Device for limiting the voltage of a consumer

Abstract

Device (100) for limiting the voltage of a consumer (200), comprising: a first connection element (102) for receiving an electrical potential (10); a second connection element (105) for delivering an electrical potential to the consumer (200); a first field effect transistor, FET, (Q4), which is designed as a normally-on FET; a second FET (Q3) which is designed as a normally-on FET; wherein the first FET (Q4) and the second FET (Q3) are connected in series between the first terminal element (102) and the second terminal element (105); wherein a drain (114) of the first FET (Q4) is connected to the first terminal element (102); wherein a source (116) of the first FET (Q4) is connected to a drain of the second FET (Q3); wherein a source of the second FET (Q3) is connected to the second terminal element (105); wherein the device (100) comprises a third FET (Q2) connected in series with the first FET (Q4) and the second FET (Q3) such that a drain of the third FET (Q2) is connected to a source of the second FET (Q3) and a source of the third FET (Q2) is connected to the second terminal element (105); further comprising a first semiconductor element (D23) with a defined breakdown voltage, which is arranged between a control electrode (112) of the first FET (Q4) and the first connection element (102); further comprising a second semiconductor element (D22) with a defined breakdown voltage, which is connected in series with the first semiconductor element (D23) between the control electrode (112) of the first FET (Q4) and the first connection element (102).

Description

Field of the InventionThe invention relates to a device for limiting the voltage of a load and to a test system for testing a voltage source having a device for limiting the voltage.BACKGROUND OF THE INVENTIONDevices for limiting the voltage are used to cause a potential difference at the device or a voltage drop along the device.A device with such a function may be necessary, for example, if a voltage source is used which provides a voltage that is too high for a load. In such a case, the device is connected between the voltage source and the load, so that a voltage divider results and the voltage present at the load is reduced.In the case of a voltage source with variable output voltage, it may be necessary to also vary the voltage drop at the device for limiting the voltage, so that a voltage at the load remains constant.If the voltage drop across the voltage limiting device is varied, voltage variations may occur for a transition time because the device must stabilize. In this context, one speaks of the necessary stabilization times of the device.DE 10 2013 107 699 A1 describes a voltage limiter with a normally on main transistor which is connected to an input for a voltage signal to be limited and an output for outputting a voltage signal. The drain terminal is connected to the input, the source terminal to the output and the gate terminal to a common reference potential.WO 93 / 01 639 A1 describes an arrangement intended to be connected in series in a line of an electrical circuit in order to protect the circuit from an overcurrent. The arrangement includes a depletion FET that switches the line current and a controller that is connected across a resistor in the line, such as the channel resistor of the FET, and biases the gate of the FET across the resistor in response to the voltage difference to turn off the FET when the arrangement is subjected to an overcurrent in the line. The arrangement has the advantage that since the FET is normally on there is no initial voltage drop across the arrangement before it becomes conductive, so that the arrangement may be substantially linear.US 2014 / 0 062 544 A1 describes a semiconductor device arrangement having a first semiconductor device which has a load path, and a multiplicity of second transistors which each have a load path between a first and a second load terminal and a control terminal. The load paths of the second transistors are connected in series and connected in series to the load path of the first transistor, the control terminal of each of the second transistors is connected to the load terminal of one of the other second transistors, and the control terminal of one of the second transistors is connected to one of the load terminals of the first semiconductor device.DE 601 22 626 T2 describes a semiconductor over-current limiter with input and output terminals, which includes a vertical depletion-type MOSFET, a lateral depletion-type MOSFET and a zener diode. A back gate of the lateral MOSFET is formed commonly with a drain electrode of the vertical MOSFET to provide the input terminal, and a gate of the vertical MOSFET is connected to an anode of the zener diode to provide the output terminal. Further, a source electrode of the vertical MOSFET is connected to source and gate electrodes of the lateral MOSFET and a cathode electrode of the Zener diode.DE 41 28 679 C1 describes a longitudinal regulator intended for current limiting. In the case of the longitudinal regulator intended for current limiting, an actuator is bridged with a low impedance in non-limiting operation. The power loss can be reduced in this way without delays occurring during the recovery of the current limitation.SUMMARY OF THE INVENTIONIt is an object of the invention to provide a device for limiting the voltage of a load, which device is distinguished by short stabilization times and a high voltage resistance.This object is achieved by the subject matter of the independent claims. Further developments of the invention are evident from the dependent claims and from the following description.The invention is defined in the claims.According to the invention, a device for limiting the voltage of a load, in particular of an electrical load, is specified. The device has a first connection element for receiving an electrical potential and a second connection element for delivering an electrical potential to the load. Furthermore, the device has a first field effect transistor, FET, and a second FET, which are each designed as a normally-on FET. The first FET and the second FET are connected in series between the first terminal and the second terminal.The device is designed to be connected to a voltage source by the first connection element and to the load by the second connection element. Depending on the configuration of the device, a potential difference or a voltage drop occurs between the first connection element and the second connection element, so that the voltage present at the load is thereby reduced or limited.The fact that the two field effect transistors connected in series are of self-conducting design has the advantage that they do not require any active activation which is therefore complicated in terms of circuitry. Rather, the field effect transistors are conductive for structural reasons without applied control voltage.By connecting two field effect transistors in series, the voltage stability of the device increases or higher potential differences can be produced between the first connection element and second connection element than would be the case for a single field effect transistor. The withstand voltage may be, for example, up to 10 kV or more.The voltage range in which the device can be used, i.e. the voltage which can be dissipated via the device, can be between 0 volts and several kilovolts.According to the invention, a drain of the first FET is connected to the first terminal element. A source of the first FET is connected to a drain of the second FET. A source of the second FET is connected to the second terminal element.The outflow of the first FET can be connected indirectly or directly to the first connection element. The source of the first FET is in particular directly connected to the outflow of the second FET. Alternatively, other components (such as diodes) can also be connected in between here. The source of the second FET can be connected indirectly or directly to the second connection element. These connections are electrical connections, for example by means of an electrically conductive element.In the case of an indirect connection, apart from an electrical connection element (electrical conductor, conductor track, etc.), one or more active or passive electrical or electronic components can be arranged between the indirectly connected elements. A direct connection is a connection between two elements, which apart from an electrical connection element has no further components between the directly connected elements.An electrical connection within the meaning of this application is a connection which enables a current flow between the connected elements.The structure of a field effect transistor is known per se. In connection with this description, all types of field effect transistors may be used as long as they are present in a normally-on embodiment. A field effect transistor, FET, usually has at least three connection elements: the control electrode (also referred to as gate), the drain (also referred to as drain or drain) and the source (also referred to as inflow or source).According to another embodiment of the invention, the first FET is a metal oxide semiconductor field effect transistor, MOSFET.MOSFETs offer a high voltage withstand capability of several hundred volts up to over 1000 V and are thus advantageously suitable for a device as described herein.According to the invention, the device comprises a third FET which is connected indirectly or directly in series with the first FET and the second FET (i.e. no other components are arranged in the series circuit between the FETs or that further components are arranged in the series circuit between the FETs), so that a drain of the third FET is connected to a source of the second FET and a source of the third FET is connected to the second connection element.The second FET is thus indirectly connected to the second connection element via the third FET. By providing the third FET, the voltage stability of the arrangement is further increased or a higher potential difference can be present between the first connection element and the second connection element without damaging or destroying the components therebetween, i.e. the first, second and third FETs.The device may also have more than three FETs connected in series, in order to increase the voltage withstand capability of the device or the maximum voltage drop across the device. Each of the FETs can be designed as a MOSFET in a normally-on configuration.According to the invention, the device as described herein further comprises a first semiconductor element having a defined breakdown voltage, wherein this semiconductor element is arranged between a control electrode of the first FET and the first connection element.Alternatively, instead of the first semiconductor element, the device may have another component at which a defined voltage drops or which causes a voltage drop.According to a further embodiment of the invention, the semiconductor element with a defined breakdown voltage is a Zener diode.The zener diode thus supplies a required voltage to the control electrode of the first FET in order to switch it from the conductive state to the blocking state. Depending on the voltage required, one or more Zener diodes can be connected in series.According to the invention, the device further comprises a second semiconductor element having a defined breakdown voltage, which is connected in series together with the first semiconductor element between the control electrode of the first FET and the first connection element.In one embodiment, the Zener diodes (the semiconductor elements with a defined breakdown voltage) associated with each individual FET are dimensioned such that the breakdown voltage or the sum of the breakdown voltages of a plurality of series-connected Zener diodes is the same for all FETs. In one embodiment, the number of Zener diodes associated with each FET may be the same for each FET and their respective breakdown voltage is identical, e.g., 200 V. The plurality of Zener diodes connected in series and assigned to one FET may be referred to as a voltage stage. The device can have a plurality of voltage stages, which can be different in terms of their dimensioning and dimensioning. The sum of the breakdown voltages of a voltage stage must not be greater than the maximum permissible voltage at the FET.According to a further embodiment of the invention, the device further comprises a fourth FET which is connected in parallel with the first FET.The fourth FET may also be a MOSFET having the same specification as the other MOSFETs. This fourth FET connected in parallel with the first FET increases the current resistance of the voltage limiting device.According to a further aspect of the invention, a test system is specified. The test system has a voltage source, a device for limiting the voltage for a load as described above, and an electrical load. The device is designed to limit a voltage applied to the electrical load to a predefinable value.The test system can be designed in particular to test the voltage source, wherein the load together with the device for limiting the voltage serves to simulate a load connected to the voltage source and its behavior.The device for limiting the voltage for a load allows, by virtue of its construction with normally-on FETs, short stabilization times and a high voltage stability, with the result that a dynamic behavior of the load with fast operating point changes can be simulated advantageously and, for example, the number of test cases per unit time can be increased, which can reduce the total duration for carrying out a plurality of test cases one after the other.According to a further embodiment of the invention, the electrical load is a current load which is designed to simulate the behavior of a traveling wave tube.Thus, with the test system in this embodiment, voltage supplies of a traveling wave tube can be tested.In the following, with exemplary reference to this embodiment, the function of the test system and the device for limiting the voltage will be described, wherein the use of MOSFETs is mentioned as FET, for example. It should be noted, however, that the device is not limited to this purpose.For starting up a voltage supply and the subsequent tests, a traveling wave tube is simulated by means of an electronic load and test cases are carried out. In the event that an electronic load varies in behavior and requires long stabilization times, the duration for conducting the test cases is increased, which may be undesirable. This behavior of the electronic load can be attributed to a disadvantageous dynamic behavior and a high duration for setting an operating point.It has been recognized that a high outlay for the actuation of individual MOSFETs is to be operated if, for example, a plurality of MOSFETs or power MOSFETs connected in series in a self-blocking embodiment are used as load. The essential challenge may be that a control voltage must be available for driving the individual normally-off MOSFETs connected in series at the respective voltage level. This voltage must be generated either from the high voltage applied to the load module itself or by a potential-free voltage source. Both variants are technically complicated. Furthermore, it has been recognized that electronic loads from load stages with normally-off power MOSFETs have a load current that is not independent of voltage and an unsatisfactory dynamic behavior. If only a single MOSFET is used, the voltage withstand capability of a corresponding device is limited to the voltage withstand capability of this single MOSFET, typically to several hundred volts up to about 1000 volts, depending on the design of the MOSFET.The device as described herein takes these findings into account and enables a high voltage strength with simultaneously short stabilization times.This is achieved by making the series-connected MOSFETs self-conducting in order to achieve the required voltage withstand capability. The advantage is that the normally-on MOSFETs do not require any active and therefore complicated control in terms of circuitry. The MOSFETs are conductive for structural reasons without applied control voltage. The required gate voltage for blocking the MOSFETs can be generated very easily by a branch connected in parallel with the load stages and having one or more Z diodes connected in series.This eliminates the need for a complicated voltage supply for each load stage, i.e. each MOSFET. The electronic load is characterized in that the load current of a few volts, for example 10 V, behaves linearly with respect to the setpoint specification in the kilovolt range, has substantially no drift, and the load current can be modulated as required into a frequency range of approximately 1 MHz sine. This means that a setpoint value for a load current can be predefined, which remains constant over the entire voltage range. Furthermore, the device is distinguished by a very simple and economically advantageous construction using elements which are usually available as standard components. An actively regulated load stage downstream of the device for limiting the voltage in the test system can be extended by any number of passive load stages (number of MOSFETs in the device), so that the need for the voltage stability of the electronic load can be very easily taken into account. The current resistance of the electronic load (the combination of the device and the load as described with reference to the test system is referred to as load) can be increased as desired by connecting load stages in parallel. The device is distinguished by a low outlay for the matching. Since the load described is distinguished by short stabilization times, the absence of times for readjustment and stabilization of the electronic load allows the test times and measurement times to be significantly reduced.Exemplary embodiments of the invention are described below with reference to the figures.Brief Description of the DrawingsFIG. 1 shows a device for limiting the voltage for a load according to an exemplary embodiment of the invention. FIG. 2 shows a device for limiting the voltage for a load according to a further exemplary embodiment of the invention. FIG. 3 shows a test system according to a further exemplary embodiment of the invention.DETAILED DESCRIPTION OF EMBODIMENTSThe representations in the figures are schematic and not true to scale. If the same reference numerals are used in the following description of the figures, these refer to the same or similar elements.FIG. 1 shows a device 100 for limiting the voltage for the load 200. The device 100 is connected to a potential 10 or a voltage source by means of the first connection element 102. The second connection element 105 of the device 100 is electrically connected to the load 200. The load is in turn connected to ground 20. In this configuration, the device 100 and the load 200 constitute a voltage divider.FIG. 2 shows a device 100 which is connected to the consumer 200. The load in this case is a current load. An operational amplifier receives a set value of the current through the resistor R2 via the terminal JP6. The voltage dropped across the resistor R2 is also fed back to the operational amplifier via the resistor R3 and can be converted into a current value with the known resistance value of R2. By comparing the current flowing through R2 and the target value, the operational amplifier outputs a signal serving as a gate voltage for the MOSFET Q1, so as to adjust the current flowing through R2 as needed.For the sake of completeness, it should be pointed out that the load 200 can be any desired load which requires a predeterminable voltage.The device 100, which is marked by the rectangle shown in dashed lines, is connected upstream of the consumer 200. The device 100 has a first load stage 110, a second load stage 120 and a third load stage 130. The load stages can also be referred to as voltage stages or voltage limiting stages. The load stages 110, 120, 130 are advantageously of identical construction, but can also be of different construction. It is also conceivable for the voltage stages in one and the same device to be constructed differently.The first load stage 110 is described by way of example, wherein the explanations relating to this apply analogously equally to the second and third load stages 120, 130 or else further load stages connected in series therewith. A high voltage is applied to the first connection element 102. The number of load stages or voltage limiting stages can be increased or reduced depending on the high voltage present. The high voltage can vary, whereas the voltage applied to the load remains constant independently of the load current absorbed.The drain 114 of the MOSFET Q 4 is connected to the first terminal 102. The voltage present at the first connection element 102 serves as the control voltage at the control electrode 112 of the MOSFET Q 4, which is reduced to a total of 800 V via four zener diodes D 20, D 21, D 22, D 23 having a breakdown voltage of, for example, 200 V in each case and is fed to the control electrode 112 via the resistor R 6. The source 116 of the MOSFET Q4 is connected to the drain of the MOSFET Q3 of the second load stage 120. A zener diode D25 is connected between the control electrode 112 and the source 116. Zener diode D25 has a protective function and prevents overvoltage between control electrode 112 and source 116.To increase the current resistance of the device 100, a MOSFET can be connected in parallel with the MOSFET Q4, for example. Likewise, one MOSFET can be connected in parallel with each of the other MOSFETs Q 3 and Q 2. The gates of the parallel-connected MOSFETs are connected to the same potential.FIG. 3 shows a test system 1 having a voltage source 2, which is designed to apply a potential to the first connection element 102 of the device 100 for limiting the voltage for the load 200. It is evident that the device 100 and the load 200 form a voltage divider. The device 100 can in particular keep a voltage applied or dropped at the load at a constant value, independently of the potential applied at the first connection element 102.List of reference characters1 Test system 2 Voltage source 10 Electrical potential 20 Ground 100 Device for limiting the voltage 102 First connection element 105 Second connection element 110 First load stage 112 Control electrode (gate) 114 Drain (drain) 116 Source (source) 120 Second load stage 130 Third load stage 200 Loads Q1 MOSFET Q2 Third FET, MOSFET Q3 Second FET, MOSFET Q4 First FET, MOSFET D20 Fourth semiconductor element, Zener diode D21 Third semiconductor element, Zener diode D22 Second semiconductor element, Zener diode D23 First semiconductor element, Zener diode D25 Zener diode JP6 Connection R2 Resistor R3 Resistor

Claims

Device (100) for limiting the voltage of a load (200), comprising: a first connection element (102) for receiving an electrical potential (10); a second connection element (105) for delivering an electrical potential to the load (200); a first field effect transistor, FET, (Q4), which is designed as a normally-on FET; a second FET (Q3), which is designed as a normally-on FET; wherein the first FET (Q4) and the second FET (Q3) are connected in series between the first connection element (102) and the second connection element (105); wherein a drain (114) of the first FET (Q4) is connected to the first connection element (102); wherein a source (116) of the first FET (Q4) is connected to a drain of the second FET (Q3); wherein a source of the second FET (Q3) is connected to the second terminal element (105); wherein the apparatus (100) comprises a third FET (Q2) connected in series with the first FET (Q4) and the second FET (Q3) such that a drain of the third FET (Q2) is connected to a source of the second FET (Q3) and a source of the third FET (Q2) is connected to the second terminal element (105); further comprising a first semiconductor element (D23) of defined breakdown voltage disposed between a control electrode (112) of the first FET (Q4) and the first terminal element (102); further comprising a second semiconductor element (D22) with a defined breakdown voltage, which is connected in series with the first semiconductor element (D23) between the control electrode (112) of the first FET (Q4) and the first connection element (102).The device (100) of claim 1, wherein the first FET (Q4) is a metal oxide semiconductor field effect transistor, MOSFET.The device (100) according to claim 1 or 2, wherein the first semiconductor element (D23) is a Zener diode.The device (100) according to any of the preceding claims, further comprising a fourth FET connected in parallel with the first FET (Q4).Test system (1), comprising: a voltage source (2); a device (100) according to one of Claims 1 to 4; an electrical load (200); wherein the device (100) is designed to limit a voltage present at the electrical load (200) to a predeterminable value.The test system (1) according to claim 5, wherein the electrical load (200) is a current load which is designed to simulate the behavior of a traveling wave tube.

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