Abstract:
A dynamic switch contact protection circuit and technique to protect a channel switch within an electrical system by limiting transients when the switch is turned on or turned off. The protection circuit comprises switching between a high resistance path and a low resistance path. The high resistance path comprises a resistor. A bypass switch is connected in parallel to the resistor to affect the low resistance path. The protection circuit can connect or disconnect switch cards to the electrical system enabling the creation of a larger switching structure. Disconnected switch cards within a switching structure preserves system bandwidth by limiting capacitive loading. Electing which switch to close last or open first can prolong the length of usage of the switches.

Description:
BACKGROUND OF THE INVENTION 
     Various types of switches are used in electronic test and measurement systems to switch or route signals between a stimulus, measuring instruments, and devices-under-test (DUTs). Often the switches that switch the fastest at the lowest currents and voltages have physical contacts that are susceptible to damage. Damage, or abuse, can be the result of electrical transient effects that occur during the make or break cycle of the switch. Measurement can be defined as the process of testing or exercising a circuit using both sources and detectors. 
     Switches common in the art are reed relays, armature relays, and electronic or solid-state switches. 
     Reed relays are used in switching platforms that employ thousands of such switches. Replacing thousands of relays at regular intervals or replacing selected worn out relays is an undesirable expense. 
     While these switch contacts can be protected by the use of current-limiting resistors in the path of the switch, the resistors sometimes add undesired impedance to a circuit. 
     A majority of the abuse happens to the switch closed last and the switch opened first for a given circuit. The abuse mechanism is different in each case. The mechanism on closure results from a voltage differential across the open contacts. As the switch closes, very high currents can flow from circuit capacitances damaging the switch. Damage on switch closure is due primarily to an excessive surge in current, also called in-rush current. This occurs when the voltage across the contacts equalizes while the instrument and channel capacitances charge or discharge. Small micro-welds develop on the surface of the switch contacts over time and the contacts become irregular and pitted. Subsequently, the contacts develop a higher resistance. Catastrophic failure during closure for most switches is a stuck close condition. 
     Damage on opening the switch is due primarily to an excessive surge in voltage causing arcing. Current flowing through the inductance of a closed circuit path cannot change instantaneously. As the switch opens, an arc forms to dissipate the energy stored in the inductance. This arc also damages the switch contacts. This arc causes contact pitting in relays as the contacts separate due to excessive power and heat in a very small area. Contact resistance for relays will typically continue to increase as they are repeatedly subjected to arcing. There is no typical failure mode for solid-state switches subjected to an arc, but it can be catastrophic. 
     One method limiting the in-rush current is to place series resistance in the switch circuit. However many applications cannot accommodate fixed resistance in the path of the switch however. Some switch cards provide both current limited and non-current limited channels while other cards provide methods defeating the current limit on selected channels by installing shorts across the current-limit resistors. Other methods of minimizing in-rush current include minimizing the voltage difference or reducing the capacitance across the open switch contacts, neither of which may be practical. Increasing the series resistance of the path decreases the peak current. This decrease in current lessens the energy stored by the inductance by the square of the current and minimizes the damage. Techniques for minimizing arcing include minimizing the circuit inductance and adding “snubber circuits” across the contacts. Snubbers are circuits that provide a transient path for the current flow immediately after the circuit is opened. Snubbers are common for switching involving high current and/or large inductance, but they are not a universal solution. They place a large residual capacitance across the switch contacts that results in in-rush current on closure and a frequency-dependent leakage path when the switch is open. 
     Configuring switch cards to form a larger switching structure is desirable when designing large systems using modular blocks. Merely adding additional switching structures in parallel can cause a problem with the additional loading the switches represent. Not all switch cards provide a feature to connect and disconnect a switch card (channel switching structure) to or from the electrical system. In some cases, this option is not desirable as it doubles the number of switch contacts in series with the measuring instruments and the amount of cabling and interconnect required to add it often makes it prohibitive. 
     Accordingly, a need exists to prevent damage to switches without permanently adding resistance to the measurement path, and providing a feature to easily disconnect a switch card from the electrical system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation of an electrical system incorporating a dynamic protection circuit; 
         FIG. 2  is a schematic of dynamic protection circuit embodiments within the electrical system; 
         FIG. 3  is a flow chart showing steps for operating the dynamic protection circuit; 
         FIG. 4  is a flow chart showing steps on using the dynamic protection to close a channel switch within an electrical system; and 
         FIG. 5  is a flow chart showing steps on using the dynamic protection to open a channel switch within an electrical system. 
     
    
    
     DETAILED DESCRIPTION 
     A dynamic switch contact protection (“dynamic protection”) circuit and technique is described herewith as a solution to the problems described above. The dynamic protection comprises switching to a high resistance path to reduce the electrical transient effects when opening and closing a channel switch. The dynamic protection also comprises switching to a low resistance path for connecting the electronic devices through the channel switch for making measurements with the electronic devices. The dynamic protection further comprises disconnecting a channel switching structure to isolate the channel switching structure from the electrical system. 
     The dynamic protection technique uses a current-limiting resistor in series with the channel switch. The added resistance of the current-limiting resistor lessens the abuse (described above), thereby protecting the channel switches of the measurement path, and increasing their usable lifetimes. When sequenced, the technique provides current-limiting protection and low path impedance. 
     An embodiment of a dynamic protection circuit comprises two switches, and a current-limiting resistance. In addition to protecting the contacts of the channel switch, the technique also provides the ability to disconnect a sub-switching structure from the electrical system, thereby isolating the channel switches from the larger switching structure. 
     With dynamic protection, peak currents are limited, thereby reducing the abuse to the switch contacts. As described above, damage to switch contacts is manifested as welds for relays or punch-throughs on solid-state devices. 
     Making the protection dynamic allows the current-limiting resistor to be available during the switching, but eliminated during steady-state operation. As some circuit applications can operate with the current-limiting resistor, the solutions described within also support such a condition. 
       FIG. 1  is a diagrammatic representation of an electrical system  101  comprising electronic devices  171 , a switching structure  105 , and second group of electronic devices  131 . The switching structure  105  comprises a dynamic protection circuit  103  and a channel switching structure  109 . The channel switching structure  109  comprises an array of channel switches (not shown). The electronic devices  171  can be measuring instruments and second group of electronic devices  131  can be DUTs. 
     The electronic devices  171  are connected to the protection circuit  103  of the switching structure  105  via a wire bus of size p. The channel switches in the channel switching structure  109  are connected to the second group of electronic devices  131  via a wire bus of size q. Generally, the electrical system  101  can accommodate 1-to-many, many-to-1, 1-to-1, or many-to-many switching configurations. 
     The protection circuit  103  identifies three paths of the dynamic protection technique; a path of high resistance  141 , a low resistance path  143 , and a state  145  that disconnects the switching structure  105  from the electrical system  101 . A measurement path is defined as a path from one electronic device  171  through the protection circuit  103 , through a channel switch in the channel switching structure  109  and to a second electronic device  131 . Measurement paths can encompass more than one channel switch. 
       FIG. 1  also describes computer readable media  161  containing code for providing instructions to and for execution by the electrical system  101 . The computer readable media  161  can be, for example, a ROM, a RAM, a DVD, a hard drive, or other computer readable media known in the art. The instructions are embedded in firmware to control the sequence of switches in the dynamic protection circuit  103  and the channel switches in the channel switching structure  109 . 
       FIG. 2  is a schematic of the electrical system  101  of  FIG. 1 . The electrical system  101  in  FIG. 2  comprises a measuring instrument  271 , and two sub-switching structures  205  and  207  within the switching structure  105 . The sub-switching structures each have a channel switching structure  209  and  211 . The channel switching structure  209  comprises n channels of at least n switches. Similarly, the channel switching structure  211  comprises m channels of at least m switches. Channel switches  213  and  215  are individual switches identified as Channel i in the channel switching structures  209  and  211  respectively. 
     A DUT  231  is connected to the output of the channel switching structures  209  and  211  via a wire bus of size of n+m. The sub-switching structures  205  and  207  in  FIG. 2  have been configured within a larger switching structure  105  to increase the number of channel switches connectable to the DUT  231 . The electrical system  101  has the flexibility of disconnecting the sub-switching structures  205  and  207  so as not to affect bandwidth performance (described in detail later). 
     Two embodiments of the dynamic protection circuits  103  of  FIG. 1  exist in  FIG. 2 , one in each sub-switching structure  205  and  207 . The dynamic protection circuits  201  and  203  each comprise two protection switches and a current-limiting resistor. The current-limiting resistor can typically be 50 Ohms, 100 Ohms, 1 kOhm or larger, to limit in-rush current. 
     The protection circuit  201  has a circuit topology wherein a current-limiting resistor  221  and a bypass switch  223  are connected in parallel. A common switch  225  is connected in series with the parallel components. 
     The protection circuit  203  has a circuit topology wherein a common switch  235  connected in series with a current-limiting resistor  231 . A bypass switch  233  connected in parallel with the series components. 
       FIGS. 3 ,  4  and  5  are flow charts showing steps for operating the dynamic protection circuits.  FIG. 3  is an overview of the dynamic protection technique and corresponds to  FIG. 1 .  FIGS. 4 and 5  provide a detailed flow of the dynamic protection technique corresponding to the protection circuits  201  and  203  of  FIG. 2 . 
     In  FIG. 3 , Block  310  describes switching the measurement path to a high resistance path  141  to reduce the electrical transient effects when opening and closing a channel switch within the channel switching structure  109 . When the high resistance path  141  is selected, the sub-switching structure is configured into the switching structure  105  of the electrical system  101 . This is also referred to as the high resistance state. 
     Block  320  describes switching to the low resistance path  143  for connecting the electronic devices  171  through the channel switch for making measurements with the second group of electronic devices  131 . When the low resistance path  143  is selected, the switching structure  105  is configured into the electrical system  101 . This is also referred to as the low resistance state. 
     Block  330  describes performing measurements on the DUT. 
     Block  335  describes connecting to the high resistance path  141  to prepare either to open a channel switch or to disconnect a sub-switching structure from the switching structure  105 . 
     Block  340  describes disconnecting the channel switching structure  109  (or sub-switching structure) from the switching structure  105  of the electrical system  101 . This is described in further detail below. This is also referred to as the disconnected state. 
       FIGS. 4 and 5  are flow charts describing steps of applying the dynamic protection technique corresponding to the two embodiments shown in  FIG. 2 . These steps can be executed manually or by an automated process. The sequence of operation to close a measurement path is illustrated in  FIG. 4 . The sequence of operation to open a measurement path is illustrated in  FIG. 5 . 
     A measurement path  217  or  219  is identified in general nomenclature by Channel i  213  and  215 . The descriptions that follow identify steps for turning on ( FIG. 4 ) and turning off ( FIG. 5 ) the channel switch Channel i  213  and measurement path  217  of the first sub-switching structure  205 . Comments for the channel switch Channel i  215  and the measurement path  219  of the second sub-switching structure  207  are in parenthesis, unless where noted otherwise. 
     In  FIG. 4 , Block  400  describes a state wherein all switches in the channels channel switching structure  209  (or  211 ) are open and the sub-switching  205  (or  207 ) is in a disconnected state. 
     Block  405  describes introducing the current-limiting resistor  221  (or  231 ) into the measurement path. This can be achieved by closing the channel switch  213  (or  215 ) and the common switch  225  (or  235 ) to enter the high resistance state. 
     Block  410  describes waiting a measured time to allow the voltage on the instrument  271  and associated capacitance to charge (or discharge) to the voltage on the DUT  231  and associated capacitance through the established measurement path  217  (or  219 ) of the channel switch  213  (or  215 ), current limiting resistor  221  (or  231 ), and the common switch  225  (or  235 ). 
     The circuit can be modeled as a pair of charged capacitors interconnected by a series resistance that exhibits a well-defined exponential voltage charge/discharge model. However, the wait time associated with this step is rarely calculated. The actual capacitances, voltages, and resistance usually may not be known, and an estimate to produce a nominal wait time sufficient to safely enter the low resistance state is acceptable. 
     Block  415  describes closing the bypass switch  223  (or  233 ) to bypass current around the current-limiting resistor  221  (or  231 ) to enter the low resistance state (analogous to Block  320  of  FIG. 3 ). 
     Block  420  describes an option of opening the common switch  235  of the second sub-switching structure  207  to minimize the power dissipation in the circuit. 
     Block  425  describes commencing the measurement on the channel using channel switch  213  (or  215 ), if desired. 
     Additional channel switches can be closed by repeatedly executing the following the steps of Blocks  430 - 460 . 
     Block  430  describes closing the common switch  235  of the second sub-switching structure  207  if the step in Block  420  was executed. 
     Block  435  describes opening the bypass switch  223  (or  233 ) to enter the high resistance state. 
     Block  440  describes closing the additional channel switch  213  (or  215 ). 
     Block  445  describes waiting a sufficient amount of time to allow a charge balance to occur between the instrument  271  and additional channel capacitance through the current-limiting resistor  221  (or  231 ). Different channels have different amounts of capacitance associated with them, but typically all close to the same. 
     Block  450  describes closing the bypass switch  223  (or  233 ) around the current-limiting resistor  221  (or  231 ) to enter the low resistance state. 
     Block  455  describes an option of opening the common switch  235  of the second sub-switching structure  207  to minimize the power dissipation in the circuit. 
     Block  460  describes continuing measuring parts of the DUT  231  connected to the channel switches closed by the steps described above. 
     In  FIG. 5 , Block  500  identifies a state wherein measurements have recently been completed utilizing measurement paths set up by more than one channel switch. As measurements have recently concluded, the channel switches and bypass switch  223  (or  233 ) are presently closed. The closed channel switch(s) and the bypass switch represents the low resistance state. 
     Block  505  describes closing the common switch  235  of the second sub-switching structure  207  if the step in Block  420  or  455  was executed. 
     Block  510  describes opening the bypass switch  223  (or  233 ) to divert the channel current through the current-limiting resistor  221  (or  231 ) to enter the high resistance state. 
     Block  515  describes waiting a sufficient time to allow the inductance in the measurement circuit to discharge to a lower current through the current-limiting resistor  221  (or  231 ). Similar to the prior wait in Block  410 , this time delay can also be accurately modeled as an exponential current decay, but is usually estimated or bounded to a nominal wait time sufficient to safely open the channel switch of the circuit. 
     Block  520  describes opening the desired channel switch  213  (or  215 ). 
     Block  525  describes closing the bypass switch  223  (or  233 ) to enter the low resistance state. 
     Block  530  describes an option of opening the common switch  235  of the second sub-switching structure  207  to minimize the power dissipation in the circuit. 
     The steps in Blocks  505 - 530  are repeated to open all but one of the remaining channel switches (that are closed). 
     The following steps describe actions to open the last channel switch  213  (or  215 ). 
     Block  535  describes continuing with measurements if required. 
     The following steps describe opening the last closed channel switch in the channel switching structure  209  (or  211 ). Block  540  describes closing the common switch  235  of the second sub-switching structure  207  if the step in Block  530  was executed. 
     Block  545  describes opening the bypass switch  223  (or  233 ) diverting the channel current through the current-limiting resistor  221  (or  231 ) to enter the high resistance state. 
     Block  550  describes waiting sufficient time to allow the inductance in the circuit to discharge to a lower current through the current-limiting resistor  221  (or  231 ). 
     Block  555  describes opening the desired channel switch  213  (or  215 ) and the common switch  225  (or  235 ), effectively disconnecting the channel switching structure  209  (or  211 ) and the sub-switching structure  205  (or  207 ), and entering the disconnected state. This last step provides detail to Block  340  of  FIG. 3 , wherein it describes disconnecting the channel switching structure  109  (or sub-switching structure) from the switching structure  105  of the electrical system  101 . 
     The order of closing the common switch  225  (or  235 ) and the channel switch  213  (or  215 ) in Block  405  of  FIG. 4  is an additional design measure to protect the contact points of the channel switch  213  (or  215 ) and the common switch  225  (or  235 ). In this instance, the switch closed last will bear the most abuse. 
     This sequence order can be implemented by firmware in an automated system. The firmware is stored on the computer readable media  161 . Described above, the firmware can control the sequence of switches in the dynamic protection circuit  103  and the channel switches in the channel switching structure  109 . 
     The abuse encountered during closure of switches can be focused on the common switch  225  (or  235 ) of the protection circuit  201  (or  203 ). With this decision, the common switch can be made more robust than the channel switches. They can subsequently be made easier to replace through initial design and preventative maintenance measures. Alternatively, this abuse can be distributed across the channel relays  217  (or  219 ) to yield uniform abuse across all the switches of the electrical system  101 . 
     A similar consideration applies for opening the relevant switches (the channel switch  213  (or  215 ) and the common switch  225  (or  235 )) when executing the steps in Block  555  of  FIG. 5 . In this instance, the switch opened first bears the most abuse. 
     The sub-switching structure  205  (or  207 ) of  FIG. 2  is disconnected from the electrical system  101  by turning off the protection switches in the protection circuit  201  (or  203 ). This option preserves bandwidth performance (reduced loading) when connecting together (in parallel) multiple sub-switching structures  205  or  207 . The bandwidth performance is preserved when the idle sub-switching structures  205  or  207  are disconnected from the electrical system  101 . This is significant as large matrices or multiplexers could have many channel relays (for example 16, 32, or 64 switches) connected to the electronic devices  271  and  231 . Without disconnecting an idle sub-switching structure, the unused channel switches would load the electrical system  101 . This would lower the bandwidth performance due to capacitance loading. 
     Channels within the switching structure  105  can be configured in parallel to form a larger switching structure using the protection circuits  201  and  203  of  FIG. 2 . For example, four 1-to-16 multiplexers in a sub-switching structure can be configured as a 1-to-64 multiplexer by paralleling the channel switches in each multiplexer. This will have approximately the same performance as each 1-to-16 multiplexer as idle multiplexers can be disconnected when not in use. 
     Paralleling channels across multiple cards can also be configured as a larger switching structure using the protection circuits of  FIG. 2 . For example, four sub-switching structures, each with four 1-to-16 multiplexers, can first be configured as 1-to-64 multiplexer on each card, as shown above. Subsequently, the four sub-switching structures can be expanded to a 1-to-256 larger switching structure by paralleling the sub-switching structures. Similarly, the 1-to-256 measuring circuit can approximate the same performance as a single 1-to-16 multiplexer when the idle cards or are disconnected from the electrical system  101 . 
     While the embodiments described above constitute exemplary embodiments of the invention, it should be recognized that the invention can be varied in numerous ways without departing from the scope thereof. It should be understood that the invention is only defined by the following claims.