Abstract:
A protection circuit provides transient protection to a solid state circuit of a line interface for wired telecommunications media at a voltage level below the power supply of the solid state circuit and prevents transient currents from entering the power supply distribution where they might cause damage to the protected interface and other circuits. The protection circuit uses a MOSFET, the gate of which is provided with a bias derived from a power supply of the solid state circuit being protected. The bias causes the MOSFET to be always on such that the MOSFET provides an immediate and high transconductance path to ground for transient currents from the line interface.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to circuit protection measures and more particularly to circuit protection measures for wired communications transmission media. 
       BACKGROUND OF THE INVENTION 
       [0002]    Solid state circuits that interface with wired communications transmission media typically require protection from transient energy appearing at the interface. This protection typically involves voltage clamping and current limiting elements. Voltage clamps that track the power supply voltage of the interface circuitry are often required. One of the challenges of using this type of protection is that some of the transient energy may find its way into the power supply distribution and cause collateral damage. Transient voltage clamping requirements become more critical for circuits with low impedance interfaces since series current limiting resistors have a detrimental affect on the interface performance. In some cases, protection at a voltage level that is below the level of supply voltage(s) and that tracks any change in voltage may provide improved protection and permit the selection of lower series resistance between the clamp and the protected circuit. Protection that tracks supply voltage changes is especially important when the supply voltage is not regulated as is the case in many telecommunications applications utilizing battery reserve. 
         [0003]    Two methods are typically employed to clamp transient voltages to a level that tracks the protected circuit power supply voltage. The simplest method is to use diodes between the line interface and the power supply that are forward biased when transients exceed the circuit supply voltage by at least one diode drop. A problem with this solution is that transient currents are diverted to the supply voltage rail. Power supplies are typically only designed to source current, and transient energy conducted to the power supply rail must be dissipated in the circuitry powered by the supply. A second method is to use a triggered thyristor device that diverts current to ground when transient voltages exceed the supply voltage by a few diode drops. Currents required to trigger this type of device are conducted to the power supply rail. This type of a device results in much lower currents at the power supply interface, but these currents can still be significant when the power supply is lightly loaded. 
         [0004]    With these protection methods, transient voltages appearing at the line interface will always exceed the supply voltage. Resistors are usually required between the protection device and the protected circuit to limit transient currents in the interface circuitry. The value of these resistors is constrained by the protection voltage and can have a detrimental affect on the circuit performance. 
         [0005]    This type of protection is least effective when transient energy finds its way to the supply rails through multiple interfaces or when the transient is repetitive as is the case with an AC power contact. Currents in the protection diodes or in trigger circuits of protective clamp devices can cause the supply voltage to be pumped up, especially when the power supply is lightly loaded. Some transient events, such as an AC power fault, can result in repetitive transient currents that must be absorbed by the power supply. If the cumulative transient current absorbed by the supply exceeds the load, the supply voltage and tracking protection clamp voltage both are increased and the protected load and possibly other circuitry powered by the supply can be damaged as a result. 
         [0006]    Transient or foreign currents find their way into the power supply circuits through multiple paths when the protection clamping voltage exceeds the supply voltage. One of these paths can be the integrated circuit that the protection circuitry is designed to protect. When currents flow through IC interfaces to the supply voltage, causing the voltage at these interfaces to exceed the supply, latch up or destructive conditions may result. If transient voltages are clamped to levels below the supply these latch up conditions can be avoided. 
         [0007]    What is required is an alternative protection circuit that will divert transient energy away from the interface circuit and its power source. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with one embodiment of the disclosure, there is provided a protection circuit for protecting at least one solid state circuit at a line interface, the protection circuit comprising at least one transistor; and at least one bias supply for supplying a bias current to said at least one transistor; wherein the supplied bias current is at least a current required to operate said transistor. 
         [0009]    In accordance with one embodiment of the disclosure, there is provided a method for protecting at least one solid state circuit at a line interface, the method comprising providing at least one transistor; providing a bias to at least one gate of said at least one transistor; receiving a current transient from said line interface; and conducting said current transient to ground through said at least one transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0011]      FIG. 1  schematically represents a circuit for providing transient protection at a line interface; 
           [0012]      FIG. 2  schematically represents a circuit diagram for providing transient protection; 
           [0013]      FIG. 3  represents a modification of the circuit of  FIG. 2 ; and 
           [0014]      FIG. 4  represents a circuit diagram for providing dual-polarity transient protection. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    With reference to  FIG. 1 , there is shown a circuit  10  in accordance with an embodiment of the disclosure. The circuit  10  provides a line interface circuit  12 , for example a solid state circuit, that is to be protected from a line interface  16 . Disposed between the line interface circuit  12  and the line interface  16  is a protection clamp  14 . The protection clamp  14  can be used to limit the voltage appearing at the interface circuit  12  to a voltage below the supply voltage(s)  15  of the interface circuit  12 . The protection clamp  14  can also isolate the line interface circuit from transient energy coming through the line interface. The protection clamp  14  is provided with a bias current  17  derived from the power supply  15  of the line interface circuit  12  which enables the protection clamp to react quickly to transients. 
         [0016]    The protection clamp  14  is thus an active circuit that consumes a small bias current  17  from the supply voltage  15  of the circuit to be protected  12  and provides a low impedance path to ground for transient energy. The bias current  17  is used to establish a protection voltage level that is below the supply voltage. Transient voltages appearing at the interface are clamped through diodes by the low impedance of the protection circuit. 
         [0017]    A simplified detailed implementation will now be described with reference to  FIG. 2 . In the depicted implementation, the line interface  16  is represented by four lines. However, it will be apparent to the person skilled in the art that the line interface  16  may have any number of lines. A protection clamp  14  interfaces with the line interface  16  through a series of diodes  25 . 
         [0018]    At the core of the protection clamp  14  is a transistor  22  such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or, more generically, an Insulated Gate Field Effect Transistor (IGFET). In operation, the MOSFET  22  is biased on with a small current. While the bias current may be derived from any suitable means, such as an independent supply, in one embodiment, the bias current is derived from the power supply  15  of the circuit to be protected, for reasons discussed below. A second transistor  24  regulates the bias current based on the value of resistor  23 . The purpose of the bias current is to keep the gate-source voltage of the MOSFET  22  at the gate threshold thus reducing the time required for the transistor to respond to a transient condition. When a transient is applied to the circuit, the gate-source voltage will increase slightly, turning the MOSFET on harder and increasing the conductance to ground  29 . The clamp has very low impedance during a transient event due to the high transconductance of the MOSFET  22 . The circuit  20  therefore has advantage over prior art circuits by ensuring that transient energy is conducted to ground  29 , rather than to the power supply  15 . 
         [0019]    Deriving the bias current from the voltage supply  15  of the protected circuit allows the clamp voltage to be set below the voltage of the supply  15  by using one or more diodes  27 ,  28  in series in the bias current path. In the embodiment depicted in  FIG. 2 , two diodes  27 ,  28  are used. The clamp voltage in this example is thus two diode drops below the power supply voltage  15  (Vbe(Transistor  24 )+V(Diode  27 )+V(Diode  28 )−V(Diode  25   a ). The MOSFET  22  source is biased three voltage drops below the power supply voltage  15 . While two diodes  27 ,  28  are shown, a single diode may be used. Alternatively, one of the diodes  27 ,  28  in series with the supply can be replaced with a zener diode, or more diodes can be used, if a lower clamp voltage is desired. 
         [0020]    In addition to setting the clamp voltage, the diodes  27 ,  28  also provide isolation between the clamp circuit and the power source. 
         [0021]    With reference now to  FIG. 3 , there is shown a modified circuit  30  in which like numerals indicate like parts. The circuit  30  has been modified by the addition of transistor  31  which connects to the gate of MOSFET  22 . The gate of transistor  31  is derived from between the drain of MOSFET  22  and a resistor  32  that is placed between the drain of MOSFET  22  and ground  29 . Transistor  31  turns the MOSFET  22  on hard during high transient current events, i.e. for high peak currents. This avalanche trip current is set by the value of resistor  32 . 
         [0022]    Some line interface circuits require protection of this type only for a single polarity and a diode to ground provides sufficient protection for the opposite polarity. The circuits  20 ,  30  depicted in  FIGS. 2 and 3  respectively provides positive polarity protection by ensuring that positive transients pass through diodes e.g. diode  25   a , while negative voltage transients pass through opposite diodes, e.g. diode  25   b  and are thus clamped to ground  29 . The person skilled in the art will readily understand that a negative clamp can be produced using the circuits  20 ,  30  depicted in  FIGS. 2 and 3  by substituting complimentary components. 
         [0023]    A circuit providing both positive and negative polarity protection is shown in  FIG. 4 . The circuit  50  includes the circuit  30  described above with reference to  FIG. 3  and an additional circuit  40  which is complementary to the circuit  30 . For example, the MOSFET  42  of circuit  40  is complementary to MOSFET  22  of circuit  30  and the diodes  47 ,  48  are shown having an opposite conducting direction to the diodes  27 ,  28 . The bias current of circuit  30  is shown derived from a power supply  15   a  of the line interface circuit while the bias current of circuit  40  is derived from a separate power supply  15   b  of the line interface circuit. In one alternative, the power supplies  15   a  and  15   b  may be the same. 
         [0024]    The operation of the circuit  40  is equivalent to the operation of circuit  30  but for negative transients. That is, negative transient voltages appearing at the line interface  16  will be conducted through MOSFET  42  to ground  29 . 
         [0025]    While the embodiments describe transistors  22 ,  42  in the form of a MOSFET or IGFET, other types of transistors may be apparent to the person skilled in the art and such transistors are intended to be encompassed herein. 
         [0026]    Although embodiments of the present invention have been illustrated in the accompanied drawings and described in the foregoing description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims. For example, the capabilities of the invention can be performed fully and/or partially by one or more of the blocks or modules. Further, although depicted in a particular manner, various modules or blocks may be repositioned without departing from the scope of the current invention. Still further, although depicted in a particular manner, a greater or lesser number of modules and connections can be utilized with the present invention in order to accomplish the present invention, to provide additional known features to the present invention, and/or to make the present invention more efficient.