CIRCUIT FOR SWITCHING AN AC VOLTAGE

Abstract

The present invention relates to a circuit for switching an AC voltage. It contains an input terminal able to be connected to an AC voltage source, an output terminal able to be connected to a load impedance, and a first series circuit. This series circuit comprises a diode and a circuit for storing electrical charges. The series circuit has a first end connection that is connected to the input terminal and a second end connection that is connected to the output terminal. The circuit for switching an AC voltage furthermore contains a DC voltage source, which is connected to an electrical connection between the diode and the input terminal or to an electrical connection between the diode and the output terminal and is designed to impress a DC current in the diode. The circuit for switching an AC voltage finally contains a first switch that is connected to an electrical connection between the diode and the circuit for storing electrical charges at one terminal. The first switch is designed to switch between a switching state in which a potential dependent on a reference potential is present at the electrical connection between the diode and the circuit for storing electrical charges, and a switching state in which an electrical floating potential is present in the electrical connection between the diode and the circuit for storing electrical charges.

Claims

1. A circuit (2) for switching an AC voltage comprising an input terminal (1) able to be connected to an AC voltage source (3), an output terminal (7) able to be connected to a load impedance (6), a first series circuit comprising a diode (5) and a circuit for storing electrical charges, wherein the first series circuit has a first end connection that is connected to the input terminal (1) and a second end connection that is connected to the output terminal (7), a DC voltage source (9) that is connected to an electrical connection between the diode (5) and the input terminal (1) or to an electrical connection between the diode (5) and the output terminal (7), and is designed to impress a DC current in the diode (5), and a first switch (16) that is connected to an electrical connection between the diode (5) and the circuit for storing electrical charges at one terminal and that is designed to switch between a switching state in which a potential dependent on a reference potential is present at the electrical connection between the diode (5) and the circuit for storing electrical charges, and a switching state in which an electrical floating potential is present in the electrical connection between the diode (5) and the circuit for storing electrical charges.

2. The circuit (2) as claimed in patent claim 1, characterized in that the first switch (16) is a first on switch or a first off switch.

3. The circuit (2) as claimed in patent claim 1, characterized in that a circuit with a low-pass filter characteristic or a circuit with a resistor is connected between the DC voltage source (9) and the electrical connection between the diode (5) and the input terminal (1) or between the DC voltage source (9) and the electrical connection between the diode (5) and the output terminal (7), and a circuit with a low-pass filter characteristic or a circuit with a resistor is connected between the first switch (16) and the electrical connection between the diode (5) and the circuit for storing electrical charges.

4. The circuit (2) as claimed in patent claim 3, characterized in that the circuit with the low-pass filter characteristic between the first switch (16) and the electrical connection between the diode (5) and the circuit for storing electrical charges is a first coil (15) and the circuit with the low-pass filter characteristic between the DC voltage source (9) and the electrical connection between the diode (5) and the input terminal (1) or the circuit with the low-pass filter characteristic between the DC voltage source (9) and the electrical connection between the diode (5) and the output connection (7) are each a second coil (13).

5. The circuit (2) as claimed in claim 1, characterized in that the circuit for storing electrical charges comprises a capacitor (30).

6. The circuit (2) as claimed in claim 1, characterized in that the circuit for storing electrical charges comprises a capacitor (30) and at least one further capacitor (30.sub.1, 30.sub.2) that are able to be connected in parallel with one another via a respective second switch (21.sub.1, 21.sub.2) arranged in an electrical connection to the diode (5) and a respective third switch (22.sub.1, 22.sub.2) arranged in an electrical connection to the output terminal (7) or to the input terminal (1).

7. The circuit (2) as claimed in patent claim 6, characterized in that the capacitor (30) and the at least one further capacitor (30.sub.1, 30.sub.2) are able to be connected in series with one another via fourth switches (23.sub.1, 23.sub.2).

8. The circuit (2) as claimed in patent claim 5, characterized in that a second series circuit comprising a third coil (19) and an interconnection (20) of a plurality of further capacitors (30.sub.1, 30.sub.2, 30.sub.3) is connected between the reference potential and an electrical connection between the diode (5) and the capacitor (30).

9. The circuit (2) as claimed in patent claim 8, characterized in that the further capacitors (30.sub.1, 30.sub.2, 30.sub.3) are able to be connected in parallel with one another via a respective second switch (21.sub.1, 21.sub.2) arranged in an electrical connection to the diode (5) and via a respective third switch (22.sub.1, 22.sub.2) arranged in an electrical connection to the reference potential.

10. The circuit (2) as claimed in patent claim 8, characterized in that the further capacitors (30.sub.1, 30.sub.2, 30.sub.3) are able to be connected in series with one another via fourth switches (23,, 23.sub.2) that are respectively connected between terminals of two further capacitors (30.sub.1, 30.sub.2, 30.sub.3).

11. The circuit (2) as claimed in patent claim 4, characterized in that a fifth switch (24) is connected between the first coil (15) and the electrical connection between the diode (5) and the circuit for storing electrical charges.

12. The circuit (2) as claimed in patent claim 11, characterized in that a sixth switch (25) is connected between an electrical connection connecting the fifth switch (24) and the first coil (15) and the reference potential, and a seventh switch (26) is connected between an electrical connection connecting the first coil (15) and the first switch (16) and the electrical connection between the diode (5) and the circuit for storing electrical charges.

13. The circuit (2) as claimed in claim 1, characterized in that further first series circuits each comprising a further diode (5.sup.2, . . . , 5.sup.n) and a further circuit for storing electrical charges are provided, wherein each further series circuit in each case has a first end connection and a second end connection, wherein each first end connection is connected to the input terminal (1) and each second end connection is connected in each case to a further output terminal (7.sup.2, . . . , 7.sup.n), each of which is able to be connected to a further load impedance

14. The circuit (2) as claimed in patent claim 13, characterized in that in each case a further first switch (16.sup.2, . . . , 16.sup.n) is connected to the electrical connection between the respective further diode (5.sup.2, . .. , 5.sup.n) and the respective further circuit for storing electrical charges at one terminal, and which is designed to switch between a switching state in which a potential dependent on the reference potential is present at the electrical connection between the respective further diode (5.sup.2, . . . , 5.sup.n) and the respective further circuit for storing electrical charges, and a switching state in which an electrical floating potential is present in the electrical connection between the respective further diode (5.sup.2, .. . , 5.sup.n) and the respective further circuit for storing electrical charges.

15. The circuit (2) as claimed in patent claim 14, characterized in that a control circuit (27), which is connected to the first switch (16) and each further first switch (16.sup.2, . . . , 16.sup.n), is designed such that it switches the first switch (16) and each further first switch (16.sup.2, . . . , 16.sup.n) on and off in each case, wherein the first switch (16) and each further first switch (16.sup.2, . . . , 16.sup.n) are in each case switched on at different times.

16. The circuit (2) as claimed in patent claim 15, characterized in that the control circuit (27) is designed such that the first switch (16) or each further first switch (16.sup.2,...,16.sup.n) is switched on only when the first switch (16) and each further first switch (16.sup.2,...,16.sup.n) are switched off.

17. The circuit (2) as claimed in patent claim 15, characterized in that the control circuit (27) is designed such that a switch-off time of the first switch or each further first switch lasts in each case at least until a voltage potential in the electrical connection between the diode (5) and the circuit for storing electrical charges or in the electrical connection between the further diode (5.sup.2, . . . , 5.sup.n) and the further circuit for storing electrical charges is in each case greater than a highest voltage of all of the DC voltage sources (9) contained in the circuit (2).

18. A controlled AC voltage source comprising an AC voltage source (3), a circuit (2) as claimed in claim 1, whose input terminal (1) is connected to the AC voltage source (3), wherein the control circuit (27) is designed such that it switches the AC voltage source (3) on and off.

19. The controlled AC voltage source as claimed in patent claim 18, characterized in that the control circuit (27) is designed such that it leaves the AC voltage source (3) switched on until the voltage potential in the electrical connection between the diode (5) and the circuit for storing electrical charges or in the electrical connection between the further diode (5.sup.2,...,5.sup.n) and the further circuit for storing electrical charges is in each case greater than a highest voltage of all of the DC voltage sources (9) contained in the circuit.

20. A motor-driving circuit having an AC voltage source (3), a circuit (2) as claimed in claim 1 and a number of electrical spark plugs, wherein the AC voltage source (3) is connected to the input terminal (1) of the circuit (2) and each electrical spark plug is connected in each case to the output terminal (7) or to one of the further output terminals (7.sup.2, . . . , 7.sup.n) of the circuit (2).

Description

INDICATION OF CONTENTS OF THE DRAWING

[0083] The present invention is explained in more detail below with reference to the exemplary embodiments specified in the schematic figures of the drawing, in which:

[0084] FIG. 1 shows an illustration of a circuit for switching an AC voltage according to the prior art,

[0085] FIG. 2A shows an illustration of a characteristic curve of a p-n or PIN diode according to the prior art,

[0086] FIG. 2B shows a timing diagram of the AC current through a PIN diode in forward operation,

[0087] FIG. 2C shows a timing diagram of a low-frequency AC current through a p-n diode in the transition to reverse operation,

[0088] FIG. 2D shows a timing diagram of a higher-frequency AC current through a PIN diode in the transition to reverse operation,

[0089] FIG. 3A shows an illustration of a first variant of the circuit according to the invention for switching an AC voltage,

[0090] FIG. 3B shows an illustration of a second variant of the circuit according to the invention for switching an AC voltage,

[0091] FIG. 3C shows an illustration of a third variant of the circuit according to the invention for switching an AC voltage,

[0092] FIG. 3D shows an illustration of a fourth variant of the circuit according to the invention for switching an AC voltage,

[0093] FIG. 3E shows a timing diagram of the AC current in a circuit according to the invention for switching an AC voltage,

[0094] FIG. 4A shows an illustration of a first development of the circuit according to the invention for switching an AC voltage,

[0095] FIG. 4B shows a timing diagram of the AC current in a first development of the circuit according to the invention for switching an AC voltage,

[0096] FIG. 5A shows an illustration of a second development of the circuit according to the invention for switching an AC voltage,

[0097] FIG. 5B shows a timing diagram of the AC current in a second development of the circuit according to the invention for switching an AC voltage,

[0098] FIG. 6A shows an illustration of a third development of the circuit according to the invention for switching an AC voltage,

[0099] FIG. 6B shows a timing diagram of the AC current in a third development of the circuit according to the invention for switching an AC voltage, and

[0100] FIG. 7 shows an illustration of a fourth development of the circuit according to the invention for switching an AC voltage.

[0101] The accompanying figures of the drawing are intended to convey a further understanding of the embodiments of the invention. They illustrate embodiments and, in association with the description, serve to clarify principles and concepts of the invention. Other embodiments and many of the advantages mentioned become apparent with regard to the drawings. The elements of the drawings are not necessarily shown in a manner true to scale with respect to one another.

[0102] In the figures of the drawing, identical, functionally identical and identically acting elements, features and components—unless explicitly stated otherwise—are provided in each case with the same reference signs.

[0103] In the following text, the figures are described in an interrelated and all-encompassing manner.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0104] The four variants of a circuit according to the invention for switching an AC voltage are explained in detail below with reference to FIGS. 3A to 3D:

[0105] An AC voltage source 3 is connected to the input terminal 1 of the circuit 2 according to the invention for switching an AC voltage. The AC voltage source 3 is preferably a voltage source for generating a high-frequency voltage. This may be for example a frequency oscillator or any other high-frequency circuit that generates a high-frequency voltage with a specific settable or fixed frequency and a specific settable or fixed amplitude.

[0106] This AC voltage source 3 or voltage source for generating a high-frequency voltage impresses a specific AC voltage or high-frequency voltage into the circuit 2 according to the invention for switching an AC voltage. As an alternative, an AC current source or a current source for generating a high-frequency current with a parallel-connected resistor are also conceivable, by way of which an AC voltage or a high-frequency voltage is also able to be impressed into the circuit according to the invention.

[0107] The input terminal 1 is connected to an input-side coupling capacitor 4 via which only an AC voltage, preferably the high-frequency voltage U.sub.HFin, is able to be coupled into the circuit 2 according to the invention. The input-side coupling capacitor 4 likewise prevents a DC voltage from being coupled out from the circuit 2 according to the invention in the direction of the AC voltage source 3.

[0108] In the first variant of the circuit according to the invention, the input-side coupling capacitor 4 is connected to the anode of the diode 5, which is preferably a PIN diode 5. The cathode of the diode 5 is connected to an output-side coupling capacitor 6. In the first variant of the circuit according to the invention, the output-side coupling capacitor 6 constitutes the capacitor 30 that serves as circuit for storing electrical charges. The series circuit comprising the diode 5 and the circuit for storing electrical charges is referred to below as first series circuit. The output-side coupling capacitor 6, that is to say the capacitor 30 serving as circuit for storing electrical charges, is finally connected to the (AC voltage) output terminal 7 of the circuit 2 according to the invention. The output-side coupling capacitor 6 couples only an AC voltage or a high-frequency voltage out from the circuit 2 according to the invention and blocks a DC voltage. No DC voltage is able to be coupled into the circuit 2 according to the invention via the output-side coupling capacitor 6 either.

[0109] The AC voltage or high-frequency voltage U.sub.HFout present at the output terminal 7 of the circuit 2 according to the invention is connected to one terminal of a load impedance 8. The other terminal of the load impedance 8 is typically connected to a ground potential. The load impedance 8 may be any complex-value impedance. For an application of the circuit 2 according to the invention for switching an AC voltage in the automotive sector, the load is for example an electrical spark plug connected to the circuit 2 according to the invention. The load impedance 8 consists in this case at least of the line impedance of the (high-frequency) supply line to the spark plug, the impedance of the inner conductor of the spark plug, stray capacitances and the impedance of the impedance respectively prevailing in the combustion chamber between the spark plug electrodes.

[0110] In order to switch the AC voltage or high-frequency voltage present at the input terminal 1 through to the output terminal 7 via the diode 5, which is preferably a PIN diode 5, a sufficient diode DC current I.sub.DCforward is required in the forward direction of the diode 5. In this case, the AC voltage or high-frequency voltage U.sub.HFin at the input is switched to an AC voltage or high-frequency voltage U.sub.HFout at the output of the circuit 2 according to the invention.

[0111] This diode DC current I.sub.DCforward is preferably generated in a DC voltage source 9. This DC voltage source 9 generates a DC voltage U.sub.DCforward at a further input terminal 10 of the circuit 2 according to the invention, a DC voltage input terminal. The further input terminal 10 is connected to the terminal of a shunt capacitor 11, whose other terminal is connected to the ground potential. The shunt capacitor 11 is connected as a stabilizing capacitor and filters low-frequency or higher-frequency interfering signals occurring at the further terminal 10 out of the DC voltage U.sub.DCforward generated by the DC voltage source 9.

[0112] In order to generate a diode DC current I.sub.DCforward, the further input terminal 10 is connected to a resistor 12. The resistor 12 is dimensioned such that, in the case of a DC voltage U.sub.DCforward generated by the DC voltage source 9, the required diode DC current I.sub.DCforward flows through the resistor 12. In addition to generating the diode DC current I.sub.DCforward in a DC voltage source 9 and a downstream resistor 12, the diode DC current I.sub.DCforward may alternatively also be generated in a DC current source.

[0113] In the first variant of the circuit according to the invention, the diode DC current I.sub.DCforward is impressed into the diode 5 by virtue of the DC voltage source 5 being connected to the electrical connection between the input terminal 1 and the anode of the diode 5. The DC voltage source 9 is connected to the electrical connection between the input terminal 1 and the anode of the diode 5 via the resistor 12 and the downstream first coil 13. As an alternative, the resistor 12 and the first coil 13 may also be interchanged in terms of their order within the series circuit 29. This first coil 13 makes it possible to feed the diode DC current I.sub.DCforward into the diode 5 with low resistance. The first coil 13 furthermore prevents the AC voltage, preferably the high-frequency voltage, from being coupled out from the circuit 2 according to the invention in the direction of the DC voltage source 9.

[0114] As an alternative, instead of the first coil, any circuit with a low-pass filter characteristic or a circuit with a resistor that has a certain inductance at higher frequencies, for example a bonding wire or a strip line implemented as a butterfly stub, may be used.

[0115] In order to allow a current flow for the diode DC current I.sub.DCforward from the DC voltage source 9 through the diode 5 up to a reference potential, the electrical connection between the diode 5 and the capacitor 30 serving as circuit for storing electrical charges should be implemented as an electrical connection to the reference potential. The reference potential in FIG. 3A is for example the ground potential of the circuit. To this end, a series circuit 14 comprising a second coil 13 and a first switch 16 is connected between the ground potential and the electrical connection between the diode 5 and the capacitor 30. In this case, as illustrated in FIG. 3A, the second coil 13 may be connected to the cathode of the diode 5 and the first switch 16 may be connected to the ground potential. As an alternative, an electrical connection of the first switch 16 to the cathode of the diode 5 and the second coil 13 to the ground potential is also possible.

[0116] The second coil 13 in turn enables low-resistance coupling-out of the diode DC current I.sub.DCforward from the circuit 2 according to the invention in the direction of the ground potential. In addition, the second coil 13 prevents the AC voltage or the high-frequency voltage from being short-circuited from the cathode of the diode 5 to the ground potential.

[0117] The first switch 16 is driven by a control circuit 27 via a signal transmission path 28.sub.1. The signal transmission path 28.sub.1 may be implemented in either wired or radio-supported form. In forward operation of the diode 5 or the PIN diode 5, the first switch 16 is closed, while it is open in reverse operation.

[0118] When the first switch 16 is closed in forward operation of the diode 5 or the PIN diode 5, this results in a diode current which results, according to FIG. 2B, from the superposition of the diode DC current I.sub.DCforward and the diode AC current IHF impressed by the AC voltage or the high-frequency voltage U.sub.HFin. In this case, there is a potential, which is dependent on the reference potential or ground potential, in the electrical connection between the diode 5 and the capacitor 30.

[0119] When the first switch 16 is open and in the case of delayed switching off of the diode AC current—what is called “afterburn” of the AC voltage or the high-frequency voltage—this results, in the transition between forward and reverse operation at a lower frequency of the fed-in AC current according to FIG. 2C, in an overall diode current that decays with a time constant and at the same time has gaps in the current flow.

[0120] At a higher frequency of the fed-in AC current, this results, in the transition between forward and reverse operation according to FIG. 2D, in an overall diode current that decays with a time constant without gaps in the current flow.

[0121] The “afterburn time” of the AC voltage or the high-frequency voltage should be suitably matched to the time constant of the decay of the overall diode current. If the AC voltage or the high-frequency voltage is switched off without an “afterburn time” and thus at the same time as the DC voltage, then only the profile of the diode DC current illustrated in dashed form in each of FIGS. 2C and 2D results as decay of the diode current. Depending on the parameters of the diode 5 or the PIN diode 5, the diode DC current may under some circumstances be too low to charge the first capacitor 6 to a sufficient capacitor voltage level to block the diode 5. In this case, a sufficient “afterburn time” of the AC voltage or the high-frequency voltage is required to block the diode 5.

[0122] The time constant of the decay of the diode current is essentially dependent on the forward resistance of the diode 5 and the capacitance of the capacitor 30. By choosing the appropriate diode 5 or PIN diode 5 and thus the forward resistance of the diode 5 and the capacitance of the capacitor 30, the time constant is able to be suitably dimensioned. It should be noted here that the choice of capacitance for the capacitor 30 for a given diode current value, that is to say the amount of charge carriers transported to the capacitor 30, determines the reverse voltage able to be achieved for the diode 5. A sensible compromise between blocking that is as fast as possible and blocking that is as reliable as possible should thus be chosen through a suitable choice of parameters.

[0123] The control circuit 27 may switch the first switch 16 on again as early as possible when the transient process from forward operation to reverse operation of the diode 5 is definitely completed. This is the case when the voltage potential in the electrical connection between the diode 5 and the capacitor 30, which represents the circuit for storing electrical charges, is greater than the DC voltage U.sub.DCforward generated by the voltage source 9. In this case, a sufficient blocking potential is present at the cathode of the diode 5. If a plurality of DC voltage sources are integrated in the circuit for switching an AC voltage, which DC voltage sources may influence blocking of the diode 5, then the first switch 16 is able to be switched on again by the control circuit 27 as early as possible only when the voltage potential in the electrical connection between the diode 5 and the capacitor 30 is greater than the highest voltage of all of the voltages that are generated by in each case all of the DC voltage sources implemented in the circuit.

[0124] The control circuit 27 switches the AC voltage source 3 on and off via the signal transmission path 28.sub.2. The control circuit 27 in particular switches off the AC voltage source 3 with a delay in relation to the opening of the first switch 16. This “afterburn” of the AC voltage is ended at the latest via the control circuit 27 by switching off the AC voltage source 3 when the voltage potential in the electrical connection between the diode 5 and the capacitor 30 is greater than the highest of all of the DC voltages generated by the DC voltage sources implemented in the circuit.

[0125] In summary, it may thus be stated that, in the first variant in forward operation of the diode 5, this results in a current flow or a DC current flow path from the voltage source 9 via the diode 5 plus external wiring elements to the ground potential or reference potential that is electrically connected to the first switch 16. After the first switch 16 has been opened, the capacitor 30 charges via the electrical charges flowing in the transient phase to a capacitor voltage, the polarity of which is illustrated in FIG. 3A and puts the diode 5 into reverse operation.

[0126] In a second variant of the circuit according to the invention for switching an AC voltage according to FIG. 3B, the DC voltage source 9 for impressing a DC current into the diode 5 is likewise connected, as in the first variant, to the electrical connection between the input terminal 1 and the diode 5. The diode 5, which is preferably designed as a PIN diode 5, has opposing polarity in the second variant compared to the first variant. The cathode of the diode 5 is in this case electrically connected to the input terminal 1 via an electrical connection and the anode is electrically connected to the output terminal 7 via an electrical connection. The DC voltage source 9 is thus electrically connected to the cathode of the diode 5. In addition, the DC voltage source 9 has opposing polarity in the second variant compared to the DC voltage source 9 in the first variant. The DC voltage U.sub.DCforward generated by the DC voltage source 9 thus drops from the ground potential to the further input terminal 10.

[0127] Thus, in the second variant in forward operation of the diode 5, this results in a current flow or a DC current flow path from the ground potential or reference potential, which is electrically connected to the first switch 16, via the diode 5 plus external wiring elements to the DC voltage source 9. After the first switch 16 has been opened, the capacitor 30 charges via the electrical charges flowing in the transient phase to a capacitor voltage, the polarity of which is illustrated in FIG. 3B and puts the diode 5 into reverse operation.

[0128] In a third variant of the circuit according to the invention for switching an AC voltage according to FIG. 3C, the DC voltage source 9 is connected to the electrical connection between the output terminal 7 and the diode 5 in order to impress a DC current into the diode 5. In the third variant, the anode of the diode 5 is electrically connected to the input terminal 1 via an electrical connection and the cathode is electrically connected to the output terminal 7 via an electrical connection. The DC voltage source 9 is thus electrically connected to the cathode of the diode 5. The first switch 16 is furthermore connected to the electrical connection between the anode of the diode 5 and the input-side coupling capacitor 4, with or without the interposition of a circuit with a low-pass filter characteristic or a circuit with a resistor. This input-side coupling capacitor 4 serves, in the third variant, as capacitor 30 for implementing the circuit for storing electrical charges. The DC voltage source 9 has a polarity such that the DC voltage U.sub.DCforward generated thereby is directed from the ground potential to the further input terminal 10.

[0129] Thus, in the third variant in forward operation of the diode 5, this results in a current flow or a DC current flow path from the ground potential or reference potential, which is electrically connected to the first switch 16, via the diode 5 plus external wiring elements to the DC voltage source 9. After the first switch 16 has been opened, the input-side coupling capacitor 4, which serves as capacitor 30 for implementing the circuit for storing electrical charges, is charged to a capacitor voltage via the electrical charges flowing in the transient phase. The polarity of the capacitor voltage is illustrated in FIG. 3C and puts the diode 5 into reverse operation.

[0130] In a fourth variant of the circuit according to the invention for switching an AC voltage according to FIG. 3D, the DC voltage source 9 for impressing a DC current into the diode 5 is likewise connected, as in the third variant, to the electrical connection between the diode 5 and the output terminal 7. The diode 5, which is preferably designed as a PIN diode 5, has opposing polarity in the fourth variant compared to the third variant. In the fourth variant, the cathode of the diode 5 is electrically connected to the input terminal 1 via an electrical connection and the anode is electrically connected to the output terminal 7 via an electrical connection. The DC voltage source 9 is thus electrically connected to the anode of the diode 5. In addition, the DC voltage source 9 has opposing polarity in the fourth variant compared to the DC voltage source 9 in the third variant. The DC voltage U.sub.DCforward generated by the DC voltage source 9 thus drops from the further input terminal 10 to the ground potential. The first switch 16 is connected, in the fourth variant, to the electrical connection between the cathode of the diode 5 and the input-side coupling capacitor 4, with or without the interposition of a circuit with a low-pass filter characteristic or a circuit with a resistor. This input-side coupling capacitor 4 serves, in the fourth variant, as capacitor 30 for implementing the circuit for storing electrical charges.

[0131] This thus results, in the fourth variant in forward operation of the diode 5, in a current flow or a DC current flow path from the DC voltage source 9 via the diode 5 plus external wiring elements to the ground potential or reference potential that is electrically connected to the first switch 16. After the first switch 16 has been opened, the capacitor 30 charges via the electrical charges flowing in the transient phase to a capacitor voltage, the polarity of which is illustrated in FIG. 3D and puts the diode 5 into reverse operation.

[0132] The timing diagram of FIG. 3D, by way of example for the first variant of the circuit according to the invention, illustrates the voltage at the cathode of the diode 5 with respect to ground potential. In this case, at the time t=10 μsec, a DC voltage of 12 V and an AC voltage with an amplitude of 150 V are switched on at the same time. When the diode 5 is switched through, a DC voltage of approx. 0.3 V is present at the cathode of the diode 5 and has superimposed on it an AC voltage with an amplitude of approx. 150 V. At the time t=100 μsec, the DC forward path is opened, while the AC voltage remains switched on until the time t=120 μsec. After the DC forward path has been opened, the first capacitor 6 is charged, with the AC voltage still switched on, to a voltage of approx. 160 V, which serves as reverse voltage for the final blocking of the diode 5.

[0133] FIG. 4A shows a first development of the circuit 2 according to the invention for switching an AC voltage:

[0134] In the first development of the circuit 2 according to the invention, a series circuit 18 comprising a third coil 19 and an interconnection 20 of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 is connected between the electrical connection that connects the cathode of the diode 5, preferably a PIN diode 5, to the capacitor 6, and a ground potential. The series circuit comprising a third coil 19 and an interconnection 20 of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 is referred to below as second series circuit. In this case, the third coil 19, as illustrated in FIG. 4A, may be connected to the cathode of the diode 5 and the capacitor 6, while the interconnection 20 of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 is connected to the ground potential. As an alternative, the interconnection 20 of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 may be connected to the cathode of the diode 5 and the first capacitor 6, while the third coil 19 is connected to the ground potential.

[0135] The third coil 19 prevents short-circuiting of the AC voltage from the cathode of the diode 5 via the interconnection 20 of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 to the ground potential. The third coil 19 furthermore enables the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 to be charged with low resistance by the diode DC current.

[0136] The capacitor 30, which is formed by the output-side coupling capacitor 6, and the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 form a circuit for storing electrical charges.

[0137] The further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 may be connected in a parallel configuration or alternatively in a series configuration. In order to implement the parallel configuration of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3, a respective second switch 21.sub.1 and 21.sub.2 is provided between the terminals of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 on the cathode side, that is to say in the electrical connection to the cathode of the diode 5, and a respective third switch 22.sub.1 and 22.sub.2 is provided between the terminals of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 on the side of the ground potential, that is to say in the electrical connection to the ground potential. In the parallel configuration of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3, the individual second switches 21.sub.1 and 21.sub.2 and the individual third switches 22.sub.1 and 22.sub.2 are closed by being driven via the control circuit 27.

[0138] A respective fourth switch 23.sub.1 and 23.sub.2 is connected between two terminals of two further capacitors 30.sub.1, 30.sub.2 and 30.sub.3. These fourth switches 23.sub.1 and 23.sub.2 are each open in the parallel configuration of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3, while they are each closed in the series configuration of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3. In the series configuration of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3, the individual second switches 21.sub.1 and 21.sub.2 and the individual third switches 22.sub.1 and 22.sub.2 are each open. The second switches 21.sub.1 and 21.sub.2, the third switches 22.sub.1 and 22.sub.2 and the fourth switches 23.sub.1 and 23.sub.2 are driven via the control circuit 27 by way of associated signal transmission paths, which are not shown in FIG. 4A for the sake of clarity.

[0139] The number of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 is not set at three capacitors, as illustrated in FIG. 4A. Rather, any other technically sensible number of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 is possible.

[0140] In forward operation of the diode 5 or the PIN diode 5, all of the second switches 21.sub.1 and 21.sub.2, all of the third switches 22.sub.1 and 22.sub.2 and all of the fourth switches 23.sub.1 and 23.sub.2 are each open. The interconnection 20 of further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 is thus decoupled from the cathode of the diode 5. Thus, starting from the time t=10 μsec, an AC voltage with an amplitude of approx. 150 V with respect to ground potential is present at the cathode of the diode 5.

[0141] After the DC forward path has been opened at the time t=100 μsec, the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 are each connected in parallel with one another and thus also connected in parallel with the capacitor 30 via the third coil 19. The capacitor 30 and the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 are thus each charged to the same capacitor voltage. Due to the parallel connection of the individual capacitors, this charging process takes place, in the first development of the circuit 2 according to the invention according to FIG. 4A, with a higher time constant than in the basic circuit 2 according to the invention according to FIG. 3A. Each capacitor voltage, and thus also the cathode voltage of the diode 5, after this charging process, at the time t=120 μsec at which the AC voltage is then also switched off, thus reaches a DC voltage value of approx. 160 V.

[0142] At the time t=130 μsec, there is a changeover between the parallel configuration and the series configuration of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3. The capacitor voltage across all three series-connected further capacitors 30.sub.1, 30.sub.2 and 30.sub.3 thereby increases threefold to approx. 480 V. Since the parallel-connected capacitor 30 still has a capacitor voltage of 160 V, this results in a balancing process between the capacitor 30 and the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3. This balancing process, which may be seen in FIG. 4B, owing to the series circuit consisting of the third coil 19 and the capacitor 30, caused by a transient process between the two times t=130 μsec and t=150 μsec, leads to a balanced capacitor voltage of the capacitor 30 and the series-connected further capacitors 30.sub.1, 30.sub.2 and 30.sub.3. This balanced capacitor voltage, which serves to block the diode 5, is approximately 250 V, as may be seen in FIG. 4B. This reverse voltage value is thus advantageously increased in comparison with that in the basic circuit according to the invention.

[0143] By dimensioning the capacitance of the further capacitors 30.sub.1, 30.sub.2 and 30.sub.3, which is preferably the same in each case, in relation to the capacitance of the capacitor 30, the level of the reverse voltage able to be achieved is able to be set. However, since the capacitances also influence the time constant of the charging process, a sensible compromise between reliable blocking and blocking that is as fast as possible should be chosen here.

[0144] FIG. 5A illustrates a second and preferred development of the circuit 2 according to the invention for switching an AC voltage:

[0145] An interconnection 20′ consisting of a capacitor 30, which is formed with the output-side coupling capacitor 6, and further capacitors 30.sub.1 and 30.sub.2 is connected between the cathode of the diode 5 or the PIN diode 5 and the output terminal 7 of the circuit 2 according to the invention. The capacitor 30 formed from the output-side coupling capacitor 6 and the further capacitors 30.sub.1 and 30.sub.2 form the circuit for storing electrical charges.

[0146] In a parallel arrangement, the capacitor 30 is able to be connected in parallel with the further capacitors 30.sub.1 and 30.sub.2. To this end, a respective second switch 21.sub.1 and 21.sub.2 is connected between the cathode-side terminals of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2, and a respective third switch 22.sub.1 and 22.sub.2 is connected between the terminals of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2 on the side of the output terminal 7. In this case, the cathode-side terminals are in each case the terminals in the electrical connection to the cathode of the diode 5 and the terminals on the side of the output terminal 7 are in each case the terminals in the electrical connection to the output terminal 7. A respective fourth switch 23.sub.1 and 23.sub.2 is connected between the terminals of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2. In the parallel configuration of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2, the second switches 21.sub.1 and 21.sub.2 and the third switches 22.sub.1 and 22.sub.2 are closed by being driven by the control circuit 27. The fourth switches 23.sub.1 and 23.sub.2 are each open in the parallel configuration.

[0147] In the series configuration of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2, the fourth switches 23.sub.1 and 23.sub.2 are each closed, while the second switches 21.sub.1 and 21.sub.2 and the third switches 22.sub.1 and 22.sub.2 are each open.

[0148] The second switches 21.sub.1 and 21.sub.2, the third switches 22.sub.1 and 22.sub.2 and the fourth switches 23.sub.1 and 23.sub.5 are likewise driven via the control circuit 27 by way of associated signal transmission paths, which are not shown in FIG. 5A for the sake of clarity.

[0149] The number of further capacitors 30.sub.1 and 30.sub.2 is not set at two capacitors, as illustrated in FIG. 4A. Rather, any other technically sensible number of further capacitors 30.sub.1 and 30.sub.2 is possible.

[0150] As may be seen from the temporal profile of the cathode voltage of the diode 5 with respect to ground potential in FIG. 5B, when the diode 5 or the PIN diode 5 is in forward operation, an AC voltage with an amplitude of 150 V with respect to ground potential is present at the cathode of the diode 5 starting from the time t=10 μsec.

[0151] When the DC voltage is switched off while the AC voltage is still switched on at the time t=100 μsec, the capacitor 30 and, in parallel, the respectively parallel-connected further capacitors 30.sub.1 and 30.sub.2 are charged. Due to the parallel connection of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2, the charging takes place with a higher time constant. The higher time constant means that the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2 are each charged, at the time t=120 μsec, the end of the charging process, to a capacitor voltage that is lower than the capacitor voltage of the capacitor 30 of the basic circuit 2 according to the invention at the same time. The cathode voltage of the diode 5 at the time t=120 μsec is approx. 150 V in the second development of the circuit according to the invention according to FIG. 5B, while the cathode voltage of the diode 5 in the basic circuit according to the invention is approx. 160 V according to FIG. 3C.

[0152] As soon as the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2 are charged at the time t=120 μsec and the AC voltage is also switched off, there is a changeover between the parallel configuration and the series configuration of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2. This changeover to the series configuration leads to the individual capacitor voltages of the capacitor 30 and the further capacitors 30.sub.1 and 30.sub.2 being summed. This thus results in a reverse voltage of approx. 450 V at the cathode of the diode 5 in the second development of the circuit 2 according to the invention according to FIG. 5B.

[0153] In a third development of the circuit 2 according to the invention for switching an AC voltage according to FIG. 6A, a fifth switch 24 is connected between the cathode of the diode 5 or the PIN diode 5 and the series circuit 14 consisting of the first coil 15 and the first switch 16. In addition, a sixth switch 25 is connected between the electrical connection that connects the fifth switch 24 to the first coil 15 and the ground potential. Finally, a seventh switch 26 connects the electrical connection that connects the first coil 15 to the first switch 16 to the cathode of the diode 5.

[0154] In forward operation of the diode 5 or the PIN diode 5, the first switch 16 and the fifth switch 25 are each closed starting from the time t=10 μsec. The sixth switch 25 and the seventh switch 26 are open starting from the time t=10 μsec.

[0155] The fifth switch 24, the sixth switch 25 and the seventh switch 26 are driven, like the driving of the first switch 16, by the control circuit 27 by way of associated signal transmission paths, which are not shown in FIG. 6A for the sake of clarity.

[0156] As may be seen in FIG. 6B, in forward operation of the diode 5 in the third development of the circuit 2 according to the invention, this results in an equivalent cathode voltage with respect to ground potential as in the basic circuit 2 according to the invention in

[0157] FIG. 3C. It corresponds to an AC voltage with an amplitude of approx. 150 V.

[0158] At the time t=120 μsec, the first switch 16 and the fifth switch 24 are opened and thus the DC forward path is opened and the AC voltage is switched off at the same time.

[0159] When the first switch 16 and the fifth switch 24 are opened at the time t=120 μsec, there is an abrupt current flow interruption in the first coil 15, which induces a voltage U.sub.ind in the first coil 15. The induction voltage U.sub.ind in the first coil 15 is directed opposite to the original current direction through the first coil 15. In order for this induction voltage U.sub.ind to be related to a reference potential after the fifth switch 24 is opened, the sixth switch 25 is preferably closed at the same time or at least close in time to the opening of the first switch 16 and the fifth switch 24.

[0160] The seventh switch 26 is furthermore preferably closed at the same time or at least close in time to the opening of the first switch 16 and the fifth switch 24 at the time t=120 μsec. When the seventh switch 26 is closed, the voltage U.sub.ind induced in the first coil 15 charges the output-side coupling capacitor 6 with the voltage induced in the first coil 15 to a capacitor voltage of approx. 240 V, as may be seen in FIG. 6B. The output-side coupling capacitor 6 in this case forms the capacitor 30 serving as a circuit for storing electrical charges.

[0161] In order that the capacitor 6 does not discharge from this capacitor voltage via the closed seventh switch 26, the first coil 15 and the closed sixth switch 25 to ground potential, the seventh switch 26 is closed again at a slightly later time t=132 μsec. The capacitor voltage of the capacitor 6 thus remains at its higher value of approx. 240 V, which is sufficient as a reverse voltage for reliably and independently blocking the diode 5 or the PIN diode 5.

[0162] FIG. 7 shows a fourth development of the circuit 2 according to the invention for switching an AC voltage:

[0163] Like the basic circuit 2 according to the invention for switching an AC voltage according to FIG. 3A, the fourth development of the circuit 2 according to the invention has an input terminal 1 to which an AC voltage source 3 is connected. At the input terminal 1, an AC voltage, preferably a high-frequency voltage U.sub.HFin, is fed into the circuit according to the invention.

[0164] The (AC voltage) input terminal 1 is connected to an input-side coupling capacitor 4 via which only an AC voltage, preferably the high-frequency voltage U.sub.Fin, is able to be coupled into the circuit according to the invention. The input-side coupling capacitor 4 likewise prevents a DC voltage from being coupled out from the circuit 2 according to the invention.

[0165] The input-side coupling capacitor 4 is in each case connected to a number n-1 of further diodes 5.sup.2, . . . , 5.sup.n, preferably further PIN diodes 5.sup.2, . . . , 5.sup.n. The interconnection of the further diodes 5.sup.2, . . . ,5.sup.n corresponds to the interconnection of the diode 5 in the first variant of the basic circuit according to the invention. The input-side coupling capacitor is thus additionally connected to the anode of the further diodes 5.sup.2, . . . ,5.sup.n. As an alternative, the other variants of the basic circuit according to the invention may also be used in the fourth development of the circuit according to the invention.

[0166] The cathode of the further parallel-connected diodes 5.sup.2, . . . , 5.sup.n is connected in each case to an associated further output terminal 7.sup.2, . . . , 7.sup.n via an associated output-side coupling capacitor 6.sup.2, . . . , 6.sup.n, which in each case constitutes the capacitor 30.sup.2, . . . , 30.sup.n forming the respective circuit for storing electrical charges. A respective output-side AC voltage U.sub.HF2out, . . . ,U.sub.HFnout, preferably an output-side high-frequency voltage U.sub.HF2out,..., U.sub.HFnout, is present at the respective further output terminal 7.sup.2, . . . , 7.sup.n. This output-side AC voltage U.sub.HF2out, . . . , U.sub.HFnout is fed to an associated further load impedance 8.sup.2, . . . ,8.sup.n. When using the fourth development of the circuit 2 according to the invention in the automotive sector, the further load impedance 8.sup.2, . . . , 8.sup.n corresponds for example to the impedance of the supply line to an electrical spark plug, the impedance of the inner conductor in the electrical spark plug and the impedance in the combustion chamber between the two spark plug electrodes.

[0167] A respective associated further series circuit 14.sup.2, . . . , 14.sup.n consisting of an associated further coil 15.sup.2, . . . , 15.sup.n and an associated further first switch 16.sup.2, . . . , 16.sup.n is connected between the cathode of the further diodes 5.sup.2, . . . , 5.sup.n and the ground potential. As an alternative to the further coil 15.sup.2, . . . , 15.sup.n, any other suitable circuit with a low-pass filter characteristic or any other suitable circuit with a resistor may also be used.

[0168] In order to switch the AC voltage or high-frequency voltage present at the input terminal 1 through to the output terminal 7 or to the further output terminal 7.sup.2, . . . , 7.sup.n via the associated diode 5 or the associated further diode 5.sup.2, . . . , 5.sup.n, a sufficient diode DC current I.sub.DCforward in the forward direction of the diode 5 or the further diode 5.sup.2, . . . , 5.sup.n is required.

[0169] This diode DC current I.sub.DCforward is generated for each diode 5 or each further diode 5.sup.2, . . . , 5.sup.n by a single DC voltage source 9 at a further input terminal 10 of the circuit 2 according to the invention. The further input terminal 10 is connected via a shunt capacitor 11 to a resistor 12, which generates a diode DC current I.sub.DCforward from the DC voltage U.sub.DCforward present at the further input terminal 10. A second coil 13 is connected between the resistor 12 and the anode of the diode 5 or the further diode 5.sup.2, . . . , 5.sup.n. As an alternative to feeding in a DC voltage U.sub.DCforward using a DC voltage source 9, the diode DC current I.sub.DCforward may also be impressed directly by a DC current source 17.

[0170] For the individual components of the fourth development of the circuit 2 according to the invention and their interconnection, what was stated above with regard to the equivalent components of the basic circuit 2 according to the invention for switching an AC voltage applies in an equivalent manner, and is therefore not explained again here.

[0171] In the fourth development of the circuit 2 according to the invention according to FIG. 7, the control circuit 27 is connected, via individual signal transmission paths 28.sub.1,28.sup.2, . . . , 28.sup.n, in each case to the first switches 16 and the further first switches 16.sup.2, . . . , 16.sup.n. The control circuit 27 closes and opens the first switch 16 or the further first switches 16.sup.2, . . . , 16.sup.n and thus puts the associated diode 5 or the associated further diode 5.sup.2, . . . , 5.sup.n into forward operation or into reverse operation.

[0172] The control circuit 27 may switch the first switch 16 or the further first switches 16.sup.2, . . . , 16.sup.n on again as early as possible when the transient process from forward operation to reverse operation of the diode 5 or the further diodes 5.sup.2, . . . , 5.sup.n is definitely completed. This is the case when the voltage potential in the electrical connection between the diode 5 or the further diodes 5.sup.2, . . . , 5.sup.n and the capacitor 30 or the further capacitors 30.sup.2, . . . , 30.sup.n, each of which constitutes the circuit for storing electrical charges, is greater than the DC voltage U.sub.DCforward generated by the voltage source 9. In this case, a sufficient reverse potential is present at the cathode of the diode 5 or the further diodes 5.sup.2, . . . 5.sup.n.

[0173] If a plurality of DC voltage sources are integrated in the circuit for switching an AC voltage, which DC voltage sources may influence the blocking of the diode 5 or the further diodes 5.sup.2, . . . , 5.sup.n, then the first switch 16 or the further first switch 16.sup.2, . . . , 16.sup.n is able to be switched on again as early as possible by the control circuit 27 only when the voltage potential in the electrical connection between the diode 5 or the further diodes 5.sup.2, . . . , 5.sup.n and the capacitor 30 or the further capacitors 30.sup.2, . . . , 30.sup.n is greater than the highest of all of the DC voltages generated by the DC voltage sources implemented in the circuit.

[0174] The control circuit 27, which switches the AC voltage source 3 on and off via the signal transmission path 28.sub.2, is able to switch the AC voltage source 3 off with a delay in relation to the opening of the first switch 16 or the further first switches 16.sup.2, . . . , 16.sup.n. This “afterburn” of the AC voltage is ended at the latest via the control circuit 27 by switching off the AC voltage source 3 when the voltage potential in the electrical connection between the diode 5 or the further diodes 5.sup.2, . . . , 5.sup.n and the capacitor 30 or the further capacitors 30.sup.2, . . . , 30.sup.n is greater than the highest voltage of all of the voltages that are each generated by DC voltage sources implemented in the circuit.

[0175] The cathode-side wiring of the diode 5 or the further diodes 5.sup.2, . . . , 5.sup.n of the fourth development of the circuit 2 according to the invention in FIG. 7, which corresponds to the cathode-side wiring of the diode 5 in the first variant of the basic circuit 2 according to the invention for switching an AC voltage according to FIG. 3A, may alternatively also be designed according to the cathode-side wiring of the diode 5 of the first, second or third development of the circuit 2 according to the invention for switching an AC voltage according to FIGS. 4A, 5A and 6A. In these cases, the control circuit 27 needs to be supplemented with the drive system for the further switches that are required in this case.

[0176] In one preferred application, the AC voltage U.sub.HFin generated in the AC voltage source 3 is cyclically switched, and thus distributed, to the individual output terminals 7,7.sup.2, . . . , 7.sup.n in cyclic operation via diodes 5,5.sup.2,...,5.sup.n that are cyclically switched to forward operation. In addition to cyclic operation, any desired technically sensible drive sequence for the individual diodes 5,5.sup.2, . . . , 5.sup.n is also possible. Finally, a plurality of diodes 5,5.sup.2, . . . , 5.sup.n may also be driven simultaneously, and the generated AC voltage U.sub.HFin may thus be switched, and thus distributed, to a plurality of output terminals 7,7.sup.2, . . . , 7.sup.n at the same time.

[0177] Although the present invention has been described completely above on the basis of preferred exemplary embodiments, it is not restricted thereto, but rather may be modified in diverse ways.

LIST OF REFERENCE SIGNS

[0178] 1 Input terminal

[0179] 2 Circuit for switching an AC voltage

[0180] 3 AC voltage source

[0181] 4 Input-side coupling capacitor

[0182] 5 Diode or PIN diode

[0183] 5.sup.2, . . . 5.sup.n Further diode or further PIN diode

[0184] 6 Output-side coupling capacitor

[0185] 6.sup.2, . . . 6.sup.n Further output-side coupling capacitor

[0186] 7 Output terminal

[0187] 7.sup.2, . . . 7.sup.n Further output terminal

[0188] 8 Load impedance

[0189] 8.sup.2, . . . 8.sup.n Further load impedance

[0190] 9 DC voltage source

[0191] 10 Further input terminal

[0192] 11 Shunt capacitor

[0193] 12 Resistor

[0194] 13 Second coil

[0195] 14 Series circuit

[0196] 14.sup.2, . . . 14.sup.n Further series circuit

[0197] 15 First coil

[0198] 15.sup.2, . . . 15.sup.n Further first coil

[0199] 16 First switch

[0200] 16.sup.2, . . . 16.sup.n Further first switch

[0201] 18 Series circuit

[0202] 19 Third coil

[0203] 20 Interconnection

[0204] 20′ Interconnection

[0205] 21.sub.1,21.sub.2, . . . , 21.sub.n Second switch

[0206] 22.sub.1,22.sub.2, . . . , 22.sub.n Third switch

[0207] 23.sub.1,23.sub.2, . . . ,23.sub.n Fourth switch

[0208] 24 Fifth switch

[0209] 25 Sixth switch

[0210] 26 Seventh switch

[0211] 27 Control circuit

[0212] 28.sub.1,28.sub.2 Signal line

[0213] 28.sup.2, . . . 28.sup.n Further signal line

[0214] 29 Series circuit

[0215] 30 Capacitor

[0216] 30.sup.2, . . . 30.sup.n Further capacitor