GB2520566A

GB2520566A - Fail Safe Switch

Info

Publication number: GB2520566A
Application number: GB1320841.8A
Authority: GB
Inventors: Lee Coulbeck; Kim Evans; Christopher Abbott; Amanda Kirbyshire
Original assignee: Zhuzhou CSR Times Electric Co Ltd; Dynex Semiconductor Ltd
Current assignee: Zhuzhou CRRC Times Electric Co Ltd; Dynex Semiconductor Ltd
Priority date: 2013-11-26
Filing date: 2013-11-26
Publication date: 2015-05-27
Also published as: GB201320841D0; CN104681359A

Abstract

A fail-safe switch connected in parallel with an electrical device comprises a mechanical actuator that is activated in response to a predetermined current passing through the electrical device and closes the switch, short circuiting the device, which may be an insulating gate bipolar transistor (IGBT). The actuator may comprise a resistive heating element 10, a spring 20, a trigger 30 comprised of shape memory alloy and a piston 40 comprising conducting plate 42, the trigger 30 holding the piston 40 in place so that the spring 20 is compressed. The heating element 10 may be in physical contact with the trigger 30 and may be made of a material with high thermal resistance that undergoes joule heating in response to a rising current; when the current reaches a predetermined level the heating element 10 heats the trigger 30 so that it releases the piston 40, which may then move under the action of spring 20 so that the conductive plate 42 contacts the terminals of the electrical device, creating a current by-pass.

Description

FAIL SAFE SWITCH

Field of the Invention

The present invention relates to power electronics.

Background to the Invention

The work leading to this invention has received funding from the Europe-an Union Seventh Framework Programme (FP7/2007-201 1) under Grant Agreement No. 283857.

Insulated Gate Bipolar Transistor (IGBT) semiconductor devices are used in the field of power electronics for switching applications, for example in high voltage direct current converters. In the event of a fault, conventional wire bonded plastic modules can fail to an open circuit if the fault current is sufficient-ly large.

In order to achieve higher blocking voltages than with individual devices, press pack modules have been developed, which are designed to be stacked is on top of one another (see US6426561, EP1403923, U52002!0060371). The pressure applied to each one ensures an appropriate electrical and thermal connection. To protect the delicate chips inside the modules, pistons or springs can be provided so that the pressure on the chips is limited. Any excess pres-sure is held by the module casing. In the case of a defect or a high current spike the silicon melts forming a conductive channel which generates a stable short-circuit. The remaining modules in the stack then take the additional load and a single defective module will not lead to malfunctioning of the entire stack. The chips may also be provided with sacrificial layers covering part of the chip. The layer may contain for example silver and together with the semiconductor mate-rial form a eutectic mixture whose melting point is below that of the two partner materials. This therefore provides a short circuit whilst saving part of the chip.

However, existing failure to short circuit IGBT modules are pressure mounted devices and provide both complex and expensive solutions to the problem of offering a fail to short capability.

There has now been developed a tail-safe switch which overcomes or substantially mitigates the above-mentioned and/or other disadvantages asso-

ciated with the prior art.

Summary of the Invention

In a first aspect ot the invention there is provided an electrical device having a fail-safe switch connected in parallel therewith, the fail-safe switch comprising a mechanical actuator activated in response to a predetermined cur-rent passing through the electrical device to close the switch, thereby torming a current by-pass of the electrical device.

The tail-safe switch (FSS) according to the invention is advantageous as it harvests a small portion of the energy contained in or associated with a cata-strophic failure event of an electrical device. The harvested energy is used to release previously stored potential energy to form a low impedance current path short-circuit. Thus the circuit path within which the electrical device is disposed remains closed even when a failure event occurs. The FSS is designed such that both normal and transient but safe tault currents do not initiate the trigger within the switch, but larger fault currents, for example due to semi-conductor tailure, will initiate the FSS action and quickly provide a sate short-circuit current path. The FSS according to the invention is further advantageous as it otfers within an industry standard footprint and outline, a plastic package that can be used in series connected applications relying on n+1 redundancy.

Electrical devices according to the invention are those where the currents experienced could be up to and in the order at several thousand amperes and the voltages experienced could be up to and in the order of several kilo Volts.

Examples of such devices can include but are not limited to power quality man- agement systems, voltage source converters, high voltage transmission sys-tems or traction inverters as used in transport vehicles. Other examples ot such devices can include but are not limited to Silicon Controlled Rectifier (SCR)/Thyristors, GTO's (Gate turn-otf Thyristors), BJT's (Bipolar Junction tran-sistors), MOSFET's (Metal Oxide Semiconductor Field Effect Transistors), JFET's (Junction Field Etfect Transistors), IGFET's (Insulated Gate Field Effect Transistors) or diodes. Preferably the electrical device is an IGBT(lnsulated Gate Bipolar Transistor).

The mechanical actuator is any mechanical means adapted to close a circuit within which the device is located. The mechanical actuator preferably comprises an energy harvest element, a trigger element, a spring and a short-ing element.

The energy harvest element is any element which harvests energy from a failure event of the electrical device and triggers the trigger element. A failure event of the electrical device is preferably where the device experiences large fault currents, for example due to semi-conductor failure. A non-failure event of the electrical device is preferably where the device experiences either normal or transient but safe fault currents. The energy harvest element may extract at least part of the energy associated with the failure event for use with the trigger element. The predetermined current is preferably current derived from or asso-ciated with a failure event of the electrical device. The predetermined current is preferably set according to a level which is likely to damage the electrical de- vice. The current level which is likely to damage the electrical device is deter- mined by the tolerance of the components of the device, pre-set component pa- rameters, the physical characteristics and/or the electrical circuitry and compo-nents.

The energy harvest element preferably increases in temperature in re-sponse to current through it. The temperature of the energy harvest element is preferably proportional to the current through it. The energy harvest element may be an element with a high thermal resistance (Rth). In this case, a high thermal resistance describes a property of the energy harvest element, which is such that when a predetermined current is passed through it, the element is caused to heat to such a degree so as to trigger the trigger element. The energy harvest element may be a structure within the FSS with an engineered imped- ance that exhibits significant Joule heating in response to a predetermined cur-rent. Preferably the energy harvest element is a high thermal resistance (Rth) metal element. The energy harvest element may also be an SCR, semiconduc-tor or silicon chip.

The spring is any element within the FSS which is capable of storing po- tential energy. The spring may therefore be a leaf spring, a distorted metal un-der tension, or a piston acting on a pressurised gas. Preferably the spring is a coiled spring. The spring preferably forces the shorting element to close the switch.

The trigger element comprises any device adapted to retain the spring in a stored energy state and release the stored energy from the spring. The trigger element preferably comprises a material which responds to the increase in tem-perature from the energy harvest element so as to trigger the spring. The trigger element preferably triggers the spring when a predetermined temperature is reached. In one embodiment, the trigger element preferably releases the energy stored within the spring when the increased temperature it experiences causes a change in its physical state sufficient to denature the trigger element, thereby removing the forces required to retain the spring in a stored energy state. Ex-amples of suitable trigger elements include but are not limited to low melting point metal, solder, deformable clips or bimetallic strips. Preferably the trigger element is a shape memory alloy. The trigger element may be a magnetic de-vice.

The shorting element comprises any device which is adapted to close the switch. The shorting element can therefore be a conductive element and con- nects electrical terminals of the electrical device and/or switch. The shorting el-ement can also be a non-conductive element but which is adapted to actuate a conducting element to create a circuit path within the electrical device and/or switch. Preferably the shorting element is adapted to close a circuit path be-tween the collector and emitter of an IGBT.

Optionally, the FSS further comprises a housing which contains all the parts of the FSS. The housing is preferably adapted to receive the portions of the electrical device adapted for electrical communication. Preferably said por-tions comprise bus bars or the like. Preferably the housing is insulated.

A preferred embodiment of the invention will now be described in greater detail, by way of illustration, with reference to the accompanying drawing.

Brief Description of the Drawings

Figure 1 shows a schematic side view of an embodiment of a FSS ac-cording to the invention at a start position and where the predetermined current has not been reached.

Figure 2 shows a schematic side view of an embodiment of a FSS ac- cording to the invention at an activated position, where the predetermined cur-rent has been reached and with a short circuit current path in place.

Figure 3 shows a three dimensional schematic view of an embodiment of a FSS according to the invention at a start position and where the predeter- -10 mined current has not been reached.

Figure 4 shows a three dimensional schematic side view of an embodi-ment of a FSS according to the invention at an activated position, where the predetermined current has been reached and with a short circuit current path in place.

is Detailed Description of the Illustrated Embodiment In Figure 1 there is shown a fail-safe switch (FSS) generally designated 1 of an electrical device. The FSS 1 is at a start position and where the predeter-mined current has not been reached. The FSS 1 comprises an energy harvest element 10, a spring 20, a trigger element 30 and a shorting element 40.

The energy harvest element 10 is an annular heating element made up of a high impedance metal connected to other portions of an electrical circuit (not shown). The energy harvest element 10 has two arcuate portions which ex-tend around the trigger element 30. The energy harvest element 10 also has an annular portion (shown in figure 3 and 4) which joins the two arcuate portions.

The spring 20 is a coiled metal spring. The shorting element 40 is an insulated piston with a narrow part that extends into the centre of the element 10 and wide part above the spring 20. The spring 20 extends around the narrow part of the shorting element 40 and pushes against the wide part of the shorting ele-ment 40 and the housing 44. The trigger element 30 is shape memory alloy in the form of a circlip which extends around a circumferential groove in the nar-row part of the piston of the shorting element 40. The shorting element 40 also has a conductive plate 42 on its upper-most surface. All the parts of the FSS 1 described above are assembled as shown in Figure 1. Also shown in the em-bodiment in Figure 1 is an insulated cylindrical housing 44 which contains all the parts of the FSS.

The FSS 1 is assembled as shown in Figure 1 and is connected in paral-lel with an electrical device (not shown). In such a position the spring 20 is compressed against the housing 44 by the wide part of the piston of the short- ing element 40. The shorting element is prevented from moving within the hous- ing by the trigger element 30 which extends around the narrow part of the short-ing element 40 and engages with the edge of the energy harvest means 10.

During a failure event of the electrical device, such as a high current fault, a portion of the energy from said high current fault within the electrical de-vice is harvested by the energy harvest element 10. The energy is harvested by carefully engineered series impedance with an increased thermal resistance in the energy harvest element 10. The predetermined current causes significant Joule heating and increased on state voltage in response to the fail current. The increased temperature triggers the trigger element 30 by changing the state of the highly temperature sensitive shape memory alloy of the trigger element 30 located around the narrow part of the shorting element 40. Once this happens the shorting element 40 is allowed to move within the housing 44. The spring 20 therefore drives the shorting element 40 to make a short circuit between the components of the electrical device (in the region of E in Figure 2) and the con-ductive plate 42. For example this may be the collector and emitter of an IGBT.

In this way the switch is closed and a current by-pass is formed around the electrical device. The level of the predetermined current is pre-set by the com-ponents of the device such that the circuit path is closed at approximately the time of failure of the electrical device. Advantageously, the electrical device (such as an IGBT) which comprises the FSS 1 can be replaced during planned maintenance. Further advantageously, where multiple electrical device are con- nected together (such as multiple IGBT modules) to control an electrical appli- cation (such as an electric motor for a train), failure of one or more of the devic- es does not lead to total failure of the electrical application. Therefore the elec-trical application continues to work, even when a failure event in one or more of the electrical devices has occurred.

In another embodiment the energy harvest element is a high Rth bridge.

The energy harvest element harvests energy from a tail event via joule heating.

The spring is as described above and is positioned around the bridge. The FSS further comprises a heat spreader which is contactable with the spring and the energy harvest element. The heat spreader preferably distributes the heat en-ergy transterred by the energy harvest element. The FSS has a circlip made from a shape memory alloy. The circlip extends around the spring and prevents it from extending, when a failure event has not been experienced. The FSS fur-ther comprises a load plate. The load plate comprises any material with a low Rth which maintains the energy in the region of/near the trigger element. On top of the load disc is a shorting element. The shorting element makes short circuit with bus bars of an IGBT or other electrical device when the switch has been actuated. That is to say in use, when a failure event is experienced by the elec-trical device and the predetermined current reached the high Rth bridge heats up and denatures the circlip, thereby releasing the retaining forces on the spring. The spring thereby forces the shorting element into contact with the electrical circuit in which the electrical device is located, and the circuit path is closed.

Claims

CLAIMS1. An electrical device having a tail-safe switch connected in parallel therewith, the fail-sate switch comprising a mechanical actuator activated in re-sponse to a predetermined current passing through the electrical device to close the switch, thereby forming a current by-pass of the electrical device.
2 An electrical device according to claim 1, wherein the electrical device is an Insulated Gate Bipolar Transistor (IGBT) device.
3. An electrical device according to claim 1 or claim 2, wherein the mechanical actuator comprises an energy harvest element, a trigger element, a -10 spring and a shorting element.
4. An electrical device according to claim 3, wherein the energy har-vest element is a high thermal resistance (Rth) metal element.
5. An electrical device according to claim 3, wherein the spring is a coiled spring.is 6. An electrical device according to claim 3, wherein the trigger ele-ment is shape memory alloy.7. An electrical device according to claim 3, wherein the shorting el-ement is adapted to close a circuit path between the collector and emitter of an IGBT.8. An electrical device according to any preceding claim, turther comprising a housing.9. An electrical device according to claim 8, wherein the housing is adapted to receive the portions of the electrical device adapted for electrical communication.10. An electrical device according to any preceding claim, wherein the level of the predetermined current is pre-set by the components ot the device such that the circuit path is closed at approximately the time of failure of the electrical device.11. An electrical device substantially as described, with reference to, and/or as shown in the drawings.Amendments to the claims have been filed as followsCLAIMS1. An electrical device having a tail-safe switch connected in parallel therewith, the fail-safe switch comprising an energy harvest element, a trigger element, a spring and a shorting element, the shorting element being released by the trigger element in response to a predetermined current passing through the electrical device to be driven by the spring to close the switch, thereby form-ing a current by-pass of the electrical device, wherein the trigger element is shape memory alloy.2 An electrical device according to claim 1, wherein the electrical device is an Insulated Gate Bipolar Transistor (IGBT) device.3. An electrical device according to claim 1 or 2, wherein the energy harvest element is a high thermal resistance (Rth) metal element.4. An electrical device according to claim 1 or 2, wherein the spring is a coiled spring.is 5. An electrical device according to claim 2, wherein the shorting el-ement is adapted to close a circuit path between the collector and emitter of an r IGBT.
6. An electrical device according to any preceding claim, further comprising a housing.
7. An electrical device according to claim 6, wherein the housing is adapted to receive the portions of the electrical device adapted for electrical communication.
8. An electrical device according to any preceding claim, wherein the level of the predetermined current is pre-set by the components of the device such that the circuit path is closed at approximately the time of failure of the electrical device.
9. An electrical device substantially as described with reference to, and/or as shown in, the drawings.