FR2682830A1 - Electrical charging device for a capacitive bank - Google Patents

Abstract

An electrical charging device for a capacitive bank is produced by using, on the one hand, a leakage transformer, known as a saturated-iron leakage transformer, because the leakage inductance of this transformer makes it possible to limit the charging current drawn by the capacitive bank, when the capacitive bank has been discharged. It is shown that the use of such a transformer makes it possible to avoid the use of an expensive and bulky series inductor. Moreover, in order to obtain reproducibility and perfect limiting of the modulation of the high voltage available on the capacitive bank, a synchronous static relay is used, driven by a drive circuit moreover providing a function of regulating the voltage available on the bank. It is shown that the use of this static relay makes it possible to reduce the number of capacitors necessary to satisfy reduced ripple conditions.

Description

DEVICE FOR ELECTRICALLY CHARGING A CAPACITIVE BENCH
The present invention relates to an electrical charging device of a capacitive bench particularly usable in the medical field for supplying a radiological mobile.

 Radiological mobiles are essentially sources of X-rays which can be moved on the ground by means of a trolley. These X-ray sources are constituted by X-ray tubes. For such tubes it is necessary, among other things, to have a high electrical voltage supplied by a generator at the time of emission. A radiological mobile must be usable on demand anywhere. This means that an electrical supply of such a mobile is normally a supply with connection to the single-phase electrical sector. The radiological mobile is thus brought into a room where a patient who is often bedridden or not transportable must undergo an radiographic examination. There, it is plugged into an electrical outlet available on one of the bedroom walls.

 As soon as this connection is made, a capacitive bench is loaded so as to store high electrical energy. When taking an X-ray picture, we use the voltage delivered by the capacitive bench to supply the X-ray tube with high voltage. In practice, we thus know how to dispose, at 80 KV, of approximately 50 mAs which allows you to take a picture in good conditions. Indeed, this energy corresponds to around 8500 joules taken from the sector and given the very short duration of the snapshot (300ms), it would be unthinkable to want to extract electrical energy from the single-phase network. In fact, the electrical outlets generally available in the rooms are domestic electrical outlets. Their available power is generally limited to 6 kilowatts while it takes at least 20 kilowatts to operate the X-ray generators.

 In a way, the capacitive bench then serves as an energy ballast. The energy stored under the nominal voltage of the bench is transformed by means of a converter, during the duration of the photograph itself, into a high supply voltage. When the picture is finished, in the most demanding cases, it happens that the voltage across the capacitive bank is reduced by three quarters: there remains only 10 to 15% of energy in the capacitive bank. Before being able to take another shot we therefore recharge the capacitive bench. This recharge is relatively slow. It lasts for example 30 seconds.

 The equipment necessary to perform these functions therefore includes a cord connected on the one hand to the single-phase sector and on the other hand to the primary of a step-up transformer. The secondary of the transformer is connected to a rectifier which flows into the capacitive bank. From a practical point of view, this can be compared to a single capacitor. The voltage delivered by this capacitor itself supplies a converter which transforms the stored high DC voltage into a very high driving voltage of the X-ray tube.

 The essential problem which thus arises is that of the charge of the capacitor at the time when the supply is connected to the mains, or better, at the time when this supply is switched to the primary of the transformer. Indeed, the capacitor being at this moment discharged, it behaves with respect to the rectifier like a true short-circuit. As a result, the inrush current is very large. Taking into account that the charged energy (8500 joules for example) is large, the duration during which the electric current is very important is itself large. It is estimated that during the first 15 seconds, if you are not careful, the electric current will be prohibitive to such an extent that it is likely to cause the capacitor to slam. If this is not the case, there is simply a risk, with the current draw, of causing the electrical supply system to be tripped.

 I1 is already known in the prior art to solve these problems by putting, in series between the rectifier and the charging terminal of the capacitor, an impedance: in general a resistor. The disadvantage presented by this resistance is that it is dissipative and that it generates an unnecessary heating of the device. In addition, the power consumption of the mobile is exaggerated compared to the needs. In another solution, it is known to replace the resistance with an inductance. However, this solution has drawbacks because the inductors are expensive components and, moreover, for it to be effective it is generally bulky.

 The object of the invention is to remedy the drawbacks mentioned by proposing a different architecture for supplying the capacitive bench.

Firstly, in principle, a logic circuit is put in place which will act, in a synchronous manner with the supply frequency of the electrical sector, to allow only a limited number of load alternations to pass. In this way, the conduction time of the circuit is reduced temporally in order to reduce on average, at least at start-up, the load current. In addition, to avoid having to add an inductance, we decide to reduce the efficiency of the transformer. This yield is no longer one. I1 is lower. I1 is for example 85%. A leakage transformer is thus used. Magnetic leakage plays a role similar to that of the added impedance in the circuit, but on the other hand, it saves space, reduces cost and better controls regulation.

 The invention therefore relates to an electrical charging device of a capacitive bench of a radiological mobile, this device comprising a transformer connected by its primary to the single-phase electrical sector, a rectifier connected to the secondary of the transformer, at least one capacitor delivering , when charged, a high voltage corresponding to an accumulated electrical charge, characterized in that it comprises a synchronous static relay interposed between this sector and this capacitor so that this transformer delivers an electrical charge to the capacitor only at the rate d 'A control of this relay, and a relay control circuit also providing a regulatory function.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are given for information only and in no way limit the invention. The figures show
- Figure 1: a schematic representation of the loading device according to the invention
- Figures 2a to 2e: time diagrams of electrical signals used in a circuit of the invention.

 Figure 1 shows an electrical charging device of a capacitive bench of a radiological mobile.

This device comprises a transformer 1, the primary 2 of which is connected to the single-phase electrical sector 3.

A rectifier 4 is connected to the terminals of the secondary 5 of the transformer 1 on the one hand, and is connected to a capacitive bank 6 here comprising a set of capacitors in parallel and in series. The capacitors comprise two groups of capacitors mounted in parallel, groups 7 and 8. In one example, each group comprises six paper capacitors, the capacity of each of which is 11,000 microfarads. Each group therefore has a capacity equal to 66 millifarads. The two groups of capacitors in parallel are placed in series with each other so that the terminals 9 and 10 of this set have a capacity between them of 33 millifarads. Putting the capacitors in series means being able to use capacitors whose nominal voltage can be half compared to the operating voltage Vcapa that they should have if each of them was connected in parallel to the output. In one example, the voltage available between terminals 9 and 10 is 720 volts, the capacitors being capacitors of nominal voltage 360 volts. This arrangement must of course be adapted if the value of the storable energy must be different. In one example, the rectifier 4 is a Graetz type diode bridge. At terminals 9 and 10 of the capacitive bench is connected a voltage-voltage converter 11 which supplies an X-ray tube 12 with very high voltage.

 One of the essential characteristics of the invention relates to the fact that this capacitive bench is supplied by means of a synchronous static relay 13 piloted by a piloting circuit 14 moreover ensuring a function of regulating the charging voltage of the capacitive bench . The static relay 13 essentially comprises a triac consisting of two thyristors 15 and 16 mounted head to tail and triggered in phase opposition. The switching conditions of the static relay 13 are as follows: it is activated when the mains voltage across the power module crosses zero (The transformer secondary).

 I1 is also blocked when the direct current crosses zero in the power module.

 According to another important characteristic of the invention, the transformer 1 is a transformer with magnetic leaks, also called a saturated iron transformer. I1 essentially comprises a secondary winding 17 constituting, with a capacitor 18, a resonant circuit of this transformer. Consequently, the cosX of this transformer is not equal to 1. In practice it is substantially equal to 0.7. This explains why the blocking conditions of the synchronous static relay are not the usual ones. The role of 11 leakage winding 17 is similar to that of an inductor which would have been added in series between one of the outputs of the rectifier 4 and one of the terminals 9, or 10, of the capacitive bench.

 One of the particular additional advantages associated with the use of a magnetic leakage transformer is the following. We realize that if the primary voltage varies in significant proportions, for example of the order of 10%, the secondary voltage, it only varies by 1%. This is entirely favorable to the use of capacitors at the limit of their nominal breakdown voltage, without having to fear at the time of an overvoltage the breakdown of these capacitors. Ultimately, for a given use, the capacitors are thus less expensive.

 The regulation circuit comprises a resistance divider bridge 20, 21 connected for example between the outputs 9 and 10 of the capacitive bench. It can otherwise be connected between one of the outputs of this bench and the midpoint of this bench. The midpoint of groups 7 and 8 of capacitors is itself connected to the midpoint of secondary 5 of transformer 1. The voltage taken from the resistor divider 20-21 is applied via a resistor 22 on l input of a high gain amplifier 23. A reference voltage Vc is applied via a resistor 24 also to the input of the amplifier 23. The output of the amplifier 23 is looped back through a resistor 25 on his entry. The resistors 22 and 24 are substantially equal to each other and 500 times weaker than the resistance 25. The voltage supplied by the resistance 22 corresponds to the voltage available between the terminals 9 and 10.

 This circuit allows the bench voltage to be compared to the voltage Vc. As it is mounted, circuit 22-25 constitutes a hysteresis comparator. Its operation will be explained later. When the voltage measured by the bridge 20-21 becomes lower than the voltage Vc, the comparator switches and delivers a signal applied to a monostable circuit 26. The signal delivered in response by the monostable 26 is transmitted as a control signal to the pilot circuit 14 The advantage of using a monostable circuit 26 is linked to the fact that one wants to avoid oscillations on the outputs 9 and 10. Indeed too frequent commissionings abnormally overheat the transformer 1, because they cause overcurrents of this transformer. The transformer heats up because it does not pass the current for storing energy storage in this transformer after a conduction stop of this transformer. In practice, the forcing time imposed by the monostable 26 is of the order of a hundred milliseconds.

The signal produced by the monostable 26 is transmitted to an OR gate 27 which also receives the signal delivered by the comparator. The signal leaving the OR gate 27 is introduced on an AND gate 28 which receives on another input logic safety signals from the system. When the other safety devices are in working order, these other logic signals are at 1. It follows that the signal delivered by the comparator 23 is transmitted as a control signal to the pilot circuit 14. In order to synchronize the circuit pilot also receives a synchronous signal from the power supply.

 In FIG. 2a, it can be seen that the voltage Vcapa oscillates between a low threshold Sb and a high threshold Sh. These two thresholds are produced by the hysteresis comparator 23-25. When the voltage Vcapa falls below the low threshold, the comparator switches and the pilot circuit 14 causes the static relay 13 to close. The transformer 1 is then supplied.

In this case, the rectifier 4 charges the capacitive bank 6.

As a result, the voltage Vcapa rises. When the tension
Vcapa reaches the high threshold Sh the comparator 23 switches again and the charging stops. Then the bank discharges slowly, all the more slowly since the capacitors are not leakage capacitors and they are of good quality. When this discharge reaches the value of the low threshold a new cycle of electric charge is undertaken.

 The control commands of the pilot circuit are therefore orders shown in FIG. 2b corresponding to the load. FIG. 2c shows the triggering of the monostable 26. In practice, the duration of maintenance of this monostable 26 is calculated to be less than the time it normally takes to recharge the capacitive bench so that its voltage rises again. Sb at Sh value.

 FIG. 2d shows the voltage supplied by the electrical sector 3. There is no reason for the instant tl at which it is decided to trip the relay 13 to correspond to a starting condition of this relay as described previously. Consequently, the actual ignition takes place at time t2 when the mains voltage changes phase and where, at the origin, the charge current (zero) is still in phase with the voltage. I1 is the same, for the date t3 crossing by the voltage Vcapa of the high threshold value Sh as for the date tl. The cut then occurs at an instant t4 after this instant t3.

 In practice, the high and low threshold values have a difference of 1.5 per thousand, that is, in an example of 2 Volts, compared to an average value Sm.

In one example, this average value Sm is worth 712 volts plus or minus 1% so that with a deviation of 1%, the nominal value of 720 volts is not exceeded. We could thus measure that if we had not used this technique of regulation with synchronous static relay it would have been necessary to use eighteen capacitors instead of twelve to guarantee a storage of 8500 joules in all the conditions of sector for the load of the capacitive bench. We also noticed that, if we do not use a hysteresis comparator but only a monostable circuit, we still have a ripple limited in relative value of the voltage of the capacitive bench. But the drawback presented by this solution is that this undulation is not reproducible. In one case, it can be less than 2 V, in another case more than 2 V. By using the hysteresis comparator one is sure to always have 2 V which in itself is a good result because it allows to choose the better the capacitors. In practice, the duration of the charge pending (without consumption by the tube X) is equivalent to approximately five periods of a sector signal at 50 hertz, or approximately 200 milliseconds. It is therefore twice the duration of the monostable 26 (100 ms).

 The invention therefore essentially consists of solving, with the synchronous static relay, problems of limitation and reproducibility of the modulation of the high voltage applied to the capacitive bench. With the leakage transformer, it is sought to limit the inrush currents of the load of the capacitive bench when it has been discharged completely, or almost. The action of relay 13 and transformer 1 also combine well to obtain automatic regulation in the event of a variation in the mains voltage

Claims (6)

 1 - Device for electrically charging a capacitive bench of a radiological mobile, this device comprising a transformer connected by its primary to the single-phase electrical sector, a rectifier connected to the transformer secondary, at least one capacitor delivering, when it is charged, a high voltage corresponding to an accumulated electrical charge, characterized in that it comprises a synchronous static relay interposed between this sector and this capacitor so that this transformer delivers an electrical charge to the capacitor only at the rate of piloting of this relay , and a relay control circuit which also provides a regulation function.  2 - Device according to claim 1, characterized in that it comprises a magnetic leakage transformer.  3 - Device according to one of claims 1 or 2, characterized in that it comprises at least two capacitors in series connected by their midpoint to a midpoint of the leakage transformer.  4 - Device according to claim 1 to 3, characterized in that the control circuit comprises a monostable circuit to force the closure of the relay for a minimum duration equal to the switching time of this monostable circuit.  5 - Device according to one of claims 1 to 4, characterized in that the control circuit comprises a hysteresis comparator comparing a voltage corresponding to the state of charge of the capacitor at two reference voltages, respectively low and high, for force closing of the relay when the charging voltage of the capacitor drops below the low reference voltage and open the relay when the charging voltage of the capacitor drops above the high reference voltage.  6 - Device according to claim 5, characterized in that the hysteresis comparator allows a modulation of 1.5 per thousand of the value corresponding to the average of the low and high references.