Electronic circuit and method for heating a filament of an x-ray tube

Abstract

Electronic circuit (20) for providing an alternating heating voltage (6) for heating a filament (3) of an X-ray tube (15). The electronic circuit (20) includes an inverter unit (8) configured to receive a direct heating voltage (10) at the input and to convert the direct heating voltage (10) into an alternating heating voltage (6) depending on a control variable (12) and to provide the alternating heating voltage (6) at the output. The electronic circuit (20) also includes a control arrangement (11) configured to measure the alternating heating voltage (6) and a heating alternating current (7) resulting from the alternating heating voltage (6) and to determine a controlled variable depending on the alternating heating voltage (6) and the heating alternating current (7). The control arrangement (11) is configured to change the control variable (12) depending on the controlled variable in order to regulate the controlled variable to a predetermined setpoint (14) in a control loop.

Description

[0001] The invention relates to an electronic circuit for providing an alternating heating voltage for heating a filament of an X-ray tube. The invention also relates to an X-ray tube system with such an electronic circuit and a medical imaging system with such an X-ray tube system. Furthermore, the invention relates to corresponding methods for providing an alternating heating voltage and for heating a filament of an X-ray tube.

[0002] An X-ray tube is a special type of electron beam tube used to generate X-rays. X-ray tubes are used in various imaging techniques and offer a wide range of applications, including in modern medicine.

[0003] To generate X-rays using an X-ray tube, free electrons are required. These electrons can be accelerated from a cathode to an anode by means of a defined high voltage applied to the tube. The released electrons, i.e., charges flowing from the cathode to the anode per unit of time, are called the tube current. To generate these free electrons, the cathode is designed as a filament, for example, in the form of a tungsten emitter. The filament is heated by an electric current to such an extent that electrons are released from the filament's metal lattice. The current flowing through the filament is subsequently referred to as the filament current.

[0004] The generated X-ray dose depends on the tube voltage and the tube current. The tube current is related to the filament current. Therefore, the filament current must be adjusted so that the desired filament temperature is reached, thus generating the appropriate tube current.

[0005] Since the cathode is often at a high-voltage potential resulting from the tube's high voltage, galvanic isolation in the form of a transformer, also called a heater transformer, is necessary for insulation purposes. This means that the filament current can only be directly measured at high-voltage potential using complex and expensive evaluation electronics. These electronics would potentially need to exchange data with the control electronics for the semiconductor switches on the primary side. Instead, in current technology, the primary-side current of the transformer, referred to as the heater current, is measured and used as the control variable.

[0006] Since the emission curve of the X-ray tube, which represents the ratio between tube and heater alternating current, can have a very large gradient, very precise measurement of this heater alternating current is necessary when regulating to the heater alternating current, which is also associated with high circuitry and cost effort.

[0007] Since certain tolerances in the measurement of the heating current and temperature dependencies in a heating power channel still exist, and the emission characteristics of an X-ray tube are also subject to a certain tolerance, the actual ratio between tube and heating current must be calibrated by test scans to meet the tube current requirements. This procedure is called filament learning. Due to the aging of the emitter resistor or filament resistor and the associated change in its resistance value, filament learning must be repeated at defined intervals to maintain sufficient accuracy. This approach is described in detail in the literature, e.g., in [Behling, 2021: Modern Diagnostic X-Ray Sources: Technology, Manufacturing, Reliability].

[0008] It is an object of the present invention to regulate the tube current more accurately and / or faster to the predefined setpoint.

[0009] This problem is solved by the respective subject matter of the independent claims. Advantageous further developments and preferred embodiments are the subject matter of the dependent claims, the following description, and the figures.

[0010] The invention is based on the idea of ​​controlling the heating of the filament using a control variable that depends on the heating alternating current and the heating alternating voltage at the output side of an inverter unit.

[0011] According to one aspect of the invention, an electronic circuit for providing an alternating heating voltage for heating a filament of an X-ray tube is described. The electronic circuit includes an inverter unit configured to receive a DC heating voltage at the input and to convert this DC heating voltage into an AC heating voltage as a function of a control variable, and to provide the AC heating voltage at the output. Furthermore, the electronic circuit includes a control arrangement configured to measure the AC heating voltage and the resulting AC heating current, and to determine a control variable as a function of the AC heating voltage, in particular the measured AC heating voltage, and the AC heating current, in particular the measured AC heating current.The control arrangement is designed to change the manipulated variable depending on the controlled variable in order to regulate the controlled variable to a predetermined setpoint in a control loop.

[0012] In other words, the electronic circuitry at the output of the inverter unit can provide an adjustable AC heating voltage, generated from the DC heating voltage, which can be used to heat the filament. The AC heating voltage can, for example, be galvanically isolated from the filament and routed via a transformer. This corresponds to an indirect supply of the AC heating voltage to the filament. The AC heating voltage can then be connected to the primary side of the transformer. The transformer can provide a filament voltage on the secondary side, which is connected to the filament. The filament voltage depends, in particular, on the AC heating voltage. The filament voltage can induce a filament current that heats the filament.The transformer can be set up so that the filament voltage has a higher voltage amplitude than the heating AC voltage; in particular, the filament voltage can also be referred to as filament high voltage.

[0013] In other embodiments, the heating AC voltage can be directly connected to the filament. In this case, the heating AC voltage can also be referred to as the filament voltage. This can be an alternative, for example, if galvanic isolation is not necessary. Galvanic isolation is particularly unnecessary if the voltage level of a tube's high voltage at the filament is equal to or close to ground level.

[0014] The inverter unit can in particular include an inverter that can receive the heating DC voltage on the input side and can convert the heating DC voltage into the heating AC voltage depending on a control variable and can make the heating AC voltage available on the output side.

[0015] The inverter can generate a variable output voltage. In this case, the output voltage is the heating AC voltage. The control variable can also be referred to as the inverter's operating parameter. The inverter can therefore generate the heating AC voltage, which can change depending on the value of the operating parameter.

[0016] In particular, the inverter's output level can be adjusted via the control variable. The inverter can, in particular, have a full bridge or two half bridges, which can be controlled, for example, via a PWM signal.

[0017] The control arrangement includes, for example, a measuring device configured to measure both the heating AC voltage and the heating AC current on the inverter's output side, i.e., a voltmeter and an ammeter. The two values ​​can be measured independently. In particular, it may be necessary to calculate an RMS value from each of the two values, i.e., an effective heating AC voltage and an effective heating AC current, or to design the measurement in such a way that the RMS values ​​can be determined directly.

[0018] The control arrangement also includes, for example, a controller configured to calculate the controlled variable and output the manipulated variable based on the controlled variable. The controlled variable can be composed of the measured values ​​of the heating voltage and heating current, or calculated based on these values—that is, dependent solely on the heating voltage and heating current, or also dependent on other variables. The calculation of the controlled variable can, in particular, involve dividing the heating voltage by the heating current, or vice versa.

[0019] The manipulated variable can be provided as an output to the control system and is connected to the inverter. A setpoint can be specified for the controlled variable, which the controller then uses to align its control. In particular, the controller can calculate the difference between the controlled variable and the setpoint and regulate to a target value where the difference is zero. It is also possible for the setpoint to change during the control process, i.e., for the setpoint to be adjusted.

[0020] A change in the control variable can cause a change in the heater voltage, as the inverter's output level can change. Once a filament is connected to the electronic circuit, either directly or indirectly, a change in the heater voltage can cause a change in the heater current or the filament current, thus altering the filament temperature. The filament temperature, in turn, can influence the tube current and thus change the X-ray dose, making it a crucial factor in the quality of an X-ray image.

[0021] An advantage of the described invention is the direct measurement of the relevant control parameters in the immediate vicinity of the filament. In particular, little or even no further knowledge of circuit components is required to draw conclusions about the filament temperature, since the values ​​for the heating voltage and heating current are directly available.

[0022] Furthermore, the gradient of the relationship between the heater current and the tube current is relatively high; that is, a small change in the heater current already has a large effect on the tube current. A conventional control system that relies solely on the heater current as the controlled variable can therefore only regulate relatively slowly and exhibit a large number of overshoots. The gradient is significantly flatter when a controlled variable is used that is composed of the heater current and heater voltage components. This is the case with the electrical circuit according to the invention, which, among other things, solves the problem mentioned above.

[0023] Ultimately, this can also reduce the exposure time and thus the applied X-ray dose of an X-ray image.

[0024] Another advantage is that the filament learning process only needs to be performed at significantly longer intervals. This is because the control, based on a single control variable consisting of heating current and heating voltage, directly reflects the defined electron emission from the filament, thus significantly reducing the flattening of the emission power as the filament ages. A key property of the filament, such as its impedance, can be estimated based on the heating voltage and current measurements and therefore does not require complex learning. In particular, estimating the filament's impedance also enables predictive maintenance of the X-ray tube.

[0025] According to at least one embodiment of the electronic circuit, the inverter unit includes an inverter configured to provide an input AC voltage on the output side, depending on the heating DC voltage. The inverter unit also includes a transformer configured to maintain the input AC voltage on the primary side. Depending on a turns ratio, in particular a predetermined turns ratio, the transformer is configured to transform the input AC voltage into the heating AC voltage and to provide the heating AC voltage on the secondary side for heating the filament.

[0026] The transformer can receive the input AC voltage that the inverter within the inverter unit provides on its output side. In particular, the transformer can ensure galvanic isolation from the filament, which is located at the high voltage potential of the tube's high voltage supply. Due to the turns ratio, the transformer can also output a higher, and in particular a significantly higher, voltage amplitude than it receives on its input side. Thus, the transformer can provide the heating AC voltage for heating the filament. In this case, the heating AC voltage can also be referred to as the filament voltage.

[0027] One advantage of using the transformer is the galvanic isolation of the circuits and the possibility of increasing the voltage amplitude. In this embodiment, the control is based directly on the current and voltage values ​​present at the filament, i.e., the heating AC voltage and heating AC current. This leads to increased control accuracy without the need for further calculations by other circuit components or, for example, estimations.

[0028] According to at least one further embodiment, the inverter unit includes an inverter configured to provide the heating AC voltage on the output side, depending on the heating DC voltage. The electronic circuit also includes a transformer configured to provide the heating AC voltage on the primary side. Depending on a turns ratio, in particular a predetermined turns ratio, the transformer is configured to transform the heating AC voltage into a filament voltage and to provide the filament voltage on the secondary side for heating the filament.

[0029] In such embodiments, the transformer is therefore not part of the inverter unit. The transformer can supply the heating AC voltage provided by the inverter unit on the output side. In particular, the transformer can ensure galvanic isolation from the filament, which is located at the high voltage potential of the tube's high voltage. Due to the turns ratio, the transformer can also output a higher, and in particular a significantly higher, voltage amplitude than it receives on the input side. Thus, the transformer can provide the filament voltage for heating the filament. Specifically, the filament voltage is an alternating voltage.

[0030] One advantage of using a transformer is the galvanic isolation of the circuits and the possibility of increasing the voltage amplitude. In this embodiment, the control is based on the current and voltage values ​​applied to the filament via the transformer, i.e., the heating AC voltage and heating AC current. Only the transformer's turns ratio needs to be considered to draw conclusions about the filament's characteristics.

[0031] According to at least one further embodiment of the electronic circuit, the control arrangement is configured to determine a heating impedance from the heating alternating voltage and the heating alternating current and to determine the controlled variable depending on the heating impedance, or to determine a heating power from the heating alternating voltage and the heating alternating current and to determine the controlled variable depending on the heating power.

[0032] In other words, the control arrangement can calculate the heating impedance from the measured values ​​of the heating AC voltage and the heating AC current, specifically as the quotient of these values, where the heating AC voltage is the dividend and the heating AC current is the divisor. Likewise, the control arrangement can first determine the RMS values ​​from the measured values, i.e., the effective heating AC voltage and the effective heating AC current, and then calculate the quotient of these RMS values. The heating impedance can also be referred to as the heating resistance. The embodiment also includes the case where the controlled variable corresponds to the heating impedance. In particular, the controlled variable can also be directly proportional to the heating impedance.

[0033] Alternatively, the control system can, for example, calculate the heating power by multiplying the measured values ​​of the heating AC voltage and the heating AC current. In this case, the controlled variable can be determined as a function of the heating power; in particular, the controlled variable can correspond to the heating power, or the controlled variable can be directly proportional to the heating power.

[0034] Here and in the following, the term impedance can refer to both a complex-valued impedance and a real-valued impedance, i.e., an ohmic resistance, unless otherwise stated.

[0035] In contrast to the steep gradient of the relationship between heating current and tube current, the relationship between heating impedance and tube current exhibits a significantly flatter profile. For this reason, control based on heating impedance can be more precise and reach the setpoint more quickly. In particular, the presented embodiment can reduce or eliminate potential overshoots in a control process.

[0036] According to at least one further embodiment of the electronic circuit, the control arrangement is designed to set the heating alternating current to a predefined initial value before an initial control loop of the control system is performed.

[0037] In other words, the control process of the closed-loop system can start with a small positive value as a predefined initial value for the heating current, which acts as a divisor in the heating impedance calculation. "Small" in this context means small compared to measured values ​​obtained in previous measurements. A heating current value close to zero or equal to zero would cause the heating impedance to rise to very large values ​​or even to infinity.

[0038] One advantage of this embodiment is improved convergence of the control loop against the setpoint, particularly in the initial phase of the measurement.

[0039] According to at least one further embodiment of the electronic circuit, the control arrangement includes a circuit part which is configured to determine the setpoint depending on a tube current of the X-ray tube, a tube high voltage of the X-ray tube and a predetermined relationship between the tube current and the controlled variable.

[0040] In other words, the setpoint can vary within a single control operation; in particular, the setpoint can be adjusted during the initial execution of the control loop. The circuitry can be configured to measure the controlled variable or to obtain the current measured value of the controlled variable. In some embodiments, the setpoint can also depend on the current measured value of the controlled variable.

[0041] Similarly, the circuit section can be configured to obtain target values ​​for the tube current and the tube high voltage. The circuit section can maintain and store the predefined relationship between the tube current and the controlled variable, for example, through calibration or other data input. This calibration can also be referred to as filament learning.

[0042] The tube current and the tube high voltage can be target parameters for a planned X-ray image.

[0043] One advantage of this embodiment is the improved accuracy of temperature adjustment of the filament based on known target values ​​for a given initial situation before the planned X-ray exposure.

[0044] According to at least one further embodiment of the electronic circuit, the heating AC voltage is a non-sinusoidal AC voltage and the control variable corresponds to the pulse-pause ratio of the heating AC voltage.

[0045] In particular, the heating voltage can contain or be composed of a sequence of pulses, especially square wave pulses. The heating voltage can also be composed of a sequence of pulses with a fixed frequency. The manipulated variable can then correspond to the pulse-pause ratio of the pulses. A high pulse-pause ratio can correspond to a high energy content of the heating voltage and thus cause a high filament current, leading to a high filament temperature. Conversely, a low pulse-pause ratio can correspond to a low energy content of the heating voltage, resulting in a lower filament current and a lower filament temperature.

[0046] This type of heating AC voltage offers the advantage of simple implementation in modern and cost-effective circuit technology and the ability to quickly and precisely adjust necessary parameters. The inverter can be implemented as a PWM inverter. This design is also advantageous in terms of switching speed and filament current control accuracy.

[0047] According to at least one further embodiment of the electronic circuit, the control arrangement includes a Pl controller or a PID controller.

[0048] The PL controller or PID controller is specifically designed to change the manipulated variable depending on the controlled variable.

[0049] The PL controller (proportional-integral controller) contains both a proportional and an integral component. In the proportional component, the relationship between an input and an output is defined by a step function with a fixed gain. The integral component exhibits a linearly increasing response between input and output. The PID controller (proportional-integral-derivative controller) additionally contains a differential component. A step response is a shock function with theoretically infinite magnitude.

[0050] One advantage of using PI controllers is the combination of a fast response time for the proportional component and precise control without any residual deviation in the integral component. The PID controller offers the additional advantage that even rapid control deviations can be corrected through a strong response from the controller. The use of PI or PID controllers simplifies the design of the electronic circuit according to the invention and increases control accuracy.

[0051] According to another aspect of the invention, an X-ray tube system, in particular an X-ray tube system for a computed tomography system, is specified, comprising an electronic circuit according to the invention and the X-ray tube.

[0052] The X-ray tube can contain a filament that can be supplied with an alternating heating voltage via the electronic circuit. Specifically, the electronic circuit is connected to the filament in such a way that the filament voltage can be applied to it. In addition to the electronic circuit and the X-ray tube, the X-ray tube system can include other components such as a housing and an X-ray anode.

[0053] Further embodiments of the X-ray tube system according to the invention follow directly from the various configurations of the electronic circuit according to the invention. In particular, individual features and corresponding explanations as well as advantages relating to the various embodiments of the devices according to the invention can be transferred analogously to corresponding embodiments of the X-ray tube system according to the invention.

[0054] According to another aspect of the invention, a medical imaging system comprising an X-ray tube system according to the invention is specified.

[0055] The medical imaging system can be, for example, an X-ray system, in particular a digital X-ray system, both a stationary and a mobile system, or a specialized X-ray device, such as a computed tomography system (CT system), a cone beam CT system, a mammography system, a dental X-ray system, a fluoroscopy system, an angiography system, a C-arm system, or even a classic X-ray device.

[0056] Further embodiments of the medical imaging system according to the invention follow directly from the various configurations of the X-ray tube system or the electronic circuit according to the invention. In particular, individual features and corresponding explanations as well as advantages relating to the various embodiments of the devices according to the invention can be transferred analogously to corresponding embodiments of the medical imaging system according to the invention.

[0057] According to a further aspect of the invention, a method for providing an alternating heating voltage for heating a filament of an X-ray tube is described. First, a direct heating voltage is converted into an alternating heating voltage depending on a control variable, and the alternating heating voltage is then provided. Furthermore, the alternating heating voltage and a heating current resulting from the alternating heating voltage are measured, particularly on the output side of an inverter unit, and a control variable is determined based on these measurements. The control variable is then changed depending on the control variable in order to regulate the control variable to a predetermined setpoint.

[0058] According to at least one embodiment of the method, the filament is heated depending on the heating alternating voltage.

[0059] According to at least one further embodiment of the method, a heating impedance is determined from the heating alternating voltage and the heating alternating current and the controlled variable is determined depending on the heating impedance, or a heating power is determined from the heating alternating voltage and the heating alternating current and the controlled variable is determined depending on the heating power.

[0060] According to at least one further embodiment of the method, the heating alternating voltage is converted into a filament voltage depending on a transformation ratio and the filament voltage is made available.

[0061] According to at least one further embodiment of the method, the setpoint is determined depending on a tube current of the X-ray tube, a tube high voltage of the X-ray tube and a predetermined relationship between the tube current and the controlled variable.

[0062] Further embodiments of the method according to the invention follow directly from the various configurations of the electronic circuit according to the invention, and vice versa. In particular, individual features and corresponding explanations as well as advantages relating to the various configurations of the electronic circuit according to the invention can be transferred analogously to corresponding configurations of the method according to the invention. In particular, the electronic circuit according to the invention is configured or programmed to carry out a method according to the invention. In particular, the electronic circuit according to the invention carries out the method according to the invention.

[0063] Further features and combinations of features of the invention will become apparent from the figures and their description, as well as from the claims. In particular, further embodiments of the invention need not necessarily include all features of any one of the claims. Further embodiments of the invention may have features or combinations of features that are not mentioned in the claims.

[0064] The invention is explained in more detail below with reference to specific embodiments and associated schematic drawings. In the figures, identical or functionally equivalent elements may be designated with the same reference numerals. The description of identical or functionally equivalent elements is not necessarily repeated with respect to different figures.

[0065] This shows FIG 1 a schematic block diagram of an exemplary embodiment of an electronic circuit according to the invention; and FIG 2 a schematic block diagram of a further exemplary embodiment of the electronic circuit according to the invention; and FIG 3 a schematic block diagram of a further exemplary embodiment of the electronic circuit according to the invention; and FIG 4 a schematic block diagram of a further exemplary embodiment of the electronic circuit according to the invention.

[0066] In FIG 1 Figure 1 shows an embodiment of an electronic circuit 20 for providing an AC heating voltage 6 for heating a filament 3 of an X-ray tube 15. The electronic circuit 20 includes an inverter unit 8, which is configured to receive an input DC heating voltage 10 and to convert the DC heating voltage 10 into an AC heating voltage 6 depending on a control variable 12 and to provide the AC heating voltage 6 as an output. Furthermore, the electronic circuit 20 has a control arrangement 11, which is configured to measure the AC heating voltage 6 and a heating current 7 resulting from the AC heating voltage 6 and to determine a controlled variable depending on the AC heating voltage 6 and the heating current 7. The control arrangement 11 is configured to change the control variable 12 depending on the controlled variable in order to regulate the controlled variable to a predetermined setpoint 14 in a control loop.

[0067] FIG 1 The figure also schematically shows an anode 2 of the X-ray tube 15 and a tube high voltage 5 applied between filament 3 and anode 2 and a tube current 1. These two quantities can be crucial for an X-ray dose that can occur during an X-ray examination and affect a patient.

[0068] The control arrangement 11 can, for example, include a measuring arrangement in the control loop, consisting of at least one measuring device, an inverter, and a controller 21. In at least one embodiment, the controller 21 can be a P1 or a PID controller. The type of controller 21 can influence the control behavior with regard to the amplitude deviation of the controlled variable and its transient response. For example, the control arrangement 11 can determine the controlled variable from the heating voltage 6 and the heating current 7 as the quotient of these two quantities. If the heating voltage 6 is the dividend and the heating current 7 is the divisor, the controlled variable can also be referred to as the heating impedance 17. Similarly, the control arrangement 11 can, for example, determine the controlled variable as the product of the heating voltage 6 and the heating current 7. In this case, the controlled variable can also be referred to as the heating power 24.

[0069] The specified setpoint 14 can serve as the input variable for the control arrangement 11 and be used within the control arrangement 11 for comparison with the controlled variable. The inverter can first invert the value of the controlled variable, and then the controller 21 can compare the inverted value of the controlled variable with the setpoint 14, in particular by calculating a difference.

[0070] The control variable 12 can be provided by the control arrangement 11 to the inverter 16 within the inverter unit 8. In at least one embodiment, the control variable 12 can correspond to a pulse-pause ratio of the heating AC voltage 6. The average heating AC voltage 6 can be adjusted using the pulse-pause ratio, and consequently, the average heating AC current 7. These two quantities can be decisive for the temperature of the filament 3.

[0071] In FIG 1 A transformer 13 is also shown, which in some embodiments can galvanically isolate the heating AC voltage 6 from a circuit of the filament 3. The transformer 13 receives the heating AC voltage 6 on its input side and can generate a filament voltage 18 from it. Depending on a turns ratio, the transformer 13 can, for example, transform the heating AC voltage 6 to a high voltage. The filament voltage 18 can, for example, be an alternating voltage. The filament voltage 18 can lead to a filament current 4, which flows through the filament 3 and can directly cause a temperature change of the filament 3.

[0072] Another embodiment of the electronic circuit 20 is shown in the block diagram in FIG 2 shown. That in FIG 2 The illustrated embodiment can analogously incorporate all features from FIG 1 exhibit, unless otherwise stated or mentioned. In FIG 2 The inverter unit 8 can contain a transformer 13. The transformer 13 can receive an input AC voltage 19, which can be generated by the inverter 16. On the output side, the inverter unit 8 can provide the heating AC voltage 6, which can result in a heating AC current 7. The heating AC current 7 can flow directly through the filament 3 and directly cause a temperature change in the filament 3. In this case, the heating AC voltage 6 can also be referred to as the filament voltage 18, since it can be applied directly to the filament 3.

[0073] The control arrangement 11 receives the heating AC voltage 6, the heating AC current 7, and the setpoint 14 as inputs and can determine a controlled variable from this. The control arrangement 11 can generate a manipulated variable 12, which can be made available to the inverter 16 within the inverter unit 8.

[0074] In FIG 3 A further schematic representation of the electronic circuit 20 according to the invention is shown. The provisions already shown in the figures apply here. FIG 1 and FIG 2 The features shown are as depicted, unless otherwise indicated.

[0075] In the exemplary embodiment shown, the control arrangement 11 includes, in addition to the controller 21 and the inverter, a control signal generator 23 and a circuit section 22. The controller 21 receives the heating AC voltage 6, the heating AC current 7, and the setpoint 14 as inputs. On its output side, the controller provides the manipulated variable 12 to the control signal generator 23. The control signal generator 23 can, for example, generate individual switch positions as input signals for the inverter 16, depending on the manipulated variable 12 applied to the input, and provide these to the inverter 16. The designations S1, S2, S3, and S4 specifically identify an embodiment in which the inverter 16 has a full bridge and contains four switches. These four switches can, for example, be controlled by the output signals of the control signal generator 23.Circuit section 22 receives the specified values ​​of the tube high voltage 5 and the tube current 1. Similarly, circuit section 22 can, for example, receive the measured values ​​of the current heater AC voltage 6 and the current heater AC current 7. From these values, circuit section 22 can determine the setpoint 14, in particular based on a predefined relationship between the tube current 1 and the controlled variable, which was determined, for example, during filament learning. Circuit section 22 can then provide the controller 21 with the setpoint 14.

[0076] In the FIG 3 A circuit section between the inverter 16 and the filament 15 is shown, for example, as a reactive network. In another embodiment, the circuit section between the inverter 16 and the filament 15 can also be implemented as a resonant network. This can be schematically indicated by a series capacitor in a supply line to the transformer 13.

[0077] Furthermore, in FIG 3 The transformer 13 is shown, which receives the heating AC voltage 6 on its input side and can provide the filament voltage 18 depending on the turns ratio. The filament voltage 18 is applied to the filament 3 and can lead to a filament current 4 flowing through the filament 3. The inverter unit 8 can be operated on its input side by a heating DC voltage 10 and a resulting heating DC current 9.

[0078] FIG 4 Figure 1 shows another embodiment of the electronic circuit 20. Within the control arrangement 11, the controller 21, the control signal generator 23, and the circuit section 22 are shown. The controller 21 receives the heating AC voltage 6, the heating AC current 7, and the setpoint 14 as inputs. From this, the controller 21 can generate the manipulated variable 12 and make it available to the control signal generator 23. The control signal generator 23 can, for example, generate individual switch positions as input signals for the inverter 16, depending on the manipulated variable 12 applied to the input, and make these available to the inverter 16. The designations S1, S2, S3, and S4 specifically identify an embodiment in which the inverter 16 has a full bridge and contains four switches. The four switches can, for example, be controlled by the output signals of the control signal generator 23.

[0079] Circuit section 22 receives the specified values ​​of the tube high voltage 5 and the tube current 1. Similarly, circuit section 22 can, for example, receive the measured values ​​of the current heater AC voltage 6 and the current heater AC current 7. From these values, circuit section 22 can determine the setpoint 14, in particular based on the specified relationship between the tube current 1 and the controlled variable, which was determined, for example, during filament learning. Circuit section 22 can then provide the setpoint 14 to the controller 21.

[0080] Furthermore, in FIG 4 The transformer 13 is shown within the inverter unit 8, which receives the input AC voltage 19 and can provide the heating AC voltage 6 depending on the turns ratio. The heating AC voltage 6 is applied to the filament 3 and can lead to a heating AC current 7 flowing through the filament 3. The inverter unit 8 can be operated on the input side by a heating DC voltage 10 and a resulting heating DC current 9.

[0081] In FIG 4 An embodiment is shown in which the heating AC voltage 6 and the heating AC current 7 are measured directly at the filament 3. In this case, the heating impedance 17 can be determined, for example, without taking into account the control variable 12, in particular the output level of the inverter 16. The algorithm described below can be used to control the temperature of the filament 3.

[0082] The control based on the heating impedance 17 has the particular advantage that the desired temperature of the filament 3 can be reached very early. From Ohm's law it follows that... R f = U f I f = ρ l s = ρ 0 1 + a T l s , where R f denotes the Ohm's resistance of filament 3, U f the voltage applied to filament 3 and I f the current flowing through filament 3, ρ the specific resistance of filament 3, l the length of the filament 3, s its cross-sectional area, ρ 0 the specific resistance of the filament 3 at 0 °C, a the temperature coefficient of the specific resistance, and T the temperature of filament 3 at 0 °C. For tungsten, for example, a = 0,0046 1 ° C and ρ 0 = 5.5 * 10 -8 < Ω m.

[0083] From this, the following equation for calculating the temperature can be derived: T = R f s l ρ 0 − 1 a = R f s a ρ 0 l − 1 a = k R f − b = k U f I f − b , where k = s aρ 0 l and b = 1 / a.

[0084] It follows that the temperature of filament 3 depends linearly on its resistance. This linearity means that the control via the inner control loop allows the temperature to be brought to the desired value more stably, precisely, and quickly.

[0085] The measurement of the two values U f and I f This can be considered equivalent to measuring the temperature of filament 3. Since directly measuring these two values ​​is technically difficult and expensive, it is advantageous to use other quantities that are easier to measure and directly related to R f stand, as is the case here for FIG 4 is achieved by the described measurement of the heating DC voltage 10 and the heating DC current 9.

[0086] The following algorithm can be used, for example: 1. Setting a new expected filament resistance R ref , which was learned through filament learning. 2. Setting a small current I d through filament 3, so that the feedback current is not exactly zero. 3. Measure the heating DC voltage 10 and the heating DC current 9. 4. Calculate the actual filament resistance. R d 5. Calculating the difference ΔR between an expected filament resistance R ref and the actual filament resistance R d 6. Using the controller 21 to calculate the manipulated variable 12 and from that the control signals for the inverter 16. ΔR 7. Repeat steps 3 to 7.

[0087] Resistance-based filament learning can be implemented, for example, according to the following algorithm: 1. Set I f on I f,llx and KV s,ls 1. Set C to 0 (false). 2. Set C = 0 and KV = KV s [0] . 3. If KV s,ls [ C ] = 1 (true), go to step 12. 4. Apply I f on the filament 3 and wait until it stabilizes. 5. Apply the current tube high voltage 5 and exposure time to the X-ray tube 15. 6. Take an X-ray image and measure the tube current 1. I T and the filament tension U f as soon as they are stable. 7. Calculate the resistance. R = U f I f 8. Store the tuple ( KV , I f , I T , U f , R ) . 9. If I f > I f,ulx , set KV s,ls set to 1 (true). 10. If I T > min ( I T,ulg , I T,ulx ), set KV s,ls set to 1 (true). 11. If I T > min P ulx KV P ulg KV , set KV s,ls set to 1 (true). 12. Set C → C + 1. 13. If C < C KV , set KV s ( C KV ) and go to step 3. 14. If all elements of KV s,ls If they are set to 1 (true), go to step 16. 15. Set I f → I f + I f,s , C = 0, KV = KV s [0] and go to step 3. 16. End.

[0088] This refers to C KV the number of X-ray voltages considered, I f,ulx an upper limit for the current through filament 3, I f,llx a lower limit for the current through filament 3, I T,ulx an upper limit for the tube current 1, I T , ulg , a maximum available tube current 1, P ulx , an upper limit for the tube power, P ulg , a maximum available tube power, I f,s a predetermined current value step, KV s an array of size C KV , which stores all the X-ray voltages considered, and KV s,ls an array of size C KV , which stores information about whether the learning process for the individual X-ray voltages is complete.

[0089] One advantage is that the controller 21 does not need to know the filament voltage 18 and the filament current 4. Furthermore, the filament current 4 can be kept more stable, as it is controlled by its resistance in relation to temperature. This is particularly helpful, for example, for time-critical X-ray imaging requiring high spatial resolution.

[0090] Furthermore, with this control system, filament 3 does not become too hot or too cold at the beginning of the X-ray exposure, which would lead to an excessively high or low dose rate. With time-controlled exposure, an image that is too bright or too dark can be avoided. Additionally, the generator and the X-ray tube can be protected from overload.

[0091] Resistance as an indicator of filament temperature is also advantageous for predictive maintenance, as deviations in filament temperatures compared to data from the scan history allow for easy detection of filament aging.

[0092] In the described invention, the filament learning process can be carried out at longer intervals. This is achieved in particular by actively mitigating the aging of the filament 3 and thus its change in heating impedance 17 by using the heating power 24 as a control variable, thereby ideally keeping the number of free electrons constant. However, since the aging of the filament 3 is not uniform, its emission characteristics still change with progressive aging. Nevertheless, this occurs at significantly longer intervals than in the conventional method.

[0093] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

1. Electronic circuit (20) for providing an alternating heating voltage (6) for heating a filament (3) of an X-ray tube (15), wherein: - the electronic circuit (20) includes an inverter unit (8) which is configured to receive an input DC heating voltage (10) and to convert the DC heating voltage (10) into an alternating heating voltage (6) depending on a control variable (12) and to provide the alternating heating voltage (6) on the output side;and - the electronic circuit (20) includes a control arrangement (11) which is configured to measure the heating alternating voltage (6) and a heating alternating current (7) resulting from the heating alternating voltage (6) and to determine a controlled variable depending on the heating alternating voltage (6) and the heating alternating current (7), and - the control arrangement (11) is configured to change the manipulated variable (12) depending on the controlled variable in order to control the controlled variable in a control loop to a predetermined setpoint (14).

2. Electronic circuit (20) according to claim 1, wherein - the inverter unit (8) includes an inverter (16) configured to provide an input AC voltage (19) on the output side, depending on the heating DC voltage (10); and - the inverter unit (8) includes a transformer (13) configured to obtain the input AC voltage (19) on the primary side; and - the transformer (13) is configured to transform the input AC voltage (19) into the heating AC voltage (6) on the secondary side, depending on a turns ratio.

3. Electronic circuit (20) according to claim 1, wherein - the inverter unit (8) includes an inverter (16) configured to provide the heating AC voltage (6) on the output side, depending on the heating DC voltage (10); and - the electronic circuit (20) includes a transformer (13) configured to obtain the heating AC voltage (6) on the primary side; and - the transformer (13) is configured to transform the heating AC voltage (6) into a filament voltage (18) on the secondary side, depending on a turns ratio, and to provide the filament voltage (18) for heating the filament (3).

4. Electronic circuit (20) according to one of the preceding claims, wherein the control arrangement (11) is configured to: - determine a heating impedance (17) from the heating alternating voltage (6) and the heating alternating current (7) and determine the controlled variable as a function of the heating impedance (17); or - determine a heating power (24) from the heating alternating voltage (6) and the heating alternating current (7) and determine the controlled variable as a function of the heating power (24).

5. Electronic circuit (20) according to one of the preceding claims, wherein the control arrangement (11) is configured to set the heating alternating current (7) to a predefined initial value before performing an initial control loop of the control circuit.

6. Electronic circuit (20) according to one of the preceding claims, wherein the control arrangement (11) includes a circuit part (22) which is configured to determine the setpoint (14) depending on a tube current (1) of the X-ray tube (15), a tube high voltage (5) of the X-ray tube (15) and a predetermined relationship between the tube current (1) and the controlled variable.

7. Electronic circuit (20) according to one of the preceding claims, wherein the heating alternating voltage (6) is a non-sinusoidal alternating voltage and the manipulated variable (12) corresponds to the pulse-pause ratio of the heating alternating voltage (6).

8. Electronic circuit (20) according to one of the preceding claims, wherein the control arrangement (11) includes a PI controller or a PID controller.

9. X-ray tube system comprising an electronic circuit (20) according to one of the preceding claims and the X-ray tube (15).

10. Medical imaging system comprising the X-ray tube system according to claim 9.

11. Method for providing an alternating heating voltage (6) for heating a filament (3) of an X-ray tube (15), wherein: - a direct heating voltage (6) is converted into an alternating heating voltage (6) depending on a manipulated variable (12) and the alternating heating voltage (6) is provided; and - the alternating heating voltage (6) and an alternating heating current (7) resulting from the alternating heating voltage (6) are measured and a controlled variable is determined depending on this, and - the manipulated variable (12) is changed depending on the controlled variable in order to control the controlled variable to a predetermined setpoint (14).

12. Method for heating a filament (3) of an X-ray tube (15), wherein a method according to claim 11 is carried out and the filament (3) is heated depending on the heating alternating voltage (6).

13. Method according to one of claims 11 or 12, wherein - a heating impedance (17) is determined from the heating alternating voltage (6) and the heating alternating current (7) and the controlled variable is determined depending on the heating impedance (17); or - a heating power (24) is determined from the heating alternating voltage (6) and the heating alternating current (7) and the controlled variable is determined depending on the heating power (24).

14. Method according to one of claims 11, 12 or 13, wherein the heating alternating voltage (6) is converted into a filament voltage (18) depending on a transformation ratio and the filament voltage (18) is made available.

15. Method according to one of claims 11 to 14, wherein the setpoint (14) is determined depending on a tube current (1) of the X-ray tube (15), a tube high voltage (5) of the X-ray tube (15) and a predetermined relationship between the tube current (1) and the controlled variable.

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