ELECTRIC CIRCUIT

Abstract: An electric circuit comprises a plurality of high power switching devices (12) connected in series and means (15) adapted to control them to be all conducting or blocking substantially simultaneously for together acting as one single switch and each holding a part of a high voltage to be held by said switch in the blocking state of the switching devices. Each switching device has an avalanche diode of SiC connected in anti-parallel therewith and adapted to start to conduct current when reverse biased as of a determined voltage limit for preventing that the voltage across the respective switching device exceeds said limit.

Description: ELECTRIC CIRCUIT TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to an electric circuit comprising a plurality of high power switching devices connected in series and means adapted to control them to be all conducting or blocking substantially simultaneously for together acting as one single switch and each holding a part of a high voltage to be held by said switch in the blocking state of the switching devices, each switching device having a rectifying element connected in anti- parallel therewith. Such electric circuits are for example used in different types of converters, rectifiers and inverters, in which high voltages are to be handled. The use of such an electric circuit in a converter in a station for converting direct voltage into alternating voltage may be mentioned as a non-limitative example. The property in common to all these applications is that the volt- ages to be held in the blocking state of said single switch are too high to be held by one single switching device, so that a plurality of such devices have to be connected in series for distributing the voltage to be held by the switch thereamong. The voltage to be held may for instance be 30-500 kV, and each switching de- vice may for example alone only hold 1 -10 kV. The problem is that it is not possible to manufacture said switching devices so that they have exactly the same properties, i.e. reacts exactly the same in every single instant. This means that an over-voltage, i.e. a voltage being higher than the average voltage to be held by the switching devices, may easily occur for an individual switching device during switching due to differences in turn-on delay times, turn-off delay times, tail currents and reverse recovery charges of opposite diodes in a phase leg of a voltage source converter in the case that said single switch forms a current valve of such a converter. It is essential to protect the different switching devices against such over-voltages, since they may destroy the device and possibly result in a drop out of a larger unit, such as a converter station with an interruption of the operation thereof being very costly as a consequence. It is not a realistic solution to this problem to arrange that many switching devices in series in such a switch without additional surrounding components that said over-voltages will always be within an acceptable limit, but it is in opposite thereto a desire to keep the number of switching devices in such a single switch as low as possible for saving costs for the equipment for controlling the switching devices, cooling them and so on. It has therefore been necessary to design the means for controlling the switching devices so that over-voltages occurring will be kept within an acceptable area with respect to magnitude and duration, so that they will not be harmful to the switching devices. This normally means that each individual switching device has to be provided with a separate control member and the control of said single switch has been made very complex and intelligent gate drive circuitry limiting the voltage-time gradient have been utilized in order to control said quotient for each individual switching de- vice. This means that the cost of those electric circuits already known is comperatively high. Another problem always existing in such electric circuits is that the power losses in different components are higher than desired. SUMMARY OF THE INVENTION The object of the present invention is to provide an electric circuit of the type defined in the introduction, which solves the problems discussed above to a large extent. This object is according to the invention obtained by providing such an electric circuit with an avalanche diode as said rectifying element adapted to start to conduct current when reverse biased as of a determined voltage limit for preventing that the voltage across the respective switching device exceeds said limit, and said avalanche diode is made of SiC. By producing such a diode of SiC this may be given the properties required for being connected in anti-parallel with a high power switching device as an over-voltage protection. Accordingly, some superior properties of SiC with respect to for example Si are then used, such that SiC has a much higher breakdown voltage than Si and such an ava- lanche diode in SiC may therefore be made much thinner, which means considerably lower power losses. Such a diode of SiC may also withstand very high temperatures. This means that such an avalanche diode may take a much higher current during a certain period of time than for instance Si when the voltage across the switching device in question reaches said determined voltage limit. This way of obtaining an over-voltage suppression utilizing an avalanche diode connected in anti-parallel with the switching device and acting as a voltage clamp constitutes a very simple solution to the problems mentioned above of over- voltages. This means that the control of the switching devices is not that delicate any longer and the means for this control may be simplified. Said snubbers may also be very much simplified. This means on one hand that the costs of the entire electric circuit with control equipment will be considerably reduced, and the reliability of the operation of the circuit will also be increased and the risks of operation interruptions of a plant or the like including such a circuit will be reduced. According to a preferred embodiment of the invention said ava- lanche diode has said determined voltage limit at a level being higher than a normal voltage to be held by the respective switching device at normal operation of the circuit and when the voltage to be held by said single switch is substantially equally distributed among the switching devices thereof, and said deter- mined voltage is preferably at least 30% higher than said normal voltage. Accordingly, when the voltage over said single switch is equally distributed among the switching devices no avalanche diode will be conducting, but such diodes are there for all the switching devices for preventing that any of them at any moment receives a current exceeding said normal voltage by more than 30% at any time, so that it is not so critical to control the different switching devices through complicated circuits for detecting different function parameters of the devices. It is advantageous that there is a considerable difference between said determined voltage limit and said normal voltage, so that the avalanche diode will not be conducting except when a condition being unnormal to a certain extent prevails at a specific switching device, since the avalanche diode may otherwise be forced to conduct at too high current during a too long time and by that itself be destroyed. According to another preferred embodiment of the invention said avalanche diode has said determined voltage limit at a level below a second voltage limit, the exceeding of which may be harmful to the respective switching device, and the level of said determined voltage limit is preferably at least 10% below said second voltage limit. This ensures that there will never be a volt- age across any switching device susceptible to completely or partially destroy the device. According to another preferred embodiment of the invention the switching devices are IGBT's. IGBT's are very preferred as switching devices in such an electric circuit, since they may very easily be turned on and off simultaneously, but the problems to be solved by the present invention are also there for such switching devices. According to another preferred embodiment of the invention the switching devices are MOSFET's. However, it is emphasized that the present invention covers electric circuits of the type defined in the introduction in which the switching devices may be of any type of power semiconductor component of turn-on and/or turn-off type. According to another preferred embodiment of the invention said circuit is a converter circuit and said single switch is forming a current valve thereof, and a free-wheeling diode is connected in anti-parallel with each switching device of the valve. This is a preferred application of the present invention. According to another preferred embodiment of the invention said free-wheeling diode in anti-parallel with the respective switching device is formed by said avalanche diode. If the forward conducting characteristics of such an avalanche diode are close to that of a conventional rectifying diode and the costs for manufacturing thereof would not be too high, this will constitute a preferred solution, in which the control of the converter is simplified to a large extent by only replacing one component in each switching device, i.e. the diode by another component, i.e. the avalanche diode. According to another preferred embodiment of the invention an avalanche diode and a separate free-wheeling diode are connected in parallel with each other and in anti-parallel with the respective switching device. This means that the avalanche diode has not to have as good forward conducting characteristics as a normal free-wheeling diode, but the task of this branch parallel to the respective switching device will be fulfilled to satisfaction. According to another preferred embodiment of the invention said switching devices are gate controlled semiconductor elements, and said controlling means include a separate gate drive unit for each said element for individually controlling the switching devices. These individual gate drive units may be controlled in a much simpler way thanks to the arrangement of the avalanche diodes of SiC. Further advantages and advantageous features of the invention will appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below follows a specific description of preferred embodiments of the invention cited as examples. In the drawing: Fig 1 is a diagram illustrating a converter, in which an elec- trie circuit according to the invention may be arranged, Fig 2 is a simplified diagram illustrating an electric circuit according to a first preferred embodiment of the invention, and Fig 3 is a view corresponding to fig 2 of an electric circuit according to a second preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig 1 illustrates schematically the structure of a VSC-converter (Voltage Source Converter) connected between a direct voltage network 1 for High Voltage Direct Current (HVDC) and a three phase alternating voltage network 2. This constitutes one possible application of an electric circuit according to the present invention. The general structure of such a converter is well known, but it will be briefly explained here for better understanding the problems solved by the present invention. Each phase leg 3-5 of the converter has two so-called current valves 6-1 1 , which are identical to said single switch discussed above and consist of units of switching devices 12 of turn-on and turn-off type connected in series, preferably in the form of IGBT's, and rectifying elements connected in anti-parallel therewith, i.e. elements being conducting in one direction and blocking in the opposite direction, in the form of so-called free-wheeling diodes 13. A great number of such units and by that IGBT's may be connected in series in one single valve so as to be turned on and turned off simultaneously so as to function as one single switch, whereby the voltage across the valve is distributed among the different IGBT's connected in series. However, these units are in fig 1 for the sake of clearness summarized through the symbol of one single unit. The control of the switching devices takes place through pulse with modulation (PWM). This converter is formed by a so-called 6-pulse bridge. If we consider for example the phase 14 and assume that the switching devices 12 belonging to the current valve 6 are turned on and a current flows from the direct voltage network 1 through these switching devices and to the alternating voltage network, a turning-off of the switching devices of this valve will cause the current to firstly continue to flow to the phase 14 from the direct voltage network, but now through the free-wheeling diodes of the valve 7. This is conventional technique, and the new features characterizing the present invention will now be described while referring to fig 2 and 3. It is illustrated in fig 2 how a valve, for example the valve 6 of fig 1 , in practice is formed by a large number of switching devices 12 connected in series and a rectifying element 13 connected in anti-parallel with each of these devices. Furthermore, a separate gate drive unit 15 is connected to the gate of each switching de- vice for individually controlling the operation thereof, i.e. the turn-on and turn-off procedure. These are all preferably connected to a main control apparatus not shown of the converter station. It is shown that avalanche diodes 13 are connected in anti-parallel with the switching devices as said rectifying ele- ments, and these diodes are made of SiC. This means that they may be made very thin and still be able to hold a sufficiently high voltage when reverse biased. Furthermore, the reverse recovery charge thereof will be small and by that the power losses will also be much smaller than for diodes of Si. The respective ava- lanche diode 13 will in the blocking state of the switching device connected in anti-parallel therewith start to conduct a current therethrough when the voltage thereacross reaches a determined voltage limit, so that the voltage over the switching device will never exceed this limit. This limit may for instance be 1 ,8 kV if 1 ,3 kV is the voltage to be held by the respective switching device at normal operation of the valve and when the voltage to be held by the valve is substantially equally distributed among the switching devices thereof. This means that the avalanche diode will seldom start to conduct when reverse biased and this voltage limit is selected so that the length of the time of conduction thereof will be shorter than the rated time limit allowed for the conduction of a current resulting from this voltage without any risk of partially or entirely destroying the diode. This time length may for example be 2-3μs. Furthermore, said voltage limit is so selected that the switching device may never be harmed thereby, and in the present example the limit for the voltage acceptable to the switching device may be 2,2 kV. An over-voltage suppression of this type makes it possible to do without any complex gate drive control and dV/dt limiting snub- bers for controlling the dV/dt of each individual switching device. Accordingly, the costs of the converter with control equipment may be considerably lower without any loss of safety and operation reliability thereof, at the same time as power losses in the diodes may be reduced with respect to the use of conventional free-wheeling diodes. A current valve according to a second preferred embodiment of the invention is shown in fig 3, and this differs from that of fig 2 by the fact that a normal rectifying diode 16 acting as a freewheeling diode and an avalanche diode of SiC 17 are connected in parallel with each other and in anti-parallel with each switching device 6. The avalanche diode is here used for the over-voltage protection of the switching device, whereas the diode 16 is used for the better on-state characteristics thereof, so that the avalanche diode may be manufactured without much concern about the on-state characteristics thereof and by that be produced to a lower cost. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.

Claims: Claims 1 . An electric circuit comprising a plurality of high power switching devices (12) connected in series and means (15) adapted to control them to be all conducting or blocking substantially simultaneously for together acting as one single switch and each holding a part of a high voltage to be held by said switch in the blocking state of the switching devices, each switching device having a rectifying element (13, 16, 17) connected in anti-parallel therewith, characterized in that said rectifying element is an avalanche diode (13, 17) adapted to start to conduct current when reverse biased as of a determined voltage limit for preventing that the voltage across the respective switching device exceeds said limit, and that said avalanche diode is made of SiC. 2. A circuit according to claim 1 , characterized in that said avalanche diode (13, 17) has said determined voltage limit at a level being higher than a normal voltage to be held by the respective switching device (12) at normal operation of the circuit and when the voltage to be held by said single switch is substantially equally distributed among the switching devices thereof. 3. A circuit according to claim 2, characterized in that said determined voltage is at least 30% higher than said normal voltage. 4. A circuit according to any of claims 1 -3, characterized in that said avalanche diode (13, 17) has said determined voltage limit at a level below a second voltage limit, the exceeding of which may be harmful to the respective switching device (12). 5. A circuit according to claim 4, characterized in that the level of said determined voltage limit is at least 10% below said second voltage limit. 6. A circuit according to any of claims 1-5, characterized in that the switching devices (12) are IGBT's. 7. A circuit according to any of claims 1-5, characterized in that the switching devices (12) are MOSFET's. 8. A circuit according to any of claims 1 -7, characterized in that it is a converter circuit and said single switch is forming a current valve (6-1 1 ) thereof, and that a free-wheeling diode (13, 16) is connected in anti-parallel with each switching device (12) of the valve. 9. A circuit according to claim 8, characterized in that said freewheeling diode (13) in anti-parallel with the respective switching device is formed by said avalanche diode (13). 10. A circuit according to claim 8, characterized in that an avalanche diode (17) and a separate free-wheeling diode (16) are connected in parallel with each other and in anti-parallel with the respective switching device (12). 1 1. A circuit according to any of claims 1-10, characterized in that said switching devices (12) are gate controlled semiconductor elements, and that said controlling means include a separate gate drive unit (15) for each said element for individually controlling the switching devices.

Inventors: AABERG ANDERS, ASPLUND GUNNAR, DANIELSSON BO, HERMANSSON WILLY, LUNDBERG PETER, OVREN CHRISTER, AABERG, ANDERS, ASPLUND, GUNNAR, DANIELSSON, BO, HERMANSSON, WILLY, LUNDBERG, PETER, OVREN, CHRISTER
Applicants: AABERG ANDERS, ABB RESEARCH LTD, ASPLUND GUNNAR, DANIELSSON BO, HERMANSSON WILLY, LUNDBERG PETER, OVREN CHRISTER, AABERG, ANDERS, ABB RESEARCH LTD., ASPLUND, GUNNAR, DANIELSSON, BO, HERMANSSON, WILLY, LUNDBERG, PETER, OVREN, CHRISTER
Application year: 1999
Main classification: H02H7
IPC codes: H02H7, H02M1, H03K17