Electricity: electrical systems and devices – Safety and protection of systems and devices – Transient responsive
Reexamination Certificate
2003-03-24
2004-11-23
Toatley, Jr., Gregory J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Transient responsive
C327S309000, C327S327000
Reexamination Certificate
active
06822842
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a switching device for switching at a high operating voltage and includes at least a low-voltage (LV) switching element having an LV cathode terminal, an LV anode terminal, and an LV grid terminal, a first high-voltage (HV) switching element having a first HV cathode terminal, a first HV anode terminal, and a first HV grid terminal, the LV anode terminal being electrically short-circuited with the first HV cathode terminal and the LV cathode terminal being electrically short-circuited with the first HV grid terminal.
A switching device is disclosed in U.S. Pat. No. 6,157,049 to Mitlehner et al. and also in European Patent Application 0 063 749 B1, corresponding to U.S. Pat. No. 4,663,547 to Baliga et al. The electronic switching device respectively disclosed is based on the special interconnection of the LV and HV switching elements mentioned. It is also referred to as a cascode circuit. The switching device serves for switching a high electric current and is also able to reliably block a high operating voltage. The LV switching element, a normally off MOSFET, includes silicon (Si) and ensures that the interconnection with the HV switching element, configured as a normally on JFET, produces a normally off unit. The HV switching element includes a semiconductor material with a breakdown field strength of more than 10
6
V/cm. In the blocking situation, the HV switching element, then, takes up the substantial part of the voltage to be blocked that is present at the cascode circuit. The semiconductor material silicon carbide (SiC) is particularly suitable as a starting material for the HV switching element.
In converter technology, which is also used, for example, in a variable-speed drive, a switching device is required that achieves a high efficiency as near to 100% as possible at a high power, i.e., generally also at a high operating voltage. If the switching device has the lowest possible static and dynamic losses, a virtually optimal utilization of energy is achieved and, accompanying this, a significant reduction in the required cooling outlay is achieved.
At the present time, converter technology usually employs, as HV switching element, a silicon Insulated Gate Bipolar Transistor (IGBT) or a silicon Gate Turn Off (GTO) thyristor in the voltage range up to 6.5 kV and a silicon thyristor in a voltage range up to about 10 kV. However, these switching elements are bipolar semiconductor components whose structure dictates that they have both a certain delay time and appreciable dynamic switching losses during switching on account of an unavoidable stored charge effect.
This problem is avoided by a cascode circuit constructed only with unipolar switching elements, that is to say, with field-effect transistors, for example. In addition to the static loss being low in any case, a unipolar switching element is also distinguished by a short switching time and by low dynamic losses on account of the lack of stored charge effects. In the case of an HV switching element configured as a junction field-effect transistor realized in SiC, the maximum permissible reverse voltage can be achieved in two different ways. First, the drift zone is lengthened. In the case of a vertical junction field-effect transistor, this is equivalent to growing a thicker epitaxial layer on the substrate used. Second, the dopant concentration within the epitaxial layer is also reduced. Both measures, which are favorable with regard to the maximum permissible reverse voltage, bring about an increase in the nonreactive drift resistance, however. In the on state (switching element closed), this leads to a higher static power loss that is dissipated thermally. Therefore, a cascode circuit that is realized using a unipolar HV switching element made of SiC is restricted at the present time to a maximum permissible reverse voltage of the order of magnitude of typically 3.5 kV. In principle, however, an even higher reverse voltage, for example, of 5 kV, is also possible.
If a switching device is required for a higher reverse voltage, at the present time this is possible only by a plurality of series-connected cascode elements, which are constructed with unipolar switching elements in the manner described, or by a series circuit including a plurality of unipolar switching elements or by the use of the bipolar silicon switching elements described. As already described, a bipolar switching element leads to a longer switching time and to higher dynamic losses.
In the case of the series circuit including a plurality of cascode circuits, a dedicated drive is required for the LV switching element of each cascode circuit. An added difficulty is that these active drives are also at a different electrical potential. This results in a not inconsiderable outlay on circuitry, which also leads to higher costs.
Furthermore, German Published, Non-Prosecuted Patent Application DE 199 26 109 A1 discloses a switching device for a higher reverse voltage based on a modified cascode circuit. In such a case, the cascode circuit is altered by an additional switching element in the form of an auxiliary transistor being inserted into the short-circuit connection originally provided between the LV cathode terminal and the first HV grid terminal. As a result, the LV switching element is intended to take up a higher reverse voltage before the HV switching element is also switched over to its blocking state so that an overall increased reverse voltage can be taken up by the modified cascode circuit. A development of such a modified cascode circuit lies' in connecting further HV switching elements in series with the first HV switching element of the modified cascode circuit. A further auxiliary transistor is in each case provided between the control terminals of the further switching elements. In addition, protection elements in the form of zener diodes may be provided for safeguarding purposes, in particular, for the purpose of limiting the potentials at the control terminals. Such a switching device is also associated with a not inconsiderable outlay on circuitry precisely on account of the auxiliary transistors characterized as essential to the invention disclosed in German Published, Non-Prosecuted Patent Application DE 199 26 109 A1.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a switching device for switching at a high operating voltage that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that has a high reverse voltage and, at the same time, can be realized with a low outlay on circuitry.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a switching device for switching at a high operating voltage, including an LV switching element having an LV cathode terminal, an LV anode terminal, and an LV grid terminal, a first HV switching element having a first HV cathode terminal, a first HV anode terminal, and a first HV grid terminal, the LV anode terminal being electrically short-circuited with the first HV cathode terminal, the LV cathode terminal being electrically short-circuited with the first HV grid terminal, a second HV switching element having a second HV cathode terminal, a second HV anode terminal, and a second HV grid terminal, the a second HV switching element being connected in series with the first HV switching element, and a first protection element being connected between the first and second HV grid terminals.
The switching device referred to in the introduction is characterized by at least in addition a second HV switching element having a second HV cathode terminal, a second HV anode terminal, and a second HV grid terminal, the second HV switching element being connected in series with the first HV switching element, and a first protection element being connected between the first and second HV grid terminals.
The invention is based on the insight that the reverse voltage strength of a cascode circuit inclu
Friedrichs Peter
Mitlehner Heinz
Benenson Boris
Greenberg Laurence A.
Locher Ralph E.
SiCED Electronics Development GmbH & Co. KG
Stemer Werner H.
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