EP0471625B1 - Device for obtaining of an adjustable d.c. voltage - Google Patents

Description

The present invention relates to devices for obtaining a DC voltage whose value is adjustable over a wide range, said devices being more particularly adapted to polarize a focus of an X-ray tube to a value chosen by the practitioner implementing a radiological installation. Such devices are known from documents US-A-4, 541, 041 and GB-A-2,045,019.

A radiological tube is generally constituted as a diode, that is to say by two electrodes, one of which, called cathode, emits electrons while the other, called anode, receives these electrons on a small surface which constitutes the X-ray source.

The cathode has a filament heated by an electric current which constitutes the source of electrons. When a high voltage, supplied by a generator, is applied across the two electrodes, so that the cathode is at a negative potential, a so-called anodic current is established through the generator and crosses the space between the cathode and the anode in the form of an electron beam.

To concentrate the electron beam, a metal part, called a concentration part, supporting the filament is isolated from the latter and brought to a negative potential, called polarization, with respect to said filament. In addition, to modify the shape and therefore the concentration of the electron beam, it is usual to modify this polarization potential over a wide range, for example between 300 and 3000 volts. In addition, it should be noted that the cathode is itself brought to a voltage of the order of -20 to -75 kilovolts relative to the mass, which poses insulation problems to apply this potential or bias voltage.

The invention relates more particularly to a device for obtaining a bias voltage of a concentrating part of an X-ray tube cathode which is variable over a wide range of values.

Such devices are known and, for information, Figure 1 gives the block diagram of a device of the prior art. It includes a supply circuit 10 which supplies a DC voltage E regulated and adjustable from an AC voltage supplied by the sector. The voltage E is applied to the terminals of an inverter circuit 11 which comprises a chopping circuit 12 and a control circuit 14.

The alternating signal supplied by the inverter circuit 11 is applied to a voltage step-up transformer 15, the secondary winding of which is connected to a rectification and filtering circuit 16. This circuit 16 provides a direct voltage V _s which is applied between the piece of concentration and filament of the x-ray tube.

It should be noted that the voltage V _s being difficult to measure due to the high common mode potential (20 to 75 kilovolts), it is preferable to measure the voltage E which is substantially proportional to it and to regulate it. For this purpose, the voltage E is measured by a resistive divider comprising the resistors R1 and R2 and the divided signal is applied to a voltage / frequency converter circuit 20 which receives, moreover, a signal V _ref corresponding to the voltage that the it is desired to obtain between the concentration piece and the filament of the x-ray tube. The converter circuit 20 supplies pulses of variable frequency and / or of variable duration which control the switches of the supply circuit 10 so as to modify the voltage of output E and therefore modify the voltage V _s to obtain V _s = V _ref .

Conventionally, the chopper circuit 12 comprises, for example, two transistors 21 and 22, the openings and closings of which are controlled by the control circuit 14.

The control circuit 14 is also a voltage / frequency converter circuit similar to the circuit 20 but whose frequency is fixed.

• de nécessiter deux convertisseurs de puissance : le premier 20 pour réguler la tension E et le deuxième 14 pour l'onduler,
• de commuter brutalement le courant dans les semiconducteurs, ce qui est source de parasites,
• d'avoir une faible gamme de tensions de sortie car la tension E que l'on règle ne peut pas tendre vers zéro à cause des limitations en rapport cyclique du circuit hâcheur.

The disadvantages of this device of the prior art which has just been described are:

• to require two power converters: the first 20 to regulate the voltage E and the second 14 to undulate it,
• suddenly switching the current in the semiconductors, which is a source of noise,
• to have a low range of output voltages because the voltage E which is regulated cannot tend towards zero because of the limitations in cyclic relation of the chopper circuit.

The object of the present invention is therefore to provide a device for obtaining an adjustable direct voltage which does not have the above-mentioned drawbacks.

• des moyens d'alimentation pour élaborer une tension continue E constante,
• des moyens pour onduler ladite tension continue E de manière à obtenir des impulsions alternatives de fréquence F correspondant chacune à une quantité d'électricité constante d'une impulsion à la suivante,
• un transformateur pour éléver la tension des impulsions alternatives,
• des moyens pour redresser et filtrer lesdites impulsions alternatives élévées de manière à obtenir ladite tension continue V_p.
• des moyens pour modifier la fréquence F desdites impulsions alternatives en fonction de la tension continue V_p que l'on souhaite obtenir.

The invention relates to a device for obtaining an adjustable direct voltage V _p characterized in that it comprises:

• supply means for developing a constant DC voltage E,
• means for undulating said direct voltage E so as to obtain alternating pulses of frequency F each corresponding to a constant quantity of electricity from one pulse to the next,
• a transformer to raise the voltage of the alternating pulses,
• means for rectifying and filtering said high alternating pulses so as to obtain said direct voltage V _p .
• means for modifying the frequency F of said alternating pulses as a function of the DC voltage V _p which it is desired to obtain.

The means for undulating said voltage E comprise an oscillating circuit whose resonant frequency is greater than the frequency F.

The frequency F is determined by calibrating the device by taking the curve V _p = f (F), curve whose characteristics are recorded by a microprocessor.

• la figure 1 est un schéma de principe d'un dispositif d'obtention d'une tension continue réglable selon l'art antérieur,
• la figure 2 est un schéma de principe d'un dispositif d'obtention d'une tension continue réglable selon l'invention,
• les figures 3-a et 3-b sont des diagrammes montrant d'une part une courbe d'étalonnage du dispositif et, d'autre part, la linéarité du dispositif selon l'invention.
• les figures 4-a à 4-f sont des diagrammes permettant de comprendre le fonctionnement du dispositif selon l'invention,

Other objects, characteristics and advantages of the present invention will appear on reading the following description of a particular embodiment, said description being made in relation to the accompanying drawings in which:

• FIG. 1 is a block diagram of a device for obtaining an adjustable direct voltage according to the prior art,
• FIG. 2 is a block diagram of a device for obtaining an adjustable direct voltage according to the invention,
• the figures 3-a and 3-b are diagrams showing on the one hand a calibration curve of the device and, on the other hand, the linearity of the device according to the invention.
• FIGS. 4-a to 4-f are diagrams making it possible to understand the operation of the device according to the invention,

• un microprocesseur 30 auquel est appliqué un signal de commande indiquant la valeur d'une tension continue V_p à obtenir et qui fournit un signal numérique N_p indiquant une fréquence F caractéristique de la tension continue V_p à obtenir,
• un compteur programmable 31 auquel est appliqué le signal numérique N_p correspondant au signal V_p fourni par le microprocesseur 30 et qui fournit des impulsions de fréquence variable F selon la valeur de N_p et donc de V_p,
• un circuit de commande 32 auquel sont appliquées les impulsions de fréquence variable F et qui fournit sur ses sorties 32-a et 32-b des impulsions de commande des interrupteurs T1 et T2 d'un circuit onduleur 35, et
• un circuit dit de puissance 33, comprenant le circuit onduleur 35, qui fournit sur ses bornes de sortie 33-a et 33-b la tension continue V_p.

In accordance with the functional block diagram of FIG. 2, the device for obtaining an adjustable direct voltage according to the invention comprises:

• a microprocessor 30 to which a control signal is applied indicating the value of a direct voltage V _p to be obtained and which supplies a digital signal N _p indicating a frequency F characteristic of the direct voltage V _p to be obtained,
• a programmable counter 31 to which the digital signal N _p corresponding to the signal V _p supplied by the microprocessor 30 is applied and which supplies pulses of variable frequency F according to the value of N _p and therefore of V _p ,
• a control circuit 32 to which the variable frequency pulses F are applied and which supplies on its outputs 32-a and 32-b control pulses for the switches T1 and T2 of an inverter circuit 35, and
• a so-called power circuit 33, comprising the inverter circuit 35, which supplies on its output terminals 33-a and 33-b the direct voltage V _p .

The power circuit 33 comprises, in addition to the inverter circuit 35, a first rectification and filtering circuit 34 which, from an alternating voltage e, supplies a regulated direct voltage E supplying the switches T1 and T2. The pulses supplied by the inverter circuit 35 are applied to the primary winding 36p of an isolation transformer 36 of the pulse type, the secondary winding 36s of which is connected to a rectification and filtering circuit 37 which supplies the required DC voltage V _p .

The inverter circuit 35 comprises, as indicated above, at least two switches T1 and T2 produced by field effect transistors according to the metal-oxide technology more known by the English abbreviation of MOSFET transistors. By construction, these transistors T1 and T2 each include in parallel a diode D1 for the transistor T1 and a diode D2 for the transistor T2, diodes including the anode is connected to the source S and the cathode connected to the drain D of the associated transistor. The gate G of the transistor T1 is connected to the output 32-a of the control circuit 32 while the gate G of the transistor T2 is connected to the output 32-b of the control circuit 32.

The inverter circuit also includes a resonant circuit consisting of capacitors C1 and C2 and a coil L. The capacitors C1 and C2 are connected in series between the drain D of the transistor T1 and the source S of the transistor T2 while the coil L is disposed in the primary circuit 36p of the transformer 36 and is connected on one side directly to the source of the transistor T1 and on the other side to the common point C of the capacitors C1 and C2 via the primary winding 36p of the transformer 36.

In a known variant, the inverter circuit may comprise only one capacitor, instead of the two capacitors C1 and C2, which would be connected for example to the negative terminal of the supply circuit 34.

The rectification and filtering circuit 37 is of the conventional type and has an output resistance R at the terminals of which the bias voltage V _{p is taken} .

The control circuit 32 comprises a first logic AND circuit 40 which has two inputs on one of which are applied the variable frequency pulses F supplied by the circuit 31 while the other input is connected to a first delay circuit 41 whose delay is Θ₁. The output of the AND circuit 40 is connected, on the one hand, to a bistable circuit 43 and, on the other hand, to the first delay circuit 41 as well as to a second delay circuit 42 whose delay is Θ₂.

The output corresponding to state 1 of the bistable circuit 43 is connected to one of the two inputs of a second logic AND circuit 44 while the output corresponding to state 0 is connected to one of the two inputs of a third AND circuit logic 45. The second input of AND circuits 44 and 45 is connected to the output of the second delay circuit 42.

c'est-à-dire qu'il donne pour chaque valeur de la tension de polarisation V_p, souhaitée par le praticien ou par le dispositif de commande, un code numérique, par exemple à huit chiffres ou digits qui, appliqué au compteur 31, conduit ce dernier à fournir des impulsions de fréquence F. Ces impulsions de fréquence F ont pour but de commander alternativement les transistors T1 et T2 par l'intermédiaire du circuit 32 de manière à créer des impulsions de courant dont le redressement et le filtrage dans le circuit 37 conduisent à la tension V_p souhaitée entre les bornes 33-a et 33-b.The microprocessor 30 performs the function: $${\text{NOT}}_{\text{p}}{\text{= f (V}}_{\text{p}}\text{)}$$

that is to say that it gives for each value of the bias voltage V _p , desired by the practitioner or by the control device, a digital code, for example with eight digits or digits which, applied to the counter 31 , leads the latter to supply pulses of frequency F. The purpose of these pulses of frequency F is to alternately control the transistors T1 and T2 by means of circuit 32 so as to create current pulses whose rectification and filtering in circuit 37 leads to the desired voltage V _p between terminals 33-a and 33-b.

In other words, the microprocessor 30 and the counter 31 carry out the function F = f '(V _p ), function which is obtained by calibration and whose pace is given by the curve 81 of the figure 3-a. This curve 81 takes account of the linearity faults of the system while the curve 80 is a theoretical curve.

The operation of the device according to the invention will now be explained using FIG. 2 and the diagrams of FIGS. 3 and 4. At a bias voltage V _p desired by the practitioner or by the device for controlling the radiological device corresponds to a numerical code N _p which, applied to counter 31, leads the latter to supply pulses 70 and 70 '(figure 4-a) at frequency F according to the correspondence given by curve 81 of figure 3-a. These pulses have for example a frequency of 30 kilohertz to obtain V _p = 3000 volts and a duration of about one microsecond. If it is assumed that the delay circuit 41 provides an opening signal 71, the pulse 70 controls the change of state of the bistable circuit 43 which passes, for example, to state 1. The pulse 70 controls the delay circuit 41 to end the opening signal 71 (figure 4-c) so that the AND circuit 40 closes for a time Θ₁. The pulse 70 also controls the delay circuit 42 so that it provides a signal T'1 of duration Θ₂ (figure 4-b) which makes passing the circuits AND 44 and 45. Only the circuit AND 44, which receives the signal state 1 of the bistable circuit 43, provides a signal T'1 making the transistor T1 conductive at time t _o (FIG. 4-d).

This signal T'1 makes and keeps the transistor T1 conductive and a current i₁ (figure 4-d) says positive, flows in the transistor T1, the coil L, the primary winding 36p of the transformer 36, the capacitors C1 and C2 and the supply circuit 34 (in fact i₁ / 2 in each capacitor).

This current i₁ gives rise to a voltage V (figure 4-e) of rectangular shape at the terminals of the primary winding 36p and this results in a current I (t) (figure 4-f) in the secondary winding 36s of the transformer 36, current of identical appearance to the current i₁ flowing in the primary winding.

The current i₁ charges the capacitor C2 and discharges the capacitor C1 and their charging voltage is opposed to the circulation of the current i₁ so that the latter is canceled at time t₁, that is to say before the end of signal T'1. The capacitor C2 then discharges while the capacitor C1 charges and a current i₂ (figure 4-d), said to be negative, flows in the capacitors C1 and C2, the primary winding 36p, the coil L, the diode D1 and the supply circuit 34 (in fact i₂ / 2 in each capacitor).

This negative current gives rise to a negative rectangular tension (figure 4-e) at the terminals of the primary winding 36p and, consequently, to a negative current I (t) (figure 4-f) in the secondary winding 53. When the current i₂ is canceled, the pulse is ended.

Before time t₂, the signal T'1 ends by the effect of the delay circuit 42 introducing a delay Θ₂ so that the AND circuits 44 and 45 are blocked.

After the time t₂ and more precisely after a delay Θ₁ from the end of the signal 71 (figure 4-c), the delay circuit 41 provides a signal 71 'which turns the AND circuit 40 on.

After a variable time defined by the frequency F, a pulse 70 'is supplied by the circuit 31 and its front edge controls the change of state of the bistable circuit 43, which goes to state 0 , as well as the resetting of the delay circuits 41 and 42.

This reset has the effect of terminating the signal 71 'and of supplying the signal T'2 which opens the AND circuits 44 and 45. As the bistable circuit 43 is at state 0 , only the AND circuit 45 provides a signal output on terminal 32-b and a pulse is applied to the control electrode of transistor T2 at time t ' _o to make it conductive. A current i'₁, said to be negative, then flows in the transistor T2, the circuit 34, the capacitors C1 and C2 (in fact i'₁ / 2 in each condenser), the primary winding 36p of the transformer 36 and the coil L. This negative current gives rise to a negative voltage V (figure 4-e) of rectangular shape across the terminals of the primary winding 36p and it results in a current I (t) negative (figure 4-f) in the secondary winding 36s of the transformer 36, current of identical appearance to the current i'₁ circulating in the primary winding.

The negative current i'₁ charges the capacitor C1 and discharges the capacitor C2 and their charging voltage is opposed to the circulation of the current i'₁ so that the latter is canceled out at time t'₁. The capacitor C1 then discharges while the capacitor C2 charges and a positive current i'₂ flows in the capacitors C1 and C2 (in fact i'₂ / 2 in each capacitor), the primary winding 36p, the coil L, the diode D2 and the supply circuit 34. This positive current gives rise to a positive rectangular voltage (FIG. 4-e) at the terminals of the primary winding 36p and, consequently, to a positive current I (t) (figure 4-f) in the secondary winding 36s. When the current i'₂ is canceled, the pulse is ended.

The pulses which are thus created by the inverter circuit 35 are applied to the transformer 36 and are rectified and filtered in the circuit 37 and it appears at the terminals of the load resistor R a voltage V _p corresponding to the frequency F determined by calibration.

This relation between the frequency F and the voltage V _p results from the fact that the electric charge contained in each pulse (figures 4-d and 4-f) is always the same whatever the point of operation provided that the frequency F is less than the frequency of the resonant circuit of the inverter circuit, which means that the inverter circuit is of the pulse hyporesonance type.

avec
   E la tension d'alimentation,
   V la tension aux bornes de l'enroulement primaire 36p,
Z= $\sqrt{\text{L/C}}$

l'impédance du circuit résonnant,
   avec C = C1 + C2,
T= 2 π $\sqrt{\text{LC}}$

on en déduit Q = 2 CE c'est-à-dire une constante si E et C sont constants, ce qui est le cas car le circuit d'alimentation 34 fournit une tension régulée et la capacité C est fixée par construction.Indeed, the electric charge Q of a pulse (figure 4-d) is given by:

with
E the supply voltage,
V the voltage across the primary winding 36p,
Z = $\sqrt{\text{L / C}}$

the impedance of the resonant circuit,
with C = C1 + C2,
T = 2 π $\sqrt{\text{LC}}$

we deduce Q = 2 CE that is to say a constant if E and C are constant, which is the case because the supply circuit 34 provides a regulated voltage and the capacitance C is fixed by construction.

de sorte que la tension V_p = RI_r = R x Q x F, ce qui signifie que V_p est proportionnel à F car R et Q sont des constantes. Ceci correspond à la courbe en pointillés 80 de la figure 4-a. Cependant, en pratique, le phénomène n'est pas parfaitement linéaire et la courbe réelle est celle référencée 81. Pour que le dispositif selon l'invention fonctionne selon la courbe 81, il est nécessaire de réaliser un étalonnage en utilisant au moins deux points de fonctionnement, par exemple ceux définis par A et B sur la courbe 81.However the current I _r which circulates in the load resistance R is given by: $${\text{I}}_{\text{r}}\text{= Q x F}$$

so that the voltage V _p = RI _r = R x Q x F, which means that V _p is proportional to F because R and Q are constants. This corresponds to the dotted curve 80 of the figure 4-a. However, in practice, the phenomenon is not perfectly linear and the actual curve is that referenced 81. In order for the device according to the invention to operate according to curve 81, it is necessary to carry out a calibration using at least two points of operation, for example those defined by A and B on curve 81.

The curves 80 'and 81' of the figure 4-b show the variations of the ratio V _p / F according to the frequency F in correspondence with the curves 80 and 81 respectively of the figure 4-a. These curves and in particular the real curve 81 ′ resulting from the calibration are linear throughout the range.

In the description of the operation of the inverter circuit 35, it was indicated that the currents i₁, i₂, i'₁ and i'₂ circulated in the capacitors C1 and C2 but it is clear that each of these currents is divided into two parts equal to the point C, one half towards the branch containing the capacitor C1 and the other half towards the branch containing the capacitor C2.

The rectangular shape of the signals of the figure 4-e is due to the fact of the presence of the rectifying and filtering circuit 37 including / understanding diodes which, while becoming conducting, realize short-circuits.

Claims (3)

1. A device for producing a variable DC voltage V_p for the concentration member of the cathode of an x-ray tube, comprising:

   - supply means (33) for deriving a constant continuous voltage E,

   - means (35) for undulating said continuous voltage E in such a manner as to obtain alternating pulses with a frequency F each corresponding to a quantity of electricity which is constant from one pulse to the next one, said means (35) consisting of an oscillating circuit whose resonant frequency is greater than the frequency F,

   - a transformer (36) for increasing the voltage of the alternating pulses,

   - means (37) for rectifying and filtering the said increased alternating pulses in such a manner as to obtain the said continuous voltage V_p,

   - means for modifying of the frequency F of the said alternating pulses as a function of the continuous voltage V_p which it is desired to obtain,

   - means (31) for determining by calibration the frequency F of the said pulses as a function of the voltage V_p to be obtained,

   - means (32) for deriving control pulses at the frequency F from information as regards the value of the said frequency F, said pulses being applied to the said means for undulating said continuous voltage E.
2. The device as claimed in claim 1, characterized in that the means for deriving control pulses at the frequency F comprise:

   - a count circuit (31) which furnishes pulses with the frequency F, and

   - a logic circuit (32) which furnishes signals for control of the means for undulating the voltage E whose duration is greater than half a cycle but less than the said resonance cycle and whose repetition cycle is at the most equal to the said resonance cycle.
3. The device as claimed in claim 2, characterized in that the logic circuit (32) comprises:

   - a first AND circuit (40) of which one of the two inputs is connected with the output of the count circuit (31),

   - a bistable circuit (43) whose control input is connected with the output of the first AND circuit (40) in such a manner as to change in state responsive to each signal furnished by the latter,

   - a second AND circuit (44) of which one of the two inputs is connected with the output of the bistable circuit (43) corresponding to the 1 state

   - a third AND circuit (45) of which one of the two inputs is connected with the output of the bistable circuit (43) corresponding to the 0 state,

   - a first delay circuit (41) of which the input is connected with the output of the first AND circuit (40) and of which the output is connected with the second input of the first AND circuit (40), and

   - a second delay circuit (42) whose input is connected with the output of the first AND circuit (40) and whose output is connected with the other input of the second and third AND circuits (44 and 45).

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