Input Circuit for a Power Supply

Abstract

An input circuit for a power supply includes an input voltage that is converted into an output voltage via periodic switching of a switch between conductive/blocked states, wherein current that charges a capacitance and supplies output voltage flows through an inductor at least during switching of the switch, and is absorbed by an active switch unit when the switch is in the blocked state and is permitted through the switch in the blocked state, upon exceeding a predefined breakdown voltage at the switch, such that during an overvoltage at the input circuit, the switch is switched into the blocked state, and current flow that then occurs is relayable into the inductor and the active switch unit is deactivatable, the switch and inductor being dimensioned such that, during an overvoltage, an “avalanche energy” occurs at the switch, upon which the switch withstands the current flowing through the inductor during the over-voltage.

Claims

1.-20. (canceled)

21. An input circuit for a power supply comprising: a switch element arranged on an input side; an inductance arranged in series with the switch element; and an active switch unit; wherein an input voltage is convertible into an output voltage via periodic switching of the switch element between a conductive state and a blocked state; wherein a current flowing at least partially flow through the inductance during a switching period of the switch element, said current charging a capacitance arranged on an output side at which the output voltage is able to be made available; wherein the active switch unit absorbs the current in a blocked state of the switch element; wherein a current flow via the switch element is permitted in the blocked state of the switch element when a predefined breakthrough voltage at the switch element is exceeded; wherein the switch element is switchable into the blocked state upon recognition of an overvoltage on the input side of the input circuit; wherein the current flow via the switch element is relayable to the inductance and switches off the active switch unit; and wherein the switch element and the inductance are dimensioned such that, upon occurrence of the overvoltage, an avalanche energy arises at the switch element, at which the switch element withstands a current flowing through the inductance for a duration of the overvoltage.

22. The input circuit as claimed in claim 21, wherein the switch element and the inductance are further dimensioned such that the predefined breakthrough voltage at the switch element and the output voltage produce at least one sum value at which the current through the inductance, upon occurrence of a maximum overvoltage to be expected, remains below a maximum predefineable value.

23. The input circuit as claimed in claim 21, further comprising: an activation unit for activating the switch element such that the switch element is supplied with activation pulses, which alternately place the switch element into the conductive state and the blocked state.

24. The input circuit as claimed in claim 21, further comprising: a comparator unit for recognizing the input-side overvoltage; wherein the comparator unit is suppliable with indirectly or directly determinable voltage value on the input side of the input circuit such that the switch element, if a predefined reference value is exceeded by the determined and supplied measured voltage value (R2), is placed into the blocked state.

25. The input circuit as claimed in claim 22, further comprising: a comparator unit for recognizing the input-side overvoltage; wherein the comparator unit is suppliable with indirectly or directly determinable voltage value on the input side of the input circuit such that the switch element, if a predefined reference value is exceeded by the determined and supplied measured voltage value, is placed into the blocked state.

26. The input circuit as claimed in claim 23, further comprising: a comparator unit for recognizing the input-side overvoltage; wherein the comparator unit is suppliable with indirectly or directly determinable voltage value on the input side of the input circuit such that the switch element, if a predefined reference value is exceeded by the determined and supplied measured voltage value, is placed into the blocked state.

27. The input circuit as claimed in claim 21, further comprising: a comparator unit for recognizing the input-side overvoltage based on a current building up as a consequence; wherein the comparator unit is suppliable with a measured current value determined by via a current sensor such that the switch element, if the predefined reference value is exceeded by the determined and supplied measured current value, is placed into the blocked state.

28. The input circuit as claimed in claim 22, further comprising: a comparator unit for recognizing the input-side overvoltage based on a current building up as a consequence; wherein the comparator unit is suppliable with a measured current value determined by via a current sensor such that the switch element, if the predefined reference value is exceeded by the determined and supplied measured current value, is placed into the blocked state.

29. The input circuit as claimed in claim 23, further comprising: a comparator unit for recognizing the input-side overvoltage based on a current building up as a consequence; wherein the comparator unit is suppliable with a measured current value determined by via a current sensor such that the switch element, if the predefined reference value is exceeded by the determined and supplied measured current value, is placed into the blocked state.

30. The input circuit as claimed in claim 21, wherein the switch element comprises a semiconductor switch.

31. The input circuit as claimed in claim 21, wherein the semiconductor switch comprises a metal oxide field effect transistor based on silicon carbide.

32. The input circuit as claimed in claim 31, wherein the semiconductor switch based on silicon carbide has a predefined minimum acceptance capability for the avalanche energy as a characteristic component value.

33. The input circuit as claimed in claim 21, wherein the switch element comprises a semiconductor switch and a unit for limiting and accepting the avalanche energy which is arranged in parallel with the semiconductor switch.

34. The input circuit as claimed in claim 21, wherein the semiconductor switch comprises a switching transistor.

35. The input circuit as claimed in claim 33, wherein the unit for limiting and accepting the avalanche energy comprises a suppressor diode, especially as a power Zener diode based on silicon.

36. The input circuit as claimed in claim 33, wherein the suppressor diode comprises a power Zener diode based on silicon.

37. The input circuit as claimed in claim 33, wherein the unit for limiting and accepting the avalanche energy comprises voltage-limiting protective circuitry which comprises at least a capacitance and a diode.

38. The input circuit as claimed in claim 21, wherein the input circuit include at least buck converter topology.

39. The input circuit as claimed in claim 21, wherein the active switch unit comprises a diode.

40. The input circuit as claimed in claim 21, wherein the active switch unit comprises a Schottky diode.

41. The input circuit as claimed in claim 40, wherein the Schottky diode is formed from silicon carbide.

42. The input circuit as claimed in claim 21, wherein the capacitance arranged on the output side comprises one of (i) a ceramic capacitor, (ii) an electrolytic capacitor and (iii) a plastic film capacitor.

43. The input circuit as claimed in claim 21, wherein the switch element is arranged in a positive voltage branch of the input circuit.

44. The input circuit as claimed in claim 21, wherein the switch element is arranged in a negative voltage branch of the input circuit.

45. The input circuit as claimed in claim 21, further comprising: a rectifier unit arranged on the input side for linking the input circuit to an at least two-phase power supply system.

46. The input circuit as claimed in claim 21, further comprising: at least one varistor arranged on the input side for limiting overvoltage that occurs.

47. The input circuit as claimed in claim 21, wherein the input circuit includes a converter stage which is arranged downstream of said input circuit on the output side, an input voltage of the converter stage being formed by the output voltage of the input circuit made available at the capacitance.

48. The input circuit as claimed in claim 21, wherein the converter stage comprises an electrically isolating converter stage.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0036] The invention will be explained below by way of examples, which refer to the enclosed figures, in which:

[0037] FIG. 1 schematically shows an exemplary embodiment of the inventive input circuit for a power supply;

[0038] FIG. 2 schematically shows a further exemplary embodiment of the inventive input circuit for a power supply;

[0039] FIG. 3 shows exemplary plots time graphs of voltages and currents in the inventive input circuit during an input-side overvoltage; and

[0040] FIG. 4 shows use of the inventive input circuit in a three-phase power supply in a schematic and exemplary manner.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0041] FIG. 1 schematically shows an exemplary embodiment of the inventive input circuit ES for a power supply, which can be linked, for example, to a single or multiphase, especially at least two- or three-phase, power supply system. The input circuit ES has at least a circuit topology of a buck converter. As an alternative, however, the input circuit can also be formed as a combination of buck converter and boost converter. Arranged on an input side of the input circuit ES is an input capacitance Ce, at which a mostly unstabilized or unregulated input voltage Ue is present. The input voltage Ue can, for example, be made available by an input stage of the power supply through which a link to the power supply system is made.

[0042] The input capacitance Ce in this case has the function of the input circuit ES for example, which operates with a much higher frequency than the system frequency of the input voltage Ue (e.g., a factor of 1000), to make the high-frequency pulse currents available. A size of the input capacitance C in this case is dimensioned, for example, so that the limit values of the system current harmonics able to be predefined are not exceeded by the overall input circuit ES.

[0043] Furthermore, the input circuit ES has a switch element SE arranged on the input side, which can be arranged in a positive voltage branch or in a negative voltage branch, for example. An inductance L is arranged directly or indirectly after the switch element SE. The input circuit ES furthermore has an active switch unit FD that, in terms of switching, for example, can be located arranged in series with the switch element SE or in parallel to the inductance. The active switch unit FD is formed as a diode or as a Schottky diode, which ideally can be switched very rapidly into a blocked state or has a very small blocking delay time. The Schottky diode FD can, for example, be formed as a semiconductor component based on silicon carbide.

[0044] Arranged on the output side is an output or intermediate circuit capacitance, at which a usually stabilized or regulated output voltage Ua of the input circuit ES is made available, for example, for a connected consumer or, with a multistage power supply, for a downstream converter stage. In circuit terms, the output capacitance Ca can be arranged in series with the inductance L and thus in parallel to the active switch unit FD. Furthermore, the capacitance Ca can, for example, be formed as a ceramic capacitor, electrolytic capacitor or a plastic film capacitor.

[0045] Furthermore, the input circuit ES has an activation unit AS, by which, based on control variables R1, R2, activation pulses GS for the switch element SE are generated. In a normal mode of operation of the input circuit ES, for example, by periodic switching of the switch element SE, the input voltage Ue present on the input side is converted into the stabilized or regulated output voltage, which is usually smaller in amount than the input voltage Ue. To this end, based on a first control variable R1 determined on the output side (e.g., output voltage value determined etc.) by the activation unit AS, activation pulses GS are created. With the activation pulses GS, the switch element SE is alternately put into a conductive state and a blocked state. That is, the switch element SE is alternately switched on and switched off during a switching period. In this case, the inductance L, at least partially during the switching period or when the switch element SE is switched on, has a current I.sub.L flowing through it, which charges the capacitance Ca arranged on the output side. The active switch unit FD is blocked when this occurs. In a blocked state of the switch element SE, the current I.sub.L commutes through demagnetization of the inductance L into the active switch unit FD, where the energy stored in the inductance L is discharged to the capacitance Ca arranged on the output side (until the inductance L is demagnetized). The switch element SE is then switched into the conductive state again by means of the activation unit AS. This switching process of the switch element SE is repeated periodically during normal operation of the input circuit ES, in order to make regulated output voltage Ua available at the output.

[0046] In the event of an overvoltage, interference from the power supply system in the form of overvoltages and/or voltage peaks occur on the input side of the input circuit ES as input voltage Ue, where such overvoltages can already be limited (e.g., to 2000V) by at least one varistor fitted to the input side. On occurrence and recognition of an input-side overvoltage Ue, the switch element SE can be put into a blocked state via an activation pulse GS of the activation unit AS. In this case, a blocking voltage falls at the switch element SE where, when a predefined breakthrough voltage is exceeded at the switch element, which lies above a continuous operating voltage of the switch element SE, through the blocking voltage Usp a current flow I.sub.AVAL is permitted via the switch element SE. A point in the circuit between switch element SE, inductance L and active switch unit FD has a voltage potential US. The current flow I.sub.AVAL via the switch element SE can be relayed into the inductance L, at which a voltage U.sub.L drops, and switches the active switch unit FD off or the active switch unit FD is blocked.

[0047] So that the input circuit ES has the necessary dielectric strength for the overvoltage case, the switch element SE and the inductance L are dimensioned such that, on occurrence of the overvoltage, “avalanche energy” arises at the switch element SE, for which the switch element withstands the current I.sub.L flowing through the inductance L for the duration of the overvoltage. Usually, such overvoltage pulses or surge voltages are only a few μs long (e.g., with a rise time of 1 to 2 μs and a half-life period of less than 10 μs). The avalanche energy at the switch element SE arises through the current flow I.sub.AVAL across the blocked switch element, e.g., in the form of heat, which the switch element must withstand in the event of an overvoltage. The current flow I.sub.AVAL and thus the avalanche energy at the switch element SE can be influenced by appropriate dimensioning of the inductance L for a breakthrough voltage able to be predefined at the switch element SE.

[0048] To this end, the switch element SE and the inductance L can additionally be dimensioned such that the predefined breakthrough voltage at switch element SE and the output voltage Ua produce at least one sum value at which the current I.sub.L, on occurrence of a maximum overvoltage to be expected, remains below a maximum value able to be predefined. That is, if an overvoltage Ue, which is limited, for example, via at least one varistor to 2000V, occurs at the input of the input circuit ES, then a voltage U.sub.L drops at the inductance L, which also produces a difference between the input voltage Ue, the predefined breakthrough voltage or the blocking voltage Usp at the switch element SE and the output voltage Ua, which is almost constant (e.g., 400V). By predefining a maximum value of the current I.sub.L flowing through the inductance L and with a determinable voltage U.sub.L at the inductance L, the inductance L can be dimensioned very easily. The breakthrough voltage Usp at the switch element SE can be predefined, for example, by choice and design of the switch element SE.

[0049] With an input-side overvoltage Ue of e.g. 2000V, which is present for 20 μs, for example, and for an output voltage Ua of e.g. 400V and also a predefined breakthrough voltage Usp at the switch element of e.g. 1200V, there remains, for example, as a voltage drop U.sub.L at the inductance L e.g. 400V, through which the inductance L is magnetized. In accordance with the relationship U.sub.L=L*di/dt, a current I.sub.L in the inductance L after reformation to I.sub.L=(U/L)*t can be determined. With a known voltage UL and a maximum value able to be predefined for the current I.sub.L, a dimensioning for the inductance L can be determined from this.

[0050] For recognition of an input-side overvoltage, a comparator unit is provided in the input circuit ES. This can be formed as an autonomous unit or can be integrated into the activation unit AS. A second control variable R2 is employed by the comparator unit, on the basis of which an activation pulse GS is created by the activation unit, through which the switch element SE is put into the blocked state. A measured voltage value or a measured current value can be used as the second control variable R2, for example.

[0051] With a measured voltage value as the second control variable R2, the voltage is, for example, determined indirectly or directly on the input side of the input circuit. The input voltage Ue or, for example, a voltage already at the output of a possible upstream stage can be determined, for example, directly at the input of the input circuit. The measured voltage value determined can then be supplied to the comparator unit such that the supplied measured voltage value is compared with a predefined reference value, which can amount to a voltage above an input voltage Ue usual for normal operation and as a maximum to the predefined breakthrough voltage of the switch element SE. If the predefined reference value is exceeded, then the switch element SE is put into the blocked state.

[0052] If as an alternative a measured current value is used for recognition of an input-side overvoltage, then to this end a current can be employed that builds up as a consequence of the overvoltage. To this end, the measured current value can be determined with a current sensor, where the current sensor is, for example, arranged in a current path, which comprises the current flow I.sub.AVAL across the switch element SE. The measured current value can then be supplied to the comparator unit such that this measured current value is compared with a predefined reference value. If the predefined reference value is exceeded by the measured current value determined and supplied to the comparator unit, then the switch element SE will be put into the blocked state via the activation unit AS.

[0053] Furthermore, it is also possible that, for recognition of an input-side overvoltage Ue, a measured voltage value is employed and is compared with a predefined reference value and additionally a current measurement is also performed. Thus, for example, as a function of a voltage form and a rise over time of the overvoltage Ue, the switch element SE can then be switched off either on the basis of the measured voltage value or on the basis of the current measurement.

[0054] The switch element SE of the inventive input circuit ES can be formed, for example, as a semiconductor switch or switching transistor S based on silicon carbide (SiC). In particular, the semiconductor switch S based on silicon carbide can be a metal oxide field effect transistor or MOS-FET or a Sic MOS-FET. This Sic MOS-FET ideally has as its component characteristic a predefined minimum acceptance capability for the avalanche energy or an “avalanche rating”, whereby the switching element SE can be dimensioned very easily for the overvoltage case.

[0055] As an alternative, the switch element SE (as shown by the dashed line in FIG. 1) can consist of a semiconductor switch S (e.g., MOS-FET based on silicon or gallium nitride) and a unit D arranged in parallel for limiting the acceptance of the avalanche energy in the event of an overvoltage. This unit D can, e.g., be formed as a suppressor diode (as is shown in FIG. 1). A power Zener diode based on silicon can be used as the suppressor diode D.

[0056] Shown schematically in FIG. 2 is a further exemplary embodiment of the inventive input circuit ES1. This form of embodiment ES1 corresponds to the form of embodiment of the inventive input circuit shown in FIG. 1 except for the configuration of the switch element SE1. In th presently contemplated alternate embodiment, the switch element SE1 consist of a semiconductor switch S (e.g., MOS-FET) and a voltage-limiting protection circuit connected in parallel thereto, through which in the event of an overvoltage, when the semiconductor switch S is blocked, a current flow I.sub.AVAL is permitted or the function of a breakthrough voltage is emulated and the semiconductor switch S is protected. Such a protective circuit can, for example, comprise at least one capacitance Cb and also a diode Db, where the capacitance Cb has a pre-charge, for example, through which the predefined breakthrough voltage can be defined. Through the diode Db the capacitance Cb can be switched in, in the event of an overvoltage. Furthermore, a discharge resistor can be arranged in parallel to the capacitance Cb, which can be formed as a suppressor diode Dz or as an ohmic resistance R.

[0057] FIG. 3 shows, by way of example and schematically, time graphs of voltages and currents in the inventive input circuit while an input-side overvoltage is occurring. To this end, the time t is plotted on the x axis. Plotted schematically on the y axis are an input voltage Ue of the input circuit ES, a voltage potential Us at the point in the circuit between switch element SE, inductance L and active switch unit FD, an output voltage Ua, a current flow I.sub.AVAL via the switch element SE, a voltage UL at the inductance L and also a current I.sub.L into the inductance L.

[0058] Before a first point in time t0 the input circuit is operating normally. That is, a continuous operating voltage (e.g., 800V) usual for normal operation is present as input voltage at the circuit. The switch element SE is switched into the blocked state, for example, where the entire input voltage Ue plus, e.g., a very small diode flux voltage of the active switch unit FD (e.g., 1V) is present at the switch element SE as the current blocking voltage Usp. The current blocking voltage Usp at the switch element SE in each case can be seen in FIG. 3, for example, from a distance between the two curves of the input voltage and the voltage Us. The inductance L is demagnetized and the current I.sub.L in the inductance L falls, where the current I.sub.L also flows via the active switch unit FD. The voltage at the potential point Us is below 0 volts by the flux voltage of the active switch unit FD. A constant output voltage Ua is made available to the capacitance arranged on the output side (e.g., 400V).

[0059] At the first point in time to, an input-side overvoltage occurs which, for example, can be limited via one or more varistors arranged on the input side to a value able to be predefined (e.g., 2000V). As a consequence of the overvoltage building up on the input side, the input voltage Ue increases. The inductance L is furthermore demagnetized. The blocking voltage Usp at the switch element SE rises, however, with the rising input voltage Ue until, at a second point in time t1, the blocking voltage Usp currently present at the switch element SE reaches or exceeds the predefined breakthrough voltage Ud. As from the second point in time t1, a current flow I.sub.AVAL ensues at switch element SE. As from this second point in time t1, the current I.sub.L, which flows through the active switch unit FD, commutes at the switch element SE or the current via the active switch unit FD switches off. That is, the entire current I.sub.L via the inductance L flows as from the second point in time t1 as I.sub.AVAL (as shown in FIG. 3) via the switch element SE. The predefined breakthrough current Ud (e.g., 1700V) is present at the switch element S at the second point in time t1 and a power loss occurs in the switch element SE through this, which is formed by the product of the blocking voltage Usp times the current I.sub.L or I.sub.AVAL.

[0060] At a third point in time t2, the overvoltage value (e.g., 2000V), able to be predefined via varistor limiting, for example, is reached by the input voltage Ue. Furthermore, up to the third point in time t2, the voltage Us as well as the voltage U.sub.L at the inductance L have also risen, e.g., to a difference between input voltage Ue and the sum of breakthrough voltage Ud at the switch element SE and output voltage Ua. For an input voltage Ue of, e.g., 2000V, an output voltage Ua of, e.g., 400V and a predefined breakthrough voltage Ud of, e.g., 1700V, a voltage U.sub.L of −100V is produced at the inductance L. That is, through the current flow I.sub.AVAL via the switch element SE, which is relayed to the inductance L, the voltage U.sub.L rises at the inductance L, which thereby receives a lower demagnetization voltage and thereby can only demagnetize itself slowly. Through this, the current I.sub.L via the inductance L also only falls slowly or thus also the current flow I.sub.AVAL via the switch element SE.

[0061] At a fourth point in time t3, such as after 20 to 30 μs, the input-side overvoltage decays and the input voltage Ue falls again, until at a fifth point in time t4 the predefined breakthrough voltage Ud at switch element SE is undershot.

[0062] Between the fourth point in time t3 and the fifth point in time t4 (with a constantly increasing breakthrough or blocking voltage at switch element SE), the voltage Us at the point in the circuit between switch element SE, inductance L and active switch element FD also falls in parallel to the input voltage Ue. In a similar way, the voltage UL at the inductance L again falls, i.e., the inductance L again receives as from the fourth point in time t3 a greater demagnetization voltage, whereby the current I.sub.L in the inductance can fall more rapidly.

[0063] If at the fifth point in time t4, the breakthrough voltage Ud of the switch element SE is reached or undershot, then the entire demagnetization voltage U.sub.L is again available to the inductance and the current I.sub.L can be discharged undisturbed to the capacitance Ca arranged on the input side. The current I.sub.L now falls with a rise as before of the overvoltage, while the current flow I.sub.AVAL via the switch element SE is ended, i.e., the switch element SE does not permit any more current to flow as from the fifth point in time t4.

[0064] At a sixth point in time t5, the input-side overvoltage has completely decayed. The input voltage Ue has again fallen to the usual continuous operating voltage (e.g., 800V). The inductance L continues to be demagnetized until, at a seventh point in time t6, the entire energy of the inductance L is discharged to the capacitance Ca. The current I.sub.L in the inductance L has then fallen to a value of 0. The input circuit ES can then be operated normally again or the switch element SE is switched via an activation pulse GS into a conductive state.

[0065] For a dimensioning of the input circuit ES, for example, first the maximum permitted avalanche energy for the switch element SE of for the overall switch element arrangement SE1 (i.e., for the components forming the switch element SE or SE1) is determined from appropriate datasheets. In this case, the maximum working temperature of the switch element SE, SE1 or of all components accepting the avalanche energy to be expected during operation are additionally to be taken into consideration. This can lead to a reduction of the permitted avalanche energy.

[0066] The maximum current flow I.sub.AVAL is then determined that, in the event of an overvoltage, predefined limit values for the switch element SE, SE1 or the individual components forming the switch element SE, SE1 may not exceed. To this end, a maximum value of the current I.sub.L through the inductance at the end of an overvoltage (i.e., at the fifth point in time t4 of FIG. 3, at which the overvoltage has decayed far enough for no more current flow I.sub.AVAL to occur through the switch element SE, SE1), is determined. For the determination, it is assumed that the inductance L, at the point in time at which an overvoltage leads to a current flow I.sub.AVAL through the switch element SE, SE1 (i.e., at the second point in time t1 of FIG. 3) at which the input voltage/overvoltage Ue exceeds a breakthrough voltage Ud of the switch element SE, SE1) caused by a preceding switching period carries a current I.sub.L. Depending on the level of the overvoltage and the limiting voltage of the switch element SE, SE1, in the course of the overvoltage that occurs, the current I.sub.L will either fall more slowly in the inductance L or will rise further. In order to cover all eventualities, the current value of the current I.sub.L at the second point in time t1 under unfavorable load conditions and also input and output voltage conditions is employed as output condition I0 for the dimensioning of the input circuit ES.

[0067] Through the occurrence of an overvoltage and the current flow I.sub.AVAL that occurs as a consequence, the voltage as from which the inductance L can demagnetize reduces. Depending on the overvoltage, the breakthrough voltage Ud of the switch element SE, SE1 and the output voltage Ua of the input circuit ES, instead of just a slowing down of the demagnetization, the result can be a magnetization of the inductance L, if, for example, during the overvoltage the voltage Us at the point in the circuit between switch element SE, inductance L and active switch unit FD rises to a higher value than the output voltage Ua. This is taken into consideration in the dimensioning in the Equations 1 and 2 specified below, wherein, as a simplification, a period of time between the points in time t1 and t2 or t3 and t4 is assumed to be extremely short. This assumption, as well as a simplification of the calculation, also represents a conservative approach for the dimensioning. System overvoltage pulses are usually hard to estimate and are rather able to be defined via an energy content. That is, voltage rise times are usually not able to be determined in advance, therefore it is sensible to establish the dimensioning of the input circuit ES for the most unfavorable case, as is described by Equations 1 and 2.

[0068] A maximum value of the current I.sub.L through the inductance L at the end of an overvoltage, i.e., at the fifth point in time t4, at which the overvoltage has decayed far enough for no more current flow I.sub.AVAL to occur through the circuit element SE, SE1, can therefore be determined as follows:

[00001]$\begin{array}{cc}{I}_L\left(t4\right)=I0+\frac{Ue-Usp-Ua}{L}*\left(t4-t1\right)& \mathrm{Eq}.1\end{array}$ [0069] I0—is the value for the current I.sub.L through the inductance L at the second point in time t1, at which a current flow I.sub.AVAL through the switch element SE, SE1 as a result of the overvoltage Ue begins; [0070] Ue—is the input or overvoltage; [0071] Usp—is the blocking voltage present at the switch element or at the overall switch element with additional circuitry in overvoltage operation, at which a current flow I.sub.AVAL flows; [0072] Ua—is the output voltage of the input circuit ES and L is the inductance L of the input circuit ES.

[0073] For the determination of the avalanche energy E.sub.AS occurring at switch element SE, SE1 between points in time t1 and t4, there can then be a determination in accordance with the following relationship:

E.sub.AS=∫.sub.t1.sup.t4Usp(t)*I.sub.VAL(t)  Eq. 2

[0074] In this case, the product of the time curve of the blocking voltage Usp at switch element SE, SE1 and the time curve of the current flow I.sub.AVAL via the switch element SE, SE1 between the points in time t1 and t4 is integrated. A curve of the current flow in this case, as can be seen in FIG. 3, corresponds to the curve of the current I.sub.L through the inductance L.

[0075] In conclusion the avalanche energy E.sub.AS determined, which occurs at switch element SE, SE1 will also be compared with a predefined maximum permitted avalanche energy from the respective manufacturer of the switch element SE, SE1 or the switch element components. If the determined avalanche energy E.sub.AS lies below the maximum permitted avalanche energy, then the switch element SE, SE1 can be used for the input circuit ES.

[0076] FIG. 4 shows an exemplary use of the inventive input circuit ES in an exemplary power supply configured as a three-phase and multi-phase supply. The exemplary power supply is linked for supply of power to a three-phase power supply system with three phases L1, L2, L3. As an alternative, a single-phase or a two-phase AC voltage can also be provided. Arranged directly at the system connection or before an input stage GL are varistors, through which overvoltages can be limited to an overvoltage value able to be predefined (e.g., 2000V). The components of the power supply arranged downstream of them are therefore protected against high supply system-side overvoltage pulses or voltage peaks of overvoltages. Arranged after the varistors is the input stage GL of the power supply, which is designed as a rectifier unit GL, e.g., with a three-phase power supply, is formed as a 6-pulse rectifier. Through the rectifier unit GL, an unstabilized or unregulated input voltage Ue is created from the AC voltage of the supply system for the stages of the power supply arranged downstream.

[0077] The input stage GL can, for example, also be arranged after a filter unit F (e.g., a transformer unit as an EMC filter). Arranged after the input stage GL or the filter unit F is the inventive input circuit ES, which uses the supply voltage rectified by the input stage GL as an unstabilized or unregulated input voltage Ue. This input voltage Ue is converted by the input circuit ES into a stabilized or regulated output voltage Ua of the input circuit. The stabilized or regulated output voltage Ua then forms the input voltage for the downstream converter stage WS, which then delivers a supply voltage for a consumer. The converter stage WS can be formed, for example, as an electrically isolating converter or as a resonant converter, and is protected by the inventive input circuit ES against input-side overvoltages.

[0078] Thus, while there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.