Electromechanical and electronic switching devices are known to be used for switching alternating current. With electromechanical switching devices a distinction is made between switching devices for operational switching (“contactors”) and for short circuit switching (“circuit breakers”). Depending on the protection concept, an overload can be switched by the circuit breaker or by the contactor.
The electronic switching devices used in practice are based on thyristor technology and are only suitable for operational switching or overload switching. In addition power semiconductors are increasingly coming into the picture: One current development known in particular is the so-called “cascode” as a combination of two antiserially switched SiC-JFETs and SI-MOSFETs.
At least one embodiment of an electronic circuit breaker is suitable for use both for operational switching on overload and also for short circuit switching. At least one embodiment of the invention combines the previous approaches of the electronic circuit breaker with newly developed functionalities.
Electronic circuit breakers must have the following functions/switching characteristics (ON, OFF):                Undelayed ON        If nec. phase-controlled ON        Undelayed operational: OFF, e.g. for resistive loads        Switching off at the zero current crossing point: operationally and on overload, especially because of switch-off or large inductances, such as of motors for example        Cascode short circuit protection: Immediate OFF        Overload cascode: Corresponds to the overload protection for electronic circuit breakers        poss. overload protection branch circuit: This corresponds to overload protection for motors, leads        Overcurrent protection for electronic circuit breakers        Isolated activation        Isolated status message        Power supply at switch potential        
The actual circuit breaker element in this case is an antiserial cascode circuit comprising the above-mentioned combination of SiC-JFET and Si-MOSFET, which is shown in detail in FIG. 1 of the drawing and is described in detail below.
With the circuit in accordance with FIG. 1 already known from the prior art however only the functions:                Undelayed ON and        Undelayed OFFcan be implemented. The other functions listed above cannot be realized with this circuit.        
The functions “phase-controlled ON” and “overload protection: branch circuit” are able to be realized in conjunction with the other phases in a soft starter control section or an overload protection unit, protecting load (motor contactor) and operating resources such as leads for example. Such a unit is already used in soft starters with thyristors and belongs per se to the prior art.
The additional circuit in the individual phase must thus fulfill the following function in addition to “undelayed ON” and “undelayed OFF”:                Switch off at zero current crossing        Cascode short circuit protection        Cascode overload protection        Overvoltage protection        Isolated activation        Isolated status message        Power supply at switch potential.        
Devices which thyristors—as already mentioned—are suitable as current valves. Thyristor switching devices consist of a control section at control potential and the respective circuit elements at primary voltage potential. Depending on the device, the following functions/switching attributes can be stored in the control section:                Undelayed ON        Phase-controlled ON (so-called “soft start”)        Operational OFF at the subsequent zero crossing by thyristors        Overload protection for power section        Overload protection for branch circuit        Isolated activation        Isolated status message, this is currently only implemented in approaches in the higher-ranking control unit.        
Operational switching off at the zero crossing point and on overload is undertaken using the component characteristic of the thyristor. A short circuit on the other hand leads to the destruction of the device, if it is not protected by a higher-ranking circuit element.
Concepts are known from the prior art which describe an electronic circuit breaker, to which end the reader is referred to DE 196 12 216 A1 and U.S. Pat. No. 5,216,352 A. These concepts employ transistors.
The above-mentioned functions are achieved by individual functional units. This generally requires the instantaneous current values to be recorded, which can be done in one of the following ways:                Current measurement: Is undertaken via converters shunts, current balancing in semiconductors.        Switch off at zero current crossing point: Is done by evaluating the current signal by means of comparators and issuing the corresponding signal to the cascode.        Cascode short circuit protection: “UCE monitoring or instantaneous current value monitoring are undertaken. This is done using known methods according to TOK (tolerant locus criterion) or the prevention of making on a short circuit by test pulse.        Cascode overload protection: Is undertaken by delayed current evaluation/characteristic curve for short-duration overload, various heating models, temperature sensor for long-term overload protection.        Overvoltage protection: Is undertaken using clamping, Zener diodes, varistors.        Isolated activation, isolated status message: Is undertaken using optocouplers, transformers if nec. as IC, via R/C voltage dividers between L and N. The signal transmission is modulated onto the power supply with pulse width modulation; possibly also MR couplers.        Power supply at switch potential:        Can be undertaken transformationally from L-N or L-L using impedance/diodes from L-N or L-L, if necessary with artificial star point using R/C voltage divider between L and N, via short-duration blocked semiconductor circuit elements and capacitor charging, using current/voltage converter signal, DC/DC converter e.g. Burr Brown, AC/DC converter, e.g. transformer, ultrasound through piezo transformers, optical using optical fiber/photo cell, combination heat/thermo element.        
Furthermore an electronic switching device is known from DE 100 62 026 A1 which includes a working circuit for application to the operating voltage, in which switching off means are present, which, in the event of danger, automatically bring the circuit element into the switched-off state by utilizing the energy contained in the operating current or the operating voltage. An SiC-JFET is available for this. In DE 196 00 807 A1 on the other hand, an intelligent, separate half-bridge power module is presented comprising at least one power transistor, with this being protected by an overvoltage clamping and de-saturation detection circuit. Each power transistor is assigned an isolating transformer in this invention, with its primary windings being connected to a control communication circuit isolated from it. The gate controller of the power transistor is connected in this case to the secondary winding of the transformer. Finally a monolithic current-controlled breaker system is known from DE 69 124 740 T2, in which switching off is undertaken if necessary using a suitable microprocessor control at the zero crossing point at which the energy is negligible.
Even if the above circuits are also to fulfill the functions of a circuit breaker, their design is extremely complex.