EP0788682A1

EP0788682A1 - Electronic switch suitable for inductive loads

Info

Publication number: EP0788682A1
Application number: EP94931494A
Authority: EP
Inventors: Reinhard Maier; Hermann Zierhut; Heinz Mitlehner
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1994-10-28
Filing date: 1994-10-28
Publication date: 1997-08-13
Also published as: WO1996013900A1; AU8056894A; US6046516A

Abstract

An electronic switch (1) has a switching element (2) of a semiconductor switch type and a mode of operation characteristic of at least one FET, i.e. also provided with an inverse operation body diode (8) required by the state of the art. A control circuit (10) for carrying out working instructions is designed in such a way that in the inverse operation the gate (5) receives a voltage having such an intensity that the gate-source voltage is sized to leave the body diode without current. A functional unit (7) of the control circuit is designed to put the switching element (2) in a conductive switched-on state depending on a switching voltage or current, i.e. in the inverse operation.

Description

description

Electronic switch, suitable for inductive loads

The invention relates to an electronic switch which is also suitable for inductive loads and which enables high switching frequencies. The switch is constructed in the manner of a semiconductor switch with the mode of operation of at least one FET, that is to say technology-related, also with an inner body diode in inverse operation, the gate being controlled by a first control circuit for operational switching on and off.

In the case of FETs, in particular MOS-FETs, the technology-related body diode in inverse operation is frequently disruptive in practice. This body diode lying parallel to the source-drain path is polarized in the reverse direction during operation. In the case of inductive loads, when the current is switched off or switched over, a commutation voltage or a commutation current occurs which can flow via the body diode. In the known MOS-FETs, for example, the forward voltage of the body diode in inverse mode is 0.8 to 1.2 V. The blocking delay time depends on the type and is approximately 150 ns for 50 V semiconductors and increases with increasing blocking voltage up to approximately 2 μs.

Since semiconductor switches in the manner of MOS-FETs are very fast switches, one would like to use them even at high switching frequencies. In the case of inductive loads such as those embodying transformers, chokes and motors, high losses occur at high switching frequencies in the body diode in inverse operation as a result of storage charges.

The losses that frequently occur in practice at high switching frequencies have led to various remedies: By means of special heavy metal doping of the FET body diode it has been achieved in the so-called FREDFETs that the storage charges become very small without influencing the other parameters of the semiconductor switch (£ ast- £ ecovery fipitaxial-diode field effect transistor = FREDFET). The losses, as they are caused by the remaining storage charges, are still so high at high switching frequencies that they reduce the switching performance in a disruptive manner. In addition, the conduction losses of the body diode are increased with the reduction in the storage charge.

In another way, an external freewheeling diode is used, which bridges a series connection of the FET and a diode which is polarized in the forward direction in the working direction of the FET. As a result, the path via the semiconductor switch is blocked in inverse mode by the diode arranged in series, and a current path is made available by the bridging diode. This thus ensures that no current can flow in the reverse operation via the semiconductor switch and thus via its body diode. Very fast diodes, for example Schottky diodes, are used for such diodes. Losses due to storage charge are greatly reduced as a result, but the additional outlay due to additional demanding diodes and in particular the switching losses in these diodes are disadvantageous.

The object of the invention is to develop an electronic switch of the type described at the outset, which is suitable for high switching frequencies even under inductive load, without having the disadvantages which had previously been accepted.

The object described is achieved by an electronic switch according to claim 1. A control circuit for fulfilling the work commands is set up for the gate to receive such a voltage in inverse operation that the gate-source voltage is set so high is that the body diode is still de-energized, whereby in Depending on a commutation voltage or a commutation current occurring when switching off under an inductive load, the semiconductor switch is controlled to be in the "ON" state.

By means of suitable semiconductor material and a control circuit which, in inverse operation by a functional unit, ensures that, contrary to previous practice, the working path of the semiconductor switch is again controlled in a conductive manner when a commutation voltage occurs, losses are largely avoided . According to the current state of technology, the semiconductor switch can advantageously be made of silicon carbide, SiC. In this way it can be ensured in a simple manner that the voltage drop across the ON resistor, RQJJ, is smaller than the threshold voltage or forward voltage of the body diode in inverse operation. The control circuit for fulfilling the work commands, for example switching on, switching off or assuming an intermediate state, is then only to be supplemented by a circuit which, depending on the voltage at the drain and source of the semiconductor switch, applies a voltage in inverse mode applies the gate, which conducts the work path, as long as the voltage of the inverse state is present. For example, voltages of one polarity can be switched with a FET and alternating voltages or the associated currents can be switched with two FETs connected in series.

Such an electronic switch avoids losses due to the otherwise usual charge storage behavior of the body diode, high switching frequencies with extremely low losses being made possible even with inductive loads.

According to a further development, the one that works is dependent on the commutation voltage or on the commutation current

Control function essentially realized by a comparator, to whose positive input the .connection of the drain and to whose negative input the connection of the source is connected. is closed, the output of the comparator being routed to the gate in terms of function. In particular, the comparator according to claim 3 can be connected with its output via a directional valve to the gate of the semiconductor switch. The directional valve, for example a diode, ensures that the semiconductor switch is controlled via the gate into the current-carrying state as long as the drain-source voltage is inversely polarized. The control function works essentially as a function of the commutation voltage.

A control function operating as a function of the commutation current can essentially be implemented according to claim 4 by a current-voltage converter which is connected in series to the semiconductor switch with its current input and is functionally connected to the gate with its voltage output. In particular, the current-voltage converter can be connected directly to the gate of the semiconductor switch with its voltage output via a directional valve.

The invention will now be explained in greater detail on the basis of exemplary embodiments shown roughly schematically in the drawing:

An electronic switch is illustrated in FIG.

2 shows a working diagram of the electronic switch.

FIG. 3 shows an exemplary embodiment of the electronic switch, the control function operating as a function of the commutation voltage being implemented by a comparator, the output of which is connected directly to the gate of the semiconductor switch via a directional valve. A further exemplary embodiment is shown in FIG. 4, the control function operating as a function of the commutation current being implemented by a current-voltage converter. A further exemplary embodiment is shown in FIG. 5, the current-voltage converter having its voltage output being connected directly to the gate of the semiconductor switch via a directional valve. 6 shows an antiserial arrangement of electronic switches with a centrally connected inductive load in the manner of a half-bridge, as it belongs to the prior art with conventional semiconductor switches and can likewise be implemented with electronic switches according to the invention.

FIG. 7 shows an antiserial arrangement of semiconductor switches according to the prior art with known measures for reducing the losses caused by storage charges.

The electronic switch 1 according to FIG. 1 works with a switching element 2 in the manner of a semiconductor switch with the mode of operation of an FET. The electron donor is to be referred to as source 3, the electron collector as drain 4 and the electrode controlling the electron flow as gate 5. Functioning of an FET is to be understood here to mean that the switching element has characteristics as are common to FETs. Technology-related, the switching element also works with an inner body diode 8 in inverse mode. A control circuit 10 works on the gate 5 with a functional unit 6 for fulfilling the work commands and a functional unit 7 in order to control the switching element 5 in the "ON" state depending on a commutation voltage or a commutation current as long as the commutation situation is present.

The functional unit 7 of the control circuit 10, which operates as a function of the commutation voltage or the commutation current, can essentially be implemented by a comparator 12, to the positive input of which the connection of the drain and to whose negative input the connection of the source is connected, the Functionally, the output of the comparator 12 is routed to the gate 5 of the switching element 2. 2 shows a characteristic field which, with the family of curves 13 in the first quadrant, the behavior of the switching element 2 for a drain-source current I ^ g ^and a drain-source voltage U ^ g as a function of the gate-source Voltage US illustrated as a parameter. The switch-off state is represented by the abscissa. With increasing gate-source voltage of the Ugg Kennli¬ reject nien 13 starting at the origin increasingly to a straight line 9, RQU ^to - ^In other approach is achieved a steep curve 9 at high gate-source voltage corresponding to general mine: ON resistance is understood because it shows a linear behavior between current and voltage. A steep straight line 9 corresponds to a low ON resistance. When two switching elements 2 are connected in series, the characteristic curves 13 and RQN run in the first and third quadrants. The body diode 8 in inverse operation results in a superimposition of the characteristic field with a characteristic 11 in the third quadrant to the characteristic field 13 in the first quadrant and a characteristic 11 in the first quadrant to the characteristic field 13 in the third quadrant.

In the case of a single switching element 2, the characteristic curves 13 in the first quadrant are supplemented by the characteristic curve 11 of the body diode in the third quadrant. This characteristic curve 11 illustrates that the body diode becomes effective in the blocking direction parallel to the working path of the formwork element. This effectiveness can be avoided by setting the gate-source voltage on the switching element in inverse mode so that the body diode remains without current. In practice, this is promoted by a switching element made of silicon carbide in the structure of an FET. Other materials that belong to the group of insulators, such as diamond, can also be used. The intrinsic conductivity of SiC in the 6H modification is 1.6 x 10 ^-6 charge carriers per cm ³ , i.e. a factor of 10 ¹⁵ less than that of silicon. With such materials, high reverse voltages can be realized, so that, for example, for 5 kV reverse voltage that can be enabled, silicon carbide has an active element. ment thick manages about 50 μ, which has a very low RθN ^result. Accordingly, the losses in the conductive state are negligible.

3, the functional unit 6 of the control circuit for fulfilling the work commands is supplemented by a functional unit 7 for actuating the switching element as a function of the commutation voltage. This functional unit consists of a comparator 12, the positive input of which is connected to drain 4 and the negative input of which is connected to source 3, and the output of which is connected to gate 5 of switching element 2 via a directional valve. If, for example, there is a positive potential at drain 4 and a negative potential at source 3, as corresponds to a normal operating situation, the comparator 12 does not output any voltage at its output. Functional unit 6 of the control circuit can then be used to set a specific conductivity of switching element 2 by means of a corresponding gate-source voltage at gate 5. If the conductivity of the switching element 2 is brought towards zero, a commutation voltage can occur in certain network states or consumers, for example inductive loads, so that negative potential is now present at drain 4 and positive potential at source 3. Under such a condition, the comparator 12 supplies a positive voltage at its output, which reaches the gate 5 via the directional valve 14, so that the switching element 2 is brought back into a conductive state as long as this inverse state is present.

In the exemplary embodiment according to FIG. 4, the functional unit of the control circuit which operates as a function of the commutation current is essentially implemented in the exemplary embodiment according to FIG. 4 by current-voltage converter 15, which is connected in series with the switching element 2 with its current input and functionally at the gate 5 with its voltage output connected. In the exemplary embodiment according to FIG. 5, a current-voltage converter 15 has its voltage output via a Directed valve 14 connected directly to the gate 5 of the switching element 2.

The known arrangement according to FIG. 6 can be advantageous with electronic switches according to the invention when AC voltages are to be switched and an inductive load 16 is connected.

The arrangement according to FIG. 7 illustrates a measure according to the prior art with a diode in the closing direction to an FET and a diode bridging the series diode and the FET. Otherwise, the arrangement according to FIG. 7 corresponds to that according to FIG. 6, in the event that it is equipped with switching elements according to the prior art.

Claims

claims

1. Electronic switch (1) with a switching element (2) in the manner of a semiconductor switch with the mode of operation of at least one FET, that is to say technology-related, also with an internal body diode (8) in inverse operation, a control circuit (10) for fulfilling the work commands is set up so that the gate (5) receives such a voltage in inverse mode that the gate-source voltage is set so high that the body diode is still de-energized, whereby a functional unit (7) depending on a commutation voltage or a commutation current, controls the switching element (2) to the ON state.

2. Electronic switch according to claim 1, characterized in that the functional unit (7) of the control circuit (10), which operates as a function of the commutation voltage or the commutation current, is essentially realized by a comparator (12), to the positive input of which the connection is made the drain (4) and the negative input of which the connection of the source (3) is connected, the output of the comparator (12) being routed functionally to the gate (5) of the switching element (2).

3. Electronic switch according to claim 2, so that the comparator (12) is connected with its output via a directional valve (14) to the gate (5) of the switching element (2) with its output.

4. Electronic switch according to claim 1, characterized in that the functional unit (7) which operates as a function of the commutation current is essentially realized by a current-voltage converter (15) which is connected in series with its current input to the switching element (2) and With its voltage output is functionally connected to the gate (5) ^' .

5. Electronic switch according to claim 4, so that the current-voltage converter (15) is connected with its voltage output via a directional valve (14) to the gate (5) of the switching element (2).

6. Electronic switch according to one of claims 1 to 5, d a d u r c h g e k e n n z e i c h n e t that the switching element (2) made of silicon carbide, SiC, is made in the structure of a FET.