Abstract:
Methods of protecting against a surge voltage and apparatus for performing the same. One embodiment of the invention is directed to a circuit to protect a device from a surge voltage. The circuit is connected between the device and first and second nodes to which the surge voltage may be applied. A charge storage device is connected between third and fourth nodes and the device is operatively connected to the third node and a fifth node. The circuit comprises a first overvoltage protection device coupled between the fourth node and a fifth node, the fourth and fifth nodes being operatively connected to the first and second nodes, respectively, and a second overvoltage protection device coupled between the third node and the fifth node. A voltage between the third and fifth nodes during the surge voltage is substantially less than a switching voltage of the second overvoltage protection device for at least a portion of the duration of the surge voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit, under 35 U.S.C. §119(e), of the filing date of U.S. provisional application Ser. No. 60/420,663 entitled “Surge Protection Circuit,” filed Oct. 23, 2002 and incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the field of overvoltage protection circuits and methods. 
   BACKGROUND OF THE INVENTION 
   Overvoltage protection circuits are used to protect electronic devices from transient voltages or “power surges.” Such a power surge may occur when power or signal lines supplying a device experience an increase in voltage above a safe or acceptable level. To give but one example, modems connected to telephone lines require protection from surges on both their power lines and the telephone lines. A conventional overvoltage protection circuit is shown in  FIG. 1 . 
   The overvoltage protection circuit of  FIG. 1  is arranged to protect a load  16  from an overvoltage occurring across nodes  14 ,  12 . The circuit comprises a zener diode  2  connected across the nodes  14 ,  12 . The current-voltage characteristics of a typical zener diode are illustrated in  FIG. 2 , which shows the current ID through the zener diode with respect to the voltage V D  across the zener diode. As shown, when the reverse-bias voltage applied to a zener diode reaches the breakdown voltage |V Z |, the current through the zener diode increases rapidly while the voltage across the diode remains substantially constant. 
   Hence, the zener diode  2  of  FIG. 1  will conduct reverse-bias current I Z  when the voltage across the diode approximates the breakdown voltage |V Z |. The breakdown voltage of the zener diode  2  may be chosen so that the diode conducts current when the voltage across nodes  14 ,  12  is at a particular threshold voltage. 
   Applicant has appreciated certain disadvantages in the overvoltage protection circuit of  FIG. 1  and related circuits. In particular, although the circuit of  FIG. 1  operates to substantially prevent the full surge voltage from appearing at the load  16 , the load  16  is exposed to the threshold voltage (i.e., breakdown voltage) of the zener diode  2  during the voltage surge. In certain applications, the exposure of the load  16  to the threshold voltage for this time period may be harmful to the device. However, if a zener diode with a lower threshold voltage is used, it may be triggered by signal and low-level noise, rather than surges alone. In addition, the load  16  will be exposed to the full surge voltage for a brief period before the voltage across the load  16  is stabilized at the threshold voltage. Exposure to the full surge voltage, even for a brief period of time, may also be harmful to the device. 
   It should be appreciated that while the exemplary conventional overvoltage protection circuit of  FIG. 1  uses a zener diode  2 , other conventional overvoltage protection circuits may use another device (e.g., a SIDACtor® device) in place of the zener diode. Such circuits also exhibit the deficiency of not preventing exposure of the device to the full surge voltage for the entire duration of the surge. 
   A further complication is presented by the reactive nature of some loads. For example, to meet the impedance specifications for loads intended for telephone line connections, a modem or other line interface often will have the configuration of  FIG. 3 , wherein a capacitance C is connected in series between (for balance) a pair of transformer windings L 1  and L 2  through which a connection is made to the tip (T) and ring (R) conductors of a standard telephone circuit. The protected device  17  is connected to one or more opposing transformer windings L 3 . The net effect of the series L-C circuit is to aggravate the problem of protecting device  17  from large surges across the tip and ring lines. If a zener diode is used, it effectively short circuits the series arrangement of L 1 , L 2  and C, forming an undamped resonant circuit that could drive a high amplitude from winding(s) L 3  into device  17 , repeatedly. 
   Accordingly, a need exists for an overvoltage protection circuit suitable for use in telephone circuits and which does not expose the protected device to voltages at or above its threshold level for a time sufficient to do permanent damage—e.g., the full duration of the surge. The threshold level may be sufficiently high so that the overvoltage protection circuit is not triggered into a protective mode by mere noise. 
   SUMMARY OF THE INVENTION 
   This need is addressed by the present invention. One embodiment of the invention is directed to a protective circuit which may be interposed between a device or apparatus requiring protection and conductors that must connect operatively to the device or apparatus. The protective circuit protects against a surge voltage on the conductors, the circuit being connected between a load coupled to the device and first and second nodes on the conductors to which the surge voltage may be applied. The circuit is connected between the device and first and second nodes to which the surge voltage may be applied. A charge storage device is connected between third and fourth nodes and the device is operatively connected to the third node and a fifth node. It should be appreciated that the terms “coupled, “connected,” and “operatively connected” are used interchangeably herein. The circuit comprises a first overvoltage protection device coupled between the fourth node and a fifth node, the fourth and fifth nodes being operatively connected to the first and second nodes, respectively, and a second overvoltage protection device coupled between the third node and the fifth node. A voltage between the third and fifth nodes during the surge voltage is substantially less than a switching voltage of the second overvoltage protection device for at least a portion of the duration of the surge voltage. An interface comprising one or more electronic components may be coupled between the third and fifth nodes. 
   Another embodiment of the invention is directed to a method of limiting exposure of a load to a surge voltage via a protective interface between the load and the surge voltage. The method comprises acts of charging a capacitor with the surge voltage, discharging the capacitor through a first overvoltage protection device, and driving the load with a discharge current of the capacitor. The method further comprises an act, when a voltage across the load exceeds a switching voltage of a second overvoltage protection device, of passing the discharge current through the second overvoltage protection device, wherein voltage across the load is substantially less than a switching voltage of the second overvoltage protection device for at least a portion of the duration of the surge voltage. 
   A further embodiment of the invention is directed to a method of limiting an exposure of a load to a surge voltage. The method comprises acts of passing current through a first overvoltage protection device and, when a voltage across the load exceeds a switching voltage of a second overvoltage protection device, passing the current through the second overvoltage protection device, wherein a voltage across the load is substantially less than a switching voltage of the second overvoltage protection device for at least a portion of the duration of the surge voltage. 
   Another embodiment of the invention is directed to a circuit to protect a device from a surge voltage. The circuit is connected between the device and first and second nodes to which the surge voltage may be applied. The circuit comprises an isolation barrier coupled between the device and third and fifth nodes and a charge storage device connected between the third node and a fourth node. The circuit further comprises a first overvoltage protection device coupled between the fourth node and a fifth node, the fourth and fifth nodes being operatively connected to the first and second nodes, respectively, and a second overvoltage protection device coupled between the third node and the fifth node. The voltage between the third and fifth nodes during the surge voltage is substantially less than a switching voltage of the second overvoltage protection device for at least a portion of the duration of the surge voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a schematic diagram of a conventional overvoltage protection circuit. 
       FIG. 2  illustrates a graph of the current-voltage characteristics of a zener diode. 
       FIG. 3  illustrates a schematic diagram of a conventional interface between a device and a surge protection circuit. 
       FIG. 4  illustrates a schematic diagram of an overvoltage protection circuit according to one embodiment of the invention. 
       FIG. 5  illustrates a graph of the current-voltage characteristics of a SIDACtor® device. 
       FIG. 6  illustrates a schematic diagram of an overvoltage protection circuit according to another embodiment of the invention. 
       FIG. 7  illustrates a schematic diagram of an overvoltage protection circuit according to a further embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In view of the foregoing, one embodiment of the invention is directed to an overvoltage protection circuit that prevents the full surge voltage from appearing at a load coupled to the circuit. In one example, the full surge voltage is prevented from appearing at the load for the full duration of the surge. Another embodiment of the invention is directed to an overvoltage protection circuit that prevents a threshold voltage (e.g., a voltage at which the circuit is triggered into its protective mode) from appearing at the load. The threshold voltage may be prevented from appearing at the load for a portion of the duration of the surge or the full duration of the surge. 
     FIG. 4  illustrates one embodiment of a protection circuit according to the invention in which a circuit  1  operates to protect device  17  from an overvoltage occurring across nodes  15 ,  13 . The surge protection circuit  1  is coupled with an isolation barrier and impedance-matching network formed by transformers  3   a  and  3   b  and capacitor  9 , although it should be appreciated that other transformer configurations may be used. The impedance-matching capacitor C of  FIG. 3  has been moved from between the windings of L 1  and L 2  on the “line” side of the barrier to a series position between one of the line conductors and the windings of transformers  3   a  and  3   b . This permits the surge protection circuit to be formed by a first overvoltage protection device  5  and a second overvoltage protection device  7 . The first overvoltage protection device  5  is connected between the node  20  and the node  13 . The second overvoltage protection device  7  is connected between the node  22  and the node  13 . The surge protection circuit  1 , together with the isolation barrier/impedance network  31 , may be connected as an interface between a circuit  18  (e.g., an interface to the public switched telephone network) and the device  17 . 
   Capacitor  9  is shown as not being part of the surge protection circuit  1  as its inclusion may be required for other purposes. For example, capacitor  9  may be required to be used with the surge protection circuit for a reason unrelated to surge protection, such as to establish the correct impedance of the circuit. Although the capacitor  9  is shown as not being part of the surge protection circuit  1 , it may be functionally involved in the surge protection operation. Therefore, it would be just as proper to define a surge protection circuit including capacitor  9 . Capacitor  9  may be connected between the first and second overvoltage protection devices  5 ,  7  at nodes  20  and  22 . The capacitor  9  may be included to adjust the impedance presented to circuit  18  in accordance with regulatory standards that define acceptable input impedance levels for certain circuits. For example, regulatory standards may set acceptable impedance specifications for the devices connected to digital subscriber lines for DSL service, requiring the inclusion of a capacitance in series with an inductance. The configuration of overvoltage protection devices  5  and  7  and capacitor  9  minimizes the appearance at the line input of device  17  of voltage spikes that otherwise damage device  17  in their absence. 
   Circuit  18  may, for example, be an interface including a signaling circuit connected across a phone line at nodes  13 ,  15 . For example, the lines connected to nodes  13  and  15  may be, respectively, the ring line and tip line of a telephone system. However, it should be appreciated that instead of ring and tip lines, nodes  13  and  15  may be connected to other communication lines or lines that conduct power, rather than signals, to or from device  17 . A fuse  11  is also illustrated in  FIG. 4  and optionally may be included along the line between node  20  and node  15  to stop the conduction of current along the line should the current increase beyond an acceptable level. 
   Device  17  may be a chip or a circuit including components sensitive to voltage surges. For example, device  17  may be an asymmetric digital subscriber line (ADSL) transceiver chip or chipset, such as an AD6493 line-interface circuit of a transceiver chipset, manufactured by Analog Devices, Inc. of Norwood, Mass., USA. 
   When the voltage applied across nodes  13  and  15  suddenly increases (surges) from a nominal level to a high, excessive level, it causes the capacitor  9  to charge. The voltage on capacitor  9  will increase until it exceeds the switching voltage of the first overvoltage protection device  5 . At that point, the device  5  “crowbars,” or switches from a high off-state impedance to a low on-state impedance and conducts current. When this occurs, the capacitor  9  discharges through the first overvoltage protection device  5 . The voltage across the right-hand (“line” side) windings of transformers  3   a ,  3   b , which are connected in parallel with the second overvoltage protection device  7 , then increases until it exceeds the switching voltage of the second overvoltage protection device  7 . At this point, the second overvoltage protection device  7  also crowbars and the discharge current of the capacitor  9  travels in a conductive loop that includes the first and second overvoltage protection devices  5 ,  7  and the capacitor  9 . 
   According to one implementation of the circuit  1 , at least one (and preferably both) of the overvoltage protection devices  5 ,  7  has a higher switching voltage than clamping voltage. In other words, the voltage required to switch the overvoltage protection devices  5 ,  7  to an on-state may be higher than the voltage across the device when the device is passing current. SIDACtor® devices manufactured by Teccor Electronics of 1800 Hurd Drive, Irving, Tex. USA, are but one example of suitable devices that may have a higher switching voltage than clamping voltage. A SIDACtor® device operates much like a switch, with its state being controlled by the voltage across it and current through it.  FIG. 5  illustrates the current-voltage characteristics of a typical SIDACtor® device. As shown, the device has an on-state in which it conducts current and an off-state in which it exhibits low leakage currents (less than 5 μA, typically). When the voltage across the SIDACtor® device exceeds the peak off-state voltage V DRM , the device exhibits characteristics similar to an avalanche diode, allowing increased current through the device. When the current supplied to the device exceeds the switching current I S , the device switches to the on-state and is able to sink large amounts of current. In this state, a voltage drop V T , lower than V DRM , exists across the device. When the current through the device falls below a minimum holding current I H , the device resets and returns to the off-state. Although embodiments of the invention may employ SIDACtor® devices as the protection devices, it should be appreciated that the invention is not limited in this respect. Other types of thyristors or other avalanche or overvoltage protection devices having a voltage drop across the device that is less than the turn-on voltage when the device is in the on-state may alternatively be used. 
     FIG. 6  shows an exemplary implementation of the surge protection circuit  1  of  FIG. 4 . In this implementation, the first overvoltage protection device  5  of  FIG. 3  is implemented with a first SIDACtor® device  19  having a switching voltage between approximately 320V and 400V, or approximately 350V (e.g., Teccor product numbers P3500SC or P3500EC). The second overvoltage protection device  7  of  FIG. 4  is implemented with a second SIDACtor® device  21  having a switching voltage between approximately 25V and 40V, or approximately 30V (e.g., Teccor product numbers P0300SB or P0300EB). Capacitor  9  may have specifications of approximately 0.027 μF/400V. In this scheme, the first SIDACtor® device  19  limits most of the surge generator energy seen by the circuit  17 . In addition, the second SIDACtor® device  21  limits the discharge energy from the capacitor  9  seen by the transformers  3   a ,  3   b . Because a SIDACtor® device experiences a low voltage drop V T  across the device relative to the switching voltage V S  when the device is in the on-state, as shown on  FIG. 5 , the device  17  of  FIG. 6  is not exposed to the switching voltage V S  for the full duration of the voltage surge. Rather, the device  17  is only briefly exposed to the switching voltage V S , for example for a duration on the order of microseconds. Once the current through the second SIDACtor® device  21  reaches I S , the voltage across the SIDACtor® device  21  drops to V T , such that the device  17  is exposed to little or no voltage from the surge. For the SIDACtor® devices having Teccor product numbers P3500SC, P3500EC, P0300SB and P0300EB, noted above, the current I S  has a maximum of approximately 800 mA and the voltage V T  has a maximum of approximately 4V. 
   The circuit in  FIG. 4  has been subjected to several rounds of lightning and power induction tests. The tests were conducted per Bellcore GR-1089 first level lightning specifications and ITU K.20 lightning and power induction recommendations. In all cases the performance of the system was compared before and after the testing, and no performance degradation was observed. Table 1, below, shows a summary of the tests performed: 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Peak Voltage 
               Peak Short-Circuit 
                 
             
             
               Test 
               (Volts) 
               Current (Amps) 
               Test Duration 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Bellcore GR-1089 
               1000 
               100 
               10/1000 uS  
             
             
               First-level test #3 
             
             
               ITU K.20 Lightning 
               1000 
               25 
               10/700 uS 
             
             
               ITU K.20 Lightning 
               4000 
               100 
               10/700 uS 
             
             
               ITU K.20 Power 
               600 
               1 
               0.2 S 
             
             
               induction 
             
             
               ITU K.20 Power 
               600 
               1 
                 1 S 
             
             
               induction 
             
             
                 
             
           
        
       
     
   
   As shown in  FIG. 7 , certain components optionally may be added to or substituted for the components of the exemplary circuit implementation shown in  FIG. 6 . For example, for applications requiring an earth ground connection, a balanced, three-terminal SIDACtor® device  25  (e.g., Teccor product numbers P3403UC or P3403AC) may used in place of the SIDACtor® device  19  of  FIG. 6 , and an additional fuse  29  may be included between nodes  24  and  13 . For the SIDACtor® devices having Teccor product numbers P3403UC and P3403AC, the switching voltage is approximately 350V, the current I S  has a maximum of approximately 800 mA, and the voltage V T  has a maximum of approximately 8V. For applications not requiring an earth ground connection, a two-terminal device may be used to provide only differential-mode protection, and a single fuse may be used. For applications requiring improved longitudinal balance, capacitors  23   a ,  23   b  may be placed between SIDACtor® devices  21  and  25  and, in particular, between nodes  20 ,  22  and nodes  24 ,  26 , respectively. Capacitors  23   a ,  23   b  may have specifications of approximately 0.056 μF/250V. 
   Having described several embodiments of the invention in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and is not intended as limiting. The invention is limited only as defined by the following claims and equivalents thereto.