Method and circuit for limiting an overvoltage

p0001The invention relates to a method and a circuit arrangement for limiting an overvoltage which acts on a load driver by switching an inductive load, such as an electric motor.

p0002When inductive loads are switched, high inductive counter-voltages occur during switch-off, which can destroy the load driver. In order to reduce the overvoltages, it is customary to switch so-called freewheeling diodes in parallel with the load driver, which derive overvoltages or current spikes from the load driver. From the<patcit id="pcit0001" dnum="DE4038199A1"><text>DE 40 38 199 A1</text></patcit> It is known to use the load drivers themselves for commutation in the case of an arrangement of four load drivers in the full-wave circuit instead of the free-wheel diodes. In this case, the load drivers are controlled in the commutation case in such a way that the two upper or the two lower load drivers of the full-bridge circuit are switched through, in contrast to normal operation, where the load drivers are connected through one of the two bridge diagonals.

p0003An application in which an inductive load is controlled is, for example, a converter circuit with which an electric motor is operated. In<figref idrefs="f0005">FIG</figref> A known converter circuit is shown schematically. A three-phase alternating voltage of an alternating voltage source 1 is first rectified by a rectifier stage 2, converted into an alternating voltage of variable output frequency by means of load drivers 3, and fed to a load 4 (here a three-phase electric motor M). The signals for generation of the output voltage are supplied by a microcontroller 5. These are converted into suitable pulses for driving the load drivers 3 with the aid of gate drivers GD. One basic element of such an arrangement is a half-bridge 6 (shown in a framed in FIG. 6).

p0004In such a converter for electric drives, the load 4 in the form of an electric motor M has inductive components. With the converter, approximately sinusoidal currents are generated in the three phases of the motor M by means of pulse-modulated switching of load drivers 3. For protection against induced overvoltages, each load driver 3 in each case has a freewheeling diode 7 arranged parallel to it. Because of the inductive components in the load 4, the current between the load driver 3 and the corresponding freewheeling diode 7 commutates back and forth within a half-bridge 6 (cf.<figref idrefs="f0005">6 and 7</figref>).

p0005Such a half-bridge 6 is shown in FIG <figref idrefs="f0005">FIG</figref> Is shown separately and has in each case two load drivers 3 connected in series which are connected between a supply potential + V_CC And a reference potential 0V. A freewheeling diode 7 is connected in parallel to each load driver. The load 4 is connected to the common connection of the two load drivers 3. This results in a bridge circuit which allows the load 4 to be controlled with an alternating voltage.

p0006The power supply V_CC Is buffer-stored in an intermediate circuit capacitor 8, which supplies the supply voltage V_CC For all half-bridges 6. An intermediate circuit is formed by the intermediate circuit capacitor 8 and a half-bridge 6.

p0007In each half-bridge 6, internal inductance inductances L act_{Σ, int} And external leakage inductances caused by the construction L_{Σ, ext} In addition to the inductance of the load 4. The two load drivers 3 are controlled in such a way that only one of the two load drivers is directed, while the other load is blocked.

p0008In <figref idrefs="f0005">FIG</figref> Is additionally the basic current profile in a half-bridge 6 upon commutation of a current from the inductive load L_L From the upper freewheel diode 7 into the lower, on-load load driver 3 (the load drivers 3 switch alternately, while the other is switched off). Before the switching-on process, the current I flowing through the upper freewheeling diode 7 flows_V By the load L_L, The upper free-field diode 7 and the two inherently scattered inductances L_{Σ, int} and L_{Σ, ext}After the commutation process has been completed, ie when the lower load driver 3 is switched on (usually used as a load driver 3 switch in the semiconductor device and therefore also referred to as a power semiconductor or semiconductor power switch), the current flows through the load L._L And by the lower load driver 3 as well as the lower leakage inductances L_{Σ, int} and L_{Σ, ext}. The electricity contribution I is indicated by dashed lines_e.g., By the transition of the free-running diode 7 from the conducting to the blocking state during the switching process.

p0009This proportion causes <figref idrefs="f0006">Figures 8A to 9B</figref> Respectively. The internal leakage inductances internal to the component and build - up_{Σ, int} or L_{Σ, ext} During the decay of the reverse current peak, an overvoltage could be greater than the intermediate circuit voltage or the voltage V_CC On the freewheeling diode 7, which may exceed the maximum permissible voltage for the load driver 3 or the free-wheel diodes 7 (free-running diode 7 can be destroyed).

p0010When the current is taken from the freewheel diode 7 into the load driver 3, the slope of the current rise in the load driver 3 is determined by the control of the load driver 3. In addition to the current from the load L_L A so-called reverse current peak occurs, which is caused by the storage charge of the free-running diode 7. The additive reverse current peak is fed from the intermediate circuit capacitor 8 and flows via the leakage inductances L_{Σ, int} or L_{Σ, est} In the intermediate circuit. Depending on the optimization of the free-wheel diodes 7, this backflow peak drops more or less rapidly. The height and the decay rate can be influenced by the slope di / dt of the current rise in the load driver 3.

p0011Depending on the size of the leakage inductances L_{Σ, int} and L_{Σ, ext} In the intermediate circuit and current steepness di / dt, a voltage drop occurs at the half-bridge 6 and the load driver 3, which is defined by the law u (L_σ) = L_σ* Di / dt is determined. At the decay of the reverse current peak, L is produced at these leakage inductances_{Σ, int} and L_{Σ, ext} A reverse voltage, which generates a voltage peak on the freewheeling diode 7. In the case of very high-speed freewheeling diodes 7, high leakage inductances L_{Σ, int} and L_{Σ, ext} Overvoltage spikes to the destruction limit.

p0012In the <figref idrefs="f0006">Figures 8A and 8B</figref> Is the voltage at component V_ce In the switching process, the switched load current I_C By the load driver 3 and the gate voltage V_GE At the switching load driver 3 '(<figref idrefs="f0006">FIG. 8A</figref>) And on the freewheeling diode 7 (with the parallel-connected, blocking load driver 3 "; <figref idrefs="f0006">FIG. 8B</figref>) Are shown schematically.

p0013On switching, the gate voltage V rises_GE At the switching load driver 3 'from -15 V to +15 V, the voltage V_ce At load driver 3 drops from + V_CC (Positive potential of the supply voltage or intermediate circuit voltage V_CC) To approximately 0V. The current through the load driver (load current I_C) Rises rapidly and takes its nominal value I after a short current peak_C on.

p0014The freewheeling current drops from the rated value I at the free-wheeling diode 7 ", which is connected in parallel with the blocking load driver 3"_C To zero (with a short overflow or negative current peak, also referred to as a back current peak). The gate voltage V there_GE Of the locked load driver 3 "remains at -15 V while the voltage V_ce At the load driver from 0 V to + V_CC (Also with a short, strong overhead, also referred to as overvoltage).

p0015Such commutation processes (current reversal or change-over) also take place in other applications with inductive loads and comparable switching operations (for example, in switching power supplies).

p0016In order to protect the free-wheel diodes 7 from such overvoltages, the load driver 3, which controls the commutation process, has so far been switched to a slower state (at least as known internally), as a result of which the reverse current peak decreases and the free-running diode 7 breaks off less steeply (cf. <figref idrefs="f0006">Figures 9A and 9B</figref>).

p0017In the <figref idrefs="f0006">Figures 9A and 9B</figref> Is the influence of overvoltage and reverse current peak by means of a slower switching of the load driver by means of a larger gate resistance R_G (<figref idrefs="f0006">FIG. 9A</figref> Shows the ratios at the switching load driver 3 'and <figref idrefs="f0006">FIG. 9B</figref> The ratios at the freewheel diode 7 "connected in parallel with the blocking load driver 3"). By way of comparison,<figref idrefs="f0006">Figures 8A</figref> Or 8B. In particular, the losses in the switching load driver are thereby reduced, as can be seen from the current / voltage curves in FIG<figref idrefs="f0006">FIG. 8A</figref> Is significantly increased.

p0018It is known, at least internally, the effective leakage inductance L_σ by a so-called Snubberbeschaltung to reduzie reindeer. If the current steepness in the load driver 3 is reduced by means of a slower drive, this results in increased, unacceptable switching losses in the load driver 3. A snubber circuit generates increased costs and also additional losses in the snubber circuit.

p0019It is an object of the invention to provide a method and a circuit arrangement for limiting an overvoltage by which additional losses are reduced. In particular, strong overvoltages and reverse current peaks are to be reduced.

p0020This object is achieved by a method for limiting an overvoltage with the features of patent claim 1 and by a circuit arrangement having the features of patent claim 7.

p0021In this case, a (semi-conductor) circuit breaker (hereinafter referred to as a load driver), which is connected in parallel with a freewheeling diode, is utilized as a limiter for the (over) voltage present during the commutation during the commutation. The limitation of the overvoltage by a parallel-connected load driver is achieved by briefly switching this load driver, just as far as possible, during the decay of a back-current peak, in such a way that an additive falling current is produced in the semiconductor switch-freewheel-diode pair. The additive, dropping current thus reduces the current slope in circuit-induced leakage inductances and reduces the induced voltages. The short-term switching-on is adapted in time to the occurrence of the back-current peak.

p0022Advantageous further developments of the invention are reproduced in the subclaims.

p0023Thus, as semiconductor switches, so-called IGBTs can be used in which the control terminal and the output terminal of the inhibited semiconductor switch are connected to each other in a high-impedance manner, whereby a short-term switching-on of the semiconductor switch is achieved when a current peak occurs. This is a very simple way of counteracting the tension tip to a certain extent.

p0024It is also advantageous to apply the control terminal of the blocked semiconductor switch to an approximately rectangular voltage pulse of a predetermined duration and an amplitude less than a predetermined threshold value during the occurrence of a current peak or at least during the decay of the current peak.

p0025It is even more advantageous to use a step-shaped voltage pulse instead of a simple square-wave pulse, the greatest amplitude occurring briefly at least during the decay of the back-current peak in order to counteract this back-current peak.

p0026The voltage at the output terminal of the other, blocked semiconductor switch can be additionally (or else alone) fed back to its control terminal via a feedback path. As a result, the blocked semiconductor switch is briefly switched on during the presence of the current peak or inductive overvoltage. This reduces the current steepness in the drop of the current peak and thus the overvoltage.

p0027This effect can be further increased if, in addition to the feedback, the control terminal of the semiconductor switch is acted upon by a voltage pulse during the pending or decaying of the current point, whereby the actually blocked semiconductor switch is partially switched on.

p0028The feedback path advantageously has at least one such a component by which the feedback path is activated only when the voltage exceeds a threshold value. Thus two antiserially switched Zener diodes can be used for this purpose. It is thereby achieved that only when a predetermined voltage is exceeded, is it possible to actuate and thus enable a partial switching-on of the semiconductor switch which is inhibited per se, and this is timely adapted to the occurrence of the overvoltage. Alternatively or additionally, the voltage rise can be fed back to the free-wheeling device, for example by means of a capacitor, on the control connection of the load drive. This influences the commutation process in such a way that the height and steepness of the back-current peak decrease. Thus, the effective reduction of inductive overvoltage and the reduction of harmful current spikes with a great current steepness is possible with simple cost-effective means. In addition, the driving of the semiconductor switch, which counteracts the overvoltage or the reverse current peak, is timed for the presence of the back-current peak. The threshold from which the feedback is to be activated can also be simply adjusted by appropriate dimensioning of the components in the feedback path.

p0029Exemplary embodiments of the invention are explained in more detail below with reference to the schematic drawings. Show it:<dl id="dl0001"><dt>Figures 1A and 1B</dt><dd>Current / voltage profiles on a switching-on semiconductor switch or on a free-running diode connected in parallel with a blocking semiconductor switch,</dd><dt>FIG</dt><dd>A first exemplary embodiment of a circuit arrangement according to the invention with a feedback path with zener diodes,</dd><dt>FIG. 3A</dt><dd>A further exemplary embodiment of a circuit arrangement according to the invention with a feedback loop with a separate capacitor,</dd><dt>FIG. 3B</dt><dd>A further exemplary embodiment of a circuit arrangement according to the invention with a feedback path for a signal proportional to the current steepness in the diode,</dd><dt>FIGS. 4 and 5</dt><dd>Current / voltage profiles on a switching-on semiconductor switch according to two exemplary embodiments of a method according to the invention,</dd><dt>FIG</dt><dd>A known converter circuit for controlling a three-phase electric motor,</dd><dt>FIG</dt><dd>A half bridge of the known converter circuit according to the invention <figref idrefs="f0004">FIG</figref> With the electrical currents occurring therein, </dd><dt>Figures 8A and 8B</dt><dd>Current / voltage profiles on a half-bridge semiconductor switch <figref idrefs="f0005">FIG</figref> and</dd><dt>Figures 9A and 9B</dt><dd>Current / voltage profiles on a half-bridge semiconductor switch <figref idrefs="f0005">FIG</figref> When using a larger gate resistance.</dd></dl>

p0030DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the figures and the exemplary embodiments, functionally similar parts have been given identical reference symbols.

p0031For controlling or switching inductive loads_L, Such as, for example, an electric motor (cf. <figref idrefs="f0005">FIG</figref>), Power semiconductor components (as a circuit breaker, a load driver 3 or a semiconductor switch) are often used as drivers. The load drivers 3 themselves are usually controlled by a control unit with a microcontroller 5. Since when switching inductive loads L_L, In particular when the loads L are switched off_L, Undesirable, high voltages are induced, a free-running diode 7 is connected in parallel with each load driver 3, which is intended to reduce an induced overvoltage.

p0032It is customary to provide a half-bridge 6 for controlling an inductive load L._L Two load drivers 3 each having parallel-connected freewheeling diodes 7 connected in series are connected in series (the switched-on load driver is indicated by 3 'and the locked load driver is denoted by 3 ") between the two load drivers 3. The output terminal for an inductive load L_L Respectively.

p0033Two IGBT power transistors are used here as load driver 3 (cf. <figref idrefs="f0001">FIG</figref>, There is shown a complete half bridge 6). In front of the control terminal (gate) of each IGBT, a gate resistor Rg and, before this, a final amplifier stage of a gate driver GD (cf.<figref idrefs="f0005">FIG</figref>). One possible embodiment of such a final amplifier stage is the emitter follower circuit shown in FIG. 9 (<figref idrefs="f0001">FIG</figref>, <figref idrefs="f0002 f0003">FIG</figref>, Circuit part 9). The gate driver GD is, as in FIG<figref idrefs="f0005">FIG</figref> - connected to a microcontroller 5. The final amplifier stage 9 of the gate driver GD is supplied with a positive supply voltage V_G+ And a negative supply voltage V_GAnd is supplied with a control signal V via a common control connection_garearive (See also FIG. 4). This control signal V_gatedrive Is ultimately connected to the gate of the IGBT.

p0034The half-bridge 6 is usually designed as an integrated component. Internal scattering inductances L_{Σ, int} And external leakage inductances L caused by the circuit configuration_{Σ, ext} During the decay of a reverse current peak caused by switching, overvoltages (voltage greater than the supply voltage V_cc Of the half-bridge 6) are induced on the free-wheeling diode 7, which may exceed the maximum permissible voltage for the components used. This overvoltage (or a current peak with a large current steepness di / dt or a steep voltage rise du / dt), which is induced during the switching of the load drivers 3 due to inductances in the circuit, must now be reduced.

p0035According to the invention, the load driver 3 (IGBT), which is parallel to the freewheeling diode 7, is utilized as a limiter for the overvoltage on the freewheeling diode 7 during the switching over. The limitation of the overvoltage by the parallel load driver 3 is achieved in that this load driver 3 is switched on briefly, just as far as the decay of a back-current peak, in such a way that an additive, decaying current is produced in the load-driver freewheel-diode pair. The additive falling current counteracts the induced current peak and thus reduces the current steepness in the leakage inductances L_{Σ, int} and L_{Σ, ext}. Thus, the induced overvoltage also decreases.

p0036The gate and the emitter of the IGBT are connected to each other in a high-impedance manner in the switched-off state of the IGBT. This causes the desired short-term switching-on of the IGBT when the parallel freewheeling diode 7 is decommitted because the break-off of the free-wheel diode current causes a steep voltage rise du / dt on the freewheeling diode 7 and the parallel IGBT in addition to the high current steepness di / dt.

p0037The voltage rise du / dt leads to an increase in the gate voltage V via the so-called mill capacitance in the IGBT_GE, Which is degraded only slowly due to the high-resistance gate-emitter connection. This partially turns on the IGBT for the duration of the voltage rise du / dt. This again reduces the voltage increase du / dt, the IGBT switches off again and is only slightly loaded with current (see corresponding current / voltage curves in FIGS. 1A and 1B).

p0038The results of the influence of overvoltages and reverse current peaks by briefly switching on the IGBT (load driver 3) parallel to the freewheeling diode 7 "is shown by the current / voltage curves in the <figref idrefs="f0001">Figures 1A and 1B</figref> Respectively. The current / voltage profiles at an IGBT (load driver 3 ') which is switched on are shown in FIG<figref idrefs="f0001">FIG. 1A</figref> And the current / voltage profiles on a free-running diode 7 "connected in parallel to the other IGBT (load driver 3") <figref idrefs="f0001">1B</figref> Respectively.

p0039When the gate voltage V_GE Of the non-switched IGBT is turned off, the gate voltage V increases_GE Within a short period of time pulse-shaped. This leads to a significant reduction in the voltage overshoot of the intermediate circuit voltage or supply voltage V_CC In comparison with the prior art (current / voltage ratios of the prior art are shown in dotted lines, cf. <figref idrefs="f0006">Figures 8A and 8B</figref>). Moreover, the current steepness di / dt of the switched load current I_C Smaller, but this leads to more switching losses.

p0040As can be seen from the comparison of the dotted and the continuous current curves of the load current I_C in the <figref idrefs="f0001">Figures 1A and 1B</figref> , The switching losses are only marginally increased as a result. The losses in the short-circuited IGBT are negligible due to the low switched current.

p0041In order to more clearly reduce the current steepness or the overvoltage, a feedback loop 11 (for each pair of IGBT freewheeling diodes 3, 7<figref idrefs="f0001">FIG</figref>) Can be provided which feeds back the voltage at the cathode of the free-run diode 7 "(or at the collector of the IGBT) to the control input of the IGBT (the control signal V_gatedrive, Which is present at the final amplifier stage 9 of the gate driver GD, is influenced), as a result of which a short-term switching-on of the IGBT which is blocked per se is effected.

p0042In the feedback path 11, zener diodes (in the exemplary embodiment according to FIG <figref idrefs="f0001">FIG</figref> A 30V Zener diode 12 connected in series with a Clamp Zener diode 13). These have the effect that a switching-on of the IGBT, which is parallel thereto, is triggered (a so-called overvoltage caging) only upon reaching a defined overvoltage at the freewheeling diode 7 ".

p0043With the aid of the Zener diodes 12, 13, when the overvoltage is reached, the clamping voltage can be increased by feedback into the final amplifier stage 9 of the gate driver GD and thus influencing the control signal V_gatedrive A short-term switching-on of the IGBT can be effected at least partially.

p0044In contrast to a direct feedback to the gate terminal of the IGBT, the Zener diodes are loaded with substantially lower currents in this circuit. In this way, a similar switching behavior can be obtained, as already described by the current / voltage curves in the<figref idrefs="f0001">Figures 1A and 1B</figref> Is shown.

p0045Instead of the two Zener diodes 12, 13, an external capacitor C can also be used in the feedback path 11_ext As, thanwie, <figref idrefs="f0002">FIG. 3A</figref> Is shown. In this case, the increase in the cathode voltage of the free-run diode 7 "is fed back to the control terminal of the IGBT connected in parallel with the free-run diode 7", whereby the IGBT is switched on briefly and at least partially, which counteracts the voltage surge and the reverse current peak.

p0046The combination of the two last-mentioned measures can also be used advantageously. Thus, with the aid of an external capacitor C,_ext In the feedback path 11, the control terminal of the power driver is biased during the voltage rise at the freewheel device to a voltage below the turn-on voltage. In this case, the parallel-connected feedback is substantially less inert when a threshold voltage is exceeded (eg via the Zener diode circuit described), since a lower voltage swing must be overcome until the switching-on operation of the load driver.

p0047Similarly, a control signal can also be fed back in proportion to the magnitude of the current gradient in the freewheel device to the control terminal of the load driver via a feedback path 14 (for example by means of a transformer Ü, an amplifier AMP and an evaluation circuit CC)<figref idrefs="f0003">FIG. 3B</figref>). The current gradient during the fall of the current in the freewheel device after exceeding the maximum of the back-current peak is fed back to the control terminal of the load drive so that it briefly at least partially starts up and limits the current drop in the back-current peak. This measure can be combined with the above or one of the measures described below for biasing the control terminal of the truck. In this way, the overvoltage at the free-wheeling device can be reduced by simple means.

p0048With suitable dimensioning, the current gradient in the free-wheeling device can be used before reaching the maximum of the reverse-current peak in order to bias the control terminal of the load driver to a voltage below the switch-on threshold. This method offers the advantage that the control voltage is increased automatically at the load driver in synchronism with the commutation process.

p0049In the simplest case, the current gradient can be detected as a voltage drop across the leakage inductances L_σint, L_σext Respectively. Likewise, a transformative detection or the detection and evaluation of the current with suitable sensors is possible. The change in the sign of the current gradient makes a rectification or evaluation of the signal necessary in a case distinction dI / dt> <0.

p0050The magnitude of the set voltage rise du / dt is determined by the systematic selection of the capacitance of the capacitor C. In this way, the slope of the voltage rise at the free-run diode 7 "can be controlled in a targeted manner and the course of the back-_ext And does not depend on the value of the mill capacitance changed during the switching process, as in the case of the first exemplary embodiment. In this way, a similar switching behavior can be effected, as already described by the current / voltage curves in FIGS<figref idrefs="f0001">Figures 1A and 1B</figref> Is shown.

p0051Instead of the feedback path 11 or also in addition to the feedback path 11 between the (cathode of the freewheeling diode 7) and the control terminal of the IGBT, a controlled control of the IGBT performed at the time of the switching operation can also be performed via the control signal V_gatedrive Be made. In this case, the IGBT parallel to the free-run diode 7 "is controlled in a targeted manner by a control signal V dependent on the time of the reverse current peak on the freewheeling diode 7"_gatedrive ( "<figref idrefs="f0002">FIG</figref>) Is controlled (at least partially switched on) during a commutation phase during the commutation phase, and is then switched off again (blocked) at a point in time at which the actual load-controlling control signal V_gatedrive Is not provided for this IGBT.

p0052The control signal V adapted in time to the reverse current peak_gatedrive Raises the gate voltage V_GE At the IGBT, which is connected in parallel to the free-wheeling diode 7 "and is closed to the IGBT, during the commutation process, for a short time.

p0053The current / voltage ratios in such a control are exemplary in the upper part of FIG <figref idrefs="f0002">FIG</figref> Compared to the one in the <figref idrefs="f0002">FIG</figref> Dotted line <figref idrefs="f0006">Figures 8A and 8B</figref> Is reproduced. The load current I_C Decreases in the case of the blocking of the IGBT and has, in the negative region, a peak or superelevation with a smaller current steepness di / dt compared to the current profile as is known from the prior art (dotted current sequence of the load current I_C). The intermediate circuit voltage V_CC Shows a considerably smaller overshoot or lower overvoltage (smaller peak), since the control of the IGBT specifically counteracts the overvoltage.

p0054Instead of a stepped control signal V_gatedrive And a short-term, additional increase in the gate voltage V associated therewith_GE Can also be a rectangular control signal V_gatedrive And a gate voltage V which is higher in average_GE ( "<figref idrefs="f0004">FIG</figref>) As compared to the gate voltage V_GE out <figref idrefs="f0002">FIG</figref> be provided. The gate voltage V_GE At the IGBT, which is in parallel with the freewheeling diode 7 ", is switched off during the commutation of the current from the freewheeling diode 7 in a predetermined, short time window during a reverse current peak of the load current I_C On the free-wheeling diode 7 is raised to a value smaller than a threshold voltage.

p0055In the same way as in the case of the first exemplary embodiment, a switching-on as a result of the voltage rise du / dt on the free-wheeling diode 7 and the reaction capacitor (capacitor C) can also be used here without a high-impedance gate-emitter connection_ext) Are carried out.

p0056The advantage of this method is that the voltage interval is reduced until the IGBT is switched on and thus the delay time until the feedback of the voltage rise du / dt can be minimized.

p0057The control signal V adapted in time to the reverse current peak_gatedrive The voltage at the IGBT rises parallel to the free-wheeling diode 7 "in a certain time window, for example by at least partially controlling the IGBT by means of the feedback capacitance, the resulting switching behavior (current / voltage profile) being in the upper part of the IGBT <figref idrefs="f0004">FIG</figref> In comparison to the behavior from the <figref idrefs="f0006">Figures 8A and 8B</figref> Respectively.

p0058The gate voltage V_GE Is less strongly increased by the constantly high control than in the previous exemplary embodiment. The collector voltage V_ce/ Voltage on the freewheel direction, however, behaves similarly to the previous exemplary embodiment, while the load current I_C Is somewhat reduced in its current steepness di / dt, but the back-current peak is still significantly less steep than the comparable back-current peak in the prior art (see dotted line for the load current I)_C in <figref idrefs="f0004">FIG</figref>).

p0059In order to limit the voltage on the freewheeling diode 7 during switching operations, circuit arrangements with a feedback path 11 can also be provided with additional, targeted control of the gate voltage V_GE Of the IGBT. Likewise, the high-impedance connection of the gate and the emitter of the IGBT can be switched off, as explained in the first exemplary embodiment, with a feedback path 11 or a specific, short-term control of the gate voltage V_GE be combined. Thus, a simple and effective reduction of the overvoltage and / or the back-current peak on the free-running diode 7 is provided. The free-wheel diodes 7 are thus largely protected against destruction.

p0060Free-throw diodes for limiting an inductive overvoltage are well known. Likewise, functionally equivalent freewheel devices are known which are not necessarily effective with only a single diode connected in parallel with a circuit breaker. For the invention it is immaterial whether a free-wheeling diode or an equivalent means is used as the free-wheeling device. It is essential, however, that an inductive overvoltage and a large current steepness are reduced according to the method according to the invention or with the circuit arrangement according to the invention.