Overcurrent protection circuit

Disclosed is an overcurrent protection circuit including an overcurrent trip and a switching element. A switched current passing through the switching element can be detected by the overcurrent trip and the switching element can be tripped to open if the switched current fulfils a tripping requirement. A current limiter is series-connected downstream of the switching element.

This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE01/02046 which has an International filing date of May 30, 2001, which designated the United States of America and which claims priority on German Patent Application number DE 100 29 418.9 filed Jun. 15, 2000, the entire contents of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to an overcurrent protection circuit. In particular, the present invention generally relates to an overcurrent release and a switching element, in which a switching current flowing through the switching element may be detected by way of the overcurrent release and opening of the switching element may be initiated if the switching current satisfies a tripping condition.

BACKGROUND OF THE INVENTION

Overcurrent protection circuits are generally known—in particular in the form of circuit breakers.

Circuit breakers generally have a thermal overload release and an electromagnetic quick-action release. If the current flowing through the circuit breaker slightly exceeds a rated current, tripping takes place with a time delay, by way of the thermal overload release. In the event of a short circuit, in contrast, when the current rises rapidly, the electromagnetic quick-action release trips the circuit breaker with virtually no delay. The tripping of the circuit breaker has two effects. First, the circuit is opened directly. Secondly, a switching mechanism is tripped, so that the circuit is not closed automatically again once the circuit breaker has tripped.

Despite tripping with virtually no delay, the electromagnetic release does have a reaction time. During the reaction time, the load current rises above the identification current at which the electromagnetic release trips. This results in a large amount of wear due to contact erosion in the contact that is to be opened. If the current rises too rapidly, a continuous arc may result. Such a continuous arc is difficult to quench, and a failure to do so may lead to the complete destruction of the circuit breaker. Furthermore, such a continuous arc may make it impossible to switch off the current.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to provide an overcurrent protection current in which an excessive rise in the current is prevented, even in the event of a short circuit in the load circuit.

An object of an embodiment of the present invention may be achieved by connecting a current limiter in series with the switching element.

If the current limiter has a steep current/voltage characteristic at low voltages and has a flat current/voltage characteristic at high voltages, this results in low power loss during normal operation of the overcurrent protection circuit, and in current limiting at a relative low value in the event of a short circuit.

In some cases, the current limiter may be in the form of a single component; however, generally it is in the form of a current limiting circuit.

Examples of a current limiter include the current limiter having two back-to-back series-connected transistors or (alternatively) two back-to-back parallel-connected series circuits, which each have one transistor and one diode.

Current limiting may also be achieved in an effective manner by the current limiter having two self-commutating transistors, each having a gate contact, a source contact and a drain contact. The source contacts may be connected to one another via a resistor. Additionally, the source contacts of the transistors may be connected to the gate contact of the respective other transistor, and the current limiter may be connected in series with the switching element via the drain contacts of the transistors.

If the transistors are in the form of SiC transistors, the current limiter has a particularly low resistance during normal operation, and can absorb a large amount of energy during short-circuit operation.

If the overcurrent release, the switching element and the current limiter are arranged in a common housing, this results in the overcurrent protection circuit being particularly compact. Furthermore, the overcurrent protection circuit may then be in the form of a unit which can be wired in advance.

Generally, the overcurrent release initiates the opening of the switching element with virtually no delay when a limit current is exceeded. If the current limiter is designed appropriately, it is also possible to open the switching element with a time delay when the limit current is exceeded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is illustrated inFIG. 1, a load1is connected to a voltage source4via an on/off switch2and an overcurrent protection circuit3. The on/off switch2can be actuated from the outside by way of an appropriate control signal S. When the on/off switch2is closed, a switching current I flows through the overcurrent protection circuit3and through the load1.

As illustrated inFIG. 2, the overcurrent protection circuit3has an overcurrent release5, a switching element6and a current limiter7. The switching element6is generally closed so that the switching current I can flow. However, the switching current I, which also flows through the switching element6, can be detected by way of the overcurrent release5. An opening of the switching element6can be initiated when the switching current I satisfies a tripping condition. As can be seen fromFIG. 2, the overcurrent release5, the switching element6and the current limiter7are connected in series and are arranged in a common housing10. The switching current I is in this case limited by way of the current limiter7.

The overcurrent release5generally has two tripping elements8,9, namely an electromagnetic quick-action release8and a thermal overload release9. The opening of the switching element6is initiated with virtually no delay by way of the quick-action release8when the switching current I exceeds a limit current that is several times a rated current, typically 12 or 19 times the rated current. Opening of the switching element6is initiated with a time delay by way of the overload release9, when the switching current I is above the rated current for a relatively long time. For example, at 1.2 times, 1.5 times, twice or 7.2 times the rated current.

The switching element6has a switch so the circuit can be opened and closed as such. Moreover, the switching element6has a switching mechanism so the switch can be fixed in its open or closed position. When one of the tripping elements8,9responds, not only is the switch of the switching element6opened, but the switching mechanism is also released. Once the switching element6has been opened, it is designed not to automatically close again.

In this case, in the event of a short circuit, initiation of the opening of the switching element6occurs with virtually no delay.

If, in contrast, the current limiter7is appropriately designed such that it copes with a short circuit until the thermal overload release9responds, there is no need for the quick-action release8. This is indicated inFIG. 2by the quick-action release8being shown only by dashed lines. Thus, in this case, the opening of the switching element6is initiated with a time delay even in the event of a short circuit.

By way of example,FIG. 3illustrates one possible refinement of the current limiter7. As illustrated inFIG. 3, the current limiter7has two self-commutating transistors11,12. In principle, the transistors11,12may be of any desired nature. In particular, they may be in the form of MOSFETs, IGBTs or else bipolar transistors. According to an exemplary embodiment, they are in the form of SiC field-effect transistors11,12. Each of the field-effect transistors11,12has one gate contact13, one source contact14and one drain contact15. The source contacts14are connected to one another via a variable resistor16. Furthermore, the source contacts14of the field-effect transistors11,12are connected to the gate contact13of the respective other field-effect transistor12,11. The current limiter7is then connected in series with the switching element6via the drain contracts15.

The current limiter illustrated inFIG. 3may be formed from discrete components11,12,16. However, as is indicated by a dashed-dotted line inFIG. 3, it may also be in the form of a monolithically integrated circuit.

The current limiter7illustrated inFIG. 3has a current/voltage characteristic as shown inFIG. 4. As can be seen fromFIG. 4, the characteristic has a steep profile at low voltages and has a flat profile at high voltages. In particular, the switching current I is substantially limited to the maximum value shown by the dashed line, even at high voltages U. The maximum value can be adjusted by appropriate adjustment of the resistor16.

Alternatively, the transistors11,12may also be connected back-to-back in series as illustrated inFIG. 5. In this case, a diode17,18is preferably connected in parallel with each of them. In another exemplary embodiment illustrated inFIG. 6, the current limiter7has back-to-back parallel-connected series circuits. Each series circuit has a transistor11or12, respectively, with which a diode17,18is connected in series. The transistors11,12are also in the form of SiC components in the embodiments shown inFIGS. 5 and 6.

The overcurrent protection circuit7according to an embodiment of the present invention is particularly suitable for use in AC voltage circuits. However, in principle, the protection circuit7may also be used in DC voltage circuits.