 0 034 279 A1 or U.S. Pat. No. 4,334,153 A. X-ray tubes are a specific type of vacuum electron tubes which in the present case serve to be able to provide an X-ray radiation for a wide variety of purposes in an intended operation. X-ray apparatuses are frequently also a component of imaging devices, as are used, for example, in medical diagnostics or also in quality assurance. The X-ray tube generally uses an operating principle in which, by suitably setting an electric voltage between the cathode electrode and the anode electrode, the electrons are strongly accelerated in the manner of an electron current and strike the anode electrode under predefined conditions. X-ray radiation is thereby released. The release of X-ray radiation can be influenced, inter alia, by an area of incidence on the anode, which can be at least partially adjusted by focusing the electron current.In X-ray tubes of the type in question, an anode-cathode voltage applied between the anode electrode and the cathode electrode can be approximately 20 kV to approximately 150 kV if the X-ray tube is embodied as single-pole. In the case of an X-ray tube of two-pole design, the same anode / cathode voltage is still present, but the voltage of the anode or cathode electrode against an electrical reference potential, for example a ground, is only half the acceleration voltage. It may be about 30 kV to about 75 kV.In the prior art, it is customary to realize the focusing and / or deflection of the electron current by means of magnetic fields which are provided by means of a corresponding magnetic field unit. In order to interrupt the provision of X-ray radiation, it has previously been customary to apply a suitable electrical potential to the at least one grid electrode, such that a grid-cathode voltage occurs between the grid electrode and the cathode electrode, which can be in a range from about a few hundred volts to about 4 kV, for example. With such a grid cathode voltage, a pinch-off of the electron current in the X-ray tube can be achieved, so that substantially no electrons can reach the anode electrode any longer. The grid-cathode voltage at which this effect occurs is sometimes also called pinch-off voltage. As a rule, the electrical potential of the grid electrode is negative with respect to the electrical potential of the cathode electrode. Moreover, the electrical potential of the anode electrode is generally positive with respect to the cathode electrode.The region of the anode electrode in which the electrons essentially strike during the generation of x-ray radiation, also called a focal spot, is advantageously adapted to respective operating modes, in particular with respect to the respective imaging method. As a result, a respective image quality can be achieved for a respective application. For this purpose, a suitable focusing can be set, or a compromise can also be set, for example, with regard to image quality and the lowest possible load on the X-ray tube.In many X-ray apparatuses, in particular angiography, this can generally be realized only with difficulty using magnetic field units because of the required overall size. Efforts are therefore being made to realize the focusing at least partially no longer by means of magnetic fields, but by focusing by means of electric fields. In this connection, US 4,361,901 discloses a multi-voltage X-ray switching system. For this purpose, it is known to apply a suitable electrical potential for focusing to the at least one grid electrode, which can be arranged, for example, at least partially between the cathode electrode and the anode electrode and / or at least partially also next to the cathode electrode. In this respect, the term "between" preferably also comprises an arrangement of the grid electrode at least partially in a region next to the cathode electrode. Thus, the grid electrode can have delimiting plates next to the cathode electrode, webs between a segmented cathode electrode and / or the like. Such a teaching is shown, for example, in DE 10 2007 042 108 A1, which discloses an electron source.Even if these teachings have proven themselves in principle in the prior art, however, at least one problem remains when discharging a generally comparatively long high-voltage cable for driving the X-ray tube when switching from the pinch-off voltage to a predefinable grid-cathode voltage for focusing the electron current.In the aforementioned teachings, the function of restricting the electron current is realized, for example, by a voltage converter with galvanic isolation for realizing potential isolation, for which purpose, for example, a transformer of corresponding design can be provided, and with which the required restricting voltage can be provided quickly. By means of a short-circuit switching element, the grid-cathode voltage can be rapidly reduced, for example to approximately zero, as a result of which a discharge of a parasitic capacitance of the connecting cable can also be achieved. In this circuit concept, actual value feedback is generally not realized because of the required technical outlay, for which reason the grid cathode voltage can be provided only with a low accuracy. For the pinch-off of the electron current, it is essentially sufficient to achieve at least the pinch-off voltage and at the same time to maintain the insulation strength of the system. However, a control for sufficiently accurate setting of the grid-cathode voltage, in particular for focusing of the electron current in the X-ray tube, is not possible with this.With respect to focusing by means of an electric field, the above-mentioned voltage converter has also been already used. Since a passive rectifier circuit is generally provided at an output terminal of the voltage converter, the grid cathode voltage can be changed only slowly. A time constant can be dependent, inter alia, on a grid-cathode capacitance and also on a discharge resistor connected in parallel therewith. However, this only permits inaccurate adjustment of the grid potential. In addition, the discharge with a discharge resistor can lead either to long time constants during the discharge, in particular with a large resistance value of the discharge resistor, or to high power losses in the discharge resistor when the pinch-off voltage is applied.The invention is based on the object of improving the use of the grid electrode not only for restricting the electron current, but in particular also for focusing the electron current.As a solution, the invention proposes a method, a circuit arrangement and an X-ray apparatus according to the independent claims.Advantageous further developments are obtained by features of the dependent claims.With respect to a method of the generic type, the invention proposes in particular that the first electrical grid potential is provided by means of an adjustable chip splitter of the focusing unit, the adjustable chip splitter is adjusted by means of a control circuit of the focusing unit by the control circuit being supplied with at least one galvanically isolated control signal of a control unit galvanically isolated from the X-ray tube, wherein the control signal depends on a predefined value for the first electrical grid potential, and an electrical power of the energy converter is adjusted depending on the predefined value for the first electrical grid potential.With respect to a circuit arrangement of the generic type, the invention proposes in particular that the focusing unit has an adjustable chip splitter and a control circuit for controlling the adjustable chip splitter, wherein the control unit is designed to provide at least one galvanically isolated control signal for the control circuit, which control signal depends on a predefined value for the first electrical grid potential, wherein the control circuit is designed to set the adjustable chip splitter depending on the at least one control signal, wherein the control unit is furthermore designed to set the electrical power of the energy converter depending on the predefined value for the first electrical grid potential.With respect to a generic X-ray apparatus, it is proposed in particular that the X-ray apparatus has a circuit arrangement according to the invention.The invention is based, inter alia, on the idea of enabling rapid adaptation of the electrical grid potential, in particular in the case of settings relating to the focusing operation. In particular, the invention uses the realization that a parasitic electrical capacitance of a connecting cable, by means of which the focusing unit is electrically coupled to the X-ray tube, is recharged or discharged. The focusing unit is intended by its construction to support the active charge reversal or discharge of the grid capacitance or the grid cathode capacitance and also of the capacitance of the connecting cable, so that the time constant can be reduced in the event of a potential change, in particular within the scope of the focusing operation. For this purpose, the focusing unit uses the adjustable voltage divider, with which the desired advantageous effect can be achieved. The adjustable voltage divider makes it possible to improve, in particular to speed up, the charge reversal or discharge of the parasitic capacitances as mentioned above. For example, a time constant in the event of a change from a pinch-off of the electron current to a focusing of the electron current and thus an influence of this potential change on properties of the focal spot can be reduced. In addition, it is possible, in particular with regard to a regulation of the grid cathode voltage or the grid potential, to couple the focusing unit to an electrical potential of the cathode electrode, as a result of which a more precise focusing of the electrode current in the X-ray tube can be achieved. The use of magnetic deflections and the associated disadvantages can be largely avoided. In particular, the overall length required for magnetic deflection can be reduced because the electrons no longer have to fly through a magnetic field. Capacitive focusing can be effected with an existing focusing element in the region of the cathode electrode, for example a Wehnelt cylinder, for which reason the overall length can be shortened. The invention takes into account, for example, the constructions and / or properties mentioned at the beginning.In addition, regulation can also be realized during the blocking operation or the cutting-off of the electron current, in which case an electrical voltage can remain limited in order to limit the voltage stress of components.In order to be able to achieve stable operation in conjunction with the energy converter for the adjustable voltage divider, the electrical power of the energy converter is adjusted by means of the control unit as a function of the predefined value for the first electrical grid potential. Preferably, it can be provided that the pinch-off potential or the pinch-off voltage is provided directly by the energy converter. In contrast, the focusing potential or the focusing voltage can be provided by means of the focusing unit, which can use an adjustable voltage divider for this purpose. For this purpose, the focusing unit can be supplied with electrical energy by the energy converter. Furthermore, it is preferably provided that the energy converter in focusing operation does not regulate to the grid potential or the grid voltage in order to avoid positive feedback. The energy converter can be adjusted with respect to the power to be provided as a function of a grid potential currently to be provided, so that reliable operation of the focusing unit in the respective operating state can be reliably ensured. As a result, for example, a power loss of the focusing unit, in particular of the voltage divider, can be kept low. The adjusting of the power can also at least partially comprise adjusting an electrical voltage provided by the energy converter. The energy converter therefore only needs to provide sufficient electrical power that the focusing unit can also reliably set the grid potential to be set. This may include a reserve for adjustment.The energy converter is preferably an electrical energy converter which provides an energy coupling between at least two electrical networks. The energy converter, sometimes also called energy converter, can be designed to couple the electrical networks galvanically separately. The energy converter serves to convert electrical energy of a first form into electrical energy of at least a second form. The energy converter can be designed to realize an energy conversion only unidirectionally. However, it can also be designed to realize an energy conversion at least partially or temporarily bidirectionally.In the present case, the focusing unit is supplied with electrical energy in a galvanically isolated manner by the energy converter. As a result, the focusing unit electrically coupled to the grid electrode and the cathode electrode can be supplied with electrical energy for the intended operation in a potential-free manner by means of the energy converter. The focusing unit can thus be part of a first electrical network. An energy source providing the required electrical energy can thus be part of a second electrical network. The first and the second electrical grid can be coupled by means of the energy converter. This makes it possible for a first electrical reference potential of the first electrical network to be different from a second electrical reference potential of the second electrical network. This is advantageous, for example, if the anode electrode is electrically coupled to an electrical ground potential, wherein a significantly lower electrical potential is generally applied to the cathode electrode during normal operation. This makes it possible to achieve a favorable energy supply, in particular for the focusing unit. As a result of the galvanic isolation, the energy converter can electrically connect at least the focusing unit with electrical energy supplies without potential. The power of the energy converter is preferably not directly controlled by the focusing unit. Rather, the power setting of the energy converter is preferably effected by the control unit. In this case, the control unit can take into account, in particular, the power currently required for the intended operation of the focusing unit and set the power of the energy converter as a function thereof. In particular, the power of the energy converter can be adjusted directly by the control unit.For the reason, this applies, of course, preferably equally to a possible separate heating energy source which can serve to supply a heater of the cathode electrode with electrical energy. The heating energy source is therefore preferably also formed to be floating. For this purpose, the heating energy source can have an electrical heating energy converter which is formed such that it is galvanically isolated. The energy converter or converters can, for example, draw electrical energy from a public energy supply network or an electrical energy store. In the present case, "free of potential" means in particular that there is no need for an electrical connection to other electrical potentials of the circuit arrangement.The control signals may be transmitted galvanically separately using isolation transformers, isolation transformers, optocouplers and / or the like. As a result, a signal-related coupling between a signal source, for example a control unit, and a signal sink, for example the focusing unit, can be realized in a potential-free manner.Electrical potentials of the X-ray tube are preferably electrically separated from electrical potentials of the control unit in order to realize the galvanic separation.The adjustable voltage regulator can have, for example, a series circuit composed of an electrical resistor and an electronic component which can be adjusted with respect to its electrical conductivity, wherein the grid electrode of the X-ray tube is electrically coupled to a central terminal of the series circuit. The electrical resistor can basically also be supplemented or replaced by a constant current circuit. The adjustable voltage regulator can basically have, for example, at least one adjustable resistive element, in particular the aforementioned electronic component, for example a transistor that is operated in linear operation, or the like. This makes it possible to be able to provide the desired grid-cathode voltage or the desired electrical grid potential for focusing the electron current using the electrical energy provided by the energy converter. It has proven to be particularly advantageous if the focusing of the electron current is regulated by means of the focusing unit, in particular the control circuit. As a result, even in the case of varying operating conditions, a substantially constant setting for generating the X-ray radiation can be achieved.For this purpose, the control circuit can have a corresponding regulating circuit which is coupled to a suitable measurement sensor. The measurement sensor can, for example, detect the emitted x-ray radiation and provide a suitable sensor signal for the control circuit or the circuit arrangement. The control circuit or the circuit arrangement can evaluate this sensor signal and can set the electrical grid potential as a function thereof. In this case, the control signal can be, for example, a setpoint value for the first electrical grid potential. A high reliability can be achieved by the series connection, because the desired function can be realized with only a few electrical or electronic components. The focusing unit, in particular its control circuit, can be communicatively or signal-wise coupled to the control unit and receive the at least one control signal therefrom.The value for the first predefined grid potential can be provided by a superordinate controller of the X-ray apparatus. This value can be dependent on an object to be examined, which is exposed to the x-ray radiation released by the x-ray tube.The focusing unit preferably has a series resistor or shunt for electrical coupling to the energy converter. The series resistor can be the aforementioned electrical resistor of the adjustable voltage regulator, which is connected in series with the transistor of the focusing unit, for example. The series resistor can make it possible to bring the focusing unit into a prescribable defined operating state, so that, with high reliability, precise regulation of the electrical grid potential of the grid electrode can be achieved.The focusing unit thus comprises at least the adjustable voltage divider and the control circuit for this purpose which adjusts the adjustable voltage divider. As a rule, the focusing unit is at least partially galvanically coupled to electrical potentials of the X-ray tube. This applies in particular to the grid electrode to which it is electrically coupled in order to be able to set the desired electrical grid potential.The control circuit is preferably an electronic hardware circuit, which can also have a program-controlled computer unit at least partially. The control circuit provides the desired functionality in order to set the adjustable voltage divider in such a way that at least the first electrical grid potential can be set, preferably in accordance with the value transmitted with the control signal for the first electrical grid potential. This setting can particularly preferably also comprise a regulation, which can have, for example, a detection of the electrical grid potential. This detected grid potential may be compared with the predetermined value for the first grid electric potential according to the control signal. Depending on the comparison, the adjustable voltage divider can then be adjusted. Basically, however, a closed-loop control of the adjustable voltage divider can also be provided. In this case, the possibility may additionally be provided of implementing a control functionality by detecting the focal spot by means of a suitable sensor and a corresponding predefined comparison value, wherein the control signal is then determined as a function of this comparison. Further configurations and combinations are conceivable.The electrical power of the energy converter is set as a function of the predefined value for the first electrical grid potential. The required electrical energy can thus be provided for the focusing unit, so that it can realize the required setting at the adjustable voltage divider and can in particular ensure that the electrical grid potential at the grid electrode substantially corresponds to the predefined value for the first electrical grid potential. For this purpose, it can be provided that the value for the first electrical grid potential is evaluated by the control unit and a corresponding energy converter signal is provided, which can be used for adjusting the power of the energy converter to be converted. The energy converter is correspondingly designed so that it adjusts the converted power depending on the energy converter signal.Overall, the invention makes it possible to significantly improve the function of the circuit arrangement and consequently also the function of the X-ray apparatus, specifically in relation to focusing the electron current using electric fields effected by means of the at least one grid electrode. The complicated use of magnetic fields can thus be significantly reduced, if not even completely avoided.It is further proposed that an electrical power of the energy converter is additionally selected depending on at least one electrical power, an electrical voltage or an electrical current of the focusing unit, which the focusing unit requires for providing the first electrical grid potential. As a result, a power supply of the focusing unit according to requirements can be achieved. It has proven to be particularly advantageous if the electrical power of the energy converter is selected depending on the electrical current of the focusing unit. In this way, in particular an oversupply and / or undersupply of electrical energy to the focusing unit can be largely avoided. In particular, the use of the adjustable voltage divider can, in the case of mutually independent regulations for the adjustable voltage divider and the energy converter, lead to an upper voltage limit for the provision of the grid potential being reached, for example, on account of positive feedback, or a large power loss can occur, in particular in the region of the adjustable voltage divider. Conversely, if the output voltage for the electrical potential of the grid electrode is too low, the grid potential can then no longer reach the predefined value. This refinement makes it possible to reduce this problem even better. Specifically, the current energy requirement or the current power can be determined from the variables on the focusing unit side, so that the energy converter can be controlled accordingly. This makes it possible to achieve good reliable coordination between the energy supply of the focusing unit and the energy requirement for the intended operation. The corresponding power requirement of the focusing unit can be determined on the basis of data tables, measured values, the at least one predefined value of the first electrical grid potential and / or the like.According to a further development, it is proposed that the electrical power of the energy converter is determined using a characteristic diagram. The energy converter is therefore not operated-as is customary in the prior art-in such a way that it provides an adjustably constant electrical voltage on the output side, but rather in such a way that, depending on the operation of the focusing unit, a setpoint value for the electrical voltage provided in particular by the energy converter is taken, which setpoint value can ensure reliable operation of the focusing unit, taking account of a control margin of the focusing unit. This setpoint value can be taken from the characteristic diagram, for example a characteristic curve or the like. The characteristic diagram can be present, for example, in the form of a file in which, depending on a respective discrete operating state of the focusing unit, required operating values of the energy converter, for example the output voltage provided by the energy converter or the like, are stored in a manner assigned thereto. Overall, the method procedure according to the invention can thereby be further improved.In addition, it is proposed that the electrical power of the energy converter is also determined as a function of an adjustment reserve predetermined for the adjustable voltage divider. The adjustment reserve serves to specify an excess value of the electrical power or electrical energy which is to be provided in order to be able to adjust the adjustable voltage divider with high dynamics as required without leaving the intended operating state of the adjustable voltage divider. This further development takes into account that the adjustable voltage divider can be adjusted at a very high speed compared to the locations of the energy converter. It is thereby possible to substantially decouple the time constant with respect to the setting of the adjustable voltage divider from the time constant of the power setting of the energy converter. The adjustment reserve can be specified as a percentage supplemental value or also as a tolerance band with respect to a specified value.It is furthermore proposed that the energy converter be operated in an operating mode in which the energy converter provides an adjustably constant electric current on the focusing unit side. As a result, the positive feedback effect explained at the beginning can be suppressed in an improved manner. The operating mode may correspond to a current source mode, in which the supplied current is set by the control unit. This allows a good decoupling from the function of the control circuit by control technology.It is furthermore proposed that the energy converter has a voltage converter coupled to an electrical energy source and a galvanically isolating resonant converter, wherein the resonant converter is electrically coupled on the input side to the voltage converter and on the output side to at least the focusing unit, wherein an input current of the resonant converter is set depending on the predefined value for the first electrical grid potential. This measure can further improve the focusing and / or narrowing of the electron current. The voltage converter can be designed as a DC / DC converter. Depending on requirements, the voltage converter can be designed, for example, as a step-up converter (booster) or else as a step-down converter (buck).Combinations thereof are of course also conceivable, for example in order to be able to realize a wide input voltage range by the voltage converter. Moreover, rectification may possibly also be additionally provided on the input side in order to enable energy supply from an AC voltage source, such as a public energy supply network. The control or regulation on the energy converter side is thus no longer voltage-based, but is advantageously at least partially current-based. For the purpose of control or regulation, it can be provided that the input current of the resonant converter is detected by means of a suitable current sensor, which provides a corresponding current sensor signal to the control unit. The control unit can evaluate this current sensor signal and control the energy converter accordingly, namely in particular the voltage converter. The resonant converter can basically also be formed by another galvanically isolating energy converter.The galvanically isolating energy converter is supplied with electrical energy by the voltage converter in that the voltage converter provides a direct voltage for this purpose. Both the voltage converter and the galvanically isolating energy converter are preferably controlled by the control unit. For example, the direct voltage provided by the voltage converter can be adjusted, in particular regulated, by means of the control unit. For this purpose, the DC voltage provided can be detected by means of a voltage sensor and a corresponding voltage signal can be transmitted to the control unit. The galvanically isolating energy converter is preferably designed as a galvanically isolating resonant converter. If the galvanically isolating energy converter is formed by a resonant converter, at least a part of a resonant inductance can be provided for this purpose by a transformer which is formed as an isolating transformer. A resonant circuit is connected, for example, to at least one half-bridge circuit, by means of which the intended resonant operation can be achieved. The functions of the voltage converter and of the galvanically isolating energy converter, in particular of the resonant converter, are known to the person skilled in the art, for which reason further detailed explanations are omitted here.By combining the voltage converter with the resonant converter, many different operating modes are possible, which allow the function of the circuit arrangement to be adapted as required for a respective specific operating situation. It is thus possible, for example, to control a respective one of the converters not only with respect to the voltage provided but also with respect to the current provided. This can be achieved, for example, with the aid of the control unit.In addition, it is proposed that a minimum current value and a maximum current value are predefined for the input current, the input current is detected and compared with at least the minimum or the maximum current value, and the electrical voltage provided by the voltage converter for the resonant converter is set as a function of the comparison. This further development makes it possible to implement tolerance band regulation. The deviation of the minimum current value or the maximum current value from a predefined average value can be selected to be the same for the minimum current value and the maximum current value. Of course, the invention is not limited to this and different differences from the mean value may be provided. This embodiment also makes it possible to implement tolerance band control. If too low a supply voltage is provided for the focusing unit, in particular for the adjustable voltage divider, the current through the voltage divider could become zero. Thus, the input current of the resonant converter would also be zero. However, the cascaded regulation can now be implemented in such a way that the electrical voltage provided by the voltage converter is increased. It is therefore no longer regulated to the electrical voltage provided by the voltage converter, but rather to the electrical current. If, on the other hand, the electric voltage and thus also the input current are increased under the effect of the positive feedback described above, the maximum input current would be correspondingly limited by the cascaded regulation. Overall, this allows reliable regulation or control to be achieved with little effort.It is further proposed that a frequency of the control signal depends on the predefined value for the first electrical grid potential and the control circuit determines the predefined value for the first electrical grid potential from the frequency of the control signal. As a result of this configuration, the value for the first electrical grid potential can be transmitted in a simple manner from the control unit to the control circuit, so that the control circuit can correspondingly set the adjustable voltage divider. The control signal in this case is preferably an AC electrical signal, the frequency of which can be adjusted by the control unit. Because the control signal is an AC voltage signal, it can be transmitted to the control circuit or the focusing unit in a galvanically isolated manner by means of a galvanically isolating transformer or transformer. As a result, it is possible in a simple manner to be able to set the electrical grid potential in accordance with the predefined value for the first electrical grid potential.In addition, it is proposed that the focusing unit is deactivated at a predefined, in particular a predefined minimum or maximum, frequency. A deactivation of the focusing unit comprises in particular at least the state that the grid potential is so negative with respect to an electrical potential of the cathode electrode that the electron current is constricted. In this operating state of the X-ray tube, substantially no X-ray radiation is emitted. For this purpose, it can be provided that in addition to the focusing unit, a switching unit is provided, which, like the focusing unit, is supplied with electrical energy by the energy converter. For example, it can be provided that the switching unit and the focusing unit are connected in series in order to be able to realize the desired functionality. For example, it can also be provided that at a frequency of the control signal which is less than the predefined minimum frequency, the switching unit enables the focusing unit to be deactivated and the grid electrode is supplied with an electrical grid potential as explained above. Basically, somewhat comparable dually can also be provided for a maximum frequency. A control characteristic can be provided inverted for this purpose, for example. Of course, this grid potential can also be realized by the interaction of the focusing unit with the switching unit. The switching unit can also be at least partially comprised by the focusing unit. Overall, the operation of the X-ray tube can be further improved.According to a development, it is proposed that the focusing unit is controlled at a predefined, in particular a predefined minimum or maximum, frequency in such a way that both the switching unit and the transistor are operated in the switched-on switching state in order to provide a grid short circuit. As a result, a third operating state can be achieved, in which the grid electrode can be short-circuited. As a result, the cutting off of the electron current and also the focusing of the electron current can be deactivated. For this purpose, the transistor is preferably operated in a switching operation which deviates from a linear operation.It is furthermore proposed that an output current of the voltage converter is set as a function of the predefined value for the first electrical grid potential. This further development uses the operation of the energy converter using a current source characteristic. This can be set separately, in particular for the focusing operation. Outside the focusing operation, on the other hand, the usual control of the energy converter can take place, in particular by providing a predefined electrical voltage. In addition, it is of course possible to change a clock frequency of the resonant converter in order to operate the resonant converter in the corresponding resonance, in particular if it is an LLCC resonant converter. At this resonant frequency, an output current is generally load-independent, in particular essentially determined only by an resonant circuit inductance and the input voltage of the resonant converter. As a result, both the voltage converter and the resonant converter can be operated in a controlled manner, because as a result of the current source characteristic, a suitable, sufficiently large output voltage for the focusing unit is generally always established. As a result, it is also possible to achieve a sufficient control reserve for the adjustable voltage divider up to a maximum load.It is furthermore proposed that the resonant converter has a full bridge circuit having two half-bridge circuits, wherein during the focusing operation one of the two half-bridge circuits is activated at least temporarily and the other of the two half-bridge circuits is deactivated. Thus, for example, a transformer primary voltage can be approximately halved, which is advantageous in particular with regard to the significantly lower focusing voltage. A deactivation of a half-bridge circuit means, in particular, that this half-bridge circuit is not actively involved in the energy conversion. Preferably, it is completely turned off. However, it can be provided that the deactivated half-bridge circuit provides, for example, a current path for a free-wheeling current or the like, without intervening here with clocks, however. This means that switching elements of this deactivated half-bridge circuit are not supplied with corresponding switching signals.The half-bridge circuits each have two switching elements connected in series, in particular semiconductor switches, by means of which the electrical direct voltage provided by the voltage converter can be converted into an electrical alternating voltage. Such a circuit topology is also called a full bridge circuit. The half bridge circuits are connected in parallel at the ends and are operated in anti-phase.A semiconductor switch in the sense of this disclosure is a preferably controllable electronic switching element, for example a transistor, a thyristor, combination circuits thereof, in particular with parallel-connected free-wheeling diodes, for example a metal oxide semiconductor field effect transistor (MOSFET), an isolated gate bipolar transistor (IGBT), preferably with integrated free-wheeling diodes, or the like.The center terminal of a half bridge circuit is a terminal electrically conductively connected to the connection point of the series-connected semiconductor switches. In inverter operation, the converted alternating voltage is usually provided at this central connection.The switching operation of the semiconductor switch in the form of a transistor means that, in a switched-on state, a very low electrical resistance is provided between the terminals forming the switching section, with the result that a high current flow is possible with a very low residual voltage. In the switched-off state, the switching path of the semiconductor switch is high-ohmic, that is to say it provides a high electrical resistance, with the result that, even when there is a high voltage present at the switching path, substantially no current flow or only a very low current flow, in particular a negligible current flow, is present. This differs in linear operation.It is furthermore proposed that the circuit arrangement has a switching unit which is designed to apply the first electric grid potential focusing the electron current to the at least one grid electrode in a first switching state and a second electric grid potential for restricting the electron current between the anode electrode and the cathode electrode in a second switching state. This development is based, inter alia, on the idea that it is possible, by a suitable combination of the switching unit with the focusing unit, to provide the possibility of rapidly switching the grid-cathode voltage or the electrical grid potential of the grid electrode from a pinch-off voltage or a pinch-off potential to a predefinable focusing voltage or a predefinable focusing potential and / or vice versa. In this case, the focusing unit can be used additionally to recharge or discharge the parasitic electrical capacitance of the connecting cable and / or grid electrode. By actively recharging the grid capacitance or grid cathode capacitance and the capacitance of the connecting cable by the switching unit and the focusing unit, a time constant in the event of a change of pinch-offs of the electron current to focus the electron current or vice versa can be reduced and thus an influence of the switching change on properties of the focal spot can be reduced. In addition, it is possible, in particular with regard to a regulation of the grid cathode voltage or the grid potential, to couple the focusing unit to an electrical potential of the cathode electrode, as a result of which a more precise focusing of the electron current in the X-ray tube can be achieved. Moreover, this further development makes it possible to integrate the circuit arrangement in an X-ray apparatus in a simple manner. Installation space and costs can be saved by the circuit arrangement according to the invention.The switching unit could basically comprise one or more suitable electromechanical switching elements in order to realize the desired switching function. As a rule, however, for example for reasons of switching speed, the switching unit has, inter alia, one or more electronic switching elements, in particular semiconductor switching elements, by means of which the desired switching function of the switching unit can be realized. The switching elements may be formed by transistors, thyristors, combinations thereof and / or the like, for example. For the intended application, it can be provided particularly advantageously that a plurality of electronic switching elements are operated essentially synchronously connected in series. As a result, it is also possible to achieve operation with an operating voltage that is significantly greater than the maximum permissible operating voltage of a respective switching element using electronic switching elements which are configured only for a portion of the voltage that occurs. The switching unit provides at least one first switching state in which the grid electrode is charged with the first electrical grid potential releasing the electron current, namely preferably the grid potential which is provided by the focusing unit. In a second switching state of the switching unit, a second electric grid potential can be applied to the at least one grid electrode in order to pinch off the electron current between the anode electrode and the cathode electrode. For this purpose, the switching unit can be electrically coupled to the energy converter, wherein the switching unit couples the energy converter to the x-ray tube in such a way that the energy converter provides at least the pinch-off voltage between the grid electrode and the cathode electrode. This can be achieved by the series connection of the switching unit to the focusing unit.Because the switching unit and the focusing unit are preferably connected in series, at least the second electrical grid potential can be provided by the focusing unit. As a result, the focusing unit can support a respective switching change of the switching unit, as a result of which the functionality can be realized more reliably.For focusing, a grid cathode voltage may be provided in a range from about zero to about 500 V. This voltage can likewise be provided by the energy converter functioning as an operating voltage source. For this purpose, the focusing unit can adjust the voltage provided by the energy converter accordingly, for example.The control circuit is preferably connected to the at least one switching element, in particular to the at least one semiconductor switching element, of the switching unit. The switching unit does not need to have its own communication interface, by means of which it is in communication connection with the control unit. As a result, a switching change of the switching unit can also be controlled by means of the control circuit.The control unit can assume or provide further functions, in particular with regard to the focusing voltage, the pinch-off voltage, the provision of an operating voltage by the energy converter and / or the like. The control unit can be designed to be electrically insulated from the circuit arrangement and is preferably connected to the latter in a galvanically isolated manner. The control unit itself can be provided as a separate structural unit. Preferably, however, it is a component of the circuit arrangement and particularly preferably arranged integrated therein.It is furthermore proposed that at least two of the aforementioned three operating states are provided at least partially statically. This enables a particularly rapid change between two successive operating states. This development proves to be particularly advantageous if pinch-off is to be changed between the focus operating state and the pinch-off operating state.The problems occurring during the change in the prior art with respect to an undefined focal spot of the electron current can be reduced to a large extent, if not even completely avoided.The advantages and effects indicated for the method according to the invention apply equally to the circuit arrangement according to the invention and to the X-ray apparatus equipped with the circuit arrangement according to the invention and vice versa. Features formulated in terms of the method can thus also be formulated in terms of the device and vice versa.The exemplary embodiments explained below are preferred embodiments of the invention. The features, feature combinations specified above in the description and also the features and feature combinations specified in the following description of exemplary embodiments and / or shown alone in the figures can be used not only in the respectively specified combination but also in other combinations. Therefore, embodiments are also encompassed by the invention or are to be regarded as disclosed which are not explicitly shown and explained in the figures, but which emerge from the explained embodiments and can be generated by separate combinations of features. The features, functions and / or effects illustrated on the basis of the exemplary embodiments can each represent, per se, individual features, functions and / or effects of the invention, which are to be considered independently of one another and which also develop the invention in each case independently of one another. Therefore, the embodiments are intended to include combinations other than those in the illustrated embodiments. In addition, the described embodiments can also be supplemented by further features, functions and / or effects of the invention already described.The following are shown: FIG. 1 shows a schematic circuit diagram illustration of an X-ray device with an X-ray tube connected to a circuit arrangement, wherein a section of a grid control of the X-ray tube is illustrated by the circuit arrangement; FIG. 2 shows a schematic block diagram illustration of the circuit arrangement including the section according to FIG. 1 ; FIG. 3 shows a schematic block diagram illustration of the circuit arrangement according to FIG. 2 in a first operating state; FIG. 4 shows a schematic block diagram illustration of the circuit arrangement according to FIG. 2 in a first operating state; FIG. 5 shows a schematic diagram illustration of electrical voltages of the circuit arrangement in the first operating state; FIG. 6 shows a schematic diagram illustration of a supply current provided by an energy converter of the circuit arrangement according to FIG. 2 in the first operating state; FIG. 7 shows a schematic diagram illustration of the electrical voltages according to FIG. 5 in the first operating state; FIG. 8 shows a schematic diagram illustration of the supply current according to FIG. 6 in the first operating state; FIG. 9 shows a schematic diagram illustration of the electrical voltages according to FIG. 4 in the first operating state; and FIG. 10 shows a schematic diagram illustration of the supply current according to FIG. 6 in the first operating state.FIG. 1 is a schematic circuit diagram of a portion of an x-ray apparatus 10 having an x-ray tube 12 that includes an anode electrode 14 and a cathode electrode 16 disposed in an evacuated vessel. Between the anode electrode 14 and the cathode electrode 16, a grid electrode 18 is disposed. The anode electrode 14 is electrically connected to one terminal 52, the grid electrode to one terminal 50, and the cathode electrode 16 to two terminals 46, 48. For heating purposes, the cathode electrode 16 has two connections, namely the connections 46 and 48, via which the cathode electrode 16 can be electrically supplied with energy in order to heat the cathode electrode 16 to a predeterminable temperature during normal operation, so that the desired electron emission can be achieved. For this purpose, the terminals 46, 48 are electrically connected to a heating electric power source 54.The terminals 48, 52 are further electrically connected to a voltage source 56 that provides an anode-cathode voltage 72 that is also substantially present between the cathode electrode 16 and the anode electrode 14. An anode potential of the anode electrode 14 is typically greater than a cathode potential of the cathode electrode 16.Depending on an electrical grid potential at the grid electrode 18, electrons emerging from a cathode material of the cathode electrode 16 are accelerated toward the anode electrode 14 forming an electron current 26. When the electrons strike the anode electrode 14, which is generally designed as a rotating electrode, X-ray radiation is generated and emitted by the X-ray tube 12.The function of the X-ray tube 12 can be influenced by the grid potential at the grid electrode 18. Thus, on the one hand, it is possible to apply a second electric grid potential to the grid electrode 18, with which a pinch-off of the electron current 26 between the anode electrode 14 and the cathode electrode 16 can be achieved. The second electrical grid potential is also referred to as pinch-off potential. Accordingly, a grid-cathode voltage results, which is accordingly referred to as pinch-off voltage. In X-ray tubes, the pinch-off voltage can be, for example, in a range from about zero to about 4 kV. In the present embodiment, the pinch-off voltage is more than about 500 V, for example, about 3.5 kV or even more. A transition region between focusing and pinch-off is undesirable because it can result in an undefined focal spot and electron current in the x-ray tube 12.As a rule, the grid potential is negative with respect to the cathode potential of the cathode electrode 16, at least for the pinch-off of the electron current 26.The second electrical grid potential is generally selected such that a reliable safe, reliable cutting-off of the electron current 26 can be achieved without damaging an electrical insulation in the X-ray apparatus 10. In many cases, the maximum permissible grid-cathode voltage is about 4 kV, which is why the x-ray apparatus 10 with its components for this voltage is designed accordingly.During the pinch-off of the electron current 26, substantially no X-ray radiation is generated because the electron current 26 is substantially suppressed.In addition, the grid electrode 18 can be supplied with a first electrical grid potential, which allows the electron current 26 to be released, in particular focused. A corresponding grid-cathode voltage is also referred to as a focusing voltage. With the focusing voltage, it is possible not only to release the electron current 26, preferably controlled, but at the same time also to control the focusing of the electron current 26 with respect to the impingement on the anode electrode 14. As a result, a focal spot 58 can be reached on the anode electrode 14, for example, in a predeterminable manner. As a result, the generation of X-ray radiation can be influenced over a wide range.A first terminal on a connecting line 20 is connected to the electrical terminals 46, 48, 50. An opposite terminal of the connecting line 20 is connected to electrical terminals 60, 62, 64. The connecting line 20 comprises in particular the high-voltage cable of the line capacitance 66 influencing the grid voltage. The electrical connections 46, 48, 50, 52 are in the present case the connections on the tube side. The electrical connections 60, 62, 64 are in the present case the connections on the generator side.The heating energy source 54 is connected to the electrical connections 60, 62. A circuit arrangement 22 is connected to the electrical connections 62, 64, by means of which circuit arrangement the electrical grid potential for the grid electrode 18 can be provided in a predeterminable manner. In FIG. 1, only a section of the circuit arrangement 22 is shown. A schematic block diagram representation of the circuit arrangement 22 is evident from FIGS. 2 to 4 explained in the following.Furthermore, it can be seen from FIG. 1 that the connecting line 20 has a line capacitance which is symbolically represented in FIG. 1 by a capacitor 66. The capacitor 66 further comprises a grid-cathode capacitance of the x-ray tube 12, which is not shown in any further detail in FIG. 1. The capacitance 66 depends on, among other things, a length of the cable and may have a capacitance value of about 4 nF, for example. For controlling the x-ray tube with respect to the pinch-off of the electron current 26 and the focusing of the electron current 26 only via the grid electrode 18, this is relevant, as will be explained in the following.For focusing, a grid-cathode voltage of approximately zero to approximately 500 V is required in the present case. Depending on the construction of the X-ray tube 12, this voltage can also be different, as can the pinch-off voltage.For providing the grid potential, the circuit arrangement 22 has an energy supply, which is explained in greater detail below and is only schematically denoted by 38 in FIG. 1. The energy supply 38 has an internal resistance 68, via which elements and assemblies of the circuit arrangement 22 are supplied with electrical energy for the intended operation.The circuit arrangement 22 further comprises a focusing unit 24 which is connected in series with a switching unit 28. This series circuit comprising the focusing unit 24 and the switching unit 28 is connected via the internal resistance 68 to the energy supply 38 and is supplied with an operating voltage by the latter.The switching unit 28 provides two switching states in the present case, namely a switched-on switching state as the first switching state and a switched-off switching state as the second switching state. The switched-on switching state corresponds to the function of focusing and is corresponding to the first operating state of the above description. In the switched-on switching state, the operating voltage is essentially present at the focusing unit 24. As will be explained in the following, the focusing unit 24 provides a grid-cathode voltage which allows the electron current 26 to be focused in a predeterminable manner.In the second switching state of the switching unit 28, in which the switching unit 28 is in the switched-off switching state, the focusing unit 24 is substantially deactivated, such that, for example, the operating voltage of the power supply 38 is provided between the grid electrode 18 and the cathode electrode 16. It should be noted that in this operating state, at least in a steady state, substantially no electric current flows. If the operating voltage is therefore approximately 3.5 kV, this operating voltage is also present between the grid electrode 18 and the cathode electrode 16 in the switched-off switching state of the switching unit 28. This voltage is negative with respect to the cathode electrode 16 in the present case so that the grid potential is smaller than the cathode potential. In this switching state, a constriction of the electron current 26 is consequently achieved, so that substantially no electrons any longer reach the anode electrode 14 and thus the generation of X-ray radiation is substantially interrupted.In the first switching state of the switching unit 28, namely the switched-on switching state, the focusing unit 24 is supplied with the operating voltage. The focusing unit 24 then provides a corresponding first electrical grid potential so that not only the electron current 26 is released, but also a corresponding predefinable focusing of the electron current 26 can be achieved when it impinges on the anode electrode 14.For this purpose, the focusing unit 24 comprises at least one series circuit of an electrical resistor 30, which can at the same time also serve as a series resistor with respect to connecting the energy supply 38, and a transistor 32, which is formed in the present case by a field effect transistor, namely a normally off n-channel MOSFET. This provides an adjustable voltage divider. Depending on the configuration, however, it is also possible here to use a different transistor, in particular also a bipolar transistor.The transistor 32 has a gate terminal in the present case, which is not designated and which is connected to a control circuit 40, which is schematically indicated in FIG. 1 and which applies a predeterminable electrical gate potential to the gate terminal, such that substantially the electrical grid potential can be provided at a central terminal 34 of this series circuit according to a predeterminable value for a first electrical grid potential. For this purpose, the transistor 32 is operated in a linear mode, so that the respective grid potential can be set at the central terminal 34 depending on the respective setting of the gate potential at the transistor 32. As can be seen from the illustration in FIG. 1, the focusing unit 24 is activated by switching on the switching unit 28 and is deactivated by switching off. In the deactivated operating state of the focusing unit 24, the pinch-off potential consequently corresponds to a predefinable value for a second electrical grid potential.FIG. 2 shows the circuit arrangement 22 now in a schematic block diagram illustration. It can be seen from FIG. 2 that the circuit arrangement 22, in addition to the elements or assemblies already explained with reference to FIG. 1, further comprises the energy converter 38 which serves for the energy supply, in particular of the focusing unit 24. For this purpose, the energy converter 38 is designed as a galvanically separated energy converter in order to be able to supply energy to the focusing unit 24 and the switching unit 28 which are electrically coupled to the grid potential and the cathode potential of the X-ray tube 12. For this purpose, the energy converter 38 is connected to a direct voltage source 80. The DC voltage source 80 can in turn be supplied with electrical energy from a public energy supply network.The energy converter 38 is connected to a control unit 74, which provides corresponding control signals for the intended operation of the energy converter 38. The control unit 74 is further communicatively coupled to a higher-order controller 90, via which operating values, in particular a value for the first electrical grid potential, can be predefined.Furthermore, an auxiliary converter 82 is provided, which is likewise supplied with electrical energy by the DC voltage source 80. The auxiliary converter 82 is likewise connected to the control unit 74 and is also supplied by the latter with corresponding control signals, not designated, for the intended operation. The auxiliary converter 82 serves for galvanically separately providing two control signals 42, 44 which serve for controlling the control circuit 40. One of the control signals 42 serves to supply the control circuit 40 with electrical energy, whereas a second of the control signals, namely the control signal 44, serves to control at least one operating value for the control circuit 40 and the units controlled by the control circuit 40, namely the focusing unit 24 and the switching unit 28. This will be explained in more detail below.The energy converter 38 in the present case has a voltage converter 76 which is electrically connected on the input side to the DC voltage source 80. On the output side, a resonant converter 78 is connected to the voltage converter 76, which provides an electrical energy supply for the focusing unit 24 and the switching unit 28 in a galvanically isolated manner. For this purpose, the resonant converter 38 has an inverter 96, which is formed in the present case from two half-bridge circuits which operate a resonant circuit, the inductance of which is formed at least partially by a primary side of a transformer 98 designed as an isolation transformer. On the secondary side, the transformer 98 is connected to a rectifier 100, which provides a corresponding DC voltage for the focusing unit 24 and the switching unit 28. A voltage sensing unit 84 senses the converted voltage provided by the voltage converter 76 and provides a corresponding voltage signal to the control unit 74. the control unit 74 also provides corresponding control signals to the inverter 96 so that the desired converter operation of the resonant converter 78 can be achieved.The transformer 98 further comprises an auxiliary winding, not shown in detail, which is connected to a transformer detection unit 86, which supplies a corresponding transformer signal to the control unit 74.The auxiliary converter 82 comprises an auxiliary inverter 88 connected to the DC voltage source 80 and an auxiliary transformer 92 connected to the auxiliary inverter 88, the auxiliary transformer 92 likewise being designed as an isolation transformer and being connected with its primary winding to the auxiliary inverter 88. A secondary winding of the auxiliary transformer 92 is connected to a rectifier 94 which provides the control signals 42, 44 to the control circuit 40. As already explained, the control signal 42 serves to supply power to the control circuit 40, whereas the control signal 44 supplies corresponding control values, for example a predefined value for the first electrical grid potential of the grid electrode 18. In the present case, it is provided that the predefined value for the first electrical grid potential is dependent on the frequency of the control signal 44. As long as the frequency of the control signal 44 is greater than a predetermined minimum frequency value, the switching unit 28 is switched in the switched-on state by the control circuit 40. At the same time, the transistor 32 is adjusted with respect to its electrical conductivity by the control circuit 40 depending on the frequency, so that a grid-cathode voltage is provided at the terminals 62, 64 in accordance with the value for the first electrical grid potential. The auxiliary inverter 88 is therefore controlled accordingly by the control unit 74, such that the corresponding control signals 42, 44 can be provided by means of the rectifier 94. Thus, the auxiliary converter 82 not only serves as an energy converter for the control circuit 40 and thus in particular for the focusing unit, which provides a potential-free energy supply, but the auxiliary converter 82 also serves at the same time as a galvanically isolating signal transformer. By way of the type of control of the auxiliary inverter 88 by the control unit 74, for example by applying a suitable modulation, it is thus possible at the same time to realize a signal functionality for transmitting data or signals, as a result of which it is possible to achieve transmission of data or signals in accordance with the control signal 44. As a result, the control unit 74 is electrically isolated from the control circuit 40, in particular from the focusing unit. At the same time, a control signal can be transmitted to the control circuit 40 in a potential-isolated manner. The control signal 42 is consequently a power signal here, which essentially serves to supply power to the control circuit 40.An electrical potential separation 102 is shown in FIG. 2, which is realized by the transformers 92, 98. As a result, the control unit 74 is electrically isolated from the X-ray tube 12, among other things.The resonant converter 78 is embodied here as an LLCC resonant converter. In alternative embodiments, a different resonant converter type or a galvanically isolating energy converter can naturally also be provided here. The invention is not limited to this.FIG. 2 shows the circuit arrangement 22 including the section according to FIG. 1. the control circuit 40 receives a setpoint value for the grid potential and a switching state for the switching unit 28 from the control unit 74. Both realize a corresponding control functionality. The control unit 74 controls or regulates the energy converter 38, wherein the control unit 74 receives corresponding control commands and data from a higher-order controller 90. A communication connection between the superordinate controller 90 and the control unit 74 is in the present case designed as a unidirectional communication connection. However, the invention need not be limited thereto. Rather, the communication connection can also be configured bidirectionally.The control circuit 40 takes over the functionality of adjusting the adjustable voltage divider 36 and controls the transistor 32 accordingly. The corresponding set values and switching states are transmitted from the control unit 74 via the auxiliary converter 82 to the control circuit 40. In this case, it is possible in principle to distinguish between two operating states, namely in the present case a first operating state in which the switching unit 28 is in the switched-on switching state and therefore the circuit arrangement 22 is in a focusing operation in which the grid potential of the grid electrode 18 is set by means of the focusing unit 24 in accordance with the predefined value for the first electrical grid potential which has been transmitted as a setpoint value from the control unit 74 to the control circuit 40. The control circuit 40 provides a control functionality in this regard and correspondingly adjusts the grid potential of the grid electrode 18. At the same time, in the present embodiment, the grid potential also serves as an electrical reference potential of the focusing unit. In alternative configurations, this can vary and, for example, the cathode potential can also be selected as the electrical reference potential. However, the function of the invention is independent of this.In a second operating state, also referred to as grid locks, the switching unit 28 is switched by the control circuit 40 into the switched-off switching state, so that the grid electrode 18 is supplied with the pinch-off potential. These operating states are set by the control circuit 40 by means of the control signal 44.In the second operating state, as is also illustrated with reference to FIG. 3, the focusing unit 24 is deactivated and the switching unit 28 is in the switched-off switching state. There is no influence on the grid potential on the high-voltage side. In this operating state, the grid cathode voltage is dependent on the voltage provided by the energy converter 38, which is dependent on the output voltage of the voltage converter 76. The voltage converter 76 can be operated in an uncontrolled manner in this operating state. A regulation can, on the other hand, take place taking into account the voltage provided by the auxiliary winding of the transformer 98 via the transformer detection unit 86. This variable may provide an actual variable that is compared to a manipulated variable provided by the output voltage of the voltage converter 76.In this operating state, resonant converter 78 is operated at a fixed frequency at the LLCC operating point with a voltage-proof output. The actual value for the regulation of the electric grid potential in this operating state is detected by measuring the voltage across an auxiliary winding of the transformer 98 on the primary side. It is thus possible to regulate the grid-cathode voltage on the primary side without direct electrical coupling to the high-voltage side.In the focusing mode or in the first operating state, the situation differs in that the adjustable voltage divider 36 loads the transformer 98 on the secondary side. As already explained, the electric grid potential is regulated by means of the adjustable voltage divider 36 in that the current through the adjustable voltage divider 36 is changed by the transistor 32 by changing its electric conductivity. In the event of an increase in current, the voltage drop across the electrical resistor 30 and across the internal resistance 68 increases. The greater this voltage drop, the lower the grid-cathode voltage at the output of the adjustable voltage divider or at the central connection 34. This is compensated for using the control unit 74, where the voltage provided by the energy converter 38 rises again. The control circuit 40 will respond by appropriately controlling the transistor 32 to adjust the boosted voltage. This results in an undesired positive feedback which can result not only in a high power loss, in particular in the adjustable voltage divider 36, but also in an overload until a component fails. This can also occur in the case of a reverse control situation.This behavior is schematically shown by the schematic diagrams according to FIGS. 5 and 6. FIG. 5 shows a schematic diagram illustration of electrical voltages of the circuit arrangement 22 in this first operating state, whereas FIG. 6 shows a corresponding schematic diagram illustration of a supply section of the section of the circuit arrangement 22 illustrated in FIG. 1 in the first operating state. In FIG. 5, the ordinate is assigned to the electric voltage and the abscissa is assigned to the time. In FIG. 6, the ordinate is assigned to the electric current and the abscissa is assigned to the time. The time axes of FIGS. 5 and 6 correspond to one another. Figures 5 and 6 are in association.FIG. 5 shows voltage characteristics, whereas FIG. 6 shows corresponding associated current characteristics. The invention is not used here. The output voltage of the transformer 98 is regulated (graph 106), namely at the element with the reference number 68 in FIG. 1, which in the present case is the leakage inductance with a wire resistance of the transformer 98 (FIG. 2 ). A target value of the lattice voltage is decreased. As a result, the control unit of FIG. 1 increases the current through transistor 32 of FIG. 1 to increase the voltage drop across element 68. As a result, the input current increases (FIG. 6 ). As a result, the input voltage of the transformer 98 rises (corresponds to transformed voltage or reference sign 38 in FIG. 1 ), resulting in positive feedback.As can be seen from FIGS. 5 and 6, an operating state in which the grid cathode voltage is approximately 250 V, which is illustrated by a graph 108 in FIG. 5, is set in a period of approximately 3 ms to approximately 4 ms. The voltage of the energy converter 38 provided upstream of the internal resistance 68 is here approximately 500 V, which is illustrated by a graph 104. The electrical voltage across the focusing unit 24 in series with the switching unit 28, which is in the switched-on state in the present case, is approximately 300 V, which is illustrated by a graph 106. In the diagram according to FIG. 6, an electric current of approximately 40 mA is provided for this period of time, which flows through the adjustable voltage divider 36. This is illustrated by a graph 110.At the time t=4 ms, the control signal 44 is used to change the setpoint value for the grid cathode voltage or the first electrical grid potential, to be precise to a grid cathode voltage of approximately 150 V, as can be seen on the basis of the graph 108 in FIG. 5. On account of the previously described positive feedback, the control unit 74 will now control the energy converter 38 in such a way that the voltage provided by it compensates for the higher load, so that a largely constant voltage is provided according to the graph 106. For this purpose, the voltage provided by the energy converter 38 is increased accordingly, to be precise to a value of approximately 1000 V. From a time of approximately t=6.5 ms, the value for the first electrical grid potential is then reset again to the value indicated before the time t=4 ms. Accordingly, the electrical voltages change according to the graphs 104, 108. It can be seen from FIG. 6 that an electric current of approximately 150 mA is present in the range from t=4 ms to t=6.5 ms. In the aforementioned period of time, this leads to a considerable power loss, which not only has to be provided by the energy converter 38 but at the same time also has to be dissipated on the high voltage side, in particular by the adjustable voltage divider 36. This can also lead to defects or faults. A scenario which reduces or avoids these problems is described below.Furthermore, a third operating state can be provided, in which both the switching unit 28 is in the switched-on switching state and the transistor 32 is operated in a switching operation in the switched-on switching state. This allows the grid electrode to be short-circuited.FIGS. 7 and 8 relate to schematic diagrams like FIGS. 5 and 6, also for the first operating state, namely with application of the invention, in which the voltage provided by the energy converter 38 is permanently set by the voltage converter 76. These two figures also belong together. The graphs again relate to the same variables as already explained above with reference to FIGS. 5 and 6. As can be seen from FIGS. 7 and 8, the voltage provided by the energy converter 38 is now fixedly set here by the control unit 74 to a fixed value of approximately 500 V. At the time t=4 ms, the change state explained with reference to FIGS. 5 and 6 is brought about again. It follows from the representations that the grid cathode voltage is adjusted accordingly. At the same time, the supply voltage for the adjustable voltage divider 36 is also reduced by a slight value. At the time of about t=6.5 ms, the change is reset again. It can be seen from FIG. 8 that the current likewise rises slightly. It can be seen from FIGS. 7 and 8 that, with the use of the invention, stable operating behavior can be realized without the great power loss compared to the use illustrated in FIGS. 5 and 6 in the first operating state.The higher-order control 90 specifies a value for the first electrical grid potential. The control unit 74 accordingly provides an output voltage through the energy converter 38. In addition, the predefined value for the first electrical grid potential is transmitted to the control circuit 40 via the auxiliary converter 82. On the basis of the frequency of the control signal 44, the control circuit 40 recognizes the switching state for the switching unit 28 and switches it into the switched-on switching state. The control circuit 40 recognizes that the frequency is greater than a predefined minimum frequency below which the switching unit 28 is to be switched in the switched-off switching state. In addition, it can be provided that a maximum frequency is specified, during the detection of which by the control circuit 40 is directly coupled to the electrical cathode potential via the switching unit 28 and the transistor 32, as a result of which virtually a short circuit between the grid electrode 18 and the cathode 16 can be achieved. Intermediate values with respect to the frequency can then be used to determine a respective value for the first electrical grid potential by assigning a respective value to a respective frequency.The function of this circuit principle can also be seen from the schematic circuit diagram illustration of the circuit arrangement 22 according to FIG. 3, in which the voltage detection unit 84 is used for the first operating state compared to the first operating state according to FIG. 2. For the second operating state, on the other hand, the unit 86 is used. In an alternative configuration to the second operating state, it can be provided that an output current of the voltage converter 76 can be used in order to simplify a characteristic diagram for open-loop or closed-loop control. In this case, control or regulation can take place on the basis of this output current.In a further alternative to the second operating state, a minimum value and a maximum value for an input current of the resonant converter 78 may be predefined. Based on this, a tolerance band regulation can then be realized. Here, for example, the voltage provided by the energy converter 38 is too small for the adjustable voltage divider 36 to set a predefined value for the first electrical grid potential; the current through the transistor 32 could become zero on account of the control functionality of the control circuit 40. Thus, the input current of the resonant converter 78 could also fall below the minimum value. In this case, it is provided that the control unit 74 carries out the regulation in such a way that the voltage provided by the voltage converter 76 is increased. It is then no longer controlled to the voltage, but instead to the current. If the aforementioned positive feedback were to increase the voltage provided by the energy converter and thus also the current for the adjustable voltage divider 36, the maximum current would be able to be limited in the same way. Overall, it can be achieved that, at least in the second operating state, the input voltage of the transformer 98 remains substantially stable when the invention is used.FIGS. 9 and 10 show schematic diagrams like FIGS. 5 and 6 of the situation for the first operating state according to a further embodiment of the invention, as explained below. The reference numerals of the graphs of FIGS. 9 and 10 correspond to the respective graphs of FIGS. 5 and 6. These two figures also belong together. In this embodiment of the invention, it is provided for the first operating state that, at least in the focusing mode, the voltage provided by the energy converter 38 is based on a current source characteristic. This can be achieved by suitable regulation by means of the control unit 74. In addition, it is possible to increase an operating frequency or clock rate of the resonant converter 78 in order to operate it in the actual resonance, in particular the LLCC resonance. At this frequency, an output current of the resonant converter 78 is substantially independent of a load and is determined only by a resonant circuit inductance and an input voltage of the resonant converter 78. As a result, both a regulation by the controller 74 and the resonant converter itself can be operated in a controlled manner, because a suitable, sufficiently high voltage can always be set at the output of the energy converter 38 on the basis of the current source characteristic. In this case, a sufficient control reserve for the intended operation of the focusing unit 24, in particular of the adjustable voltage divider 36, can be realized. This is shown by the schematic block diagram illustration according to FIG. 4, in which the transformer detection unit 86 does not need to be used for the focusing operation. This makes it possible to avoid the undesired positive feedback.As can be seen from FIGS. 9 and 10, the current according to the graph 110 is substantially at a value of approximately 10 mA almost independently, independently of the performance of the changes with respect to the value for the first electrical grid potential. It is apparent from FIG. 9 that the respective voltages are varied. As compared to FIG. 5, it can be seen that the voltage provided by the energy converter 38 is even smaller in the third operating state in the time period of about 4 ms to about 6.5 ms than in the time period before t=4 milliseconds. This allows proper operation with particularly low power to be achieved. This is advantageous for the construction of the circuit arrangement, in particular of the X-ray apparatus 10, because on the one hand significantly less power is required compared to the first operating state and on the other hand, due to the significantly lower power, the components can be configured more favorably and compactly during intended operation.In order to be able to achieve further advantages with regard to the intended operation, it can be provided that in particular the inverter 96 can be operated dynamically with regard to its operating mode. It can be taken into account here that the pinch-off voltage is generally in a range of several kilovolts. The focusing voltage in the focusing operation, on the other hand, is generally only a few 100 V. Therefore, it may be advantageous to deactivate one of the half bridge circuits of the inverter 96 in the focusing operation. Thus, the voltage transfer ratio of the resonant converter 78 can be reduced accordingly, substantially halved.In an alternative embodiment, it can also be provided that two separate transducers are provided. In the event of a changeover between the provision of the pinch-off voltage and the focusing voltage, it is possible to switch over between outputs of the converters. Short switching times, in particular large edge steepnesses, for example, can thus be realized, for example, when switching from focusing after blocking or cutting off the electron current. A switching time of blocking or blocking of the electron current after focusing can be determined by the focusing unit. Each of the converters can be designed in an optimized manner for its output voltage range.The exemplary embodiments serve exclusively to explain the invention and are not restrictive for it.

Claims

Method for operating an X-ray tube (12) which has at least one grid electrode (18) arranged between an anode electrode (14) and a cathode electrode (16), wherein - an electron current (26) is focused from the cathode electrode (16) to the anode electrode (14) by means of a focusing unit (24) by the focusing unit (24) applying a first electrical grid potential to the grid electrode (18) at least in a focusing operation in order to focus the electron current (26), - the focusing unit (24) is supplied with electrical energy in a galvanically isolated manner by means of an energy converter (38), - the first electrical grid potential is provided to the focusing unit (24) by means of an adjustable chip splitter (36), and - the adjustable chip splitter (36) is adjusted by means of a control circuit (40) of the focusing unit (24) by the control circuit (40) being supplied with at least one galvanically isolated control signal (42, 42), 44), the control signal (42, 44) being dependent on a predetermined value for the first electric grid potential, characterized in that - an electric power of the energy converter (38) is set dependent on the predetermined value for the first electric grid potential.Method according to Claim 1, characterized in that an electrical power of the energy converter (38) is additionally selected as a function of at least one electrical power, an electrical voltage or an electrical current of the focusing unit (24), which the focusing unit (24) requires for providing the first electrical grid potential.Method according to one of the preceding claims, characterized in that the electrical power of the energy converter (38) is determined using a characteristic diagram.Method according to one of the preceding claims, characterized in that the energy converter (38) is operated in an operating mode in which the energy converter (38) provides an adjustably constant electric current on the focusing unit side.Method according to one of the preceding claims, characterized in that the electrical power of the energy converter (38) is also determined as a function of an adjustment reserve which is predetermined for the adjustable chip splitter (36).Method according to one of the preceding claims, characterized in that the energy converter (38) has a voltage converter (76) coupled to an electrical energy source (80) and a galvanically isolating resonant converter (78), wherein the resonant converter (78) is electrically coupled on the input side to the voltage converter (76) and on the output side to at least the focusing unit (24), wherein an input current of the resonant converter (78) is set as a function of the predefined value for the first electrical grid potential.Method according to Claim 6, characterized in that a minimum current value and a maximum current value are predefined for the input current, the input current is detected and compared with at least the minimum or the maximum current value, and the electrical voltage provided by the voltage converter (76) for the resonant converter (78) is set as a function of the comparison.Method according to one of the preceding claims, characterized in that a frequency of the control signal (44) depends on the predefined value for the first electrical grid potential and the control circuit (40) determines the predefined value for the first electrical grid potential from the frequency of the control signal (44).Method according to Claim 8, characterized in that the focusing unit (24) is deactivated at a predefined, in particular a predefined minimum or maximum, frequency.Method according to Claim 8 or 9, characterized in that an output current of the voltage converter (76) is set as a function of the predefined value for the first electrical grid potential.Method according to one of Claims 6 to 10, characterized in that the resonant converter (78) is always operated in resonant mode at least during the focusing mode.Method according to one of Claims 6 to 11, characterized in that the resonant converter (78) has a full bridge circuit having two half-bridge circuits, wherein during the focusing operation one of the two half-bridge circuits is activated at least temporarily and the other of the two half-bridge circuits is deactivated.Circuit arrangement (22) for operating an X-ray tube (12), which has at least one grid electrode (18) arranged between an anode electrode (14) and a cathode electrode (16), having: - a focusing unit (24) for focusing an electron current (26) from the cathode electrode (16) to the anode electrode (14), wherein the focusing unit (24) is designed to apply a first electrical grid potential to the grid electrode (18) at least in a focusing operation in order to focus the electron current, - an energy converter (38) for supplying the focusing unit (24) with electrical energy in a galvanically isolated manner, and - a control unit (74), galvanically isolated from the X-ray tube (12), for adjusting an electrical power of the energy converter (38), wherein the focusing unit (24) has an adjustable chip divider (36) and a control circuit (40) for controlling the adjustable chip divider (36), wherein the control unit (74) is designed to provide at least one galvanically isolated control signal (42, 44) for the control circuit (40), which control signal depends on a predetermined value for the first electrical grid potential, wherein the control circuit (40) is designed to adjust the adjustable voltage splitter (36) depending on the at least one control signal (42, 44), characterized in that the control unit (74) is further designed to adjust the electrical power of the energy converter (38) depending on the predetermined value for the first electrical grid potential.Circuit arrangement (22) according to Claim 13, characterized bya switching unit (28) which is designed to apply the first electric grid potential focusing the electron current (26) to the at least one grid electrode (18) in a first switching state and a second electric grid potential for blocking off the electron current (26) between the anode electrode (14) and the cathode electrode (16) in a second switching state.X-ray apparatus (10) having an X-ray tube (12) which has at least one grid electrode (18) arranged between an anode electrode (14) and a cathode electrode (16), and having a circuit arrangement (22), which is connected to the X-ray tube (12) by means of a connecting line (20), for operating the X-ray tube (12), characterized in that the circuit arrangement (22) is designed according to one of Claims 13 or 14.

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