Low-capacitance linear surge protection circuit

Abstract

Low-capacity overvoltage protection circuit (72), comprising: - an overvoltage protection device (44) having at least two terminals, the overvoltage protection device (44) being responsive to an overvoltage to provide a low impedance path between the two terminals; - wherein the overvoltage protection device (44) is characterized by a rated surge current; - a diode bridge configured for connection to a communication line (12, 14), having at least four nodes, wherein the diode bridge comprises a first diode (52) connected between a first node (68) and a third node, a second diode (54) connected between the third node and a second node (69), a third diode (58) connected between the second node (69) and a fourth node, and a fourth diode (56) connected between the first node (68) and the fourth node, and wherein the overvoltage protection device (44) is connected between the first node (68) and the second node (69) of the diode bridge; - a bias voltage source (48) for biasing the overvoltage protection device (44) through a first and / or a second insulation resistor (64, 66) to bring the overvoltage protection device (44) into an operating range in which a change in the capacitance of the overvoltage protection device (44) is less dependent on the voltage applied across the overvoltage protection device (44); - wherein one or more of the diodes (52, 54, 56, 58) has an associated junction region configured to conduct the rated surge current not substantially greater than that of the overvoltage protection device (44), thereby minimizing a capacitance of the diode bridge; and - wherein the polarity of the bias voltage source (48) is such that the diodes (52, 54, 56, 58) of the diode bridge are reverse biased during normal operation of the communication line (12, 14).

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates generally to surge suppressors and to low capacitance surge suppressor circuits, and more particularly to low capacitance surge suppressor circuits that provide reduced capacitance changes in circuits experiencing varying voltages. BACKGROUND OF THE INVENTION

[0002] Many circuits in electronic devices provide protection against the dangerous effects of overvoltages, overcurrents, etc. These protection circuits are often designed as an integrated part of the general electronic circuit, but can also be added to it as additional devices or circuits.

[0003] Protection circuits can often be constructed on silicon substrates, such as bipolar transistors, diodes, or thyristors. Bipolar silicon devices can carry very large currents and are therefore very well suited for use in protecting electronic circuits from damage caused by overvoltages or overcurrents. Bipolar semiconductor devices constructed with junctions have an inherent capacitance that is a function of the width of the junction region. The junction region in a semiconductor junction acts as the "dielectric" layer of a capacitance. Since the width of the junction region varies with the voltage across the junction, the capacitance of the junction of a bipolar semiconductor varies as a function of the voltage across the junction. Capacitances whose values ​​vary with voltage are inherently nonlinear devices.In other words, a bipolar surge protector connected across a circuit to be protected can negatively impact the circuit's operation, even if the surge protector remains in its off state. This nonlinearity can lead to suboptimal channel performance and intermodulation noise.

[0004] The adverse effects of the above-described processes have been observed in many applications, including communications lines, where surge suppression circuits are routinely used to protect the transmit and receive circuits from overvoltages inadvertently coupled to the communications lines. Many devices in the thyristor family can be used to respond to the overvoltage condition and provide a low-impedance path between the communications line and ground, or another path through which the energy is safely dissipated.

[0005] The adverse effects of using bipolar silicon surge protectors may not arise from the fact that such devices have inherent capacitance, but rather from the characteristic that capacitance varies as a function of the voltage across the device. For example, many communication lines are designed to carry high-speed digital signals of various protocols, including ADSL, T1, E1, ADSL2+, ADSL2++, 10BaseT, VDSL, VDSL2, T3, 100BaseT, and others. Some of these protocols are transmitted between remote locations using modems or other transmit and receive circuits.To optimize high-speed data transmission, some modems use an initialization process to select the correct matching components so that digital signals can be transmitted at the highest speed possible, given the frequency response of the line and the circuitry associated with the line. The matching parameters selected by the modem are those prevailing at the time the matching check is performed—usually once when the modem is first commissioned or at each reboot after an initialization process. It can be seen that if the electrical condition of the line changes after the matching process, the transmission data rate cannot be optimized, and therefore transmission errors may occur.

[0006] An example of transmission inefficiencies can occur in connection with the following example. A modem connected to a line or in operation is programmed to automatically perform an alignment process to determine the best electrical parameters for operation to optimize high-speed data transmission. The modem is connected to the communications line, such as a DSL telephone line, which is configured to transmit VDSL or other data signals. An on-hook state (of the telephone device) of the DSL line for transmitting digital signals is typically 48 volts. After the modem completes the alignment process, it is in a state to effect optimal transmission of the VDSL signals based on the electrical characteristics of the DSL communications line that existed during the alignment process.

[0007] During an actual communication session by a user transmitting high-rate VDSL signals, it is assumed that the user's telephone device, connected to the same DSL communication line, is in an off-hook state. In other words, the user is simultaneously using the DSL communication line for voice transmissions with the telephone device and for data communications using the modem. This off-hook state results in a different set of voltages on the communication line. The communication line transitions from an on-hook state of 48 volts to an off-hook state of approximately 10 volts. Therefore, the capacitance of the surge suppressors, and possibly other devices, changes, altering the electrical characteristics of the lines for which the modem was calibrated.Because the communication line now has different electrical characteristics, the effective transmission rate may decrease, but the modem maintains transmission at the rate optimized during the calibration process. As a result, the data receiver or modem at the receiving end of the communication line may detect errors resulting from transmitting data at a rate greater than the line can reliably transmit in the off-hook state. The excessive error rate may cause the modem to relearn, resulting in a temporary loss of service during the relearning process, which is generally unacceptable.

[0008] From the above discussion, it is clear that there is a need for a technique to make surge suppressors and surge protection circuits less susceptible to changes in capacitance as a function of voltage, thereby reducing the variation in the electrical characteristics of the devices or circuits connected to the lines. There is also a need for an efficient method for assembling surge suppressors so that simple voltages can be applied to their pins or terminals to make the capacitance / voltage characteristics more linear.

[0009] US 6,628,497 B1 discloses a surge protection device with a diode bridge circuit, in which the bridge elements are each formed by two series-connected diodes. US 5,539,820 A describes a surge protection device for a telephone line comprising a bridge circuit with four bridge diodes, but does not disclose that none of the bridge diodes is forward-biased during normal operation when the surge protection device is not in a low-voltage line state. JP H05-199 655 A generally relates to protecting a circuit with asymmetrically biased conductors by means of a surge protection circuit consisting of three protection components.EP 0 148 577 A1 discloses a low-capacitance overvoltage protection circuit comprising a relatively large-area semiconductor overvoltage suppression chip connected between a first and a second power supply voltage conductor. One of the electrodes of a low-capacitance PIN diode is connected to one electrode of the overvoltage suppression chip, and the other electrode of the PIN diode is connected to a high-speed signal conductor. During normal operation, the diode is always reverse-biased, resulting in a very low capacitance load on the high-speed signal conductor. During a transient overvoltage, the PIN diode is forward-biased by the overvoltage and forces a junction breakdown of the suppression chip to occur, thereby limiting the amplitude of the overvoltage. SUMMARY OF THE INVENTION

[0010] The present invention relates to a low-capacity overvoltage protection circuit according to claim 1, a low-capacity overvoltage protection circuit according to claim 10, a method for protecting a communication line according to claim 12, and a method for constructing a low-capacity overvoltage protection circuit according to claim 15. Preferred embodiments are defined in the respective subclaims.

[0011] According to an important aspect, the use of a surge protection device to provide overvoltage protection for a communication line and a bias circuit to bias the surge protection device through a pair of isolation resistors is disclosed. The bias source places the surge protection device into an operating range where the change in the device's capacitance is less dependent on the voltage across the device.

[0012] According to a further feature, a surge protection circuit having at least two terminals is disclosed. The surge protection device responds to an overvoltage to effect a low-impedance path between the two terminals. The surge protection device is characterized by a surge current. A diode bridge having at least four diodes and a first and a second node is provided. The surge protection device is connected between the first and second nodes of the diode bridge. Some of the diodes have junction regions to carry the surge currents of the surge protection device and not significantly larger surge currents, thereby minimizing the capacitance of the diodes.

[0013] According to yet another embodiment, a surge protection circuit is disclosed that includes a diode bridge configured for connection to a communication line. A surge protection device is connected to the nodes of the diode bridge so that a current resulting from surges of different polarity on the communication line flows in one direction through the surge protection device. A first resistor and a second resistor are connected to different terminals of the surge protection device. The resistors are configured to be connected to a bias voltage source.

[0014] Another important feature is a method for protecting a communication line using a surge protection circuit. The method involves biasing a surge protection device with a bias voltage to reduce the surge protection device's voltage. The surge protection device is coupled to a diode bridge such that, when the diode bridge is connected to a communication line, currents of different polarities resulting from respective surges flow in one direction through the surge protection device.

[0015] According to yet another feature, a method of constructing a surge protector is disclosed. The method includes the steps of selecting a surge protector having a desired peak current and a desired breakover voltage. Further included is the step of selecting diodes for a bridge such that at least some of the bridge's diodes have a peak current that is not significantly greater than that of the surge protector. The surge protector and the bridge's diodes are arranged in series, thereby providing a low-capacitance surge protector. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Further features and advantages will become apparent from the following detailed description of preferred and further embodiments of the invention, which are illustrated in the accompanying drawings, in which like reference numerals generally designate the same parts, functions or elements throughout the views and in which: Fig. 1a illustrates a first method for providing overvoltage protection for a communication line; Fig. 1b illustrates a second method for providing overvoltage protection for a communication line; Fig. Figure 1c illustrates a third method for providing overvoltage protection for a communications line; Fig. Figure 2 graphically shows a conventional capacitance versus frequency characteristic of a bipolar semiconductor overvoltage protection device, where the voltage across the device is a variable parameter; Fig. 3 shows, in electrical representation form, a drawing of a biased surge protection device according to an embodiment of the invention; Fig. 4 is an electrical schematic diagram showing an overvoltage protection circuit according to an embodiment of the invention; Fig. Figure 5 graphically shows the capacitance / voltage characteristics of a number of semiconductor overvoltage protection devices; Fig. 6 shows another embodiment of an overvoltage protection circuit according to the invention; Fig. 7 shows yet another embodiment of an overvoltage protection circuit according to the invention; and Fig. 8 shows another example of an overvoltage protection circuit not belonging to the invention. DETAILED DESCRIPTION

[0017] With reference to Fig. 1a of the drawings, an environment 10 is shown in which the invention may be advantageously practiced. Here, the communication line includes a ground conductor (tip conductor) 12 and a ring conductor 14. A surge suppressor 16 is connected between ground, or some voltage source, and the ground conductor 12 to provide surge protection for this conductor 12. The ring conductor 14 is similarly connected to the surge suppressor 18 to provide surge protection for it. In this circuit 10, both surge suppressors 16 and 18 are preferably bidirectional devices constructed on the same silicon chip so as to provide balanced electrical characteristics for both conductors 12 and 14 of the communication line.The bidirectional characteristics of the surge protectors 16 and 18 provide surge protection for both polarities of surges. Furthermore, it is preferred that both surge protectors 16 and 18 provide breakover voltages (V). BO ) that are substantially equal. If either the ground conductor 12 or the ring conductor 14, or both, experiences an overvoltage, the associated overvoltage protection device 16 or 18, or both, is controlled into a conductive state, thereby providing a low-resistance path to ground. The electronic circuits (not shown) connected to the communication line are thereby protected.

[0018] Surge suppressors 16 and 18 have a high impedance to conductors 12 and 14 of the communication line when in the off state, but still contain intrinsic capacitances related to them. Surge suppressors 16 and 18 are typically 4-layer thyristor devices constructed with bipolar junctions. The capacitance of such bipolar devices can be less than 100 picofarads. Sidactor ® Surge suppressors, available under the brand name TECCOR from Littelfuse, Des Plaines, Illinois, are widely used in industry as high-speed, high-surge current, low-overshoot devices in surge suppression circuits.

[0019] Fig. 1b shows another example 20 in which a single bidirectional surge protection device 22 is connected between the ground conductor 12 and the ring conductor 14 of a communication line to provide surge protection for the line. In this protection configuration, an overvoltage present on one communication line causes the surge protection device 22 to become conductive and couple the power to the other communication line. The downstream circuits connected to the communication line are thus protected. Although not shown, the bidirectional surge protection device 22 can be replaced with unidirectional surge protection devices and a diode bridge.

[0020] Fig. Figure 1c shows a surge protection configuration in which three surge protection devices 24, 26, and 28 are used to protect the ground conductor 12 and the ring conductor 14 of the communication line. This configuration represents a balanced protection circuit for the communication line. The cumulative breakover voltage of the series-connected devices 24 and 28 is preferably equal to the cumulative breakover voltage of the series-connected devices 26 and 28. The devices can be selected in a manner as explained in US Patent No. 4,905,119 (Webb). The series-connected arrangement of the two devices of Fig. 1c reduces a change in the voltage in each semiconductor junction, thereby reducing the amount of change in capacitance.

[0021] Fig. Figure 2 graphically shows the capacitance / frequency characteristics of a typical bipolar surge protective device as a function of various voltages applied across the device. Note that the horizontal frequency axis is on a logarithmic scale. In particular, the diagram shows the electrical characteristics of a Sidactor ®Surge suppressor, part number P3100SCMS, available from Teccor Electronics. Line 30 shows the capacitance / frequency characteristics of the device at an applied voltage of 0 volts. Line 32 shows the capacitance / frequency characteristics of the device at an applied voltage of 1 volt. Line 34 shows the capacitance / frequency characteristics of the device at an applied voltage of 2 volts. Line 36 shows the capacitance / frequency characteristics of the device at an applied voltage of 5 volts. Line 38 shows the capacitance / frequency characteristics of the device at an applied voltage of 15 volts. Line 40 shows the capacitance / frequency characteristics of the device at an applied voltage of 40 volts. These electrical characteristics are those of the device in a test circuit, with no other communication circuitry connected to the device.

[0022] It should be noted that when higher voltages are applied across the bipolar surge suppressor circuit, a smaller change in capacitance as a function of frequency prevails. This is generally the case with most bipolar surge suppressors. However, the existing problem is that the voltage across a surge suppressor is not generally known or predictable at all times when it is connected to communication circuits or lines. Therefore, if the voltage across such a device is low, the other communication circuits must operate with the changes in the capacitance of the surge suppressor.When VDSL and other high-speed digital signals are transmitted over a DSL line, this either means reducing the transmission speed to a rate lower than what would otherwise be required to compensate for the increased capacity of the surge protector, or accepting a higher error rate. Neither of these solutions is acceptable to communications providers or users.

[0023] With further reference to Fig. 2, it should be noted that at low voltages across the surge protector, namely between about 1 and 5 volts, the capacitance of the device varies significantly more than at higher voltages across the device. For the specific device of the example, and for a voltage of 1 volt applied across the device, the capacitance of the device varies by about 5 pF, from about 40 pF to about 35 pF, in the frequency range of 1 MHz to 10 MHz. This represents a change in the capacitance of the device of about 12.5%. With a potential of 1 volt applied across the device, the capacitance at 100 MHz is about 33 pF. Considering the same surge protector with an applied voltage of 40 volts, it is noted that the change in capacitance between 1 MHz and 10 MHz is about 1 pF (28 pF - 27 pF), which corresponds to a change in capacitance of about 3.6%.At 100 MHz, the capacitance of the device is approximately 24 pF with an applied bias of 40 volts. The small amount of change in capacitance is shown in . Fig. 2 by the linear line at 40 volts. The other lines in the graph show that lower voltages across the device exhibit much greater nonlinearity. It can also be seen that by ensuring a voltage greater than the voltage normally applied across the device is applied, there is a corresponding reduction in the capacitive change in the device. As noted above, at larger voltages applied across a bipolar semiconductor junction, there is a wider junction region between the conductive regions (the capacitor plates) of the device, and thus a lower capacitance.

[0024] According to an important feature of the invention, a bias voltage is applied to the surge protector so that it continuously operates at a lower capacitance, allowing the communication lines to operate at optimal speed and bandwidth. To ensure that at least a predetermined voltage is always present across the surge protector, it must be ensured that the connected communication line experiences a minimal amount of capacitance change—at least the capacitance to which the surge protector contributes.

[0025] It will now Fig. 3, which illustrates an embodiment of the invention in which an overvoltage protection device 44 is connected to a communication line 12 and 14 via a diode bridge 46. Furthermore, a bias voltage 48 is applied to the overvoltage protection device 44 to reduce the capacitance variations of the device in a manner as described above. An important advantage of the overvoltage protection circuit 42 of Fig. 3 is that only a single surge protection device 44 is required, and because of the diode bridge 46, the device 44 only needs to be a unidirectional device. The diode bridge 46 couples each polarity of surge from the communication lines 12 and 14 in such a way that the corresponding current flows in the same direction through the surge protection device 44.

[0026] Another important advantage of using a diode bridge is that the overall capacitance of the overvoltage protection circuit 42 is reduced. The capacitance of the bridge 46 diodes in series with the capacitance of the overvoltage protection device reduces the overall capacitance of the circuit 42. As described in more detail below, the selection of the bridge diodes is important to obtain a low-capacitance overvoltage protection circuit.

[0027] Fig. Figure 4 shows a biased surge protection circuit according to one embodiment of the invention. Here, a Sidactor surge protection device 44 is a unidirectional bipolar device connected to a communication line including a ground conductor 12 and a ring conductor 14. A ground 50 is also provided. The ground conductor 12, the ring conductor 14, and the ground 50 of the circuit are connected to nodes 68 and 69 of the surge protection device 44 via corresponding diode pairs of bridge 46. The ground conductor 12 is connected to the surge protection device 44 by the diode pair 52 and 54, while the ring conductor 14 is connected to the surge protection device 44 by the diode pair 56 and 58. The overvoltage protection device 44 is connected to ground 50 via nodes 68 and 69 and via the diode pair 60 and 62.In particular, the cathode of diode 62 is connected to ground 50, and the anode of diode 60 is connected to ground 50. As explained above, overvoltages of either polarity can be conducted in a conventional manner from either the ground conductor 12 or the ring conductor 14, or from both conductors, through the various diodes of bridge 46 to ground 50. As can be seen, the various current paths through the surge suppressor 44 include a first diode, the surge suppressor 44, and then a second diode. The three components are all connected in series, thereby reducing the effective capacitance seen by the communication lines 12 and 14 in the surge suppressor 10.

[0028] According to an important feature of the invention, a bias voltage is applied across the terminals of the surge protection device 44. The voltage is continuously applied to the surge protection device 44. Alternatively, the bias voltage may be applied only during the time that the communication lines 12 and 14 are active with respect to transmitting communication signals.

[0029] The voltage is applied by the bias voltage source 48 to the surge suppressor 44 through a pair of resistors 64 and 66. The resistors 64 and 66 preferably have a sufficiently high resistance to provide isolation between the voltage source 48 and the surge suppressor 44 when the latter is in a conductive state. In practice, the resistances may be on the order of 1 megohm or greater each. However, in certain applications, the value of each resistor 64 and 66 may be as low as a few hundred ohms. During the time periods when an overvoltage on the communication line 12 and 14 switches the surge suppressor 44 to the conductive state, the breakover voltage or other electrical characteristics of the surge suppressor 44 are not otherwise adversely affected by the presence of the bias voltage.

[0030] It should be noted that in the embodiment of Fig. 4, the bias voltage source 48 has ground as its reference, since the negative terminal is connected to ground. Node 69 of the diode bridge 46 is connected to ground through the insulation resistor 66. This biases the surge protection device 55 to an operating point at which the device 44 exhibits a lower total capacitance and a lower change in capacitance as a function of frequency. Fig. Figure 5 graphically shows the electrical characteristics of a number of different Sidactor surge suppressors available under the brand name Teccor from Littelfuse, Des Plaines, Illinois. These electrical characteristics are typical of bipolar devices, in that larger reverse junction bias voltages result in wider junction regions. It can be seen that the capacitance of the surge suppressors becomes significantly small at bias voltages of about 10 volts or more. It should be noted that when thyristors are used as surge suppressors for low-voltage applications, such as data communication lines, the doping level of the junction must be higher to reduce the device's breakover voltage. However, devices with higher doping levels have increased junction capacitance due to the resulting narrower junction regions.A thyristor with a low breakover voltage and also with a low capacitance is therefore difficult to realize, but such a device is what is needed to protect high-speed data communication lines.

[0031] With further reference to Fig. 4, the polarity of bias source 48 is preferably such that the diodes of bridge 46 are reverse biased during normal operation of communication lines 12 and 14. Although bias source 48 is shown as grounded at the negative terminal, in other communication line configurations, the overvoltage protection circuit may also be configured to operate when the positive terminal of bias source 48 is connected to ground.

[0032] When the overvoltage protection circuit is operating Fig. 4, it is assumed that the voltage of source 48 is greater than the highest operating voltage of the communication line, but less than the breakover voltage of the surge suppressor. The capacitance of the surge suppressor remains unchanged due to the constant bias voltage across the device. Changes in the line voltage will change the capacitance of the bridge diodes because the voltage across the diodes changes with the line voltage. As noted above, during high data rate operation or in other environments where circuit capacitance changes are critical and undesirable, biasing the surge suppressor can improve communication line performance and reduce data transmission errors.

[0033] According to yet another alternative, where a variable bias voltage source is desired, the overvoltage protection circuit 72 may be made of Fig. 6. In this embodiment 72, the negative terminal of the voltage source 48 is connected to the node 69 of the diode bridge 46. Only one isolation resistor 64 is shown, but an optional second isolation resistor 66 may be used between the negative terminal of the bias voltage source 48 and the node 69 of the diode bridge 46. Again, only a single overvoltage protection device 44 is used in conjunction with the diode bridge 46. Other configurations of the circuit 72 are possible, including omitting the resistor 64 and using the resistor 66.

[0034] As noted above, the diodes of bridge 46 in series with the surge suppressor 44 provide a low-capacitance protection circuit. Such an arrangement is particularly suitable for use in protecting high-speed digital or other types of communication lines. The protection circuit has a characteristically low capacitance because the total capacitance of such a circuit is less than the capacitance of the lowest-capacitance device in the series arrangement. By using a low-capacitance device in the protection circuit, it is thus ensured that the total capacitance is at least as low as that of the low-capacitance device. In the surge suppressor circuits described above, the protection devices 44 may have low capacitances of approximately 20-30 pF.However, the diode bridges have a much simpler construction and can have capacitances lower than those of the overvoltage protection device 44, namely as low as about 16 pF.

[0035] The capacitance of a semiconductor diode is a function of the junction area and the doping level of the junction areas. Diodes constructed with small junction areas have lower capacitances, but are then limited in their current carrying capabilities. Diodes with lightly doped semiconductor regions have low capacitances and correspondingly high reverse breakover voltages. Consequently, to obtain a low-capacitance diode, it is preferable to select the manufacturing parameters to use small junction areas and lightly doped semiconductor regions.

[0036] According to an important feature of the invention, the current-carrying capacity of bridge diodes 52, 54, 56, and 58 is determined by the maximum current-carrying capacity of the surge suppressor 44. In other words, there is no need to construct bridge diodes with large junction areas for conducting currents significantly greater than those of the surge suppressor 44. Therefore, if the surge current of the surge suppressor 44 is approximately 200 amperes, then the bridge diodes 52, 54, 56, and 58 should be constructed or selected with junction areas merely large enough to safely conduct similar surge currents. Making the junction area of ​​the bridge diodes larger than necessary only unnecessarily increases the overall capacitance of the protection circuit.

[0037] Bridge diodes 60 and 62 are preferably structured or selected to conduct approximately twice the current as diodes 52, 54, 56, and 58. This is recommended when overvoltages are present simultaneously on both communication lines 12 and 14. Due to the compact semiconductor devices, it is preferable not to make the junction regions of diodes 60 and 62 larger than necessary.

[0038] A high reverse breakover voltage is desirable for all bridge diodes, and therefore a low doping level in the semiconductor regions is preferred. This also reduces the junction capacitance.

[0039] Fig. Figure 7 shows a surge protection circuit for a communication line, which is very well designed for minimizing the capacitive loading of the line. This protection circuit includes a bridge equipped with four diodes, with nodes 78 and 80 connected to the ground conductor 12 and the ring conductor 14 of the communication line, respectively. Nodes 82 and 84 of the diode bridge are connected to resistors 64 and 66, respectively. Resistors 64 and 66 are coupled to a bias source 48 for applying a bias voltage above a threshold device 86, such as a Zener diode. Zener diode 86 can be selected to provide a reverse breakover voltage suitable for the application in question. The bias voltage achieves the same function as described above in connection with Fig. 4 and Fig. 6. In addition, diodes 52, 54, 56, and 58 may be either configured or selected for low capacitance in a manner as described above.

[0040] Fig. Figure 8 shows another example 90 which is similar to the one shown in Fig. 6, but without the bias voltage or the isolation resistors. This configuration also provides a lower-capacitance overvoltage protection circuit 90 than previously available by carefully selecting the diodes for low capacitance in the manner described.

[0041] The above explanations disclose a surge protection circuit for biasing a surge protection device into an operating range in which the capacitance of the device is reduced, since the change in capacitance is a function of frequency and voltage. By using a surge protection device, the associated communication line can be optimized for high-speed data transmission and transmission rates. The surge protection circuit does not require a grounded bias source and uses only a single unidirectional surge protection circuit. A pair of isolation resistors isolates the bias source from the surge protection device and the communication line.

[0042] The various embodiments have been described in connection with the use of a Sidactor surge suppressor to protect a ground conductor and a ring conductor of a communication line. The components are for illustrative purposes only, as other surge suppressors and lines may be used with equal effectiveness. For example, the biased surge suppressor circuit of the invention may utilize other two- or three-terminal thyristors, including SCRs, TRIACs, etc. Communication lines other than those with ground and ring conductors may be used with the surge suppressor circuit of the invention.

[0043] Although the present invention has been described above in connection with various embodiments, it is to be understood that this disclosure has been made by way of example only, since many changes may be made in the details and structure of the invention without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] Low-capacity overvoltage protection circuit (72), comprising: - an overvoltage protection device (44) having at least two terminals, the overvoltage protection device (44) being responsive to an overvoltage to provide a low impedance path between the two terminals; - wherein the overvoltage protection device (44) is characterized by a rated surge current; - a diode bridge configured for connection to a communication line (12, 14), having at least four nodes, wherein the diode bridge comprises a first diode (52) connected between a first node (68) and a third node, a second diode (54) connected between the third node and a second node (69), a third diode (58) connected between the second node (69) and a fourth node, and a fourth diode (56) connected between the first node (68) and the fourth node, and wherein the overvoltage protection device (44) is connected between the first node (68) and the second node (69) of the diode bridge; - a bias voltage source (48) for biasing the overvoltage protection device (44) through a first and / or a second insulation resistor (64, 66) to bring the overvoltage protection device (44) into an operating range in which a change in the capacitance of the overvoltage protection device (44) is less dependent on the voltage applied across the overvoltage protection device (44); - wherein one or more of the diodes (52, 54, 56, 58) has an associated junction region configured to conduct the rated surge current not substantially greater than that of the overvoltage protection device (44), thereby minimizing a capacitance of the diode bridge; and - wherein the polarity of the bias voltage source (48) is such that the diodes (52, 54, 56, 58) of the diode bridge are reverse biased during normal operation of the communication line (12, 14). [2] The overvoltage protection circuit (72) of claim 1, wherein the first isolation resistor (64) is connected to the first node (68) of the diode bridge and the bias voltage source (48) is connected to the first node (68) via the first isolation resistor (64) such that the bias voltage is applied to the overvoltage protection device (44) to reduce the capacitance thereof. [3] The overvoltage protection circuit (72) of claim 2, wherein the second isolation resistor (66) is connected to the second node (69), wherein the first and second isolation resistors (64, 66) are configured to be connected to the ground (50) referenced bias voltage source (48). [4] The overvoltage protection circuit (72) of claim 3, wherein the first and second isolation resistors (64, 66) have a resistance of about 1 megohm or more. [5] The overvoltage protection circuit (72) of claim 1, wherein the overvoltage protection device (44) comprises a 4-layer semiconductor device with only two terminals. [6] The overvoltage protection circuit (72) of claim 5, wherein the 4-layer semiconductor device includes a Sidac. [7] The overvoltage protection circuit (72) of claim 1, wherein the diode bridge additionally comprises a fifth diode (60) and a sixth diode (62) to thereby form three diode pairs (52, 54; 56, 58; 60, 62), a cathode and an anode of the additional diode pair (60, 62) being configured for connection to a ground (50). [8] The overvoltage protection circuit (72) of claim 1, wherein the overvoltage protection device (44) operates in a unidirectional manner such that currents resulting from respective overvoltages flow in only one direction between the two terminals. [9] The overvoltage protection circuit (72) of claim 1, wherein the diodes (52, 54, 56, 58) have junction regions configured to conduct no more than 150% of the surge current of the overvoltage protection device (44). [10] Low-capacity overvoltage protection circuit (72) comprising: a diode bridge configured for connection to a communication line (12, 14) and having at least four nodes and at least four diodes (52, 54, 56, 58), wherein the diode bridge has a first diode (52) connected between a first node (68) and a third node, a second diode (54) connected between the third node and a second node (69), a third diode (58) connected between the second node (69) and a fourth node, and a fourth diode (56) connected between the first node (68) and the fourth node, and wherein the overvoltage protection device (44) is connected between the first node (68) and the second node (69) of the diode bridge; an overvoltage protection device (44) characterized by a rated surge current and connected to the first and second nodes (68, 69) of the diode bridge such that a current resulting from overvoltages of any polarity on the communication line (12, 14) flows in one direction through the overvoltage protection device (44), wherein one or more of the diodes (52, 54, 56, 58) has an associated junction region configured to conduct a rated surge current not substantially greater than that of the overvoltage protection device (44), thereby minimizing a capacitance of the diode bridge; and a first insulation resistor (64) and a second insulation resistor (66), each insulation resistor (64, 66) being connected to a different terminal of the overvoltage protection device (44), the insulation resistors (64, 66) being connected to a bias voltage source (48) for biasing the overvoltage protection device (44) through the first and second insulation resistors (64, 66) to bring the overvoltage protection device (44) into an operating range in which a change in the capacitance of the overvoltage protection device (44) is less dependent on the voltage applied across the overvoltage protection device (44), the polarity of the bias voltage source (48) being such that the diodes (52, 54, 56, 58) of the diode bridge are reverse biased during normal operation of the communication line (12, 14). [11] The overvoltage protection circuit (72) of claim 10, wherein the diode bridge comprises at least six diodes (52, 54, 56, 58, 60, 62). [12] A method for protecting a communication line (12, 14) using a low-capacity overvoltage protection circuit (72), comprising the steps of: - biasing an overvoltage protection device (44) characterized by a rated surge current with a bias voltage to reduce the change in capacitance of the overvoltage protection device (44) as a function of a voltage applied to the overvoltage protection device (44); and - coupling the overvoltage protection device (44) to a diode bridge having at least four diodes (52, 54, 56, 58) such that, when the diode bridge is connected to the communication line (12, 14), currents of different polarities resulting from respective overvoltages on the communication line (12, 14) flow in one direction through the overvoltage protection device (44), wherein one or more of the diodes (52, 54, 56, 58) has an associated junction region configured to conduct a rated surge current not substantially greater than that of the overvoltage protection device (44) to thereby minimize a capacitance of the diode bridge, wherein the diode bridge comprises a first diode (52) connected between a first node (68) and a third node, a second diode (54) connected between the third node and a second node (69), a third diode (58),which is connected between the second node (69) and a fourth node, and a fourth diode (56) connected between the first node (68) and the fourth node, and wherein the overvoltage protection device (44) is connected between the first node (68) and the second node (69) of the diode bridge; and, - biasing the overvoltage protection device (44) with a bias voltage source (48) through a first insulation resistor (64) and a second insulation resistor (66), each insulation resistor (64, 66) being connected to a different terminal of the overvoltage protection device (44) to bring the overvoltage protection device (44) into an operating range in which a change in the capacitance of the overvoltage protection device (44) is less dependent on the voltage applied across the overvoltage protection device (44), and wherein the polarity of the bias voltage is such that the diodes (52, 54, 56, 58) of the diode bridge are reverse biased during normal operation of the communication line (12, 14). [13] The method of claim 12, further comprising biasing the overvoltage protection device (44) by means of a variable bias voltage source (48). [14] The method of claim 12, further comprising coupling two terminals of the bias voltage source (48) to different terminals of the overvoltage protection device (44) without connecting the terminals of the bias voltage source (48) to ground (50). [15] A method of constructing a low-capacity overvoltage protection circuit (72) comprising the steps of: - selecting an overvoltage protection device (44) with a desired rated surge current and a desired breakover voltage; - selecting diodes for a diode bridge having at least four diodes (52, 54, 56, 58) such that at least some of the diodes (52, 54, 56, 58) of the diode bridge have a rated surge current that is not significantly greater than that of the overvoltage protection device (44); - arranging the overvoltage protection device (44) and the diodes (52, 54, 56, 58) in the bridge circuit to thereby provide a low-capacitance overvoltage protection circuit (72), and configuring the diodes (52, 54, 56, 58) of the diode bridge and the overvoltage protection device (44) so ​​that currents resulting from overvoltages of both polarities flow through at least one diode (52, 54, 56, 58) of the diode bridge and the overvoltage protection device (44), wherein one or more of the diodes (52, 54, 56, 58) has an associated junction region configured to conduct a rated surge current not significantly greater than that of the overvoltage protection device (44), thereby minimizing a capacitance of the diode bridge; and - biasing the overvoltage protection device (44) with a bias voltage source (48) through a first insulation resistor (64) and a second insulation resistor (66), each insulation resistor (64, 66) being connected to a different terminal of the overvoltage protection device (44) to bring the overvoltage protection device (44) into an operating range in which a change in the capacitance of the overvoltage protection device (44) is less dependent on the voltage applied across the overvoltage protection device (44), the polarity of the bias voltage being such that the diodes (52, 54, 56, 58) of the diode bridge are reverse biased during normal operation of the communication line (12, 14), the diode bridge comprising a first diode (52) connected between a first node (68) and a third node, a second diode (54) connected between the third node and a second node (69), a third diode (58),which is connected between the second node (69) and a fourth node, and a fourth diode (56) connected between the first node (68) and the fourth node, and wherein the overvoltage protection device (44) is connected between the first node (68) and the second node (69) of the diode bridge. [16] The method of claim 15, further comprising selecting the diodes (52, 54, 56, 58) with a surge current not exceeding about 150% of the surge current of the overvoltage protection device (44). [17] The method of claim 15, further comprising constructing the diodes (52, 54, 56, 58) with a junction having a narrow area to reduce the capacitance of the overvoltage protection device (44) and to allow a peak current to be conducted. [18] The method of claim 15, further comprising selecting a diode (52, 54, 56, 58) having a capacitance of no greater than about 20pF at a bias voltage of 0V. [19] The method of claim 15, further comprising selecting the overvoltage protection device (44) and at least two diodes (52, 54, 56, 58) of the diode bridge to achieve a total capacitance of the overvoltage protection circuit (72) of no more than about 20pF at a bias voltage of 0V.

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