Abstract:
A switch protection circuit includes a discharging circuit and, optionally, a clamping circuit. The discharge circuit operates prior to the switch completing the switching action to discharge capacitance from a signal line of a cable connected to a device under test to a ground voltage. When not discharging, the discharge circuit presents low leakage to a measurement circuit so as not to interfere with such measurement. If present, the clamping circuit clamps a signal line of the cable to a guard structure of the cable so that the discharge circuit can couple both the signal line and the guard structure to ground. The protection circuit operates without significantly worsening low current performance of the measurement instrument.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 61/915,910, filed Dec. 13, 2013, entitled “RELAY HOT SWITCH PROTECTION CIRCUIT.” 
    
    
     FIELD OF THE INVENTION 
     This disclosure is directed to protection circuits, and, more particularly, to protection circuits for switch systems that use switches such as electromechanical relays to connect test instruments to devices under test (DUTs). 
     BACKGROUND 
     Typically, test systems that use switches such as electromechanical relays to connect test instruments to DUTs have many more device terminals than instruments, and a given DUT only has a few terminals. Therefore, any given test typically has many unused pins. These unused pins are still connected to DUT terminals, but the pins are not driven by instruments during the test. Each unused pin has capacitance due to cables and DUT terminals. For a variety of reasons relating to DUT arrangement, there may be undesired parasitic connections between the test pins and unused pins, which can cause these unused pin capacitances to be charged up to high test voltages. 
     If the voltage on an unused pin is not discharged before initiating the next test, and the pin is used in the next test in a test sequence, as the relay opens or closes, the energy stored in this capacitance will suddenly discharge through the relay contact. This is called “hot-switching.” Hot-switching can deteriorate the relay contact, leading to early relay failure. 
     Ordinarily, an instrument could discharge this capacitance before running a test. However, in such a switch system there is no way of connecting an instrument to a charged pin without causing the relay to hot switch. Also, there is no easy way to know that the other side of the relay is at a voltage sufficient to induce hot switching. 
     Since the energy stored in an unused test system cable or DUT capacitance increases in proportion to the square of the voltage, and this energy is directly responsible for damaging the relay contacts, this is an important problem in higher voltage test systems, which are becoming more widely used to test high voltage devices. 
     Embodiments of the invention prevent hot-switching without degrading low current performance, and address other limitations of the prior art. 
     SUMMARY OF THE DISCLOSURE 
     Embodiments of the invention include a protection circuit coupled to a switch in a system coupled to a device under test through a cable, such as a coaxial or triaxial cable, each of which includes a signal line. The protection circuit includes a trigger signal receiver that receives a signal before the switch changes states. The protection circuit includes a discharge circuit that electrically couples the signal line of the cable to a pre-determined voltage, in response to receiving the trigger signal and before the switch finishes changing operating states. 
     In embodiments where the system is coupled to a triaxial cable, the protection circuit may further include a clamping circuit structured to electrically couple the signal line of the cable to the guard structure of the cable after the clamping circuit receives the trigger signal. 
     When not in a discharge mode, the protection circuit may instead isolate the signal line from the pre-determined voltage. 
     Other embodiments include methods of operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating the principle causing switch or relay damage that embodiments of the invention address. 
         FIG. 2  is a partial circuit diagram of a hot switch protection circuit according to embodiments of the invention. 
         FIG. 3  is a functional block diagram illustrating how embodiments of the invention support low-current measurements. 
         FIG. 4  is a detailed circuit diagram illustrating a hot switch protection circuit according to embodiments of the invention. 
         FIG. 5  is a detailed circuit diagram illustrating a hot switch protection circuit according to other embodiments of the invention. 
         FIG. 6  is a detailed circuit diagram illustrating a relay switch protection circuit according to yet other embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention prevent hot-switching by automatically discharging the voltage present on one side of the relay, thereby eliminating the possibility of hot switching. This is accomplished by use of a secondary solid state switch, which is not damaged by hot switching. This solution prevents such hot switching while at the same time does not significantly worsen low current performance. This is important since typically one test system must be capable of testing both high voltage and low current, and also must have reliable relay contacts. 
       FIG. 1  depicts the problem addressed by embodiments of the invention. A capacitor  101  is coupled to a relay  110 , which includes an electrical contact  112  and a movable armature  114 . The armature may be energized by a coil  116 , when the coil is driven by a voltage source  120 . If the capacitor  101  is charged to a voltage when the contact  112  in relay  110  closes, then the capacitor  101  will suddenly discharge through the relay as the contact opens or closes, causing arcing and deterioration of the contact  112  in the relay  110 . Note that the capacitor  101  may represent an actual capacitor, or it may instead represent the parasitic capacitance present on cables connected to the relay contact. 
     One method to prevent capacitive charging would be to place a resistor in parallel with the capacitor  101 . However, this will compromise measurement performance when the terminal is connected to a DUT during a test if the resistance is too low, and will be unable to discharge the capacitor  101  quickly if the resistance is too high. 
     Another solution would be to replace the relay  110  with a relay capable of hot switching. Unfortunately, low-current test systems as well as high-voltage test systems often require relays that are not capable of hot switching. Additionally, relays capable of hot switching are often physically larger than relays that are not. Therefore, using these kinds of relays is a disadvantage in a high density switch matrix. 
     A third solution would be to simply detect that there is voltage present across the capacitor  101  and prevent relay switching until this voltage decays to a low enough level. The first problem with this solution is that, in some cases depending on design of a coil control circuit, delaying the coil signal may not be possible. Second, even if it were possible, the time required for the voltage on the capacitor  101  to naturally decay will cause the overall switch system to have undesirably slow switching performance. 
       FIG. 2  is a partial circuit diagram of a relay hot switch protection circuit  200  according to embodiments of the invention.  FIG. 2  depicts the case of a normally open relay  210 . A voltage from voltage source  220  changes from low to high voltage to energize a coil  216  of the relay  210  and close a primary contact  212 . This change serves as a trigger to a trigger and isolation circuit  230 , which, upon triggering, closes a secondary discharge switch  240  that is not damaged by hot-switching. In this embodiment, transistors  242  and  252  discharge the capacitor  201  to a ground  260 . The entire discharge action happens through the secondary switch  240  between the time it takes for the voltage from the voltage source  220  to change and for the relay  210  to actually close the contact  212  by moving the armature  214 . 
     Another important feature of the circuit  200  to support low current measurements is that, in the embodiment shown in  FIG. 2 , which uses transistors  242 ,  252  as the discharge switches of the discharge switch  240 , the circuit to drive the transistors  242 ,  252  is isolated from the trigger and coil circuitry of the relay  210 . If the circuit were not isolated, the large voltage difference across the discharge switch  240  could induce a significant offset current on a guard of a triaxial cable coupled between the instrument and the DUT. In such a case, the offset current between the signal pin and guard of the triaxial cable would compromise low current measurement performance. 
     An additional important feature of this example circuit is that it is supports low-current measurements.  FIG. 3  shows this arrangement. Here a relay  310  is a two-contact relay that switches a guard and center pin of a low-current triaxial cable  370 . The contact  312  is coupled to a node  332 , which is coupled to the signal pin of the triaxial cable  370 , while the contact  313  is coupled to a node  334 , which is coupled to the guard of the triaxial cable  370 . To automatically discharge the effective capacitance of the triaxial cable  370 , while not contributing to leakage of the triaxial center pin, a low leakage clamp circuit  380  connects the center pin to the triaxial guard. 
     For low current performance during testing, the switch  310  is closed and an instrument  390  drives the node  334  to a voltage very close to the signal at the node  332 . During this time, the clamp circuit  380  should preferably not activate, and therefore incur only low leakage between the nodes  332 ,  334 . When not testing, the switch  310  is open, and no instrument is directly driving the node  334 . However, the center pin may be connected through a DUT and charged to a voltage. Just before the switch  310  closes again for another test, the clamp circuit  380  activates, thereby ensuring that the guard voltage moves along with the center pin. At approximately the same time, the discharge circuit within the trigger and discharge circuit  360  engages, and the clamp circuit  380  and the discharge circuit within the trigger and discharge circuit  360  together provide a low impedance path for discharge of the both the center pin and the guard to ground. 
     There are several potential embodiments of the low leakage clamp circuit  380  of  FIG. 3 .  FIG. 4  shows one embodiment in which a low leakage clamp circuit is constructed with a MOSFET switch in parallel with a gas discharge tube.  FIG. 5  shows an embodiment where the low leakage clamp circuit is formed from low-leakage, anti-parallel diodes. There are other possible clamp circuit embodiments. Thus, embodiments of the invention discharge the guard pin, and also the center pin, to ground when triggered by a relay control signal. 
     In more detail, the clamp and discharge circuit  400  of  FIG. 4  is a detailed example of a clamp and discharge circuit that was illustrated in  FIG. 3 . The clamp and discharge circuit  400  includes a node  432  that may be coupled to the center wire or signal pin of a triaxial cable (not illustrated), as well as a node  434  that may be coupled to a guard of the triaxial cable. In other embodiments the clamp and discharge circuit may be instead coupled to a coaxial cable, such as described with reference to  FIG. 6  below. 
     The clamp and discharge circuit  400  includes four sections, or portions, with each section performing a different function. A clamp circuit portion  420  operates in concert with a discharge circuit portion  460  to make a protection circuit for a switch or relay coupled to the signal and guard nodes  432 ,  434 . In a protection, or discharge mode, the clamp circuit portion  420  electrically couples the signal and guard nodes  432 ,  434  together, or closely together, and the discharge circuit  460  couples the nodes to a reference voltage, such as a ground voltage. In a low-leakage mode, the clamp circuit  420  and discharge circuit  460  operate to minimize any current leakage between the signal and guard nodes  432 ,  434  to ground. A clamp activating circuit  450  drives the clamp circuit  420 , while a discharge activating circuit  451  drives the discharge circuit  460 . In the embodiment illustrated in  FIG. 4 , the activating circuits  450 ,  451  are identical, although in some embodiments they may be different. 
     In operation, when the signal and guard nodes  432 ,  434  are driven by an instrument, the transistors  422 ,  424  are in a low-leakage state between the nodes  432 ,  434 . The gas discharge tube  440  similarly provides low-leakage. In this mode, transistors  422  and  424  are OFF, and the nodes  432 ,  434  are driven by the instrument to nearly the same voltage, resulting in very little current leakage. Instead, when the circuit  400  is in a discharge mode, transistors  422 ,  424  turn ON, causing the circuit  400  to short the signal node  432  to the guard node  434  through the transistors  422 ,  424 . In the low-leakage mode, the gas discharge tube  440  protects transistors  422 ,  424  by limiting the voltage across them when the DUT charges up the signal and guard nodes  432 ,  434 , while adding little additional current leakage at low voltage. 
     As mentioned above, the clamp circuit  420  and discharge circuit  460  of the clamp and discharge circuit  400  operates in two modes. In the discharge mode, a trigger signal in the clamp activating circuit  450  and in the discharge activating circuit  451  transitions state, for example from LOW to HIGH. This causes optically coupled switches  454  and  455  to turn ON, which in turn provides voltage stored on a capacitor  456  to gates of transistors  422  and  424  in the clamp circuit  420 , and further provides voltage stored on a capacitor  457  to gates of transistors  462  and  464  in the discharge circuit  460 . This causes the transistors  422 ,  424 ,  462 , and  464  to turn ON, which closely couples the signal and guard nodes  432 ,  434  to one another through the clamp circuit  420 , and further closely couples the nodes to a ground voltage through the discharge circuit  460 . Capacitors  456  and  457  are sized large enough to drive their respective transistors without significant loss of voltage. 
     In this way, the discharge mode operates to protect the switch or relay that is coupled between the instrument and the DUT that is connected to the signal and guard nodes  432 ,  434 . More specifically, the discharge mode is activated as the relay is closing, or before the relay closes, so that any stray voltage from the DUT is discharged to ground before the relay closes. This protects the relay contacts from degradation. When used to protect a switch, the discharge mode drains any stray voltage from the switch itself so that high current or high voltage does not exceed the capacity of the switch when it is being turned on or off. The discharge mode operates within a short time window, starting just before the relay or switch changes state and ending after the switch finishes changing state. In one example embodiment, where the switch is a mechanical relay, it takes the relay somewhere between several hundred microseconds and several milliseconds to change states. In such a case, the discharge circuit  460  begins discharging before the relay switches states. In some embodiments the discharge circuit  460  may change from the discharge state to the low-level state before or as the relay finishes switching states. In this way the discharge circuit  460  has completed reducing voltage on the relay contact to ground or near ground before the relay contact closes. In other embodiments the discharge circuit  460  may remain in the discharge state for several hundreds of microseconds up to several milliseconds after the relay has completed switching states, which ensures protection during a time the relay may be experiencing bounce. 
     When the discharge mode is not activated, the clamp circuit  420  and discharge circuit  460  operate in a low-leakage mode. In the low-leakage mode, the clamp circuit  420  and discharge circuit  460  operate to minimize any effect their presence has on signals from the DUT. More specifically, in the low-leakage mode, optically coupled switches  454  and  455  open, which disconnects capacitors  456  and  457  from their respective transistors  422 ,  424  and  462 , 464 . Over time, gate voltage will decay from the transistors  422 , 424 ,  462 ,  464  which turns the transistors OFF. 
     Another potential embodiment of the low leakage clamp  380  illustrated in  FIG. 3  is a clamp circuit portion  520  of the circuit  500  shown in  FIG. 5 . Here, the clamp includes a pair of anti-parallel diodes,  542 ,  544  coupled between a signal node  532  and a guard node  534 . In this arrangement, when an instrument coupled to the nodes  532 ,  534  drives the signal node  532 , the instrument will also drive the guard  534  to a voltage nearly the same as the signal node  532 . The low-leakage diodes  542 ,  544  will add little leakage between the nodes  532 ,  534 . In the discharge mode, when signal node  532  is not being used for testing but still charged up by virtue of being connected to a DUT connection, one of the two diodes  542 ,  544  will conduct, effectively connecting the center pin coupled to the signal node  532  to the guard node  534 , and allowing the circuit to discharge the center pin of a triaxial cable coupled to the instrument. There may be a small, residual, voltage on the diode remaining, but such a voltage usually does not represent enough energy to damage a switch or a relay contact. 
     As with the embodiment illustrated in  FIG. 4 , a discharge activating circuit  550  drives a discharge circuit  560 . Unlike  FIG. 4 , however, only one activating circuit  550  is illustrated, although implemented embodiments may include two such circuits, as illustrated in  FIG. 4 . 
       FIG. 6  is a detailed circuit diagram illustrating a hot switch protection circuit  600  according to yet other embodiments of the invention. The protection circuit  600  is coupled to a coaxial cable, rather than the triaxial cables illustrated in  FIGS. 3-5 . Because in a coaxial cable there is no guard structure, there is no need to include a clamp circuit in the protection circuit  600 . The protection circuit  600  couples a signal node  634  to the signal or center pin of the coaxial cable. In some embodiments, such as illustrated in  FIG. 6 , the shield of the coaxial cable may be coupled to a ground voltage at a node  636 . 
     Although specific embodiments of the invention have been illustrated and described for purposes if illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.