Supply circuit for an X-ray emitter usable in radiology

#1. Field of the Invention

#The present invention relates to a supply circuit for an X-ray emitter usable in radiology. In radiology, it is necessary to supply an X-ray source or emitter for a predetermined time. This time corresponds to the exposure time of a radiosensitive plate. In cineradiology, it is also necessary to modify the activity of the X-ray emitter as a function of the timing of recording, so that the radiation dose received by a patient is reduced during such an examination.

#2. Description of the Prior Art

#The invention is described in greater detail hereinafter relative to a non-limitative embodiment and the attached drawings, wherein show:

#FIG. 1: A general diagram of a supply circuit according to the invention.

#FIGS. 2a and 2b: Waveshapes at different locations of the preceding circuit.

#FIG. 1 shows a supply circuit according to the invention comprising a transformer 1, whose primary circuit 2, 3, 4 is connected to the three-phase power supply 5. The secondary circuit 6, 7, 8 of said transformer is connected to an X-ray emitter 9. The connection of said secondary circuit has a current rectifier 10. The primary circuit is put into operation by means of a switch 11, which receives its control or command on two input terminals 12, 13. This switch has a set 14 of thyristors ensuring the closing of the switch whilst virtually short-circuiting points A and B of said switch. Switch 11 also has a correctly charged capacitor 15 connectable to the primary circuit for reversing the direction of passage of the current into the set 14 of thyristors. When the current in the thyristors is reversed, they are turned off and reestablish the opening of the circuit between switch points A and B. Under these conditions, the primary no longer supplies and the X-ray emitter 9 can no longer emit. What characterizes the invention is that capacitor 15 is connected to the thyristor set 14 by a switching circuit 50 having thyristors 16 to 19. These thyristors are controlled in pairs (16-17 and 18-19) by the common control terminals respectively 20, 21. By selecting one or other of said terminals, it is possible to reverse the connection direction of the capacitor to set 14.

#Windings 2, 3, and 4 are the three primary windings of the three-phase transformer. They are magnetically coupled to the three secondary windings of transformer 1. On the opposite side to the three-phase power supply, windings 2, 3 and 4 are connected to six diodes 22 to 27. Each end of each of the windings is connected to the centre respectively of the series-connected diode pairs 25-22, 26-23, and 27-24. The three pairs are connected in parallel between switch points A and B. For putting the primary into operation, it is merely necessary to short-circuit point A, connected in common to the three cathodes of diodes 22 to 24, to point B connected in common to the three anodes of diodes 25 to 27. When the circuit is open, the d.c. voltage V.sub.R between points A and B is equal to .sqroot.2 times the voltage distributed by power supply or network 5. To ensure that the putting into operation of the primary does not lead to overvoltages at the secondary, the connection of point A to point B takes place in two periods.

#In the set of thyristors 14, a thyristor 28 is first opened at a time t.sub.1 (FIG. 2a) by delivering a short-duration pulse to its gate by control terminal 12. A resistor 29 connected in series with thyristor 28 leads to a voltage drop .DELTA.V, which is also represented in FIG. 2a. Then at the end of a short time (the time separating t.sub.2 from t.sub.1), a main thyristor 30 is fired by a short pulse applied to its control terminal 13. The two thyristors 28, 30 are connected in parallel between points A and B, the first via resistor 29 in series and the second via a choke 31 in series. Choke 31 limits the current variation in such a way that voltage V.sub.AB then tends exponentially towards zero (FIG. 2a). The transformer is then energized. In radiology, this constitutes the start of exposure. Bearing in mind the fact that thyristor 30 is conductive, thyristor 28 in series with resistor 29 is naturally blocked or rendered non-conductive. The current liable to pass there is below its holding current.