JP4247771B2 - electric circuit - Google Patents

Description

[0001]
In the present invention, the high-power switch elements connected in series and the switch elements are cut off or conducted at substantially the same time to act as one switch as a whole. And an electric circuit having a rectifying element in which each switch element is connected in antiparallel.
[0002]
The electric circuit as described above is used in, for example, various converters, rectifiers and inverters for handling high-voltage current. For example, the above-described electric circuit of a conversion device that converts a DC voltage into an AC voltage can be given as an example, but the present invention is not limited to this.
[0003]
A feature common to all these applications is that the voltage to be held in the single switch in the cut-off state is too high for one switch element to hold, and the multiple elements are connected in series. The voltage must be shared among the elements. The voltage that must be held is, for example, 30 to 500 kV, and each switch element can hold a voltage of 1 to 10 kV alone. The problem is that it is impossible to produce switch elements that have exactly the same characteristics, i.e. always operate in the same way. This means that if such a switch constitutes a converter current valve, the on-switching delay time, off-switching delay time, tail current and inversion recovery characteristics of the reverse diode used in the phase leg of the power converter Since it differs depending on the element, it means that an overvoltage, that is, a voltage larger than the average voltage that the switch element should hold is easily applied to each switch element in the switch operation. Since overvoltages can damage switch elements and cause larger unit shutdowns, such as substations, resulting in very costly shutdowns, switch elements that differ from such overvoltages It is very important to protect.
[0004]
It is not a realistic solution to connect many switch elements in series and connect other elements so that the overvoltage is always within the allowable range. Conversely, the switch elements used in such a switch are not suitable. There is a need to reduce the number of switches as much as possible to save on the cost of controlling switch elements, cooling and other devices. Therefore, it has been necessary to design a means that does not harm the switch element by controlling the generated overvoltage to be within an allowable range with respect to the voltage and duration. This usually involves providing a separate control device for each switch element, which makes the control of the switch extremely complex, and the intelligence to control the voltage-time gradient to control the quotient of each switch. A gate drive circuit was used. This means that the cost of such previously known electrical circuits is quite high. Another problem that such electrical circuits always have is that the power loss of the various components is greater than desired.
[0005]
SUMMARY OF THE INVENTION
The object of the present invention is to provide an electrical circuit as described in the introduction section, which solves most of the problems mentioned above.
[0006]
In the case of the present invention, this object is achieved by using an avalanche diode as a rectifying element configured to start flowing a current when a predetermined reverse bias voltage is applied to such an electric circuit, and a switching element. The above-described voltage is prevented from being applied to both sides of the semiconductor device, and the avalanche diode is made of SiC. By using SiC as the diode, it is possible to provide the necessary characteristics for anti-parallel connection to the high-power switch element and use as protection against overvoltage. In other words, SiC uses characteristics superior to SiC, such as a breakdown voltage much higher than that of Si, and therefore SiC avalanche diodes are much thinner and can control power loss much smaller. To do. This type of SiC diode can withstand very high temperatures. This means that the avalanche diode can pass a much larger current compared to the Si diode while the potential difference between both sides of the target switch element reaches the predetermined limit voltage. Using an avalanche diode that is connected in antiparallel with the switch element and acts as a voltage clamp to prevent overvoltage in this way is a very simple solution to the above overvoltage problem. This means that the control of the switch element is no longer a very sensitive issue and the means for control can be simplified. This shock absorber can also be greatly simplified. This means, on the one hand, that the cost of all electrical circuits including the control device will be greatly reduced, the reliability of the operation of the circuit will be improved, and the risk of interruption of operation of the plant using the circuit will be reduced. become.
[0007]
According to a preferred embodiment of the present invention, the predetermined limit voltage of the avalanche diode is a state in which the voltage to be held by the switch is approximately uniformly distributed between the switch elements during normal operation of the circuit. Higher than the normal voltage of the switch element at, preferably at least 30% higher. Therefore, if the voltage applied to the switch is evenly distributed to the switch elements, no avalanche diode is conductive, and the diode is a voltage higher than the normal voltage by 30% or more for any switch element. As a result, it is unnecessary to detect various functional parameters of the element using a complicated circuit and control the switch element. There is a considerable difference between the predetermined limit voltage and the normal voltage so that the avalanche diode does not damage itself by passing too much current for too long, and therefore a particular switching element In this case, it is preferable that the avalanche diode is not energized unless an abnormal state appears to a certain extent.
[0008]
According to another preferred embodiment of the invention, the predetermined limit voltage of the avalanche diode is below a second limit voltage, preferably exceeding the second limit voltage, which is harmful to the switching element. Is at least 10% lower than the threshold voltage. This ensures that no voltage is applied to any switch element that would damage the element in whole or in part.
[0009]
According to another preferred embodiment of the invention, the switch element is an IGBT. The IGBT can very easily perform simultaneous opening and closing of the power supply. Therefore, the IGBT is very preferable in such an electric circuit, but there is a problem to be solved by the present invention even in such a switching element. In another preferred embodiment of the invention, the switch element is a MOSFET. However, it should be emphasized that the present invention is intended for all types of switching elements of power semiconductors that conduct and cut off current as described in the introduction.
[0010]
According to another preferred embodiment of the invention, the circuit is a converter circuit, the switch is its current valve, and the freewheel diode is anti-parallel connected to the valve switch element. This is one of the preferred applications of the present invention.
[0011]
According to another preferred embodiment of the present invention, the freewheel diode antiparallel connected with the corresponding switch element is an avalanche diode. If the characteristics of the avalanche diode current flow direction are close to those of the conventional rectifier diode and if the manufacturing cost is not too high, this can be a preferred solution and the control of the converter is greatly simplified. Only one element is provided for each switch element, that is, an avalanche diode is provided.
[0012]
According to another preferred embodiment of the invention, the avalanche diode and another freewheeling diode are connected in parallel to each other and anti-parallel to the corresponding switch elements. This means that even if the avalanche diode is not as excellent in current-carrying characteristics as a normal free wheel diode, the role of the branch provided for the corresponding switch element is sufficiently fulfilled.
[0013]
According to another preferred embodiment of the present invention, the switch elements are gate control semiconductor elements, and the control means controls the switch elements individually by providing each semiconductor element with a separate gate drive unit. These individual gate drive units can be controlled in a much simpler manner by providing SiC avalanche diodes.
[0014]
Further advantages and preferred features of the invention will be apparent from the following description and other dependent claims. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0015]
Detailed Description of the Preferred Embodiments of the Invention
FIG. 1 shows the concept of a VSC-converter (voltage power converter) provided between a DC voltage network 1 and a three-layer AC voltage network 2 for supplying high voltage direct current (HVDC). This is one application of the electrical circuit according to the invention. The general configuration of this type of converter is well known, but will be briefly described to help understand the problem solved by the present invention. Each phase leg 3-5 of the converter has two so-called current valves 6-11, which are the same as the above-described switches, and are composed of open / close type switch elements 12 connected in series. The element is preferably an IGBT type, and a rectifying element is anti-parallel connected thereto. That is, the element passes a current in one direction and does not pass a current in the opposite direction, and is in the form of a so-called free wheel diode 13. These units, which may be of the IGBT type, are connected in series to form a single valve, which conducts and shuts off at the same time, functions as a single switch, and the voltage on both sides of the valve is Distributed to different IGBTs connected in series. However, in FIG. 1, it is represented by a symbol of one unit for the sake of clarity. The switch element is controlled by a modulation pulse (PWM). This converter is constituted by a so-called 6-pulse bridge. For example, if attention is paid to the phase 14 and the switch element 12 belonging to the current valve 6 becomes conductive, current flows from the DC voltage network 1 through these switch elements to the AC voltage network. , The current first flows from the DC voltage network to phase 14, but then flows through the freewheeling diode of valve 7. This belongs to the prior art, and the features that characterize the present invention will be described with reference to FIGS.
[0016]
FIG. 2 is a diagram illustrating a state in which, for example, the valve 6 illustrated in FIG. 1 is configured by a number of switch elements 12 connected in series to each other and rectifying elements 13 anti-parallel connected to each element. Further, another gate drive unit 15 is connected to the gate of each switch element and individually controls it, that is, opens and closes. Preferably, they are all connected to a central controller (not shown) of the conversion station. The avalanche diode 13 is anti-parallel connected to the switch element in the same manner as the rectifying element described above, and it is shown that the diode is SiC. This means that these thicknesses are very small and nevertheless can withstand a sufficiently high reverse bias voltage. Furthermore, the reverse recovery charge is small, and therefore the power loss is much smaller than that of the Si diode. The corresponding avalanche diode 13 that is anti-parallel connected to the switch element is initially in a blocked state, but starts conducting when the potential difference on both sides reaches a predetermined limit voltage, so the potential difference on both sides of the switch element. Does not exceed this limit voltage. If the voltage to be borne by the corresponding switch element is 1.3 kV when the voltage to be borne by the valve is substantially uniformly distributed to the switch element during normal operation of the valve, this limit is assumed. The voltage is, for example, 1.8 kV. This means that an avalanche diode is selected so that the length of time during which the limiting voltage is applied under reverse bias is shorter than the time it takes for the diode to be partially or totally damaged at that voltage. If it is, it means that it may be rarely conducted. The length of this time is, for example, 2-3 μs. Further, the limit voltage is selected such that the switch element is never damaged by the voltage. In this example, the limit of the voltage allowed for the switch element is 2.2 kV.
[0017]
This type of overvoltage suppression eliminates the need for complex gate drive control and dV / dt limiting buffers that limit the dV / dt of each switch element. Therefore, the cost of the converter having the control device is greatly reduced without any loss of safety and operation reliability, and at the same time, the power loss due to the diode can be reduced as compared with the conventional free wheel diode.
[0018]
The current valve according to the second preferred embodiment of the present invention shown in FIG. 3 is different from the embodiment shown in FIG. 2 in that a normal rectifier diode 16 acting as a freewheel diode and an avalanche diode 17 made of SiC are mutually connected. The difference is that they are connected in parallel and anti-parallel connected to the switch element 6. Here, an avalanche diode is used as an overvoltage protection element of the switch element, and the diode 16 is used for improving the conduction characteristic, and as a result, the conduction characteristic of the avalanche diode is low and the cost is low. Can be manufactured.
[0019]
The present invention is of course not limited in any way to the preferred embodiments described above, and many modifications will be apparent to those skilled in the art without departing from the basic concept of the invention as defined in the appended claims. is there.
[Brief description of the drawings]
FIG. 1 is a diagram showing a converter having an electric circuit according to the present invention.
FIG. 2 is a simplified drawing showing an electrical circuit according to a first preferred embodiment of the present invention.
FIG. 3 is a drawing showing an electric circuit according to a second preferred embodiment of the present invention.

Claims (2)

A plurality of high-power switch elements (12) connected in series and all of them are turned on or off almost simultaneously and act as one switch as a whole, and in the cut-off state, a part of the high voltage is supplied to each switch element. And a control means (15) for controlling so as to be shared, each switch element having an rectifying element (13) connected in antiparallel,
The rectifying element is an avalanche diode (13) made of SiC which is turned on when a reverse bias exceeds a predetermined limit voltage so that a voltage applied to a corresponding switch element does not exceed a limit voltage.
The electrical circuit is a converter circuit, the switch is a current valve (6-11) of the converter, and a freewheel diode is anti-parallel connected to each switch element (12) of the current valve;
The predetermined limit voltage of the avalanche diode (13) is at least 30 than the voltage held by the corresponding switch element (12) when the circuit is operating normally and the plurality of switch elements share the voltage equally. % High voltage,
The predetermined threshold voltage of the avalanche diode (13) is a voltage that is at least 10% lower than a second limit voltage, which is harmful to the corresponding switch element (12);
The electric circuit characterized in that the free wheel diode anti-parallel connected to the corresponding switch element is composed of the avalanche diode (13) . The switch element (12) is a gate control semiconductor element, and the control means provided for each element has a distinguished gate drive unit (15) to individually control the switch elements. Item 2. The electric circuit according to Item 1.

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