Circuit for direct control of anode current of an x-ray tube with a unipolar or bipolar power

Abstract

The invention relates to a circuit intended to directly control the anode current of a cold cathode X-ray tube with a feedback signal proportional to the current (IA) of the anode itself by automatically adjusting the gate current (IG) of the X-ray tube, the control being carried out by a closed loop comprising: a closed loop control circuit (CLR1) generating an amplified error signal (SMOD), the amplified error signal is an amplified difference between a signal proportional to the anode current (IA) and the demand current (IDEM) of the anode; a digital sequencer circuit that converts the received signal into a plurality of digital control signals; a gate current control circuit receiving a digital control signal from the digital sequencer circuit and including at least one group formed by an inverter (INV) and an optional converter (DC-DC CONV); and a voltage and current adaptive transformer (TR), the rectified output of which provides the gate current (IG) necessary to obtain the desired anode current (IA). The invention avoids the need to calibrate and / or characterize the curve of the anode current (IA) with the gate current (IG), making the X-ray tube maintain a perfect state throughout its service life.

Description

Technical Field

[0001] As the title of the invention indicates, the object of the present invention is a circuit which directly controls the anode current of a cold cathode X-ray tube by automatically adjusting the grid current.

[0002] The present invention is characterized by the special design and configuration of each element as part of the control circuit, so that the current (IA) of the X-ray anode can be controlled by closed-loop control, wherein a feedback signal proportional to the anode current (IA) is used for automatic adjustment of the grid current (IG) of the X-ray tube.

[0003] In this way, the need to calibrate and / or characterize anode current (IA) versus grid current (IG) curves is avoided.Anode current (IA) is a fundamental parameter that must be controlled very accurately, as it is responsible for the number of X-ray photons emitted by the anode of the tube.

[0004] Due to the characteristics of the circuit, a higher accuracy and stability of the radiation emitted during the entire life of the X-ray tube is achieved, without the need for regular recalibration due to changes in the characteristics of the X-ray tube caused by the degradation suffered in daily use, because the degradation is compensated by the closed control loop of the circuit of the invention.

[0005] Therefore, the present invention belongs to the scope of X-ray devices, in particular cold cathode devices. Background Art

[0006] As is known in the art, the energy of X-ray radiation depends on the kV applied between the anode and cathode, while the amount of radiation depends on the current of the anode and the exposure time.

[0007] Low-temperature or room-temperature cathode X-ray tubes are currently being developed, as distinguished from thermionic emission tubes, which require an incandescent filament to heat the cathode and allow it to emit electrons toward the anode. With this new cold cathode technology, control of electron emission is achieved through a cathode formed from an array of carbon nanotubes (at room temperature), with a grid inserted between the cathode and the anode. Between the grid and the cathode, a sufficiently high electric field is applied to extract the electrons attracted and accelerated by the anode. The electrons striking the anode cause X-ray photons to be released, which are emitted by the anode with an energy determined by the voltage applied between the anode and the cathode. In order to control the number of electrons striking the anode, the electric field between the grid and the cathode must be controlled.

[0008] Figure 1A schematic diagram of a cold cathode X-ray device is shown, wherein the X-ray tube has a unipolar power supply, includes an anode (A) and a cathode (K), wherein the cathode (K) is formed by a series of carbon nanotubes and is arranged opposite to the anode (A), a grid (G) is inserted between the anode (A) and the cathode (K), wherein there is a first loop formed by the anode (A), the cathode (K) and a power supply (KV) located between the anode (A) and the cathode (K). Figure 1a The same tube is shown, in this case using a bipolar supply where half the voltage is supplied to the anode (A) and the other half to the cathode (K), with the center of both sources connected to ground to reduce insulation stress on the sources themselves and on the X-ray tube relative to ground. There is also a second loop formed by the gate (G), the cathode (K) and a voltage or current controllable power supply (V). The two loops share a common portion, so that current IA flows through the first loop, current IG flows through the second loop, and current IA+IG flows through the common portion.

[0009] Figure 2 shows a graph showing that the anode current (IA) varies linearly with the gate current (IG), while Figure 3 It is shown that the relationship between the anode current (IA) and the gate current (IG) with respect to the gate voltage (VG) is exponential, so controlling the anode current (IA) by the gate voltage is very complicated.

[0010] Figure 4 A diagram of a circuit for controlling anode current (IA) is shown, wherein the anode is grounded via a power supply (VAG), the gate (G) is directly grounded, and the cathode (K) is grounded via a power supply (Vsup) and a MOSFET switch, wherein there is a voltage drop (Vcont) across the MOSFET switch, and the MOSFET switch is controlled by a demand signal (IKDemand) for the cathode current.

[0011] Figure 4 The control scheme shown satisfies the following relationship:

[0012] VGK = Vsup – Vcont;

[0013] VAK=VAG+VGK;

[0014] VAK=VAG+Vsup-Vcont.

[0015] This control scheme performs linear control of the cathode current (IK) in a very complex manner. Furthermore, it requires a MOSFET capable of operating at several kilovolts, where the voltage drop between source and drain is very large.

[0016] Another major disadvantage of this type of control is that the voltage between the anode and cathode of the X-ray tube depends on the grid voltage (which varies as the X-ray tube ages), thus directly influencing the energy of the emitted radiation.

[0017] Figure 5 and 5a The X-ray exposure is controlled by controlling the grid current, keeping the voltages between the anode and cathode (VAK unipolar or bipolar) independent; however, in order to obtain the required anode current, the grid current must be known in advance. The grid current is either calibrated in advance or by Figure 2 The IA is determined by the characterization of the IA vs. IG curve shown, which can be modified by daily use of the X-ray tube and can also vary between tubes of the same type due to tolerances in its production process. The advantage of this type of controller is that when the required IG current is known in advance, said IG can be preloaded in the DC-DC converter before starting the X-ray exposure, thus shortening the start of the X-ray exposure by a few microseconds. Reducing this time is very important for scanning technology and making the X-ray exposure at the exact moment required, such as in batch production where the objects to be X-rayed are traveling on a conveyor belt. Considering that for more than 90% of radiation applications this time is not critical, it is preferred to focus on accuracy, stability and shorter downtime of the equipment for making a new recalibration.

[0018] The object of the present invention is therefore to develop a circuit which directly controls the anode current of a cold cathode X-ray tube by automatically adjusting the grid current (i.e. without the need to know said current by prior calibration or characterization). Furthermore, the control circuit which supplies the grid is isolated from the circuit which supplies the circuit between the anode and the cathode, which establishes a constant voltage VAK (unipolar or bipolar) between the anode and the cathode which is independent of the grid control, resulting in a control circuit such as that described below. Summary of the invention

[0019] The object of the invention is essentially contained in the independent claim, while different embodiments are contained in the dependent claims.

[0020] The circuit object of the present invention is designed to directly control the anode current (IA) of the X-ray exposure through a feedback signal proportional to the anode current, which is used to automatically control the grid current (IG) of the X-ray tube to maintain the required precise and stable anode current (IA) (IDEM) and is ultimately responsible for the number of emitted X-ray photons.

[0021] The control of the anode current (IA) is performed by a circuit that automatically provides the gate current (IG) through a closed-loop control and includes:

[0022] a closed-loop control circuit in which a comparison is made between a signal proportional to the anode current (IA) and a demand signal for the anode current (IDEM) to obtain a signal which is an amplified difference between the signal proportional to the anode current and the demanded anode current;

[0023] a digital sequencer circuit connected to the closed loop control and receiving a signal of an amplified difference between a signal proportional to the anode current and a required anode current, and converting the received signal into a plurality of digital control signals;

[0024] A gate current control circuit receives the digital control signal from the digital sequencer circuit and includes at least one inverter and an optional DC-DC converter, the digital control signal being applied to the DC-DC converter and the inverter through an adaptation transformer and a rectifier at its output to achieve a gate current, thereby obtaining an anode current almost equal to the required current.

[0025] The closed-loop control circuit preferably includes, but is not limited to, a comparator having a first input terminal and a second input terminal, wherein the first input terminal receives a signal (IFB) proportional to the anode current, and the second input terminal is a demand signal (IDEM) for the anode current. The two signals are compared and amplified at a gain G(s), and a signal is provided at its output terminal, which is the amplified difference between the signal proportional to the anode current and the demand current of the anode.

[0026] In a possible non-limiting embodiment, the gate current (IG) control circuit includes a connection to a supply voltage (Vsup), which is connected to a DC-DC converter (DC-DC CONV) known as a "buck converter", which is in turn connected to an inverter, the output of which is connected to a transformer, the output of which is connected to a rectifier, the output of which is connected to the gate and ground.

[0027] It should be noted that the DC-DC converter and the inverter can be integrated into a single inverter with a different topology than that described and controlled by voltage and / or current, thereby obtaining similar results to those described in the present invention. The same is true for closed-loop control, where the closed-loop control can be analog or digital, with almost the same results.

[0028] In one possible embodiment, when the digital sequencer circuit receives an exposure command (exp), it generates two signals, namely, a first signal (S1) for the DC-DC converter and a second set of control signals (S2) for the inverter (INV).

[0029] In a preferred but non-limiting embodiment, the DC-DC converter consists of a capacitor connected in parallel with the first switch and the first diode, a coil, and a return diode arranged in parallel with the assembly formed by the first switch and the coil.

[0030] The inverter is formed by a bridge including a second switch, a third switch, a fourth switch and a fifth switch, wherein the second switch and the fifth switch are connected in series to form a first branch, and the third switch and the fourth switch are connected in series to form a second branch, and the two branches are connected in parallel to each other.

[0031] Each branch is connected to a transformer in the middle, the output end of the transformer is connected to the rectifier bridge, and the necessary gate current is obtained from the rectifier bridge, while the lower ends of the two branches of the inverter and the DC-DC converter are grounded.

[0032] Thanks to the control circuit described, anode current control can be achieved directly by controlling the power supply to the grid via the transformer TR. If necessary, the transformer TR can provide a grid voltage of up to 15 kV or more. Since the grid control circuit is independent of the anode-cathode circuit, the VAK voltage is completely unaffected by the grid control. Since the VAK (whether unipolar or bipolar) voltage is responsible for the emitted radiation (photon) energy, the overall emission dose of X-rays is more accurate and stable.

[0033] The direct anode current control circuit has very important advantages which are fundamental in any fixed or portable radiology equipment and can be perfectly applied in equipment with tomography and / or tomosynthesis with one or more independently controlled X-ray tubes:

[0034] First, control of the gate does not change the anode-cathode voltage at all.

[0035] Second, the gate control circuit can provide 15KV or higher voltage if necessary.

[0036] Thirdly, the device does not require any type of calibration, characterization and / or maintenance, maintaining optimal accuracy throughout its lifetime.

[0037] Unless otherwise specified, all technical and scientific elements used in this specification have the meanings commonly understood by those skilled in the art to which the invention belongs. In the practice of the present invention, methods and materials similar or equivalent to those described in this specification can be used.

[0038] In the specification and claims, the word "comprise" and its variations are not intended to exclude other technical features, additions, components or steps. For those skilled in the art, other purposes, advantages and features of the present invention will be inferred in part from the specification and in part from the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] To supplement the description herein, and to help better understand the characteristics of the invention, according to a preferred practical embodiment of the invention, said specification is accompanied as an integral part thereof by a set of drawings, in which the following are given in an illustrative and non-limiting manner:

[0040] Figure 1 A schematic diagram of a cold cathode X-ray device using a unipolar power supply to power the X-ray tube is shown.

[0041] Figure 1a The same schematic diagram of a cold cathode X-ray device is shown, where in this example the X-ray tube is powered by a bipolar power supply.

[0042] Figure 2 A graph is shown showing that the anode current (IA) varies linearly with the gate current (IG).

[0043] Figure 3 The relationship between the anode current (IA) and the gate current (IG) and the gate voltage (VG) is shown.

[0044] Figure 4 A schematic diagram of a prior art circuit is shown for controlling the anode current (IA) via the cathode current (IK), which in turn affects the voltage (VAK) between anode and cathode (and therefore also the energy of the emitted radiation).

[0045] Figure 5 and 5a Unipolar and bipolar power supplies are shown respectively for a circuit for controlling X-ray exposure via a gate current (IG), which requires a process of characterizing and / or calibrating said current to obtain the desired anode current (IA).

[0046] Figure 6 A block diagram is shown of the control of a unipolar power supply with an X-ray tube used in an embodiment of the present invention, the control including direct control of the anode current (IA) by automatically controlling the grid current (IG).

[0047] Figure 6a The same block diagram is shown, but using a bipolar power supply for the X-ray tube.

[0048] Figure 7A non-limiting preferred embodiment of a gate current control circuit is shown, wherein a DC-DC converter is formed by a capacitor (C) connected in parallel with a first switch (Q1) and a diode (D1), a coil (L), and a return diode (D2) connected in parallel with the assembly formed by the first switch (Q1) and the coil (L). The output of the DC-DC converter powers an inverter formed by transistors Q2, Q3, Q4 and Q5, which in turn provides high frequency alternating current to a transformer TR1. The output of TR1 is rectified to deliver direct current to the gate of the X-ray tube.

[0049] Figure 8 An illustrative example of a digital sequencer operating cycle is shown, using analog type closed loop control operation before, during and at the end of an X-ray exposure.

[0050] Fig. 9 A possible embodiment of a current control circuit with a pulse width modulator PWM is shown, which acts directly and digitally on a DC-DC converter starting from an analog control input (SMOD). DETAILED DESCRIPTION

[0051] In view of the accompanying drawings, preferred embodiments of the present invention are described below.

[0052] Figures 1 to 5 and Figure 5a Corresponding to the description of the prior art of X-ray exposure control.

[0053] Figure 6 and 6a A block diagram of a direct anode current control circuit (IA) is shown, wherein the anode (A) is connected to a power supply (VAK with a unipolar or bipolar power supply) which is grounded via an anode current (IA) meter, while the cathode (K) is also grounded; on the other hand, the grid (G) is connected to the anode current via a closed-loop control circuit, wherein in a preferred but non-limiting embodiment, the control circuit comprises:

[0054] A closed-loop control circuit (CLR1) similar to or equivalent to PID, preferably but not limited to having a first input terminal and a second input terminal, wherein the first input terminal receives a current signal (IFB) proportional to the anode current (IA), and the second input terminal is a demand signal (IDEM) of the anode current. The two signals are compared and amplified by a gain amplifier G(s), and an amplified error signal (SMOD) is provided at the output terminal of the closed-loop control circuit (CLR1), and the amplified error signal (SMOD) is the amplified difference between the signal proportional to the anode current and the demand current (IDEM) of the anode;

[0055] A digital sequencer circuit, wherein there is a first input and a second input, the first input is the output of the closed-loop control circuit, in particular the amplified error signal (SMOD), and the second input is the X-ray exposure sequence signal (EXP). When receiving the X-ray exposure signal (EXP), the digital sequencer generates a first output (S1) and a second set of signals (S2), the first output (S1) controls the DC-DC converter through a PWM signal, and the second set of signals (S2) sequences the operation of the inverter (INV);

[0056] A gate current (IG) control circuit, which preferably but not limitedly includes a connection to a power supply (Vsup), which is connected to a DC-DC converter (DC-DC CONV) called a "buck converter", which is in turn connected to an inverter (INV), the output of which is connected to a transformer (TR), the output of which is connected to a rectifier (RECT), and the output of which is connected to a gate (G) and ground.

[0057] exist Figure 6 In the example, the power supply is a unipolar supply from a power source (VAK), where the anode (A) is connected to the power source (VAK) which is grounded through a shunt to measure the anode current IA, and the cathode (K) is also grounded.

[0058] exist Figure 6a In the example, the power supply is a bipolar supply where the anode (A) is connected to a supply (VAK / 2) connected to ground via a shunt to measure the anode current (IA), while the cathode (K) is connected to another supply (VAK / 2) also connected to ground.

[0059] Figure 7 A non-limiting preferred embodiment of the above elements of the gate current control circuit is shown, wherein a DC-DC converter is formed by a capacitor (C) connected in parallel with a first switch (Q1) and a diode (D1), a coil (L), and a return diode (D2) connected in parallel with the assembly formed by the first switch (Q1) and the coil (L).

[0060] The inverter is formed by a bridge including a second switch (Q2), a third switch (Q3), a fourth switch (Q4) and a fifth switch (Q5), wherein the second switch (Q2) and the fifth switch (Q5) are connected in series to form a first branch, and the third switch (Q3) and the fourth switch (Q4) are connected in series to form a second branch, and the two branches are connected in parallel to each other.

[0061] Each branch is connected to a transformer (TR1) in the middle, the output end of the transformer (TR1) is connected to a rectifier bridge, and a gate current (IG) is obtained from the rectifier bridge, while the lower ends of the two branches of the inverter and the DC-DC converter are grounded.

[0062] Figure 8 An example of the operation of the digital sequencer in different clock cycles (C1 to C12), the states of the different switches (Q1, Q2, Q3, Q4, Q5), and the exposure sequence input (EXP) and the error signal are shown; in this example, the error signal is an amplified analog signal (SMOD).

[0063] In cycle (C1), there is no exposure sequence, so all five transistors (Q1, Q2, Q3, Q4, and Q5) are off.

[0064] In cycle (C2), an exposure command (exp) is received and transistor (Q1) starts modulating according to the amplified error signal (SMOD) compared with a sawtooth signal (DS) synchronized with a clock signal (CLOCK) and generated in the digital sequencer itself. At the same time, a command is given to switch on the diagonal formed by transistors (Q2) and (Q4), and to keep (Q3) and (Q5) off.

[0065] In cycle (C3), transistor (Q1) continues to be modulated in the same manner as in the previous cycle, i.e. controlled by signals (SMOD) and (DS). At this time, the diagonal lines of transistors (Q2) and (Q4) are turned off, and the opposite diagonal lines (Q3 and Q5) are turned on, generating an AC signal to power the transformer (TR), wherein the output of the transformer (TR) is rectified to provide the gate DC (IG) necessary to achieve the required value of the anode current.

[0066] From cycle (C2) to (C11), the same control is continued in transistor (Q1). The alternation on the diagonal of the inverter is also maintained, that is, when (Q2) and (Q4) are turned on, transistors (Q3) and (Q5) are turned off, and the conduction state of the four transistors of the inverter is reversed in the next cycle.

[0067] Eventually (in this example, in cycle (C12)), the signal (EXP) issues a command to terminate the exposure. At this point, the five transistors (Q1, Q2, Q3, Q4, and Q5) are immediately turned off simultaneously. At this moment, the current stops flowing to the gate, instantly shutting down the X-ray exposure. The energy stored in the inductor (L) is transferred to the power supply (Vsup) through the diodes (D1) and (D2), and everything returns to the initial state in cycle (C1).

[0068] at last, Fig. 9A circuit is shown that converts the amplified analog error signal (SMOD) into a digital signal that controls a transistor (Q1) and, in turn, the output current (ISUP) of the DC-DC converter. It can be seen that the circuit includes a positive edge triggered flip-flop (FF1), which has: an activation input terminal, to which an AND gate (AND1) is connected, and the AND gate (AND1) has a clock signal (CLOCK) and an exposure start signal (EXP) as inputs; and a reset signal (RES), to which a comparator (COMP1) is connected, and the comparator (COMP1) has the instantaneous value of the amplified error signal (SMOD) and the sawtooth signal (DS) as inputs, so that when the inputs of the gate (AND1) are both "1", the flip-flop (FF1) will activate its output (Q) to turn on the transistor (Q1), and when it reaches and / or exceeds the value of the amplified error signal (SMOD) in the comparator (COMP1) and the instantaneous value of the sawtooth signal (DS), it will cause the output of the flip-flop (FF1) to become "0", thereby restarting the output (Q) of the flip-flop (FF1) and turning off the transistor (Q1) until the transistor (Q1) is turned on again in the next clock cycle (CLOCK).

[0069] It is worth noting that although the grid current (IG) ripple is proportional to the anode current, it has no effect on the dose control of X-ray exposure, because the dose control of X-ray exposure depends on the amount of photons generated in the anode of the tube, that is, the integral of the anode current during the exposure time, internationally known as mAs. On the contrary, the ripple of the voltage (VAK) is very important because it produces photons of different energies, which has a very important direct impact on the emission dose.

[0070] After fully describing the characteristics of the present invention, in addition to the way of putting it into practice, it should be noted that, in essence, the present invention can also be put into practice in other embodiments that are different in details from those shown in the examples, and the scope of protection is equally applicable to these embodiments, as long as the main principles thereof are not changed, varied or modified.

Claims

1. A control circuit for directly controlling the anode current in an X-ray tube by automatically adjusting the grid current, wherein the anode (A) is connected to a power supply (VAK) or (VAK / 2), which is grounded via an anode current meter, the cathode (K) is also grounded, and the grid (G) is connected to the anode current via a closed control loop, wherein in the preferred but non-limiting illustrated embodiment, the closed control loop comprises: a closed-loop control circuit (CLR1), the closed-loop control circuit (CLR1) comprising a comparator, in which a comparison is made between a signal proportional to the anode current (IA) and a demand signal (IDEM) of the anode current to obtain an amplified error signal (SMOD), the amplified error signal (SMOD) being an amplified difference between the signal proportional to the anode current (IA) and the demand current (IDEM) of the anode; a digital sequencer circuit, the digital sequencer circuit being connected to the closed-loop control circuit (CLR1), receiving the amplified error signal (SMOD) and converting the received signal into various digital control signals; a gate current control circuit, which receives the digital control signal from the digital sequencer circuit and comprises at least one group formed by an inverter (INV) and an optional converter (DC-DC CONV), the digital control signal being applied to the converter (DC-DC CONV) and the inverter (INV) supplying a transformer, the rectified output of the transformer providing a gate current (IG) so as to obtain an anode current almost equal to the required current (IDEM).

2. The control circuit according to claim 1, wherein: The closed-loop control circuit (CLR1) further comprises a gain amplifier G(s) arranged at the output end of the comparator.

3. The control circuit according to claim 1 or 2, wherein: The gate current (IG) control circuit also includes a connection to a supply voltage (Vsup), which is connected to an inverter component (INV) and optionally to the DC-DC converter (CONV DC-DC), wherein the output of the inverter (INV) is connected to a transformer (TR), and the output of the transformer (TR) is connected to a rectifier (RECT), and the output of the rectifier (RECT) is connected to the gate and ground.

4. The control circuit according to claim 3, wherein: The DC-DC converter is formed by a capacitor (C) connected in parallel with a first switch (Q1) and a diode (D1), a coil (L), and a return diode (D2) connected in parallel with the assembly formed by the first switch (Q1) and the coil (L), and the inverter is formed by a bridge including a second switch (Q2), a third switch (Q3), a fourth switch (Q4) and a fifth switch (Q5), wherein the second switch (Q2) and the fifth switch (Q5) are connected in series to form a first branch, and the third switch (Q3) and the fourth switch (Q4) are connected in series to form a second branch, and the two branches are connected in parallel to each other so that each branch is connected to a transformer (TR1) in the middle, the output end of the transformer (TR1) is connected to a rectifier bridge, and a gate current (IG) is obtained from the rectifier bridge, and the lower ends of the two branches of the inverter and the DC-DC converter are grounded.

5. The control circuit according to claim 1 and 3, wherein: The converter (DC-DC) and the inverter (INV) are integrated in a single inverter and controlled in terms of voltage and / or current, thereby obtaining very similar results.

6. A control circuit according to any one of the preceding claims, wherein: The digital sequencer circuit generates two signals upon receiving an exposure command (EXP), namely, a first signal (S1) for the converter (CONV DC-DC) and a second set of control signals (S2) for the inverter (INV).

7. The control circuit according to claim 6, wherein: The digital sequencer circuit comprises a positive edge triggered flip-flop (FF1), the flip-flop (FF1) having: an activation input terminal, an AND gate (AND1) connected to the activation input terminal, the AND gate (AND1) having a clock signal (CLOCK) and an exposure start signal (EXP) as inputs; and a reset signal (RES), a comparator (COMP1) connected to the reset signal (RES), the comparator (COMP1) having an amplified error signal (SMOD) and an instantaneous value of a sawtooth signal (DS) as inputs, so that when the inputs of the gate (AND1) are both "1", the flip-flop (FF1) will activate its output (Q), and when it reaches and / or exceeds the value of the amplified error signal (SMOD) in the comparator (COMP1) and the instantaneous value of the sawtooth signal (DS), the output of the flip-flop (FF1) will be "0", thereby restarting the output (Q) of the flip-flop (FF1) until it is turned on again in the next clock cycle (CLOCK).

8. A control circuit according to any one of the preceding claims, wherein: The power supply is a unipolar power supply from a power source (VAK), wherein the anode (A) is connected to the power source (VAK) which is grounded through a shunt to measure the anode current IA, and the cathode (K) is also grounded.

9. The X-ray exposure control circuit by gate current according to any one of claims 1 to 7, wherein: The power supply is a bipolar power supply, where the anode (A) is connected to a power supply (VAK / 2) connected to ground through a shunt to measure the anode current (IA), while the cathode (K) is connected to another power supply (VAK / 2) also connected to ground.

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