Abstract:
A switching circuit for an electromagnetic source for generating acoustic waves has at least one first capacitor connected in parallel with a series circuit formed by a second capacitor and an electronic switch. The switching circuit is connected to a coil of the electromagnetic source, and the first and second capacitors are switched so as to both discharged into the coil, thereby supplying the coil with current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a switching circuit for an electromagnetic source for the generation of acoustic waves of the type having a capacitor that is switched in parallel with at least one series circuit composed of another capacitor and a first diode. 
     2. Description of the Prior Art 
     A switching circuit for an electromagnetic pressure wave source of the above type is known from German OS 198 14 331. It has two LC oscillators connected in series. Of these, the first switching circuit has a first capacitor and, in parallel to this, a semiconductor power switch formed by a triggerable thyristor and a recovery diode switched antiparallel to the thyristor, as well as a subsequent inductance. Part of this first switching circuit, switched in series with the semiconductor power switch and the inductance, as well as parallel to the first capacitor, is a second capacitor that likewise belongs to the second switching circuit. Connected parallel to it is a saturable inductor and an electromagnetic pressure wave source fashioned as an inductive load. As soon as the thyristor of the semiconductor power switch has been triggered in the conductive state, the first capacitor charged with the capacitor charge device is connected to the second, initially uncharged capacitor, such that its charge passes into the second capacitor. The inductor and both capacitors are dimensioned such that the saturable inductor goes into saturation (and thus is of low inductance) only at the point in time when practically the same charge has been loaded from the first capacitor to the second capacitor. At this moment, due to the discharge voltage of the second capacitor with a time constant predetermined by the second switching circuit, a high discharge current flows through the inductive load of the electromagnetic pressure wave source, where an acoustic pulse is generated. 
     The switching circuit disclosed in Soviet Union 17 188 patent for the inductivity of an electrodynamic radiator has a common voltage source to which are connected a number of parallel branches with, respectively, one diode at the input, a storage capacitor connected to ground and an output-side commutator, i.e. switch. The diodes are thereby polarized such that the storage capacitors of the individual parallel branches always remain separated (i.e. unconnected) with regard to their charge voltages, such that transfer or transient effects of these charge voltages among one another are prevented. At the mutual discharging of storage caps, the commutators of all parallel branches are collectively, i.e. simultaneously, closed. During this discharging event, the storage capacitor of the respective branch is switched in parallel to its input-side diode. 
     A further switching circuit according to the prior art is shown in  FIG. 1 . The switching has a direct voltage source  1 , a switch  2  that is normally executed as a discharger, a capacitor C as well as a coil L that is part of a sound generating unit of the electromagnetic source. In addition to the coil L, the acoustic wave generation unit of the electromagnetic source has a coil carrier (not shown) upon which the coil is arranged and an insulated membrane (likewise not shown) arranged on coil L. Upon the discharge of capacitor C via the coil L, a current i(t) flows through coil L, whereby an electromagnetic field is generated that interacts with the membrane. The membrane is thereby repelled in an acoustic propagation medium, whereby source pressure waves are emitted in the acoustic propagation medium as a carrier medium between the acoustic wave generation unit of the electromagnetic source and a subject to be acoustically irradiated. Shock waves can arise, for example, via non-linear effects in the carrier medium of the acoustic source pressure waves. The design of an electromagnetic source, especially of an electromagnetic shock wave source, is, for example, specified in European Application 0 133 665, corresponding to U.S. Pat. No. 4,674,505. 
     Shock waves are used, for example, for non-invasive destruction of calculi inside a patient, for instance for the destruction of a kidney stone. The shock waves directed at the kidney stone produce cracks in the kidney stone. The kidney stone finally breaks apart and can be excreted in a natural fashion. 
     If the switching circuit shown in  FIG. 1  is operated for the generation of acoustic waves, during the discharge event of the capacitor C via the coil L (for which a short circuit is generated by means of the switch  2 ) the curves of the voltage u(t) (exemplarily plotted in  FIG. 2 ) (curve  3 ) over the coil L and of the current i(t) (curve  4 ) result via the coil L. The decaying current i(t) flowing through the coil  4  is, as mentioned already, causes the generation of acoustic waves. 
     The acoustic waves generated by the electromagnetic shock wave source are proportional to the square of the current i(t) (curve  5  in  FIG. 2 ). Subsequently originating from the discharge event of the capacitor C are a first acoustic source pressure wave from the first acoustic source pressure pulse (1st maximum) and further acoustic source pressure waves from the abating sequence of positive acoustic source pressure pulse. The first source pressure wave and the subsequent source pressure waves can, as mentioned already, form into shock waves with short, intensified positive portions and subsequently long, negative pressure troughs via non-linear effects in the carrier medium and a non-linear focusing which normally ensues with a known acoustic focusing lens. 
     Via the frequency of the current i(t) flowing through the coil L, characteristics of the shock wave (such as, for example, its focal radius) can be altered. With a variable current frequency, and thus a variable frequency of the shock wave, the size of the effective focus can, for example, be modified and adjusted to the subject to be treated dependent on the application. For instance, in a lithotripter the effective focus can be selected corresponding to the respective stone size, such that the acoustic energy is utilized better for the disintegration of the stone and the surrounding tissue is stressed less. 
     Due to the relatively high short circuit capacity up to the 100 MW range, a variable capacitance of the capacitor C and a variable inductance of coil L are costly. In order to vary the shock wave, in generally only the charge voltage of the capacitor C is therefore varied, whereby the maxima of the current i(t) changes via the coil L and the voltage u(t) to the coil L. However, the curve shapes of the current i(t) and the voltage u(t) remain essentially the same. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a switching circuit of the type initially described wherein the generation of acoustic waves is improved. 
     According to the invention this object is achieved by a switching circuit of the previously cited type wherein the first switching component is switched such that, after the charging of both capacitors during the discharge of the first capacitor, it blocks as long, as the first capacitor is charged with a greater voltage than the second capacitor and is conductive as soon as the charge voltage of the initially discharged first capacitor achieves substantially the charge voltage of the second capacitor, whereby the second capacitor begins to discharge and both discharging capacitors feed the coil of the electromagnetic source with current. 
     The invention furthermore concerns an electromagnetic source with an inventive switching circuit as well as a lithotripter with such an electromagnetic source. 
     The first switching component (that, according to a preferred embodiment of the invention, is a first diode or a first diode module) is switched such that it blocks after the charging of both capacitors, thus preventing transient effects between both capacitors. In a preferred variant of the invention, the first capacitor can be charged with a greater charge voltage than the second capacitor prior to the discharge of both capacitors. For the generation of the acoustic wave by the electric circuit, the discharge of the first capacitor, thus with the capacitor with the greater charge voltage, is first begun via the coil of the electromagnetic source. As soon as the charge voltage of the first capacitor is substantially equal to the charge voltage of the second capacitor, the first switching component becomes conductive, so that both capacitors discharge and both capacitors feed the coil of the electromagnetic source with current. Consequently the switching circuit has the capacity of the first capacitor before the second capacitor begins to discharge. While both capacitors discharge, the switching circuit has a capacitance that corresponds to the sum of the capacitances of both capacitors. Thus a temporally variable capacitance of the circuit arises, whereby the curve form of the current flowing through the coil of the electromagnetic source can be influenced. By a variation of the charge voltages of both capacitors, the curve form of the current can thus be modified by the coil, and in turn the properties of the shockwave of the electromagnetic source can be varied. The curve form of the discharge current can be further varied when the switching circuit has a number of switching component capacitor pairs switched in series that are switched in parallel to the first capacitor and are charged with different charge voltages. 
     The first diode module can be formed, for example, as a series circuit and/or a parallel circuit of a number of diodes. 
     According to an embodiment of the invention, prior to the discharge the first capacitor can be charged with a first direct voltage source and the second capacitor can be charged with a second direct voltage source. According to a preferred embodiment of the invention, the first capacitor and the second capacitor are charged with only one direct voltage source, and the direct voltage source is disconnected from the second capacitor with a switching element as soon as the second capacitor has achieved its charge voltage. According to an embodiment of the invention, the switching element is at least one semiconductor element. 
     According to a preferred embodiment of the invention, the parallel circuit composed of the second capacitor/first switching component and first capacitor is switched in parallel to with a second switching component. According to an embodiment of the invention, the second switching component is a second diode or a second diode module. 
     A temporal extension of the first source pressure pulse is achieved by the parallel connection of the second switching component to the capacitors given the discharge. Moreover, the subsequently decaying source pressure pulses dependent on the impedance of the second switching component are significantly damped. The damping can be so great that the subsequent source pressure pulses disappear entirely. Via the temporal extension of the first source pressure pulse, a stronger first acoustic wave (thus a stronger first shock wave) is generated, and an amplification of the volume results in an improved effect for the disintegration of calculi. Since only a few weak source pressure pulses, or even no source pressure pulses at all, occur subsequent to the first source pressure pulse, the tissue-damaging cavitation caused by shockwaves from the subsequent source pressure pulses and following the first shockwave is prevented. The lifespan of the first and the second capacitors is thereby increased by the conditionally reverse voltage reduced dependent on the second switching component. In addition, given such a generation of shock waves less audible sound waves are produced, so that a noise reduction results. The total area under the curve of the current is a determining factor in the generation of audible sound waves during the generation of shock waves. In the case of the present invention, this is reduced overall by the omission of the source pressure pulse normally following the first source pressure pulse. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a known switching circuit for generation of acoustic waves. 
         FIG. 2  illustrates the curve of the voltage u(t), the current l(t) and the square of the current i 2 (t) over time during the discharge of the capacitors of the switching circuit of  FIG. 1 . 
         FIG. 3  schematically illustrates an electromagnetic shockwave source. 
         FIG. 4  shows an inventive switching circuit for generation of acoustic waves. 
         FIG. 5  illustrates the curve of the current i′(t) over time during the discharge of the inventive switching circuit. 
         FIGS. 6 through 8  respectively show further embodiments of the inventive switching circuit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Partly in section and party in the form of a block diagram,  FIG. 3  shows an electromagnetic shockwave source in the form of a therapy head  10  that, in the exemplary embodiment, is a component of a lithotripter (not shown in detail). The therapy head  10  has a known sound generation unit (designated with  11 ) that operates according to the electromagnetic principle. In  FIG. 3 , the sound generation unit  11  has (in a manner not shown) a coil carrier, a flat coil arranged thereon and a metallic membrane insulated from the flat coil. To generate shockwaves, the membrane is repelled in an acoustic propagation medium  12  by electromagnetic interaction with the flat coil, whereby a source pressure wave is emitted into the propagation medium. The source pressure wave of the acoustic lens  13  is focused on a focus zone F, whereby the source pressure wave is intensified into a shockwave during its propagation in the acoustic propagation medium  12  and after introduction into the body of a patient P. In the exemplary embodiment shown in  FIG. 3 , the shockwave serves to disintegrate a stone ST in the kidney N of the patient P. 
     The therapy head  10  is allocated to an operation and care unit  14  that, except for the flat coil, has the inventive switching circuit shown in  FIG. 4  for generation of acoustic waves. The operation and care unit  14  is electrically connected with the sound generation unit  11  via a connection line  15  shown in  FIG. 3 . 
     The inventive switching circuit shown in  FIG. 4  for an electromagnetic shockwave source for generation of acoustic waves has direct voltage sources DC 0 , DC 1  and DC 2 , a switching means S, capacitors C 0 , C 1  and C 2  and the flat coil  23  of the electromagnetic sound generation unit  11  of the therapy head  10 . In the exemplary embodiment, a diode D 1  is switched in series with the capacitor C 1  and a diode D 2  is switched in series with the capacitor C 2 . The series switching circuits made from capacitor C 1 /diode D 1  and capacitor C 2 /diode D 2  are moreover switched parallel to the capacitor C 0 . 
     For charging the capacitors C 0  through C 2 , the switching element S is opened. The capacitor C 0  is therefore charged with the direct voltage U 0  of the direct voltage source DC 0  and the polarity shown in  FIG. 4 . The capacitor C 1  is charged with the direct voltage U 1  of the direct voltage source DC 1  and the polarity shown in  FIG. 4 . In the exemplary embodiment, the voltage U 1  of the direct voltage source DC 1  is smaller than the voltage U 0  of the direct voltage source DC 0 . The diode D 1  is switched such that it blocks as long as the capacitor C 0  is charged with a greater voltage u 0 (t) than the capacitor C 1 . The diode D 1  thus prevents a transient effect between the capacitors C 0  and C 1  charged with the voltages U 0  or U 1 , which is why, at the end of the charging, the capacitor C 0  is charged with the higher voltage U 0  than the capacitor C 1 , which is charged with the voltage U 1  at the end of the charging. The capacitor C 2  is furthermore charged with the direct voltage U 2  of the direct voltage source DC 2  and the polarity shown in  FIG. 4 . In the exemplary embodiment, the direct voltage U 2  is smaller than the direct voltage U 1 . The diode D 2  is likewise switched such that it blocks as long as the voltage u 2 (t) of the capacitor C 2  is smaller than the voltage u 0 (t) of the capacitor C 0 . It is thus possible to charge the capacitors C 0  through C 2  with voltages of different sizes. 
     For the generation of the shockwaves, the switching element S is closed. The capacitor C 0  begins to discharge via the coil  23 , whereby the voltage u 0 (t) of the capacitor C) sinks and a current i′(t) flows through the flat coil  23 . The voltage applied to the flat coil  23  is designated with u′(t). If the voltage u 0 (t) of the capacitor C 0  achieves the value of the voltage U 1  of the charged capacitor C 1 , the diode D 1  is conductive and the current i′(t) through the flat coil  23  is fed by both capacitors C 0  and C 1 . If the voltage u 0 (t) of the capacitor C 0  and the voltage u 1 (t) of the capacitor C 1  achieve the voltage U 2  of the charged capacitor C 2 , the diode D 2  is conductive and the current i′(t) through the flat coil  23  is fed by the three capacitors C 0  through C 2 . This thus represents a temporally variable capacitance of the switching circuit, whereby the curve shape of the current i′(t) flowing through the flat coil  23  can be influenced. By further combinations (not shown in  FIG. 4 ) of capacitors/diodes switched in parallel with the capacitor C 0 , the capacitors of which combinations being charged with voltages of different amounts that are less than the voltage U 0  of the direct voltage source DC 0 , the curve shape of the current i′(t) can be further influenced by the flat coil  23  during the discharge. 
     As an example,  FIG. 5  shows curves of currents i′(t) through the flat coil  23  during the discharge, when the switching circuit shown in  FIG. 4  comprises only the capacitors C 0  and C 1 . By a suitable selection of the voltages U 0  and U 1  of the direct voltage sources DC 0  and DC 1 , the current maxima have equal values. 
       FIG. 6  shows a further embodiment of an inventive switching circuit. In the exemplary embodiment, the switching circuit shown in  FIG. 6  comprises capacitors C 0 ′ through C 2 ′, switching elements S′, S 1  and S 2 , diodes D 1 ′ and D 2 ′, a direct voltage source DC 0 ′ and the flat coil  23 . 
     The diode D 1 ′ and the capacitor C 1 ′ as well as the diode D 2 ′ and the capacitor C 2 ′ are switched in series. The series switching circuits made from capacitor C 1 ′/diode D 1 ′ and capacitor C 2 ′/diode D 2 ′ are switched parallel to the capacitor C 0 ′. The diodes D 1 ′ and D 2 ′ are polarized such that they block as long as the capacitor C 0 ′ is charged with a voltage u 0 ′(t) according to the polarity indicated in  FIG. 6 , which is greater than the voltage u 1 ′(t) of the capacitor C 1 ′ or the voltage u 2 ′(t) of the capacitor C 2 ′ according to the indicated polarity. 
     During the charging of the capacitors C 0 ′ through C 2 ′, the switching element S′ is opened. At the beginning of the charging, the switches S 1  and S 2  are closed. Since the capacitors C 1 ′ and C 2 ′ should be charged with charging voltages U 1 ′ and U 2 ′, which are smaller than the voltage U 0 ′ of the direct voltage DC 0 ′, the switches S 1  and S 2  are opened when the capacitors C 1 ′ and C 2 ′ are charged with the desired voltages U 1 ′ and U 2 ′. Since, in the case of the present exemplary embodiment, the capacitors are charged with relatively low currents (less than 1 ampere), switching precisions of the switches S 1  and S 2  in the millisecond range are sufficient in order to charge the capacitors C 1 ′ and C 2 ′ with sufficient precision. The voltages u 1 ′(t) and u 2 ′(t) of the capacitors C 1 ′ and C 2 ′ are monitored with measurement devices (not shown in  FIG. 6 ) during the charging. 
     At the end of the charging, the switching elements S 1  and S 2  are therefore open, the capacitor C 0 [ is charged with the voltage U 0 ′ of the direct voltage source DC 0 ′, and the capacitors C 1 ′ and C 2 ′ are charged with the voltages U 1 ′ and U 2 ′. Moreover, in the exemplary embodiment the voltage U 2 ′ of the charged capacitor C 2  is smaller than the voltage U 1 ′ of the charged capacitor C 1 . 
     For discharging the capacitors C 0 ′ through C 2 ′, the switching element S′ is closed and the capacitor Co′ begins to discharge via the flat coil  23 , whereby a current i′(t) flows through the flat coil  23 . As long as the voltage u 0 ′(t) of the capacitor C 0 ′ is greater than the voltage U 1 ′ of the charged capacitor C 1 ′, the diodes D 1 ′ and D 2 ′ block. If the voltage u 0 ′(t) of the capacitor C 0 ′ achieves the value of the voltage U 1 ′ of the charged capacitor C 1 ′, the diode D 1 ′ is conductive and the current i′(t) through the flat coil  23  is fed by both capacitors C 0 ′ and C 1 ′. If the voltages u 0 ′(t) and u 1 ′(t) of the capacitors C 0 ′ and C 1 ′ achieve the voltage U 2 ′ of the charged capacitor C 2 ′, the diode D 2 ′ is conductive and the current i′(t) through the flat coil  23  is fed by the capacitors C 0 ′ through C 2 ′. 
       FIG. 7  shows a further inventive switching circuit that has an additional diode in comparison to the switching circuit shown in  FIG. 4 . The diode D 3  is switched in parallel and in the blocking direction relative to the charging voltage U 0  of the capacitor C 0 . 
       FIG. 8  shows yet another inventive switching circuit that has an additional diode D 3 ′ in comparison to the switching circuit shown in  FIG. 6 . The diode D 3 ′ is switched in parallel and in the blocking direction by the charging voltage U′ 0  of the capacitor C 0 ′. 
     Instead of the diodes D 1  through D 3  and D 1 ′ through D 3 ′, in particular diode modules composed of a series switching circuit and/or parallel switching circuit of a number of diodes can also be used. The switching elements S, S′, S 1  and S 2  can be a series switching circuit of known thyristors that, for example, are offered by the company BEHLKE ELECTRONIC GmbH, Am Auerberg 4, 61476 Kronberg, in their catalog “Fast High Voltage Solid State Switches” of June 2001. 
     Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventor to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of his contribution to the art.