Patent Application
20080308394
Embodiments of the invention relate generally to switching devices for switching on/off a current in current paths, and more particularly to micro-electromechanical system based switching devices.
To switch on/off current in electrical systems, a set of contacts may be used. The contacts may be positioned as open to stop current, and closed to promote current flow. Generally, the set of contacts may be used in contactors, circuit-breakers, current interrupters, motor starters, or similar devices. However, the principles of switching current on/off may be understood through explanation of a contactor.
A contactor is an electrical device designed to switch an electrical load ON and OFF on command. Traditionally, electromechanical contactors are employed in control gear, where the electromechanical contactors are capable of handling switching currents up to their interrupting capacity. Electromechanical contactors may also find application in power systems for switching currents. However, fault currents in power systems are typically greater than the interrupting capacity of the electromechanical contactors. Accordingly, to employ electromechanical contactors in power system applications, it may be desirable to protect the contactor from damage by backing it up with a series device that is sufficiently fast acting to interrupt fault currents prior to the contactor opening at all values of current above the interrupting capacity of the contactor.
Previously conceived solutions to facilitate use of contactors in power systems include vacuum contactors, vacuum interrupters and air break contactors, for example. Unfortunately, contactors such as vacuum contactors do not lend themselves to easy visual inspection as the contactor tips are encapsulated in a sealed, evacuated enclosure. Further, while the vacuum contactors are well stated for handling the switching of large motors, transformers, and capacitors, they are known to cause undesirable transient overvoltages, particularly as the load is switched off.
Furthermore, the electromechanical contactors generally use mechanical switches. However, as these mechanical switches tend to switch at a relatively slow speed, predictive techniques are employed in order to estimate occurrence of a zero crossing, often tens of milliseconds before the switching event is to occur, in order to facilitate opening/closing near the zero crossing for reduced arcing. Such zero crossing prediction is prone to error as many transients may occur in this prediction time interval.
As an alternative to slow mechanical and electromechanical switches, fast solid-state switches have been employed in high speed switching applications. As will be appreciated, these solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, because solid-state switches do not create a physical gap between contacts as they are switched into a non-conducing state, they experience leakage current. Furthermore, due to internal resistances, if solid-state switches operate in a conducting state, they experience a voltage drop. Both the voltage drop and leakage current contribute to the generation of excess heat under normal operating circumstances, which may affect switch performance and life. Moreover, due at least in part to the inherent leakage current associated with solid-state switches, their use in circuit breaker applications is not practical.
Furthermore, switching currents on or off during current flow may produce arcs, or flashes of electricity, which are generally undesirable. As described above, contactors may switch alternating current (AC) near or at a zero-crossing point where current flow is reduced compared to other points on an alternating current sinusoid. In contrast, direct current (DC) typically does not have a zero-crossing point. As such, arcs may occur at any instance of interruption.
Therefore, direct current interruption imposes different switching requirements compared to alternating current interruption. For example, if there is a significant amount of current or voltage, an alternating current interrupter may wait for an AC sinusoidal load or fault current to reach a naturally occurring zero before interruption. In contrast, DC interrupters do not experience a naturally occurring zero, and therefore must force a lower current or voltage in order to reduce arcing. Electronic devices such as transistors or field-effect transistors may force DC current to lower levels, but have the drawback of having high conducting voltage drop and power losses.
Accordingly, there exists a need in the art for a direct current control device and/or interrupter arrangement to overcome these drawbacks.
An embodiment of the invention includes a current control device. The current control device includes control circuitry integrally arranged with a current path and at least one micro electromechanical system (MEMS) switch disposed in the current path. The current control device further includes a hybrid arcless limiting technology (HALT) circuit connected in parallel with the at least one MEMS switch facilitating arcless opening of the at least one MEMS switch, and a pulse assisted turn on (PATO) circuit connected in parallel with the at least one MEMS switch facilitating arcless closing of the at least one MEMS switch.
Another embodiment of the invention includes a method of controlling an electrical current passing through a current path. The method includes transferring electrical energy from at least one micro electromechanical system (MEMS) switch to a hybrid arcless limiting technology (HALT) circuit connected in parallel with the at least one MEMS switch to facilitate opening the current path. The method further includes transferring electrical energy from the at least one MEMS switch to a pulse assisted turn on (PATO) circuit connected in parallel with the at least one MEMS switch to facilitate closing the current path.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein;
An embodiment of the invention provides an electrical interruption device suitable for arcless interruption of direct current. The interruption device includes micro electromechanical system (MEMS) switches. Use of MEMS switches provide fast response time. A Hybrid Arcless Limiting Technology (HALT) circuit connected in parallel with the MEMS switches provides capability for the MEMS switches to be opened without arcing at any given time regardless of current or voltage. A Pulse-Assisted Turn On (PATO) circuit connected in parallel with the MEMS switches provides capability for the MEMS switches to be closed without arcing at any given time.
As illustrated in
In a presently contemplated configuration as will be described in greater detail with reference to
Turning now to
In accordance with further aspects of the present technique, a load circuit 40 may he coupled in series with the first MEMS switch 20. The load circuit 40 may include a voltage source VBUS 44. In addition, the load circuit 40 may also include a load inductance 46 LLOAD, where the load inductance LLOAD 46 is representative of a combined load inductance and a bus inductance viewed by the load circuit 40. The load circuit 40 may also include a load resistance RLOAD 48 representative of a combined load resistance viewed by the load circuit 40. Reference numeral 50 is representative of a load circuit current ILOAD that may flow through the load circuit 40 and the first MEMS switch 20.
Further, as noted with reference to
In one embodiment, the first MEMS switch 20 may be coupled in parallel across midpoints of the balanced diode bridge 28. The midpoints of the balanced diode bridge may include a first midpoint located between the first and second diodes 30, 32 and a second midpoint located between the third and fourth diodes 34, 36. Furthermore, the first MEMS switch 20 and the balanced diode bridge 28 may be tightly packaged to facilitate minimization of parasitic inductance caused by the balanced diode bridge 28 and in particular, the connections to the MEMS switch 20. It may be noted that, in accordance with exemplary aspects of the present technique, the first MEMS switch 20 and the balanced diode bridge 28 are positioned relative to one another such that the inherent inductance between the first MEMS switch 20 and the balanced diode bridge 28 produces a di/dt voltage less than a few percent of the voltage across the drain 22 and source 24 of the MEMS swatch 20 when carrying a transfer of the load current to the diode bridge 28 during the MEMS switch 20 turn-off which will be described in greater detail hereinafter. In one embodiment, the first MEMS switch 20 may be integrated with the balanced diode bridge 28 in a single package 38 or optionally, the same die with the intention of minimizing the inductance interconnecting the MEMS switch 20 and the diode bridge 28.
Additionally, the arc suppression circuitry 14 may include a pulse circuit 52 coupled in operative association with the balanced diode bridge 28. The pulse circuit 52 may be configured to detect a switch condition and initiate opening or the MEMS switch 20 responsive to the switch condition. As used herein, the term “switch condition” refers to a condition that triggers changing a present operating state of the MEMS switch 20. For example, the switch condition may result in changing a first closed state of the MEMS switch 20 to a second open state or a first open state of the MEMS switch 20 to a second closed state. A switch condition may occur in response to a number of actions including but not limited to a circuit fault or switch ON/OFF request.
The pulse circuit 52 may include a pulse switch 54 and a pulse capacitor CPULSE 56 coupled to the pulse switch 54. Further, the pulse circuit may also include a pulse inductance LPULSE 58 and a first diode DP 60 coupled in series with the pulse switch 54. The pulse inductance LPULSE 58, the diode DP 60, the pulse switch 54 and the pulse capacitor CPULSE 56 may be coupled in series to form a first branch of the pulse circuit 52, where the components of the first branch may be configured to facilitate pulse current shaping and timing. Also, reference numeral 62 is representative of a pulse circuit current IPULSE that may flow through the pulse circuit 52.
In accordance with aspects of die present invention, the MEMS switch 20 may be rapidly switched (for example, on the order of picoseconds or nanoseconds) from a first closed state to a second open state while carrying a current albeit at a near-zero voltage. This may be achieved through the combined operation of the load circuit 40, and pulse circuit 52 including the balanced diode bridge 28 coupled in parallel across contacts of the MEMS switch 20.
Reference is now made to
In accordance with one aspect of the invention, the soft switching system 11 may be configured to perform soft or point-on-wave (PoW) switching whereby one or more MEMS switches in the switching circuitry 12 may be closed at a time when the voltage across the switching circuitry 12 is at or very close to zero, and opened at a time when the current through the switching circuitry 12 is at or close to zero. By closing the switches at a time when the voltage across the switching circuitry 12 is at or very close to zero, pre-strike arcing can be avoided by keeping the electric field low between the contacts of the one or more MEMS switches as they close, even if multiple switches do not all close at the same time. Similarly, by opening the switches at a time when the current through the switching circuitry 12 is at or close to zero, the soft switching system 11 can be designed so that the current in the last switch to open in the switching circuitry 12 falls within the design capability of the switch. As alluded to above and in accordance with one embodiment, the control circuitry 72 may be configured to synchronize the opening and closing of the one or more MEMS switches of the switching circuitry 12 with the occurrence of a zero crossing of an alternating source voltage or an alternating load circuit current.
Turning to
Although for the purposes of description,
The exemplary MEMS switch 20 may include three contacts. In one embodiment, a first contact may be configured as a drain 22, a second contact may be configured as a source 24, and the third contact may be configured as a gate 26. In one embodiment, the control circuitry 72 may be coupled to the gate contact 26 to facilitate switching a current state of the MEMS switch 20. Also, in certain embodiments, damping circuitry (snubber circuit) 33 may be coupled in parallel with the MEMS switch 20 to delay appearance of voltage across the MEMS switch 20. As illustrated, the damping circuitry 33 may include a snubber capacitor 76 coupled in series with a snubber resistor 78, for example.
Additionally, the MEMS switch 20 may be coupled in series with a load circuit 40 us further illustrated in
As previously noted, the detection circuitry 70 may be configured to detect occurrence of a zero crossing of the alternating source voltage or the alternating load current ILOAD 50 in the load circuit 40. The alternating source voltage may be sensed via the voltage sensing circuitry 80 and the alternating load current ILOAD 50 may be sensed via the current sensing circuitry 82. The alternating source voltage and the alternating load current may be sensed continuously or at discrete periods for example.
A zero crossing of the source voltage may be detected through, for example, use of a comparator such as the illustrated zero voltage comparator 84. The voltage sensed by the voltage sensing circuitry 80 and a zero voltage reference 86 may be employed as inputs to the zero voltage comparator 84. In turn, an output signal 88 representative of a zero crossing of the source voltage of the load circuit 40 may be generated. Similarly, a zero crossing of the load current ILOAD 50 may also be detected through use of a comparator such as the illustrated zero current comparator 92. The current sensed by the current sensing circuitry 82 and a zero current reference 90 may be employed as inputs to the zero current comparator 92. In turn, an output signal 94 representative of a zero crossing of the load current ILOAD 50 may be generated.
The control circuitry 72, may in turn utilize the output signals 88 and 94 to determine when to change (for example, open or close) the current operating state of the MEMS switch 20 (or array of MEMS switches). More specifically, the control circuitry 72 may be configured to facilitate opening of the MEMS switch 20 in an arc-less manner to interrupt or open the load circuit 40 responsive to a detected zero crossing of the alternating load current ILOAD 50. Additionally, the control circuitry 72 may be configured to facilitate closing of the MEMS switch 20 in an arc-less manner to complete the load circuit 40 responsive to a detected zero crossing of the alternating source voltage.
In one embodiment, the control circuitry 72 may determine whether to switch the present operating state of the MEMS switch 20 to a second operating state based at least in part upon a state of an Enable signal 96. The Enable signal 96 may be generated as a result of a power off command in a contactor application, for example. In one embodiment, the Enable signal 96 and the output signals 88 and 94 may by used as input signals to a dual D flip-flop 98 as shown. These signals may he used to close the MEMS switch 20 at a first source voltage zero after the Enable signal 96 is made active (for example, rising edge triggered), and to open the MEMS switch 20 at the first load current zero after the Enable signal 96 is deactivated (for example, falling edge triggered). With respect to the illustrated schematic diagram 19 of
As previously noted, in order to achieve a desirable current rating for a particular application, a plurality of MEMS switches may be operatively coupled in parallel (for example, to form a switch module) in lieu of a single MEMS switch. The combined capabilities of the MEMS switches may be designed to adequately carry the continuous and transient overload current levels that may be experienced by the load circuit. For example, with a 10-amp RMS motor contactor with a 6X transient overload, there should be enough switches coupled in parallel to carry 60 amps RMS for 10 seconds. Using point-on-wave switching to switch the MEMS switches within 5 microseconds of reaching current zero, there will be 160 milliamps instantaneous, flowing at contact opening. Thus, for that application, each MEMS switch should be capable of “warm-switching” 160 milliamps, and enough of them should be placed in parallel to carry 60 amps. On the other hand, a single MEMS switch should be capable of interrupting the amount or level of current that will be flowing at the moment of switching.
However, example embodiments are not limited to arcless switching of alternating current and/or sinusoidal waveforms. As depicted in
In a presently contemplated configuration as will be described in greater detail with reference to FIG, 6, the MEMS based switching circuitry 111 may include one or more MEMS switches. Additionally, the arc suppression circuitry 110 may include a balanced diode bridge and a pulse circuit and/or pulse circuitry. Further, the are suppression circuitry 110 may be configured to facilitate suppression of an arc formation between contacts of the one or more MEMS switches by receiving a transfer of electrical energy from the MEMS switch in response to the MEMS switch changing state from closed to open (or open to closed). It may be noted that the arc suppression circuitry 110 may be configured to facilitate suppression of an arc formation in response to an alternating current (AC) or a direct current (DC).
Turning now to
In accordance with further aspects of the present technique, a load circuit 140 may be coupled in series with the first MEMS switch 123. The load circuit 140 may include a voltage source VBUS. In addition, the load circuit 140 may also include a load inductance 117 LLOAD, where the load inductance LLOAD 117 is representative of a combined load inductance and a bus inductance viewed by the load circuit 140. Reference numeral 116 is representative of a load circuit current ILOAD that may flow through the load circuit 140 and the first MEMS switch 123.
Further, as noted with reference to
In one embodiment, the first MEMS switch 123 may be coupled in parallel across midpoints of the balanced diode bridge 141. The midpoints of the balanced diode bridge may include a first midpoint located between the first and second diodes 124, 125 and a second midpoint located between the third and fourth diodes 126, 127. Furthermore, the first MEMS switch 123 and the balanced diode bridge 141 may be tightly packaged to facilitate minimization of parasitic inductance caused by the balanced diode bridge 141 and in particular, the connections to the first MEMS switch 123. It may be noted that, in accordance with exemplary aspects of the present technique, the first MEMS switch 123 and the balanced diode bridge 141 are positioned relative to one another such that the inherent inductance between the first MEMS switch 123 and the balanced diode bridge 141 produces a di/dt voltage less than a few percent of the voltage across the drain 120 and source 122 of the first MEMS switch 123 when carrying a transfer of the load current to the diode bridge 141 during the MEMS switch 123 turn-off/on which will be described in greater detail hereinafter. In one embodiment, the first MEMS switch 123 may be integrated with the balanced diode bridge 141 in a single package 119 or optionally, the same die with the intention of reducing the inductance interconnecting the first MEMS switch 123 and the diode bridge 141.
Additionally, the arc suppression circuitry 110 may include pulse circuits 138 and 139 coupled in operative association with the balanced diode bridge 141. The pulse circuit 139 may be configured to detect a switch condition and initiate opening of the MEMS switch 123 responsive to the switch condition. Similarly, pulse circuit 138 may be configured to detect a switch condition and initiate closing of the MEMS switch 123 responsive to the switch condition. As used herein, the term “switch condition” refers to a condition that triggers changing a present operating state of the MEMS switch 123. For example, the switch condition may result in changing a first closed state of the MEMS switch 123 to a second open state or a first open state of the MEMS switch 20 to a second closed state. A switch condition may occur in response to a number of actions including but not limited to a circuit fault or switch ON/OFF request.
The pulse circuit 138 includes a pulse switch 133 and a pulse capacitor CPULSE1 129 series coupled to the pulse switch 133. Further, the pulse circuit 138 may include a pulse inductance LPULSE1 137 coupled in series with the pulse switch 133. The pulse inductance LPULSE1 137, the pulse switch 133, and the pulse capacitor CPULSE1 129 may he coupled in series to form a first branch of the pulse circuit 138, where the components of the first branch may be configured to facilitate pulse current shaping and timing. Pulse current shaping and timing may be determined from the initial voltage across the capacitor Cpulse1 (generated by a charging circuit) and from the capacitance and inductance values of Cpulse1 and Lpulse1 respectively. Therefore, pulse current shaping and timing may be facilitated through choosing different values of initial voltage, capacitance of Cpulse1 and inductance of Lpulse1. Also, reference numeral 136 is representative of a pulse circuit current IPULSE1 that may flow through the pulse circuit 138.
The pulse circuit 138 may be operatively connected to a capacitance charging network 142 including resistors 128 and voltage source 150. The capacitance charging network may transfer electric charge to the pulse capacitor 129. In a switching event, discharge of the pulse capacitor 129 may facilitate transfer of energy from the MEMS switch 123 to the pulse circuit 138. Thus, the pulse circuit 133 may be a pulse assisted turn on (PATO) circuit to facilitate arcless closing of the first MEMS switch 123.
The pulse circuit 139 includes a pulse switch 132 and a pulse capacitor CPULSE2 131 series coupled to the pulse switch 132. Further, the pulse circuit 139 may include a pulse inductance LPULSE2 134 coupled in series with the pulse switch 132. The pulse inductance LPULSE2 134, the pulse switch 132 and the pulse capacitor CPULSE2 131 may be coupled in series to form a first branch of the pulse circuit 139, where the components of the first branch may be configured to facilitate pulse current shaping and timing. Also, reference numeral 135 is representative of a pulse circuit current IPULSE2 that may flow through the pulse circuit 52.
The pulse circuit 139 may also be operatively connected to a capacitance charging network 142 including resistors 128 and voltage source 130. The capacitance charging network 142 may transfer electric charge to the pulse capacitor 131. In a switching event, discharge of the pulse capacitor 131 may facilitate transfer of energy from the MEMS switch 123 to the pulse circuit 139. Thus, the pulse circuit 139 may be a hybrid arcless limiting technology (HALT) circuit to facilitate arcless opening of the first MEMS switch 123.
As noted above, the pulse circuits 138 and 139 may include pulse inductances 137 and 134. However, in some example embodiments the pulse circuits 138 and 139 may share an inductance, thereby reducing the number of components in the are suppression circuitry.
In accordance with aspects of the present invention, the first MEMS switch 123 may be rapidly switched (for example, on the order of picoseconds or nanoseconds) from a first closed state to a second open state while carrying a current albeit at a near-zero voltage. This may be achieved through the combined operation of the load circuit 140, and pulse circuits 138, 139 including the balanced diode bridge 141 coupled in parallel across contacts of the first MEMS switch 123. For example, energy may be transferred from the first MEMS switch 123 to the pulse circuit 138. This may be facilitated through discharge of the pulse capacitance 129. Similarly, energy may be transferred from the first MEMS switch 123 to the pulse circuit 139. This may be facilitated through discharge of the pulse capacitance 131. It is appreciated that the resistors 128 and voltage source 130 facilitate charging of the pulse capacitors 129 and 131. Therefore, arcless operation of the MEMS switch 123 is possible through embodiments of the present invention.
However, example embodiments are not limited to current control devices including a single MEMS switch. For example, a plurality of MEMS switches may be used to achieve a different voltage rating, or different current handling capabilities, compared to a single MEMS switch. For example, a plurality of MEMS switches may be connected in parallel to achieve increased current handling capabilities. Similarly, a plurality of MEMS switches may be connected in series to achieve a higher voltage rating. Furthermore, a plurality of MEMS switches may be connected in a network including combinations of series and parallel connections to achieve a desired voltage rating and current handling capabilities. All such combinations are intended to be within the scope of example embodiments or the present invention.
As further illustrated in
In an example embodiment, the current control device 164 may include a final isolation device 161. The final isolation device 161 may provide air-gap safety isolation of an electrical load on the current path 154. For example, the final isolation device may include a contactor or other interruption device, which may be opened in response to the MEMS array 160 changing switch conditions.
In another example embodiment, the current control device 164 may further include an electronic bypass device 162. A bypass device may include one or more electronic components which shunt overload current away from the MEMS switches for a duration of the current overload. For example, the electronic bypass device 162 may receive overload current from the current path 153 in response to current overload. Therefore, the electronic bypass device 162 may extend the temporary overload rating of the current control device 164. It is noted that the current control device 164 may include either or both of the final isolation device 161 and electronic bypass device 162 without departing from example embodiments of the invention.
As described hereinbefore, a current control device according to example embodiments may be used to interrupt current flow for both direct and alternating currents. Turning to
Therefore, current control devices as described herein may include control circuitry integrally arranged with a current path, at least one micro electromechanical system (MEMS) switch disposed in the current path, a hybrid arcless limiting technology (HALT) circuit connected in parallel with the at least one MEMS switch facilitating arcless opening of the at least one MEMS switch, and a pulse assisted turn on (PATO) circuit connected in parallel with the at least one MEMS switch facilitating arcless closing of the at least one MEMS switch.
Furthermore, example embodiments provide methods of controlling an electrical current passing through a current path. For example, the method may include transferring electrical energy from at least one micro electromechanical system (MEMS) switch to a hybrid arcless limiting technology (HALT) circuit connected in parallel with the at least one MEMS switch to facilitate opening the current path. The method may further include transferring electrical energy from the at least one MEMS switch to a pulse assisted turn on (PATO) circuit connected in parallel with the at least one MEMS switch to facilitate closing the current path. Therefore, example embodiments of the present invention provide arcless current control devices, and methods of arcless current control.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.
