Patent Application
20080137238
Embodiments of the invention relate generally to a switching system for switching a current to a selectable current path, and more particularly to micro-electromechanical system based switching devices.
A circuit breaker is an electrical device designed to protect electrical equipment from damage caused by faults in the circuit. Traditionally, most conventional circuit breakers include bulky electromechanical switches. Unfortunately, these conventional circuit breakers are large in size thereby necessitating use of a large force to activate the switching mechanism. Additionally, the switches of these circuit breakers generally operate at relatively slow speeds. Furthermore, these circuit breakers are disadvantageously complex to build and thus expensive to fabricate. In addition, when contacts of the switching mechanism in conventional circuit breakers are physically separated, an arc is typically formed therebetween which continues to carry current until the current in the circuit ceases. Moreover, energy associated with the arc may seriously damage the contacts and/or present a burn hazard to personnel.
As an alternative to slow electromechanical switches, relatively 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, since solid-state switches do not create a physical gap between contacts when they are switched into a non-conducting state, they experience leakage current. Furthermore, due to internal resistances, when 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 be detrimental to 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 possible.
U.S. patent application Ser. No. 11/314,336 filed on Dec. 20, 2005, (Attorney Docket No. 162711-1), which is incorporated by reference in its entirety herein, describes micro-electromechanical system (MEMS) based switching devices including circuitry and techniques adapted to suppress arc formation between contacts of the micro-electromechanical system switch.
The switching devices may be part of a current limiting protection device that may have to absorb or withstand a surge current associated with starting up a motor or other industrial equipment. This surge current often comprises multiple times (e.g., six times or more) the value of the steady state load current and can last up to ten seconds. One known technique of handling this current is to combine a number of MEMS switches (e.g., six times the number of switches required for handling the steady state current) in parallel. This technique perhaps will become cost-effective at some point in the future as the cost and yield of MEMS switch arrays improve, but presently to use six times the number of MEMS switches would increase the cost by six times, and these additional MEMS switches would only function for just about 10 seconds of operation. Accordingly, it is desirable to provide circuitry and/or techniques that reliably and cost-effectively handle such a surge current while still able to use the MEMS switches for the steady state operation and for addressing fault conditions that may arise.
Generally, aspects of the present invention provide a switching system including electromechanical switching circuitry. The system may further include solid state switching circuitry coupled in a parallel circuit with the electromechanical switching circuitry, and a controller coupled to the electromechanical switching circuitry and the solid state switching circuitry. The controller may be configured to perform selective switching of a load current between the electromechanical switching circuitry and the solid state switching circuitry in response to a load current condition appropriate to an operational capability of a respective one of the switching circuitries.
Further aspects of the present invention provide a switching system including a micro-electromechanical system switching circuitry. The system may further include solid state switching circuitry. A first over-current protection circuitry may be connected in a parallel circuit with the micro-electromechanical system switching circuitry and the solid state switching circuitry, wherein the first over-current protection circuitry may be configured to suppress arc formation between contacts of the micro-electromechanical system switching circuitry. A controller may be coupled to the electromechanical switching circuitry, the solid state switching circuitry, and the first over-current protection circuitry. The controller may be configured to perform selective switching of a load current between the electromechanical switching circuitry and the solid state switching circuitry in response to a load current condition appropriate to an operational capability of a respective one of the switching circuitries.
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:
In accordance with one or more embodiments of the present invention, systems and methods for micro-electromechanical system based arc-less switching is described herein. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments of the present invention. However, those skilled in the art will understand that embodiments of the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, well known methods, procedures, and components have not been described in detail.
Furthermore, various operations may be described as multiple discrete steps performed in a manner that is helpful for understanding embodiments of the present invention. However, the order of description should not be construed as to imply that these operations need be performed in the order they are presented, nor that they are even order dependent. Moreover, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may. Lastly, the terms “comprising”, “including”, “having”, and the like, as used in the present application, are intended to be synonymous unless otherwise indicated.
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 be 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 switch 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 over current protection 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 of 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 series 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 the present invention as will be described in further detail hereinafter, the MEMS switch 20 may be rapidly switched (e.g., 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.
Moreover, for discussion of this example operation of the arc-less MEMS based switching system 18, it may be assumed that a resistance associated with the MEMS switch 20 is sufficiently small such that the voltage produced by the load current through the resistance of MEMS switch 20 has only a negligible effect on the near-zero voltage difference between the mid-points of the diode bridge 28 when pulsed. For example, the resistance associated with the MEMS switch 20 may be assumed to be sufficiently small so as to produce a voltage drop of less than a few millivolts due to the maximum anticipated load current.
It may be noted that in this initial condition of the MEMS based switching system 18, the pulse switch 54 is in a first open state. Additionally, there is no pulse circuit current in the pulse circuit 52. Also, in the pulse circuit 52, the capacitor CPULSE 56 may be pre-charged to a voltage VPULSE, where VPULSE is a voltage that can produce a half sinusoid of pulse current having a peak magnitude significantly greater (e.g., twice) the anticipated load current ILOAD 50 during the transfer interval of the load current. It may be noted that CPULSE 56 and LPULSE 58 comprise a series resonant circuit.
The pulse circuit 52 may be configured to detect the switch condition to facilitate switching the present closed state of the MEMS switch 20 to a second open state. In one embodiment, the switch condition may be a fault condition generated due to a voltage level or load current in the load circuit 40 exceeding a predetermined threshold level. However, as will be appreciated, the switch condition may also include monitoring a ramp voltage to achieve a given system-dependent ON time for the MEMS switch 20.
In one embodiment, the pulse switch 54 may generate a sinusoidal pulse responsive to receiving a trigger signal as a result of a detected switching condition. The triggering of the pulse switch 54 may initiate a resonant sinusoidal current in the pulse circuit 52. The current direction of the pulse circuit current may be represented by reference numerals 66 and 68. Furthermore, the current direction and relative magnitude of the pulse circuit current through the first diode 30 and the second diode 32 of the first branch 29 of the balanced diode bridge 28 may be represented by current vectors 72 and 70 respectively. Similarly, current vectors 76 and 74 are representative of a current direction and relative magnitude of the pulse circuit current through the third diode 34 and the fourth diode 36 respectively.
The value of the peak sinusoidal bridge pulse current may be determined by the initial voltage on the pulse capacitor CPULSE 56, value of the pulse capacitor CPULSE 56 and the value of the pulse inductance LPULSE 58. The values for the pulse inductance LPULSE 58 and the pulse capacitor CPULSE 56 also determine the pulse width of the half sinusoid of pulse current. The bridge current pulse width may be adjusted to meet the system load current turn-off requirement predicated upon the rate of change of the load current (VBUS/LLOAD) and the desired peak let-through current during a load fault condition. According to aspects of the present invention, the pulse switch 54 may be configured to be in a conducting state prior to opening the MEMS switch 20.
It may be noted that triggering of the pulse switch 54 may include controlling a timing of the pulse circuit current IPULSE 62 through the balanced diode bridge 28 to facilitate creating a lower impedance path as compared to the impedance of a path through the contacts of the MEMS switch 20 during an opening interval. In addition, the pulse switch 54 may be triggered such that a desired voltage drop is presented across the contacts of the MEMS switch 20.
In one embodiment, the pulse switch 54 may be a solid-state switch that may be configured to have switching speeds in the range of nanoseconds to microseconds, for example. The switching speed of the pulse switch 54 should be relatively fast compared to the anticipated rise time of the load current in a fault condition. The current rating required of the MEMS switch 20 may be dependent on the rate of rise of the load current, which in turn is dependent on the inductance LLOAD 46 and the bus supply voltage VBUS 44 in the load circuit 40 as previously noted. The MEMS switch 20 may be appropriately rated to handle a larger load current ILOAD 50 if the load current ILOAD 50 may rise rapidly compared to the speed capability of the bridge pulse circuit.
The pulse circuit current IPULSE 62 increases from a value of zero and divides equally between the first and second branches 29, 31 of the balanced diode bridge 28. In accordance with one embodiment, the difference in voltage drops across the branches 29, 31 of the balanced diode bridge 28 may be designed to be negligible, as previously described. Further, as previously described, the diode bridge 28 is balanced such that the voltage drop across the first and second branches of the diode bridge 28 are substantially equal. Moreover, as the resistance of the MEMS switch 20 in a present closed state is relatively low, there is a relatively small voltage drop across the MEMS switch 20. However, if the voltage drop across the MEMS switch 20 happened to be larger (e.g., due to an inherent design of the MEMS switch), the balancing of the diode bridge 28 may be affected as the diode bridge 28 is operatively coupled in parallel with the MEMS switch 20. In accordance with aspects of the present invention, if the resistance of the MEMS switch 20 causes a significant voltage drop across the MEMS switch 20 then the diode bridge 28 may accommodate the resulting imbalance of the pulse bridge by increasing the magnitude of the peak bridge pulse current.
Referring now to
Once the amplitude of the pulse circuit current IPULSE 62 becomes greater than the amplitude of the load circuit current ILOAD 50 (e.g., due to the resonant action of the pulse circuit 52), a voltage applied to the gate contact 26 of the MEMS switch 20 may be appropriately biased to switch the present operating state of the MEMS switch 20 from the first closed and conducting state to an increasing resistance condition in which the MEMS switch 20 starts to turn off (e.g., where the contacts are still closed but contact pressure diminishing due the switch opening process) which causes the switch resistance to increase which in turn causes the load current to start to divert from the MEMS switch 20 into the diode bridge 28.
In this present condition, the balanced diode bridge 28 presents a path of relatively low impedance to the load circuit current ILOAD 50 as compared to a path through the MEMS switch 20, which now exhibits an increasing contact resistance. It may be noted that this diversion of load circuit current ILOAD 50 through the MEMS switch 20 is an extremely fast process compared to the rate of change of the load circuit current ILOAD 50. As previously noted, it may be desirable that the values of inductances L1 84 and L2 88 associated with connections between the MEMS switch 20 and the balanced diode bridge 28 be very small to avoid inhibition of the fast current diversion.
The process of current transfer from the MEMS switch 20 to the pulse bridge continues to increase the current in the first diode 30 and the fourth diode 36 while simultaneously the current in the second diode 32 and the third diode 34 diminish. The transfer process is completed when the mechanical contacts 22, 24 of the MEMS switch 20 are separated to form a physical gap and all of the load current is carried by the first diode 30 and the fourth diode 36.
Consequent to the load circuit current ILOAD being diverted from the MEMS switch 20 to the diode bridge 28 in direction 86, an imbalance forms across the first and second branches 29, 31 of the diode bridge 28. Furthermore, as the pulse circuit current decays, voltage across the pulse capacitor CPULSE 56 continues to reverse (e.g., acting as a “back electro-motive force”) which causes the eventual reduction of the load circuit current ILOAD to zero. The second diode 32 and the third diode 34 in the diode bridge 28 become reverse biased which results in the load circuit now including the pulse inductor LPULSE 58 and the bridge pulse capacitor CPULSE 56 and to become a series resonant circuit.
Turning now to
Additionally, a significant switch contact voltage difference from the drain 24 to the source 26 of the MEMS switch 20 may now rise to a maximum of approximately twice the VBUS voltage at a rate determined by the net resonant circuit which includes the pulse inductor LPULSE 58, the pulse capacitor CPULSE 56, the load circuit inductor LLOAD 46, and damping due to the load resistor RLOAD 48 and circuit losses. Moreover, the pulse circuit current IPULSE 62, that at some point was equal to the load circuit current ILOAD 50, may decrease to a zero value due to resonance and such a zero value may be maintained due to the reverse blocking action of the diode bridge 28 and the diode DP 60. The voltage across the pulse capacitor CPULSE 56 due to resonance would reverse polarity to a negative peak and such a negative peak would be maintained until the pulse capacitor CPULSE 56 is recharged.
The diode bridge 28 may be configured to maintain a near-zero voltage across the contacts of the MEMS switch 20 until the contacts separate to open the MEMS switch 20, thereby preventing damage by suppressing any arc that would tend to form between the contacts of the MEMS switch 20 during opening. Additionally, the contacts of the MEMS switch 20 approach the opened state at a much reduced contact current through the MEMS switch 20. Also, any stored energy in the circuit inductance, the load inductance and the source may be transferred to the pulse circuit capacitor CPULSE 56 and may be absorbed via voltage dissipation circuitry (not shown). The voltage snubber circuit 33 may be configured to limit voltage overshoot during the fast contact separation due to the inductive energy remaining in the interface inductance between the bridge and the MEMS switch. Furthermore, the rate of increase of reapply voltage across the contacts of the MEMS switch 20 during opening may be controlled via use of the snubber circuit (not shown).
It may also be noted that although a gap is created between the contacts of the MEMS switch 20 when in an open state, a leakage current may nonetheless exist between the load circuit 40 and the diode bridge circuit 28 around the MEMS switch 20. This leakage current may be suppressed via introduction of a secondary mechanical switch (not shown) series connected in the load circuit 40 to generate a physical gap. In certain embodiments, the mechanical switch may include a second MEMS switch.
Referring now to
As indicated by block 114, a current level in a load circuit, such as the load circuit 40 (see
If the sensed current level or sensed voltage level varies or departs from an expected value, a switch condition may be generated as indicated by block 118. As previously noted, the term “switch condition” refers to a condition that triggers changing a present operating state of the MEMS switch. In certain embodiments, the switch condition may be generated responsive to a fault signal and may be employed to facilitate initiating opening of the MEMS switch. It may be noted that blocks 114-118 are representative of one example of generating a switch condition. However as will be appreciated, other methods of generating the switch condition are also envisioned in accordance with aspects of the present invention.
As indicated by block 120, the pulse circuit may be triggered to initiate a pulse circuit current responsive to the switch condition. Due to the resonant action of the pulse circuit, the pulse circuit current level may continue to increase. Due at least in part to the diode bridge 28, a near-zero voltage drop may be maintained across the contacts of the MEMS switch if the instantaneous amplitude of the pulse circuit current is significantly greater than the instantaneous amplitude of the load circuit current. Additionally, the load circuit current through the MEMS switch may be diverted from the MEMS switch to the pulse circuit as indicated by block 122. As previously noted, the diode bridge presents a path of relatively low impedance as opposed to a path through the MEMS switch, where a relatively high impedance increases as the contacts of the MEMS switch start to part. The MEMS switch may then be opened in an arc-less manner as indicated by block 124.
As previously described, a near-zero voltage drop across contacts of the MEMS switch may be maintained as long as the instantaneous amplitude of the pulse circuit current is significantly greater than the instantaneous amplitude of the load circuit current, thereby facilitating opening of the MEMS switch and suppressing formation of any arc across the contacts of the MEMS switch. Thus, as described hereinabove, the MEMS switch may be opened at a near-zero voltage condition across the contacts of the MEMS switch and with a greatly reduced current through the MEMS switch.
Response curve 142 represents a variation of amplitude of the load circuit current as a function of time. A variation of amplitude of the pulse circuit current as a function of time is represented in response curve 144. In a similar fashion, a variation of amplitude of gate voltage as a function of time is embodied in response curve 146. Response curve 148 represents a zero gate voltage reference, while response curve 150 is the reference level for the load current prior to turn-off.
Additionally, reference numeral 152 represents region on the response curve 142 where the process of switch opening occurs. Similarly, reference numeral 154 represents a region on the response curve 142 where the contacts of the MEMS switch have parted and the switch is in an open state. Also, as can be seen from the second section 138 of the graphical representation 130, the gate voltage is pulled low to facilitate initiating opening of the MEMS switch. Furthermore, as can be seen from the third section 140 of the graphical representation 130, the load circuit current 142 and the pulse circuit current 144 in the conducting half of the balanced diode bridge are decaying.
Aspects of the present invention comprise circuitry and/or techniques that reliably and cost-effectively enable to withstand a surge current (e.g., during a start up event or a transient condition) with solid state (e.g., semiconductor-based) switching circuitry while able to, for example, utilize MEMS-based switching circuitry for steady state operation and for addressing fault conditions that may arise.
As will be appreciated by one skilled in the art, the surge current may arise when starting up an electrical load, such as a motor or some other type of electrical equipment, or may arise during a transient condition. The value of the surge current during a start up event often comprises multiple times (e.g., six times or more) the value of the steady state load current and can last for several seconds, such as in the order of ten seconds.
A controller 208 may be coupled to MEMS-based switching circuitry 202, solid-state switching circuitry 204, and over-current protection circuitry 206. Controller 208 may be configured to selectively transfer current back and forth between the MEMS-based switching circuitry and the solid state switching circuitry by performing a control strategy configured to determine when to actuate over-current protection circuitry 206, and also when to open and close each respective switching circuitry, such as may be performed in response to load current conditions appropriate to the current-carrying capabilities of a respective one of the switching circuitries and/or during fault conditions that may affect the switching system. It is noted that in such a control strategy it is desirable to be prepared to perform fault current limiting while transferring current back and forth between the respective switching circuitries 202 and 204, as well as performing current limiting and load de-energization whenever the load current approaches the maximum current handling capacity of either switching circuitry.
A system embodying the foregoing example circuitry may be controlled such that the surge current is not carried by MEMS based switching circuitry 202 and such a current is instead carried by solid-state switching circuitry 204. The steady-state current would be carried by MEMS based switching circuitry 202, and over-current and/or fault protection would be available during system operation through over-current protection circuit 206. It will be appreciated that in its broad aspects the proposed concepts need not be limited to MEMS-based switching circuitry. For example, a system comprising one or more standard electromechanical switches (i.e., not MEMS-based electromechanical switching circuitry) in parallel with one or more solid state switches and a suitable controller may similarly benefit from the advantages afforded by aspects of the present invention.
Below is an example sequence of switching states as well as example current values in the switching system upon occurrence of a load starting event. The letter X next to a number indicates an example current value corresponding to a number of times the value of a typical current under steady state conditions. Thus, 6× denotes a current value corresponding to six times the value of a typical current under steady state conditions.
As will be appreciated by one skilled in the art, the voltage drop across an inverse-series connected pair of MOSFETs is the IR drop of two Rdson (on-resistance) switches, in lieu of one Rdson plus a voltage diode drop, as would be the case in an inverse-parallel arrangement. Thus, in one example embodiment an inverse-series configuration of MOSFETs may be desirable since it has the capability of providing a relatively lower voltage drop, hence lower power dissipation, heat, and energy loss.
It will be further appreciated that in one example embodiment wherein solid state switching circuitry 204 comprises a bidirectional thyristor (or an inverse-parallel pair of thyristors) while this arrangement may incur relatively higher losses at lower currents, such an arrangement would have the advantage of being able to withstand a relatively higher short-term current surge because of the relatively lower voltage drop at high currents, and the transient thermal response characteristics.
After the initial start-up current has subsided to a suitable level, MEMS-based switching circuitry 202 may be turned on using a suitable MEMS-compatible switching technique, or by closing into the voltage that is dropped across the solid-state switching circuitry provided such voltage drop comprises a relatively small voltage. At this point, FET switches 210 and 219 can be turned off.
As will be appreciated by one skilled in the art, MEMS-based switching circuitry should not be closed to a conductive switching state in the presence of a voltage across its switching contacts nor should such circuitry be opened into a non-conductive switching state while passing current through such contacts. One example of a MEMS-compatible switching technique may be a pulse-forming technique as described and/or illustrated in the context of
Another example of a MEMS-compatible switching technique may be achieved by configuring the switching system to perform soft or point-on-wave switching whereby one or more MEMS switches in the switching circuitry 202 may be closed at a time when the voltage across the switching circuitry 202 is at or very close to zero, and opened at a time when the current through the switching circuitry 202 is at or close to zero. For readers desirous of background information regarding such a technique reference is made to patent application titled “Micro-Electromechanical System Based Soft Switching”, U.S. patent application Ser. No. 11/314,879 filed Dec. 20, 2005, (Attorney Docket No. 162191-1, which application is incorporated herein by reference in its entirety.
By closing the switches at a time when the voltage across the switching circuitry 202 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. As alluded to above, control circuitry may be configured to synchronize the opening and closing of the one or more MEMS switches of the switching circuitry 202 with the occurrence of a zero crossing of an alternating source voltage or an alternating load circuit current. Should a fault occur during a start up event, over-current protection circuitry 206 is configured to protect the down stream load as well as the respective switching circuitries. As illustrated in
It is noted that although electromechanical and solid-state switching circuitry when viewed at a top level may in concept appear to behave substantially similar to one another, in practice, however, such switching circuitry may exhibit respective distinct operational characteristics since they operate based on substantially different physical principles and thus the over-current protection circuitry may have to be appropriately configured to account for such characteristics and still appropriately actuate the switching circuitry. For instance, a MEMS switch generally involves a mechanical movement of a cantilever beam to break contact, whereas a solid-state switch generally involves movement of charge carriers in a voltage-induced channel. The time it takes to clear the channel of the carriers is called the recovery time, and this recovery time can range from a time of <1 μs to a time >100 μs. For instance, if the solid-state switch is closed into a fault, then over-current protection circuitry 206 should be able to absorb the fault current and protect the solid-state switch and the down stream load until the switch's channel is fully cleared and the switch is fully open. In the event over-current protection circuitry 206 comprises a pulse circuit 52 and a balanced diode bridge 31, it can be shown that the pulse characteristics (such as the width and/or height of a pulse formed by the pulse circuit) could affect the quality of down stream protection. For example, over-current protection circuitry 206 should be able to generate a pulse having sufficient width and/or height to accommodate the recovery time of the parallel solid-state switching circuitry as well as accommodate the fault protection for the MEMS based switching circuitry.
As will be appreciated by those skilled in the art, there are two general categories of solid state switching circuitry, with regard to fault current interruption. Some solid state switches (such as FETs) can inherently force a zero current condition when turned off. Others (such as SCRs) cannot force such a zero current condition. Solid state switching circuitry that can force a zero current condition may not need the aid of over-current protection circuitry 206 to perform current limiting during a fault. Solid state switching circuitry that cannot force a zero current condition will generally require an over-current protection circuitry 206.
As previously mentioned, a suitable control technique should be implemented to selectively transfer current back and forth between the MEMS-based switching circuitry and the solid state switching circuitry. In one example embodiment, such a control technique may be based on a respective electrical loss model for each switching circuitry. For instance, electrical losses (and concomitant temperature rise) in MEMS-based switching circuitry are generally proportional to the square of the load current, while losses (and concomitant temperature rise) in solid state switching circuitry are generally proportional to the absolute value of load current. Also, the thermal capacity of solid state devices is generally greater than that of MEMS-based switching circuitry. Accordingly, for normal values of load current, it is contemplated that the MEMS-based switching circuitry will carry the current, while, for temporary overload currents, it is contemplated for the solid state switching circuitry to carry the current. Thus, it is contemplated to transfer current back and forth during transient overload situations.
We will discuss below, three example techniques for selectively transferring load current back and forth between the MEMS-based switching circuitry and the solid state switching circuitry. One example technique contemplates use of dual over-current protection circuitry, such as shown in
It is noted that if the switching system uses just a single over-current protection circuitry 206, then such a single over-current protection circuitry would be activated upon a switching event in connection with the MEMS-based switching circuitry. However, if shortly thereafter a fault were to occur, then the single over-current protection circuitry 206 may not be ready to be reactivated to protect the switching circuitry. As described above, over-current protection circuitry 206 operates based on pulsing techniques, and thus such circuitry would not be instantaneously ready to operate shortly upon a pulse firing. For example, one would have to wait some period of time to recharge the pulse capacitor in pulse circuit 52.
The technique involving dual over-current protection circuitry ensures leaving one over-current protection circuitry (e.g., circuitry 2062) free and ready to assist current limiting in the event of a fault, even when the other over-current protection circuitry 2061 has just performed a pulse-assisted switching in connection with a normal switching event (non-fault driven switching event). This technique is believed to provide substantial design flexibility with a relatively simpler control, but requires dual over-current protection circuitry instead of a single over-current protection circuitry. It is noted that this technique is compatible with any type of solid state switching circuitry.
It will be appreciated that in an example embodiment that comprises dual over-current protection circuitry, then such circuitry should include dual pulse circuits 52 but need not include dual balanced diode bridges 31. For example, if the first over-current protection circuitry comprises a respective pulse circuit 52 and a respective balanced diode bridge 31, then the second over-current protection circuitry may just comprise a respective pulse circuit 52 configured to apply a suitable pulse current (when needed) to the balanced diode bridge 31 of the first over protection circuit. Conversely, if the second over-current protection circuitry comprises a respective pulse circuit 52 and a respective balanced diode bridge 31, then the first over-current protection circuitry may just comprise a respective pulse circuit 52 configured to apply a suitable pulse current (when needed) to the balanced diode bridge 31 of the second over protection circuit.
A second example technique is to time the execution of the transfer to coincide with a current zero. This eliminates the need for a second over-current protection circuitry, and is also compatible with any type of solid state switching circuitry. However, this technique may involve relatively more elaborate control and could require a complete shut-off of the system in some cases. A third example technique is to perform the current transfer by coordinating the opening and closing of the MEMS switching circuitry and the solid state switching circuitry. As will be appreciated by one skilled in the art, this technique can be used provided the solid state switching circuitry has a relatively small voltage drop.
In any case, it should be appreciated that the control strategy may be configured to determine when to operate the over-current protection circuitry (either single or dual over-current protection circuitry) and to determine when to open and close the respective switching circuitries, such as in response to load current conditions appropriate to the current-carrying capabilities of a respective one of the switching circuitries. The general concept is to be prepared to perform fault current limiting while transferring current back and forth between alternate current paths, as well as performing current limiting and circuit de-energization when the load current approaches the maximum capacity of either load current carrying path. One example control strategy may be as follows:
Use the solid state switching circuitry to energize the load, on the expectation that there will be a large initial current. Transfer the load over to the MEMS-based switching circuitry after the current falls within the rating of the MEMS-based switching circuitry.
When it is desired to de-energize the load under normal conditions, do so with whatever switching circuitry is carrying the current at that time. If it is the MEMS-based switching circuitry, use point-on-wave switching to turn off at current zero.
Based on simulated or sensed temperatures, determined the respective temperature of both the MEMS-based switching circuitry and the solid state switching circuitry. If any of such temperatures is determined to be approaching a respective thermal ratings limit, or if the load current is approaching a respective maximum current carrying capability, (such as under fault conditions or a severe overload) perform an instantaneous current interruption (assisted with the over-current protection circuitry and open both the MEMS-based switching circuitry and the solid state switching circuitry. This action would pre-empt any other control action. Wait for a reset before allowing a re-close switching action.
Under normal operation, the respective thermal conditions of each respective switching circuitry may be used to determine whether to pass current through the MEMS-based switching circuitry or through the solid state switching circuitry. If one switching circuitry is approaching its thermal or current limit while the other switching circuitry still has thermal margin, a transfer may be automatically made. The precise timing would depend on the switching transfer technique. For instance, in a pulse-assisted transfer, the transfer can take place essentially instantaneously as soon as the transfer is needed. In a transfer based on point-on-wave switching, such a transfer would be performed (e.g., deferred) until a next available zero crossing of the current occurs. For a deferred transfer, there should be some margin provided in the setting for the decision to transfer in order to make it likely that the transfer can be successfully deferred until the next current zero.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
