The invention relates generally to the area of electrical components. More specifically, the invention relates to the area of reliability of electrical components such as electrical switches and electrical switch arrays.
Micro-electro-mechanical systems (MEMS) represent an integration of mechanical elements, with electrical elements on a substrate through micro-fabrication technology. While the electrical elements are typically fabricated using integrated circuit fabrication processes, the mechanical components are typically fabricated using compatible micromachining processes, such as lithographic, metallization, or etching processes. The ability to employ such processes is a key advantage of MEMS fabrication technology as it allows for enhanced control of the characteristic “micro-scale” dimensions typical of MEMS devices. Such processes also enable efficient production of MEMS devices by enabling batch fabrication of the MEMS devices on a common substrate die.
MEMS technology is suited to fabricate components, such as actuators or switches, that require a limited range of motion for their operation. Switch arrays may also be realized based on MEMS technology.
One type of MEMS includes a suspended connecting member, which connecting member may be in the form of a movable beam, such as a cantilever. Such a device may further include an actuation mechanism, which actuation mechanism may be electrostatic, to cause a movement of the suspended connecting member. The movement enables an electrical communication between any two or more parts of the MEMS by causing a “making” and “breaking” of an electrical contact between a surface of the suspended connecting member and a surface of an adjacent part of the MEMS.
Unacceptably high voltages can develop across a MEMS switch array when the MEMS switch array “opens” from an energized state. The development of such voltages can result in arcing between electrical contacts of the individual switches of the switch array. Therefore, an issue affecting reliability of performance of a MEMS switch array concerns the possibility of arcing between, or welding of, the two components under question during the opening. This arcing and/or welding may result in uncontrolled variation in the electrical contact resistance, and indeed, may also result in temporary or permanent seizure of any one or more electrical contacts.
Development of reliable and cost-effective MEMS and MEMS arrays is one of the challenges facing MEMS technology. High reliability MEMS and MEMS arrays would therefore, be highly desirable.
Embodiments of the invention are directed to systems and methods to enhance reliability of electrical components such as electrical switches.
A method to reduce an inductive voltage surge across a switch array, said method comprising the steps of, (a) directing at least a portion of an electric current away from at least a portion of said switch array; and (b) independently opening different portions of the switch array.
A method of varying resistance of a switch array, said method comprising the step of independently opening different portions of the switch array at independent gating voltages.
A system to reduce an inductive voltage surge across an electrical device comprising a current bypass circuit, said system comprising, a subsystem capable of independently opening different portions of the electrical device according to shaped distribution of opening times.
A system to reduce an inductive voltage surge across a switch array, comprising, a current bypass circuit; and a control system comprising, a signal generator; wherein, the control system is capable of independently toggling any one or more individual switches of the switch array between an open state and a closed state in response to a control signal generated by the signal generator.
These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.
In the following description, whenever a particular aspect or feature of an embodiment of the invention is said to comprise or consist of at least one element of a group and combinations thereof, it is understood that the aspect or feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
As used herein, the term “switch” refers to a device that can be used to connect and disconnect two parts of an electrical component. The mechanism of operation of such switches may be mechanical, or it may be electrical, or it may be chemical, or it might be a combination of the above. A suitable non-limiting example of such a switch is a micro-electro-mechanical system switch.
As used herein, the term “switch array” may refer to an array of switches that have been fabricated on a single die or it may refer to an array of dies each of which includes multiple switches.
Systems and methods to protect an electrical device, such as a switch array are known. Non-limiting examples of switches include micro-electro-mechanical systems (MEMS). One known “protective” system includes a hybrid arc limiting technology (HALT) circuit (see, for instance, Kumfer et al., U.S. Patent Publication Number 2009/0115255A1; Kumfer et al., U.S. Patent Publication Number 2008/0310056A1; Howell, U.S. Pat. No. 4,723,187). The HALT circuit shields the electrical device from arcing during an interruption of a load current and/or of a fault current. In one non-limiting example, the array of MEMS may service, for instance, a motor-starter system.
During a fault condition, an electrical device is typically required to “open” expeditiously. The resulting and correspondingly sudden change in an amount of electric bias, results in a development of a bias across the device, and presents a damage risk for the electrical device due to a possibility of the bias induced, arcing across electrical contacts within the electrical device. A protective circuit, for example, a HALT circuit, works by substantially preventing a resultant bias, for example, a voltage, across the electrical contacts from exceeding a so called “melt voltage” of the electrical contacts, as they are opening. There are multiple factors contributing to the voltage. A first contribution is due to a static unbalanced voltage of a diode bridge in the HALT circuit. The static unbalanced voltage is substantially a result of a simultaneous flow of both a load electric current, and an electric current pulse that is produced through the diode bridge by the HALT circuit. Systems and methods that address mitigation strategies of the static unbalanced voltage are known (see, for instance, the documents referenced above). It has been ascertained that, a second contribution to the voltage, is substantially a result of an inductive voltage surge in the wiring of the HALT circuit diode bridge.
Previous approaches to mitigate the inductive voltage surge have involved appropriate modifications to the basic HALT circuit design. Such approaches potentially are adequate at relatively low values of fault or load electric currents due to correspondingly low levels of inductive voltage surge. At relatively higher fault or load current values, the inductive voltage surge is correspondingly higher, and therefore, additional strategies to mitigate the inductive voltage surge may be useful. Aspects of the invention disclosed herein include systems and methods to control the inductive voltage surge within the HALT circuit. The disclosed methods include appropriately distributing the opening of any one or more individual switches belonging to switch array.
As discussed herein, embodiments of the electric device 126 include a MEMS switch array and operate as a motor starter. During a fault condition a motor starter is required to perform a function of opening the individual switches of the switch array, that is, of interrupting the flow of electric current through the electric device 126. Similarly, during a load condition, a motor starter is required to perform the function of closing the individual switches of the switch array, that is, of energizing the load 102 by initiating a flow of electric current through it via the electric device 126. However, as discussed above, even with the aid of a HALT circuit 106 to relieve the switch array 126 as they are opened or closed, the stray inductance of the HALT circuit 106 itself may present an obstacle to arcless operation of the switch array 126. As will be discussed in relation to
It may be evident that, during a fault condition, with the progressive opening of the electric device 126, the resistance of the electric device 126 will also rise in a corresponding manner. The corresponding rise in voltage across the electric device 126 drives the load current IL(t) 112 into the HALT circuit diode bridge 110 against the resistance and inductance of the HALT circuit diode bridge 110. This substantially results in an inductive voltage surge across the electric device 126.
It may be evident that, during a time window wherein the time-dependent electric current pulse ID(t) 114 substantially exceeds the time-dependent load current IL(t) 112, alternate flow path 116 represents a low resistance shunting path for the time-dependent load current IL(t) 112. This time window then, represents a “time window of opportunity” to open the electric device 126. In the interest of clarity, the discussions herein will substantially consider a switch array as a non-limiting example of an electric device 126.
It has been determined, as discussed in relation to
A first possibility 206 for the opening time distribution of individual switches of the switch array represents a typical situation as is encountered in switch arrays. It will be evident that all switches are opened substantially simultaneously, i.e., a first time duration 208 over which the switches are opened is very small. Quite generally, in the discussions herein, opening time distributions of type 206 will be referred to as “fast” opening time distributions.
Simulations were performed in order to ascertain the time dependence of the inductive voltage surge for different opening time distributions of the electric device “switch array” 126 shown in
It seems therefore, that if individual switches in the switch array 126 are opened as quickly as possible (for example, as per the first opening time distribution 206), the resulting high rate of change of electric current flowing through the stray inductance of the HALT circuit 106 may generate enough voltage to cause arcing and destruction of an unacceptable number of individual switches of the switch array 126.
A comparison of
In order to explore the consequences of the above insight, a “shaped” third opening time distribution 214 was used.
Non-limiting embodiments of this invention manage the inductive voltage surge that would otherwise occur, during a typical prior art distribution of opening times (for instance, of type 206) by causing the opening of individual switches in the array to be spread over a suitable time interval. Those skilled in the art will recognize that the present invention includes any scheme used to shape the opening time distribution of the individual switches of a switch array in a manner so as to mitigate the inductive voltage surge within the HALT circuit, during the duration over which the individual switches are opening, to a value that is below the value of melt voltage of any individual switch. The openings time distributions 206 and 214 constitute two non-limiting examples of such a scheme.
In accordance with one embodiment of the invention therefore, depicted via a flow chart in
At step 602, the method 600 includes the step of directing at least a portion of an electric current away from at least a portion of the switch array. In one embodiment of the invention, said portion of electrical current flows through a HALT circuit (for instance, of type 106). In one embodiment of the invention, the HALT circuit includes a diode bridge (for instance, of type 110). The inductive voltage surge occurs, for instance, across the diode bridge within the HALT circuit. In one embodiment, the method 600 is capable at least of mitigating, during said opening, development of the inductive voltage surge across the HALT circuit. At step 604, the method includes independently opening different portions of the switch array. In one embodiment of the invention, any one or more individual switches of the switch array can be toggled between an open state and a closed state in response to independent gating voltages. In one embodiment of the method 600, the opening of different portions of the switch array is performed substantially continuously in time. A non-limiting example of such a continuous time distribution is the second possibility of opening time distribution 210. In one embodiment of the method 600, the opening of different portions of the switch array is performed substantially step-wise in time. A non-limiting example of such a continuous time distribution is the third possibility of opening time distribution 214. In one embodiment, the method 600 is capable of mitigating the inductive voltage surge across a switch array, during said opening, to a value that is substantially less than the melt voltage “Vm”.
In accordance with another embodiment of the invention, depicted via flow chart 700 in
In accordance with one embodiment of the invention, shown via schematic diagram 800 in
In accordance with an embodiment of the invention, shown via schematic diagram 900 in
In one embodiment of the invention, the switch array is disposed within a hermetically sealed chamber. In one embodiment of the invention, an environment within the hermetically sealed chamber includes an inert gas. In one embodiment of the invention, an environment within the hermetically sealed chamber includes a vacuum. In one embodiment of the invention, the switch array services an electrical power device. In one embodiment of the invention the electrical power device is a motor starter.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
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