FR2709396A1 - Grid control device for an X-ray tube and implementation method - Google Patents

Abstract

The invention relates to a device for controlling the grid of an X-ray tube, having a capacitor (Cp) located between the grid and the cathode, which comprises a source intended to establish and remove a high voltage across the terminals of the said capacitor (Cp) for a very short period of time, for blocking and reestablishing X-ray emissions, respectively, this source consisting of - a four-diode bridge (16) constituted by two branches connected together and each constituted by two diodes (D5, D7) and (D6, D8) mounted in series, one (D8) of which is connected to the terminals of the said capacitor (Cp): - an electronic circuit intended to supply and remove an electric charge to and from the said capacitor, connected at its output to the mid-points (M1, M2) of the two branches of the bridge (16); - an energy receiver (19), placed at the output of the diode bridge (16) and maintaining the voltage across the terminals of the capacitor (Cp) at the determined value. It also relates to its implementation method and applies particularly to radiology equipment.

Description

GRID CONTROL DEVICE
X-RAY TUBE
AND METHOD OF IMPLEMENTING
The invention relates to a grid control device of an X-ray tube particularly used in radiology equipment, especially for cardiovascular examinations, as well as its method of implementation. During an operation of the heart, surgeons may wish to visualize firstly, in real time, the heartbeat, which requires to make cinema - 24 frames per second - and secondly, in slow motion, the video recording of the intervention, without jerky scrolling images. This last mode of visualization supposes to increase considerably the frequency of recording of the poses - approximately 150 Hz -.

Currently, as schematically shown in FIG. 1, a radiological equipment comprises a high voltage generator 1 supplying an X-ray tube 2, a patient support 3, an image receiver 4 and an image processing system 5. The X-ray tube consists of a cathode 9 and an anode 7, placed in a vacuum enclosure 8 and between which the very high voltage is applied. The cathode 9 is made by a thermoelectronic emitter, such as a tungsten filament, housed in a metal piece of concentration 10, and which emits a beam 11 of electrons when it is heated, focused on the anode 7. This The latter is generally constituted by a massive graphite disk covered with a layer of high atomic number material emitting an X-ray beam 12 when it is bombarded by an electron beam 11. The anode is brought to a high potential positive - several tens of kilovolts - with respect to the cathode and the electric field thus created between the two electrodes accelerates the electrons emitted by the cathode and striking the anode on a small surface or area of impact of said electron beam on the anode , which is the focus of emission of X-radiation. The high voltages that must be applied to the electrodes are provided by the power generator 1 said high voltage.

The energy expended to produce the electron beam is transformed into X-rays on the one hand but especially into heat. Therefore, to avoid excessive heating of the anode, due in particular to the small dimensions of the focus - about 1 millimeter - during operation of the tube, the anode is rotated to distribute the heat flow on a so-called crown focal length whose surface is much larger than that of the focus. This rotation is done using an asynchronous electric motor powered by the high voltage generator.

To cool an X-ray tube dissipating a large amount of energy during operation, it is placed inside an insulating sheath 13 filled with a cooling fluid, usually electrically insulating oil. This sheath is opaque to X-rays except at a window 14 through which the X-ray beam 12 passes through to pass through a patient to be examined. This beam is then converted to photons by means of a fluorescent plate to obtain an X-ray or a pose.

Obtaining a minimum of 24 exposures per second, to make cinematographic captures of the surgical procedure, is done by switching the emission of X-rays, not by the control of the high voltage generator but by the control of a grid placed between the cathode and the anode. This grid is constituted by the concentration piece 10 electrically isolated from the filament of the cathode 9. This grid blocks the emission of electrons towards the anode when it is brought to a sufficiently negative potential with respect to that of the cathode and restores the emission when the potentials of the cathode 9 and the gate are identical.

However, the gate control uses high-voltage forced switching using high-voltage controllable switches to be oversized, associated with bulky rectifying and filtering circuits. To reduce this size and substantially increase the number of poses per second - from 24 to 120 for example the invention aims to eliminate any forced switching under high voltage, which has the additional advantage of reducing the cost of the device.

From experience, there is the existence of a capacity
Cp located between the cables of the gate and the cathode, and typically evaluated at 6nF in a non-limiting example.

The object of the invention is to design a grid control device of an X-ray tube comprising a voltage source intended to supply and remove, to the capacitance existing between the gate and the cathode, the charge Q necessary for the voltage at the terminals of this capacitance reaches a high value, about 3000 Volts in the case of a capacity of 6 nF, that is to say the value of the voltage to be applied between the gate and the cathode to block the X-ray emission, for a very short time, less than 150 As. The power supplied by this source is reactive. Since the cathode can have a potential equal to -75,000 volts, the load Q must be supplied through a transformer.

For this, the object of the invention is a device for controlling the grid of an X-ray tube, this gate being placed between the cathode and the anode and having a capacitance located between said gate and the cathode, characterized in that it comprises a source intended to establish and remove a voltage across the capacitance, for a very short given duration, of a determined value for respectively blocking and re-establishing the emission of X-rays by the tube, this source comprising: a bridge of four diodes consisting of a first and
a second branch connected to each other and
each made from two diodes mounted in
series, one of the diodes of the second branch being
connected to the terminals of the capacity; - an electronic circuit intended to provide and remove
electrical charge with parasitic capacitance, whose
exit is connected to the middle points of the two branches
bridge; - a receiver of energy, placed at the exit of the bridge of
diodes and now the voltage across the
parasitic capacitance at the determined value.

Other features and advantages of the invention will appear on reading the following description of a particular embodiment, said description being made in relation to the accompanying drawings in which, in addition to Figure 1 already described and which is a diagram in longitudinal section of a radiological equipment - FIG. 2 is a schematic diagram of the device of
grid control of an X-ray tube according to
the invention; FIG. 3 is the electrical diagram of the device
grid control of an X-ray tube according to
the invention; - Figure 4 is the chronogram of the different
sequences applied to the commands of the elements of the
device according to the invention; FIGS. 51 to 58 are each the electrical diagram
of the control device on which the
different phases of operation of the device
command.

The elements bearing the same references in the different figures perform the same functions in view of the same results.

Figure 2 is the block diagram of the gate control device according to the invention. It comprises a bridge 16 of four diodes Dg, D6, D7 and D8 of which one of the diodes D8 is connected in parallel with the capacitance
Cp, to allow said capacitance Cp to reach the potential necessary to stop the flow of electrons emitted by the cathode, about 3000 volts, to keep it constant for a certain time and to put it back to 0 Volt quickly. It also comprises a DC voltage converter 17, for example resonant type LC series, connected to a DC voltage source E and providing a charge Q to the capacitor Cp through a transformer 18 connecting said bridge from diodes 16 to the converter which operates in pulse mode. A power receiver 19 placed at the output of the diode bridge 16 is intended to limit the voltage of the capacitor Cp and to absorb the excess charge supplied by the resonant converter 17. It also comprises a demagnetizing circuit 20 of the transformer 18 , avoiding the saturation thereof and finally a programmable control logic circuit 26 connected to the converter 17 and the demagnetizing circuit 20.

In Figure 3, which is the electrical diagram of the grid control device according to the invention, are detailed the various elements that compose it. The diode bridge 16 comprises four diodes D5, D6, D7 and D8 placed in series in pairs in two branches interconnected. The energy receiver 19 is connected to both ends of the branch constituted by two of the diodes D6 and D8. The parasitic capacitance Cp must be charged under high voltage - about 3000 volts - to block the flow of X-rays and then unloaded quickly, without resorting to forced commutation.

This is why the two branches of the bridge are connected to the two terminals of the secondary 22 of the transformer 18 and the diode D8 is placed across the capacitor Cp to be loaded.

As has been said before, the voltage across the capacitor Cp must be maintained at a high value, about 3000 Volts, by a circuit 19 which operates as a receiver, providing no energy to the assembly and having to absorb the energy which does not not used to load the capacity. For this, the circuit consists of a Zener diode maintaining this high voltage across the capacitor Cp and dissipating the additional energy supplied by the converter 17.

In FIG. 3 appears the electronic circuit diagram of the AC series resonant AC voltage direct voltage converter 17, responsible for providing a charge Q at capacitance Cp in a very short time, in less than 150 ps. Since the capacity Cp has been experimentally evaluated at 6.10-9 F, the converter must provide a load of at least 18 pC. The converter has a capacitance Cr and an inductor
Lr in series with the primary 23 of a transformer 18, whose characteristics are determined, in particular its ratio m, and two switches 24 and 25 connected in series on the terminals of a DC voltage source E. The role of the transformer is both to raise the voltage and to ensure the galvanic isolation, in continuous mode, between the primary 23 and the secondary 22. A resistor R is placed in parallel on the resonant capacitance Cr to allow the discharge of this capacitance Cr . The converter 17 is powered by the DC voltage source E, taken directly on the general supply voltage for example, this voltage can vary between 200 V and 375 V.

The converter 17 operates in hyporesonant mode.

The interest of its structure resides on the one hand in the possibility to reduce the size of the transformer because the working frequency is high and on the other hand in the assurance that the current in the resonant branch is null when one opens the switches 24 and 25.

Said switches 24 and 25 each consist of a transistor T1, respectively T2, in antiparallel with a diode D1, respectively D2, mounted between the collector and the emitter of the transistors. The bases of the transistors are connected to the control logic circuit 26 which supplies the switching signals of the transistors. The role of these power transistors, in series with the voltage source E, is to convey the power absorbed by the assembly. In addition, it is not necessary to use a radiator because the switching losses of these switches are negligible. Indeed, the resonant circuit LC imposes a zero current at the time of closing of the switch and its opening is performed by the diode in series with the switch, so that the current is already zero when blocking the transistor.

During the charging phase of the capacitance Cp between the instants t0 and t1 already mentioned, the transistor T1 is saturated and the transistor T2 is blocked, as indicated in FIG. 4 where their control sequences appear.

In permanent blocking or unblocking mode, always positive or always negative current pulses are sent on the transformer 18, which could saturate the magnetic core. To demagnetize this core in the case of these steady state, the control device according to the invention comprises two arms 26 and 27, said degaussing, each comprising a switch T3, respectively T4, connected in series with a diode D3, respectively D4 these two arms being mounted in opposite directions to one another.

The operation of the arm 27 (D4, T4) is similar to the operation of the arm 26 (D3, T3) and used when sending negative current pulses. The control of the switch T3, respectively T4, must always be stopped before closing the switch T2, respectively T1, otherwise the current pulse is diverted through the demagnetization circuit and the power supply is then short-circuited. circuit. The voltage drop across the arm 26 causes a voltage of the order of 3 volts on the primary 23 of the transformer, which is found on the secondary 22 multiplied by the transformation ratio m. In order to prevent this electromotive force at the secondary 22 from discharging the capacitance Cp, a diode is interposed in series and in the opposite direction with the diode D5.
Zener Z5 on the one hand and with the diode D7 a diode
Zener Z7 on the other hand, such that the voltage at their terminals is greater than V2 in this case.

The logic circuit 26 for controlling the power transistors of the converter 17 and the demagnetization circuit 20 is conventionally composed of transistors that turn them on or block them.

On the timing diagram of FIG. 4 are successively represented the state of transistors T1 and T3 and of diode D11 the resonant current 1r 'the state of transistors T2 and T4 and of diode D2, the voltage across the capacitors Cp1 the current Iz in the receiver 19, the current Is in the secondary 22 of the transformer 18, the voltage V2 across the secondary 22 of said transformer.

FIGS. 51 to 58 explain the different operating phases of the gate control device according to the invention, assuming that initially no diode is passing and the parasitic capacitance is not charged.

The X-ray emission blocking period corresponds to phases 1 to 4 and the X-ray emission period corresponds to phases 5 to 8.

FIG. 51 corresponds to the phase 1 of loading the capacitance Cp between times t0 and t1. When a blocking command is initiated, the transistor T1 of the converter 17 and the transistor T3 of the demagnetizing circuit are saturated and a positive current arc 1r appears in the resonant branch LrCr. The implementation of the transistor T3 prevents the flow of a current in the arm 26 of the demagnetizing circuit 20 of the transformer 18 when the transistor T1 leads, the diode D3 remains blocked. At the secondary 22 of the transformer 18, the current Is flows through the capacitor Cp which is charged, therefore its voltage Vcp increases. The voltage V2 across the secondary 22 increases, the current passes through the diode D7, and parasitic capacitance Cp is loaded. The other diodes are all blocked because the voltage at their terminals is negative
VD5 = - Vz - VZ7
VD6 = - Vz - VCp
VD8 = - VCp <0
Figure 52 corresponds to the phase 2 of maintaining the voltage of the capacitance Cp at 3000 volts. Between times t1 and t2, the control voltage V2 across the secondary 22 of the transformer 18 is maintained at 3000 volts. At time t1, the voltage V2 is equal to the voltage of the source 19 is 3000 volts; so the diode
D7 remains on while the diode D6 becomes busy, and the two diodes D5 and D8 remain blocked. As the Zener diode 19 turns on in reverse, the current Is is then deflected through the diode D6 passing to the Zener diode 19.
V2 = VZ = VCp
VD7 = 0
VD6 = V2 - VZ = 0
VD5 = - VZ - VD7 = - VZ <0
VD8 = - VCp = - 3000 V <0
FIG. 53 also corresponds to the phase of maintaining the parasitic capacitance at 3000 volts between times t2 and t3 where the control voltage V2 is cut off. In this case, the diode D7 is blocked, the diodes D8 and D5 remain blocked, the diode D6 is then blocked because of the discharge of the capacitor Cp.

When the current in the primary 23 of the transformer 18 is zero, the diodes D6 and D7 are locked at the secondary. At the same time, the diode D3 becomes conductive and the secondary becomes isolated because the diode D6 is blocked. The magnetizing inductance Lm which had accumulated energy until then, will tend to restore it, creating a magnetizing current Im in the primary of the transformer and allowing its demagnetization. The sum between the resonant current 1r and the magnetizing current Im is then established in the demagnetizing arm 26.

At the end of 256 ps after the beginning of phase 1, between times t2 and t4 of zero crossing of the current Irt is blocked transistor T1 at time t3 so that the resonant current Irt which vanishes a second time at time t4, remains zero. The objective of this phase is to discharge the resonant capacitance Cr, as shown in Figure 54.

In permanent blocking or unblocking mode, the demagnetization phase continues long enough for the transformer 18 to be completely demagnetized, then the transistor T3 is blocked at time t5. When the magnetizing current Im is canceled, the diode D3 is blocked. The transformer is demagnetized. The resonant capacitance Cr discharges through resistor R placed in parallel, as shown in FIG. 54. This lasts throughout the phase between times t4 and t5. The capacity Cp always remains at the same potential of 3000 Volts because it remains isolated since phase 3. The OFF state ends. On the other hand, in normal cinematographic mode, where the blocking of the X-rays emitted by the tube lasts 40 ms and is followed by their emission which lasts itself 40 ms - these times being reduced to 7 ms in accelerated cinema mode -, the transformer does not need to be demagnetized because the flow that passes through it during the blocking is compensated for by the flux crossing it during the emission. In this case, the saturation control of transistors T2 and T4 can intervene before.

Figure 55 corresponds to the discharge phase 5 of the capacitance Cp between times t6 and t7. For this, the transistors T2 and T4 are now saturated. A resonant current arc 1r is established in the resonant branch LrCr such that, at the secondary of the transformer, the control voltage V2 decreases and becomes negative. The diode D5 is unlocked, the other diodes D6, D7 and D8 are blocked, the secondary current flows through the diode Dg, the receiver 19 and the capacitance Cp, which causes the discharge of the capacitor Cp.

Figure 56 corresponds to the phase 6 of maintaining the voltage across the capacitor Cp at 0 volts between times t7 and t8. The control voltage
V2 = -3000 Volts at time t7 so the voltage Vcp is zero. The diode D8 is unlocked then imposing a zero voltage on the capacitance Cp. Similarly, the diode D5 is unlocked allowing the converter 17 to evacuate the energy it provides in addition to the receiver.

The potential VCp on the capacitance Cp then remains zero.

FIG. 57 corresponds to the phase of maintenance of the parasitic capacitance at 0 volts, between instants t8 where the control voltage passes from -3000 volts to 0 volts and tg.

At the secondary of the transformer 18, the four diodes of the bridge D5 to D8 are blocked. The resonant current Ir is canceled at time t8 and the secondary of the transformer is isolated. Since the transistor T4 is saturated and the diode D4 becomes conducting, the current resulting from the sum of the resonant current 1r and the magnetizing current Im flowing at the primary of the transformer 18 flows through the demagnetizing arm 27.

Finally, FIG. 58 corresponds to phase 8 during which the resonant capacitance discharges through the resistor R. Between the two zero crossings of the current Irt at times t8 and t1o, T2 is blocked.

When the magnetizing current Im is zero, the diode D4 is blocked and the transformer is demagnetized. The capacitance Cp is always at zero potential. Then the transistor T4 is blocked at time t1l, while the transformer 18 is completely demagnetized. When the duration of the ON state is reached, one goes back to phase 1, to realize the cinema mode.

All of these operating phases of the gate controller are applicable to the normal or accelerated movie mode. To obtain the continuous OFF mode, that is to say the blocking of the X-ray beam permanently, simply repeat phases 1 to 4 with a duration of phase 4 equal to 128 ms for example, time required for demagnetize the transformer 18 as explained previously. To obtain the continuous ON mode, that is to say the passage of the X-ray beam permanently, simply repeat phases 5 to 8 with a duration of phase 8 equal to 128 ms for example.

The invention also relates to the method of implementing the device according to the invention, which firstly comprises, to ensure the blocking period of the emission of X-rays by the tube, the steps in the following order: a) from time t0, the transistors T1 and
T3 are saturated, the transistors T2 and T4 being
blocked, so that at time t1 the capacity Cp is
charged and its voltage is equal to 3000 Volts; b) transistor T1 is blocked at time t3 included
between the instants t2 and t4 switching to 0 of the current
1r circulating in the resonant branch LrCr of
converter 17, so that said current remains zero; c) the transistor T3 is blocked at time t5, when
the transformer 18 is totally demagnetized; d) repeating steps a) to c).

The method of implementing the device according to the invention comprises, on the other hand, to ensure the period of emission of X-rays by the tube, the steps in the following order: e) from time t6 , transistors T2 and
T4 are saturated, the transistors T1 and T3 being
blocked, so that at the moment t7 the capacity Cp is
discharged and its voltage is equal to 0 volts; f) the transistor T2 is blocked at the instant tg included
between times t8 and t10 of changing to 0 of
current 1r flowing in the resonant branch LrCr
of the converter 17, so that said current remains
no; g) transistor T4 is blocked at time t1l, when
the transformer 18 is totally demagnetized; h) repeating steps e) to g).

Finally, to ensure the blocking and the emission of X-rays by the tube in a cinematographic mode, the method of implementation of the device comprises the steps in the following order: a) from time t0, the transistors T1 and
T3 are saturated, the transistors T2 and T4 being
blocked, so that at time t1 the capacity Cp is
charged and its voltage is equal to 3000 Volts; b) transistor T1 is blocked at time t3 included
between the instants t2 and t4 switching to 0 of the current
Ir circulating in the resonant branch LrCr of
converter 17, so that said current remains zero; c) the transistor T3 is blocked at time t5, when
the transformer 18 is totally demagnetized; e) from time t6, the transistors T2 and
T4 are saturated, the transistors T1 and T3 being
blocked, so that at the moment t7 the capacity Cp is
discharged and its voltage is equal to 0 volts; f) the transistor T2 is blocked at the instant tg included
between times t8 and t10 of changing to 0 of
current 1r flowing in the resonant branch LrCr
of the converter 17, so that said current remains
no; g) transistor T4 is off at time t11, when
the transformer 18 is totally demagnetized; h) repeating steps a) to g).

According to the invention, on the one hand, the impedance of the resonant circuit LrCr must be sufficiently low for the converter 17 to provide a current sufficient to charge the capacitor Cp and, on the other hand, a resonance frequency of this same circuit resonant must be high enough that the load of the capacitance Cp is fast. These two constraints lead to choosing a capacity Cr equal to 1.8 pF and an inductance Lr equal to lmH, for example.

The device according to the invention does not use forced switching under high voltage, has a smaller footprint compared to the devices of the prior art and offers the advantage of a much lower realization cost.

Claims (9)

1. Device for controlling the grid of an X-ray tube, this grid being placed between the cathode and the anode of the tube and having a capacitance (Cp) located between said gate and the cathode, characterized in that comprises a source intended to establish and remove a high voltage across said capacitance (Cp), for a given very short duration, of determined value to block and restore respectively the emission of electrons by the cathode towards the anode, this source consisting of: - a bridge of four diodes (16) consisting of a first  and a second connected branches and  each consisting of two diodes respectively  (D5, D7) and (D6, D8) connected in series, one (D8) of  diodes of the second branch being connected to the terminals  said capacitance (Cp); - an electronic circuit intended to provide and remove  electric charge to said capacity, whose output  is connected to the middle points (M1, M2) of the two  branches of the bridge (16); an energy receiver (19) placed at the bridge exit  diodes (16) and maintaining the voltage across  the capacity (Cp) at the determined value. 2. Device according to claim 1, characterized in that the electronic circuit is constituted by - a converter circuit (17) of DC voltage in  alternating voltage resonant type (LC) series,  operating in pulse mode, connected to the output at  the primary winding (23) of a transformer (18)  whose output is connected between the midpoints  (M1, M2) of the two branches of the diode bridge (16) and  input fed by a voltage source  alternative created from a voltage source (E)  continuous by at least two switches (24 and 25); a logic circuit (26) for controlling the switches  (24 and 25) of the AC voltage source. 3. Device according to claim 1, characterized in that the energy receiver (19) is constituted by a Zener diode maintaining the voltage at the terminals of said capacitance (Cp) existing between the gate and the cathode of the tube, at the value determined high. 4. Device according to claim 2, characterized in that it further comprises a demagnetizing circuit (20), constituted, - across the primary (23) of the transformer (18),  two arms (26 and 27) mounted in opposite directions, one of  the other and each comprising a switch (T31 T4)  in series with a diode (D3, D4), said  switches being controlled by the circuit (26)  control logic, and - at the secondary of the transformer, two Zener diodes  (Zg and Z7), placed in series and in opposite directions  respectively with the two diodes (Dg and D7) of the  first branch of the diode bridge (16). 5. Device according to claim 2, characterized in that the resonant-type voltage converter circuit (17) comprises an inductance (Lr) in series with a capacitance (Cr), which is connected in parallel with a resistor (R) of discharge. 6. Device according to claim 2, characterized in that the AC voltage source is constituted from a DC voltage source (E) whose two output terminals are each connected in series with a switch (24 and 25). constituted by a transistor (T1), respectively (T2) and a diode (D1), respectively (D2) mounted in antiparallel between the collector and the emitter of said transistor, the gate of each transistor being connected to the control logic circuit 7. A method of implementation of the device according to one of claims 1 to 6 for controlling the grid of an X-ray tube, said grid being placed between the cathode and the anode of the tube and having a capacitance (Cp ) located between said gate and the cathode, characterized in that it comprises, to ensure the blocking period of the emission of X-rays by the tube, the steps in the following order: a) from the instant t0, the transistors (T1) and  (T3) are saturated, the transistors (T2) and (T4) being  blocked, so that at time t1 the capacity (Cp) is  charged and its voltage is equal to 3000 Volts; b) the transistor (T1) is blocked at time t3 included  between the instants t2 and t4 switching to 0 of the current  (Ir) circulating in the resonant branch (LrCr) of the  converter (17), so that said current remains zero; c) the transistor (T3) is blocked at instant ts, when  the transformer (18) is totally demagnetized; d) repeating steps a) to c). 8. A method of implementation of the device according to one of claims 1 to 6 for controlling the grid of an X-ray tube, this gate being placed between the cathode and the anode of the tube and having a capacitance (Cp ) located between said gate and the cathode, characterized in that it comprises, to ensure the period of emission of X-rays by the tube, the steps in the following order: e) from time t6, the transistors (T2) and  (T4) are saturated, the transistors (T1) and (T3) being  blocked, so that at the moment t7 the capacity (Cp) is  discharged and its voltage is equal to 0 volts; f) the transistor (T2) is blocked at the instant tg included  between times t8 and t10 of changing to 0 of  current (Ir) flowing in the resonant branch  (LrCr) of the converter (17), so that said current  remains zero; g) the transistor (T4) is blocked at time t11, when  the transformer (18) is totally demagnetized; h) repeating steps e) to g). 9. A method of implementing the device according to one of claims 1 to 6 for controlling the grid of an X-ray tube, this gate being placed between the cathode and the anode of the tube and having a capacitance (Cp ) located between said gate and the cathode, characterized in that it comprises, to ensure the blocking and the emission of X-rays by the tube in a cinematic mode, the steps in the following order: a) from the instant t0, the transistors (T1) and  (T3) are saturated, the transistors (T2) and (T4) being  blocked, so that at time t1 the capacity (Cp) is  charged and its voltage is equal to 3000 Volts; b) the transistor (T1) is blocked at time t3 included  between the instants t2 and t4 switching to 0 of the current  (Ir) circulating in the resonant branch (LrCr) of the  converter (17), so that said current remains zero; c) the transistor (T3) is blocked at time t5, when  the transformer (18) is totally demagnetized; e) from time t6, the transistors (T2) and  (T4) are saturated, the transistors (T1) and (T3) being  blocked, so that at the moment t7 the capacity (Cp) is  discharged and its voltage is equal to 0 volts; f) the transistor (T2) is blocked at the instant tg included  between times t8 and t10 of changing to 0 of  current (Ir) flowing in the resonant branch  (LrCr) of the converter (17), so that said current  remains zero; g) the transistor (T4) is blocked at time t111 when  the transformer (18) is totally demagnetized; h) repeating steps a) to g).