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

Abstract

Electronic circuit (22) for providing an AC heating voltage (6) for heating a filament (3) of an X-ray tube (15). The electronic circuit (22) includes an inverter (8) 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) on the output side. Furthermore, the electronic circuit (22) includes a control arrangement (11) configured to measure the DC heating voltage (10) and a DC heating current (9) resulting from the DC heating voltage (10) and to determine a controlled variable depending on the DC heating voltage (10) and the DC heating current (9). 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 an inner 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.

[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] X-rays are generated using an X-ray tube, requiring free electrons that can be accelerated from a cathode to an anode by means of a defined high voltage applied within the tube. The electrons released per unit of time, i.e., charges, flowing from the cathode to the anode 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. This current flow 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, a very precise measurement of the heater alternating current is necessary, which is also associated with high circuitry and cost effort.

[0007] Currently, known applications aim for the most precise possible measurement of the RMS value of the heating alternating current. For this purpose, an additional measuring transformer is required for primary-side measurement of the heating alternating current, which also ensures galvanic isolation from the control electronics. The secondary-side current of this transformer is converted into a voltage in the form of a high-frequency signal via a measuring resistor. Evaluating this high-frequency signal requires a comparatively complex integrated analog circuit to determine a DC signal corresponding to the RMS value. As an alternative to the analog implementation of the RMS value measurement, a suitable digital filter with comparable functionality can be used. This requires a high degree of oversampling of the high-frequency signal to achieve sufficient accuracy in the RMS value, which also necessitates a comparatively expensive signal processing chain.

[0008] 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].

[0009] It is an object of the present invention to regulate the tube current more accurately and / or faster to the predefined setpoint.

[0010] 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.

[0011] The invention is based on the idea of ​​controlling the heating of the filament using a control variable that depends on the heating DC current and the heating DC voltage at the input side of an inverter.

[0012] According to one aspect of the invention, an electronic circuit for providing an AC heating voltage for heating a filament of an X-ray tube is described. The electronic circuit includes an inverter configured to receive a DC heating voltage at its 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 its output. Furthermore, the electronic circuit includes a control arrangement configured to measure the DC heating voltage and the DC heating current resulting from it, in particular at the input of the inverter, and to determine a control variable as a function of the DC heating voltage and the DC 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 an inner control loop.

[0013] In other words, the electronic circuit at the inverter output can provide a controllable 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 transformer on its primary side. The transformer can provide a filament voltage on its 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.

[0014] In other embodiments, the heating AC voltage can be directly connected to the filament. This can be an alternative, for example, if galvanic isolation is not necessary. Galvanic isolation is particularly unnecessary if the voltage level of the tube's high voltage at the filament is equal to or close to ground level.

[0015] The term "inner control loop" can be understood to mean either that the inner control loop is the only control loop relevant for the corresponding embodiment of the invention, or that the inner control loop is subordinate to an outer control loop. If the inner control loop is the only control loop, the term "inner control loop" can be equated with the term "control loop." Similarly, the term "inner control loop" can then be equated with the term "control loop."

[0016] As an alternative to the described arrangement of the inner and outer control loops, it is also possible, for example, to operate a first control loop for the heating power in parallel with a second control loop for the tube current. This structure is called a step-down controller structure, characterized by the fact that the controller output with the lower value is used as the control factor.

[0017] In the event that the electronic circuit has an external control loop, this may in particular be designed to be slower than the internal control loop and / or, for example, be set up to control the setpoint of the internal control loop.

[0018] The inverter can generate a variable output voltage. In this case, the output voltage is the AC heating voltage. The control variable can also be referred to as the inverter's operating parameter. The inverter can therefore generate the AC heating voltage, which can change depending on the value of the operating parameter. In particular, the inverter's output level can be varied via the control variable. The inverter can, in particular, have a full bridge or a half bridge in combination with a capacitive voltage divider that halves the DC heating voltage. The full bridge or the half bridge can, for example, be controlled via a PWM signal.

[0019] The control arrangement includes, for example, a measuring device configured to measure both the heating DC voltage and the heating DC current at the inverter's input, i.e., a voltmeter and an ammeter. The two values ​​can be measured independently. 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 DC voltage and current, or calculated based on these values—that is, dependent solely on the heating DC voltage and current, or also dependent on other variables.

[0020] The calculation of the controlled variable may, in particular, involve multiplying the heating DC voltage by the heating DC current or dividing the heating DC voltage by the heating DC current or vice versa.

[0021] 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.

[0022] 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.

[0023] One advantage of the described invention is the simple measurement of the relevant control parameters in the DC voltage or DC current range without the need for complex measuring instruments or galvanic isolation. Measuring these output variables is simpler and more cost-effective than measuring them in the AC voltage or AC current range. Furthermore, no RMS value calculations are necessary, as the output variables are directly available. The measurement also allows for a simpler design with regard to component cooling; in particular, heat sinks, which are required for measurements in the AC voltage or AC current range, can be omitted.

[0024] 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 on the heater current as the controlled variable can therefore only regulate relatively slowly and contain a large number of overshoots. The gradient is significantly flatter when a controlled variable is used that is composed of the components heater current and heater voltage. This is the case with the electrical circuit according to the invention, which, among other things, solves the problem mentioned above.

[0025] Ultimately, this can also reduce the exposure time and thus the applied X-ray dose of an X-ray image.

[0026] Another advantage is that the filament learning process only needs to be performed at significantly longer intervals. This is because the dependence of the emission on the heating voltage and current has a considerably lower gradient. Therefore, the tolerance range with regard to filament aging is greater. A key property of the filament, such as its impedance, can be estimated based on measurements of the heating voltage and current, as well as knowledge of the other circuit components, and thus does not require complex learning. In particular, estimating the filament's impedance also enables predictive maintenance of the X-ray tube.

[0027] According to at least one embodiment of the electronic circuit, the control arrangement is configured to determine a heating power from the heating DC voltage and the heating DC current. Furthermore, the control arrangement is configured to determine the controlled variable as a function of the heating power.

[0028] In other words, the control arrangement can calculate the heating power from the measured values ​​of the heating DC voltage and heating DC current, in particular as the product of these values. The embodiment also includes the case where the controlled variable corresponds to the heating power. Alternatively, the controlled variable can also be directly proportional to the heating power, or another defined relationship between heating power and the controlled variable can exist.

[0029] In contrast to the steep gradient of the relationship between heating current and tube current, the relationship between heating power and tube current exhibits a significantly flatter profile. For this reason, control based on heating power is more precise and can reach the setpoint more quickly. In particular, the presented embodiment can reduce or eliminate potential overshoots in the control process.

[0030] According to at least one further embodiment of the electronic circuit, the control arrangement is designed to regulate the predetermined setpoint of the controlled variable depending on the heating power and the manipulated variable in an external control loop.

[0031] In such embodiments, the control of the controlled variable to the setpoint can be understood as an inner control loop with corresponding inner control loops, and the control of the setpoint as an outer control loop with corresponding outer control loops.

[0032] The outer control loop can be superimposed on the inner control loop and regulate the setpoint of the inner control loop based on the heating power and the manipulated variable. The heating power can be determined from the heating DC voltage and current. In particular, several inner control loops can be executed during an outer control loop.

[0033] An additional control variable for the outer control loop can be determined depending on the heating power and the manipulated variable. The heating power and the manipulated variable have a direct influence on the filament temperature. Therefore, in some embodiments, this additional control variable can correspond to the effective impedance of the filament or an impedance equivalent of the filament.

[0034] One advantage of this design is the acceleration of the control process, particularly during the start-up or warm-up phase of a recording process. The maximum manipulated variable can be explicitly limited to a higher value, for example, to transfer a larger amount of energy into the filament and thus shorten the warm-up phase.

[0035] This can be particularly advantageous because, after the start of an X-ray exposure, adjusting the tube current by increasing or, more importantly, decreasing the heating current is only possible against the backdrop of a thermal time constant of the emitter resistance, i.e., the filament, which can be on the order of the exposure time of the X-ray image. Tube current only flows once the high voltage is switched on. It is therefore desirable that the tube current at the start of the exposure is as close as possible to the target value.

[0036] According to at least one further embodiment of the electronic circuit, the control arrangement is configured to determine a heating impedance from the heating DC voltage and the heating DC current. Furthermore, the control arrangement is configured to determine the controlled variable as a function of the heating impedance.

[0037] In particular, the control arrangement can be configured to determine the heating impedance from the heating DC voltage, the heating DC current and the output level of the inverter or the manipulated variable.

[0038] In particular, the heating impedance can also be referred to as the heating impedance equivalent, since the heating impedance of the filament cannot be directly determined, but can be determined, for example, from the quantities of the heating DC voltage, the heating DC current and the manipulated variable.

[0039] In other words, the control arrangement can calculate the heating impedance from the measured values ​​of the heating DC voltage and the heating DC current, specifically as the quotient of these values, where the heating DC voltage is the dividend and the heating DC current is the divisor. 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.

[0040] 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.

[0041] 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 is more accurate and can reach the setpoint more quickly. In particular, the presented embodiment can reduce or eliminate potential overshoots in a control process.

[0042] According to at least one further embodiment of the electronic circuit, the control arrangement is designed to set the heating direct current to a predefined initial value before an initial inner control loop of the inner control circuit is carried out.

[0043] In other words, the control process of the inner control loop can start with a small positive value as a predefined initial value for the heating DC 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 DC current value close to zero or equal to zero would cause the heating impedance to rise to very large values ​​or even to infinity.

[0044] One advantage of this embodiment is improved convergence of the inner control loop against the setpoint, particularly in the initial phase of the measurement.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] According to at least one further embodiment, the electronic circuit includes a transformer configured to provide the heating AC voltage on the primary side. Furthermore, the transformer is configured to provide a filament voltage on the secondary side, depending on a turns ratio, for heating the filament. The transformer can also be referred to as a heating transformer.

[0049] The transformer can maintain the heating AC voltage supplied by the inverter's output. 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 its turns ratio, the transformer can also output a higher, and in particular a significantly higher, voltage amplitude than it receives at the input. Thus, the transformer can provide a filament voltage for heating the filament. Specifically, this filament voltage is an alternating voltage.

[0050] One advantage of using a transformer is the galvanic isolation of the circuits and the possibility of increasing the voltage amplitude.

[0051] According to at least one further embodiment of the electronic circuit, the control arrangement includes a PI controller or a PID controller.

[0052] The PL controller or PID controller is specifically designed to change the manipulated variable depending on the controlled variable.

[0053] 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.

[0054] One advantage of using proportional (PL) 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 PL or PID controllers simplifies the design of the electronic circuit according to the invention and increases control accuracy.

[0055] According to a further aspect of the invention, an X-ray tube system, in particular an X-ray tube system for a medical imaging system using X-rays, is specified, comprising an electronic circuit according to the invention and an X-ray tube.

[0056] 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.

[0057] 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.

[0058] According to another aspect of the invention, a medical imaging system comprising an X-ray tube system according to the invention is specified.

[0059] 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.

[0060] 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.

[0061] 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 manipulated variable, and the alternating heating voltage is then provided. Furthermore, the direct heating voltage and a direct heating current resulting from the direct heating voltage are measured, particularly on the input side of an inverter, and a controlled variable is determined based on these measurements. The manipulated variable is then changed depending on the controlled variable in order to regulate the controlled variable to a predetermined setpoint, particularly within an internal control loop.

[0062] According to at least one embodiment of the method, the filament is heated depending on the heating alternating voltage.

[0063] According to at least one further embodiment of the method, the predetermined setpoint of the controlled variable for a given tube high voltage depends on a relationship between a predetermined tube current and the controlled variable.

[0064] The relationship between the specified tube current and the controlled variable can be determined through a preliminary calibration. Upon completion, the calibration can produce a table of values ​​that can be stored for later use, for example, in a data processing unit.

[0065] According to at least one further embodiment of the method, a heating power is determined from the heating DC voltage and the heating DC current. Furthermore, the control variable βe is determined as a function of the heating impedance.

[0066] According to at least one further embodiment of the method, a heating impedance is determined from the heating DC voltage and the heating DC current. Furthermore, the controlled variable is determined as a function of the heating impedance.

[0067] In particular, in a further embodiment of the method, the heating impedance can be determined from the heating DC voltage, the heating DC current and the output level of the inverter or the manipulated variable.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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 an electronic circuit according to the invention; and FIG 3 a schematic representation of a relationship between relative heating current and relative heating power with respect to the tube current; and FIG 4 a schematic block diagram of a further exemplary embodiment of an electronic circuit according to the invention; and FIG 5 a schematic block diagram of a further exemplary embodiment of an electronic circuit according to the invention.

[0072] In FIG 1 Figure 1 shows an embodiment of an electronic circuit 22 according to the invention for providing an AC heating voltage 6 for heating a filament 3 of an X-ray tube 15. The electronic circuit 22 has an inverter 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 at the output of the inverter 8. Furthermore, the electronic circuit 22 has a control arrangement 11, which is configured to measure the DC heating voltage 10 and a DC heating current 9 resulting from the DC heating voltage 10 and to determine a control variable depending on the DC heating voltage 10 and the DC heating current 9.The control arrangement 11 is designed to change the manipulated variable 12 depending on the controlled variable in order to control the controlled variable in a control loop, for example in an inner control loop, to a predetermined setpoint 14.

[0073] FIG 1 Figure 1 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 the X-ray dose that can occur during an X-ray examination and acts on an object being examined.

[0074] The control arrangement 11 can include in its inner control loop a measuring arrangement comprising at least one measuring device for measuring the heating DC voltage 10 and the heating DC current 9, an internal controller 20, and optionally an inverter 21. In at least one embodiment, the internal controller 20 can be a P1 or a PID controller. The type of controller 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 DC voltage 10 and the heating DC current 9 as a product or as a quotient of these two quantities. In the case that the heating DC voltage 10 is multiplied by the heating DC current 9, the controlled variable can thus be referred to as heating power 16.In the case where the heating DC voltage 10 is the dividend and the heating DC current 9 is the divisor, the controlled variable can also be referred to as the heating impedance 17. The specified setpoint 14 can serve as the input variable of the control arrangement 11 and can be used within the control arrangement 11 for comparison with the controlled variable. For example, the inverter 21 can first invert the value of the controlled variable, and then the internal controller 20 can compare the inverted value of the controlled variable with the setpoint 14, in particular by calculating a difference.

[0075] The control variable 12 can be provided to the inverter 8 by the control arrangement 11. 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 through the filament 3. These two quantities can be decisive for the temperature of the filament 3.

[0076] In FIG 1 A transformer 13 is also shown, which can galvanically isolate the heating AC voltage 6 from a circuit of the filament 3. In some embodiments, the transformer 13 is part of the electronic circuit 22. The transformer 13 receives the heating AC voltage 6 as an input 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 therefore directly cause a temperature change of the filament 3.

[0077] Another exemplary embodiment of the electronic circuit 22, which is based on the embodiment of the FIG 1 based, is in the block diagram of FIG 2 The heating DC current 9 can be measured, for example, via a measuring resistor 23 and detected, for example, using an optional low-pass filter 24, which can be used to smooth the heating DC current 9. In addition, the heating DC voltage 10 can be measured and likewise detected using an optional low-pass filter 25, which can be used to smooth the heating DC voltage 10. Smoothing in this context can be understood as averaging the measured value over a predetermined time interval. Such smoothing can, for example, remove short-term current or voltage spikes from a measurement to improve control. The controlled variable can be determined from both smoothed signals, namely the smoothed heating DC current 9 and the smoothed heating DC voltage 10. In this embodiment, the controlled variable is represented as heating power 16, which results from multiplying the two signals.

[0078] Inverter 8 is in FIG 2 This is exemplified by a full-bridge inverter. Full-bridge topologies with or without a resonant circuit can be used, for example, with power semiconductors with reverse voltages below 100 V, preferably in SMD packages. These could be, for example, Si-MOSFETs or GaN-FETs. Phase shift modulation or switching frequency modulation can be particularly suitable as a control method. Other types of inverters are also possible.

[0079] FIG 3 Figure 1 shows a functional diagram illustrating the fundamental relationship between a relative heating current 7' and a relative heating power 16' with the tube current 1, denoted here as It and plotted in milliamperes on the ordinate axis. In this context, "relative" is defined as the quotient of the value of the heating current 7 and its maximum value, or as the quotient of the value of the heating power 16 and its maximum value. The relationship is shown for various tube voltages 5, in particular for tube voltages 5 of 40 kV, 80 kV, and 125 kV. The relationship may depend on various factors in a medical imaging system, for example, the design and type of filament 3, and may deviate from the representation shown for certain embodiments.The graph shows that the three curves representing the relative heating current 7' exhibit a relatively steep slope, i.e., a high gradient, especially compared to the three curves relating to the relative heating power 16'. A small change in the relative heating current 7' therefore has a large effect on the tube current 1, I t. The relationship between the relative heating power 16' and the tube current 1, I t is significantly flatter; a small change in the relative heating power 16' thus has a small effect on the tube current 1, lt. In particular, the difference in the gradients can be, for example, a factor of 3.5 smaller.

[0080] Another exemplary embodiment of the electronic circuit, based on that of the FIG 1 based, is in FIG 4 Figure 11 illustrates an exemplary AC power supply 27 connected to a rectifier 26. The rectifier 26 can supply the electronic circuit 22 with the heating DC voltage 10. This heating DC voltage 10 can be supplied to the inverter 8 and cause a heating DC current 9. The control arrangement 11 is configured to measure the heating DC current 9 and the heating DC voltage 10 and to calculate the controlled variable from them. In this embodiment, the controlled variable is, for example, a heating impedance 17. The heating impedance 17 can be the quotient of the heating DC voltage 10 and the heating DC current 9. In at least this embodiment, the heating impedance 17 can represent the controlled variable for the internal controller 20 of the control arrangement 11. Additionally, Figure 11 illustrates the following: FIG 4 An inverter 21 is shown, which can invert the received value of the heating impedance 17 for comparison with the setpoint 14. The inverted value of the heating impedance 17 can be compared with the setpoint 14 in the internal controller 20. The internal controller 20 can be designed as a PL controller or as a PID controller with the features already described above.

[0081] Based on the sum of the inverted value of the heating impedance 17 and the setpoint 14, a value of the manipulated variable 12 is obtained, which the internal controller 20 can provide to a control signal generator 8'. The control signal generator 8' can, for example, generate individual switch positions as input signals for the inverter 8 and provide them to the inverter 8. The heating AC voltage 6 can be provided at the output of the inverter 8, in particular to the transformer 13. Depending on the turns ratio, the transformer 13 can generate a filament voltage 18 from the heating AC voltage 6 and provide this to the filament 3. The filament voltage 18 can then induce the filament current 4.

[0082] In the FIG 4 Furthermore, the X-ray tube 15 is shown, which contains the anode 2 in addition to the filament 3.

[0083] 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 .

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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 internal controller 20 to calculate the manipulated variable 12 and from this the control signals for the inverter 8. ΔR 7. Repeat steps 3 to 7.

[0088] 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. 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.

[0089] 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.

[0090] One advantage is that the internal controller 20 does not need to know the filament voltage 18 and the filament current 4. Furthermore, the filament current 4 can be kept more stable because 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.

[0091] 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 15 can be protected from overload.

[0092] 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.

[0093] In FIG 5 A further schematic representation of another exemplary embodiment of the electronic circuit 22 is shown, which is based on that of the FIG 1 The features already shown in the other figures apply here, unless otherwise indicated. In the exemplary embodiment shown, the control arrangement 11 includes, in addition to the inner controller 20, the inverter 21, and the control signal generation 8', an outer controller 19, which can be superimposed on the inner controller 20. The outer controller 19 receives the current values ​​of the tube high voltage 5, the tube current 1, and the values ​​of the controlled variable, in this case the heater power 16, and the value of the manipulated variable 12, in this case the pulse-pause ratio as the output level of the inverter 8. From these values, the outer controller 19 can calculate an effective impedance of the filament 3, which can also be referred to as the impedance equivalent. The outer controller 19 can provide the inner controller 20 with the setpoint 14, which in this case can also be referred to as an external manipulated variable.

[0094] The remaining parts of the electronic circuit 22 can be identical to those already described in the FIG 1 und FIG 2 the depicted parts. In particular, in FIG 5 The control signal generation unit 8' is also shown, which can generate the individual switch positions as input signals for the inverter 8. The designations S1, S2, S3, and S4 specifically identify an embodiment in which the inverter 8 has a full bridge and contains four switches. The four switches can, for example, be controlled by the output signals of the control signal generation unit 8'.

[0095] In an embodiment where the inverter 8 contains a full bridge and the heating DC voltage 10 is in the range of a minimum of 24 V and a maximum of 80 V, a very low-loss and compact design can be realized due to the available very small components, the smaller voltage gaps and the full bridge topology; for example, heat sinks, which are necessary for control via quantities in the AC voltage or AC current range, can be omitted.

[0096] 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 16 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.

[0097] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

1. Electronic circuit (22) for providing an alternating heating voltage (6) for heating a filament (3) of an X-ray tube (15), comprising: - an inverter (8) 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 manipulated variable (12) and to provide the alternating heating voltage (6) on the output side; and - a control arrangement (11) configured to measure the DC heating voltage (10) and a DC heating current (9) resulting from the DC heating voltage (10) and to determine a controlled variable depending on the DC heating voltage (10) and the DC heating current (9); and - the control arrangement (11) configured to change the manipulated variable (12) depending on the controlled variable in order to control the controlled variable in an inner control loop to a predetermined setpoint (14).

2. Electronic circuit (22) according to claim 1, wherein the control arrangement (11) is configured to determine a heating power (16) from the heating DC voltage (10) and the heating DC current (9); and to determine the controlled variable depending on the heating power (16).

3. Electronic circuit (22) according to claim 2, wherein the control arrangement (11) is configured to control the predetermined setpoint (14) of the controlled variable depending on the heating power (16) and the manipulated variable (14) in an external control loop.

4. Electronic circuit (22) according to claim 1, wherein the control arrangement (11) is configured to determine a heating impedance (17) from the heating DC voltage (10) and the heating DC current (9); and to determine the controlled variable as a function of the heating impedance (17).

5. Electronic circuit (22) according to claim 4, wherein the control arrangement (11) is configured to set the heating direct current (9) to a predefined initial value before performing an initial inner control loop of the inner control circuit.

6. Electronic circuit (22) 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).

7. Electronic circuit (22) according to one of the preceding claims, wherein - the electronic circuit (22) includes a transformer (13) configured to provide the heating AC voltage (6) on the primary side; and - the transformer (13) is configured on the secondary side, depending on a turns ratio, to provide a filament voltage (18) for heating the filament (3).

8. Electronic circuit (22) 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 (22) according to one of the preceding claims and an X-ray tube (15).

10. Medical imaging system comprising an 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 direct heating voltage (10) and a direct heating current (9) resulting from the direct heating voltage (10) 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 for heating a filament (3) of an X-ray tube (15) according to one of claims 11 or 12, wherein the predetermined setpoint (14) of the controlled variable for a predetermined tube high voltage (5) is determined by a predetermined relationship between a tube current (1) and the controlled variable.

14. Method according to one of claims 11, 12 or 13, wherein - a heating power (16) is determined from the heating DC voltage (10) and the heating DC current (9); and - the controlled variable is determined as a function of the heating power (16).

15. Method according to one of claims 11, 12 or 13, wherein - a heating impedance (17) is determined from the heating DC voltage (10) and the heating DC current (9); and - the controlled variable is determined as a function of the heating impedance (17).

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