Protection devices and methods for preventing the flow of undesirable differential mode transients

A differential mode surge protection apparatus includes first and second solid state protection devices in series between respective input and output connection points. The apparatus may be provided in conjunction with a common mode protection apparatus such as an isolation transformer. Alternatively, the apparatus may be provided integrated into a single miniaturized protection package. In a preferred embodiment, the apparatus includes a surge arrestor arranged to supplement protection conferred by the first and second solid state protection devices. The surge arrestor is capable of being triggered prior to breakdown of either the first or second solid state protection devices. The apparatus may be incorporated into digital processing cards such as LAN cards and also into cables to conveniently provide differential mode protection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from PCT Application Number PCT/AU2003/001326 filed Oct. 8, 2003, which claims priority from Australian Patent Application Number 2002951888 filed Oct. 8, 2002. Both applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to electrical surge protection devices, and more particularly to protection devices and methods for preventing the flow of undesirable differential mode transients.

BACKGROUND TO THE INVENTION

Wherever electronic circuitry is coupled to an external cable run, a risk of damage to the circuitry, due to the transmission of transient overvoltages by the cable run, may occur. Such overvoltages may be due to any one of several factors. For example, lightning, electrostatic discharge, or malfunction of equipment at a remote end of the cable may be responsible. Several techniques exist for isolating circuitry from potentially damaging surges. These include, inductive coupling as shown inFIG. 1, capacitive coupling, and opto-isolation as shown inFIG. 2. Isolation transformers are usually used to implement inductive coupling.

FIG. 1is a schematic diagram of a typical arrangement whereby a cable including a pair of conductors2and4terminate on an isolation transformer6which is connected to a load12. In the event of a common mode voltage surge, the potential on both conductors2and4typically varies in the same manner, thereby causing substantially zero net current to flow through the primary coil8of transformer6. As a result, the common mode transient does not induce a fault current in secondary coil10, and so isolation transformer6provides common mode protection to load12.

However, in the event that a voltage transient affects one of conductors2and4substantially more than the other, then a surge current typically flows through primary coil8and induces a transient voltage across the output terminals of secondary coil10. The surge current may damage the isolation device and may also damage the equipment that is intended to be protected. Consequently, while an isolation transformer provides a good measure of protection from common mode transients, it does not provide protection from differential mode surges.

One area where isolation transformers are used significantly is in the implementation of local area computer data networks (LANs). With reference toFIG. 3, a digital processing card in the form of a network interface card (NIC), or as it is often called a “LAN card,” couples to J8-45 plug22of LAN cable16by means of J8-45 socket18. LAN card14includes an isolation transformer module20that couples socket18to a data processing chip24, which in turn communicates with the central processor26of a workstation by means of PCI slot connectors28. Those PCI slot connectors28are received into a PCI connector,30which is in turn mounted on a mainboard32of the workstation.

FIG. 4is a schematic diagram of the prior art card ofFIG. 3, wherein like indicia are used to refer to like components. The card receives and transmits data pulses by means of respective balanced twisted pair wires that are enclosed in LAN cable16and that terminate on the RX+ and RX−pins and TX+ and TX− pins of J8-45 socket18.

Isolation module20includes a pair of transformers20A and20B which respectively provide isolation for processing chip24from transients on the transmit and receive twisted pairs. As previously explained, isolation transformers20A and20B usually provide considerable immunity from damaging common mode transients but not from differential mode transients. For example, in the event of a voltage transient occurring at the RX+ pin but not on the RX− pin, that transient most likely will be transmitted across the isolation transformer and may damage processing chip24.

It is therefore desirable to provide a convenient means for addressing the problems posed by differential mode transients as discussed above.

SUMMARY OF THE INVENTION

According to an embodiment, a differential mode surge protection apparatus comprises: a pair of input connection points and a corresponding pair of output connection points; a first unipolar solid state protection device in series between a first one of the pair of input connection points and a first one of the corresponding pair of output connection points; and a second unipolar solid state protection device in series between a second one of the pair of input connection points and a second one of the corresponding pair of output connection points.

The first and second unipolar solid state protection devices may be integrated into a single miniaturized protection package. A surge arrestor may be arranged to supplement protection conferred by the first and second unipolar solid state protection devices. It may be capable of being triggered prior to breakdown of either the first or second unipolar solid state protection devices. The surge arrestor may be connected across the input connection points.

The first and second unipolar solid state protection devices may comprise a matched pair of unipolar transient blocking units (TBUs), each unipolar transient blocking unit including two series connected field effect transistors.

The differential mode surge protection apparatus may further comprise a common mode isolation device disposed between the first unipolar solid state protection device and the second unipolar solid state device and the output connection points. The common mode isolation device may comprise an isolation transformer. Alternatively, the common mode isolation device may comprise another suitable device such as an opto-isolator.

According to another embodiment, the differential mode protection apparatus may be provided in combination with a digital signal processing card such as a LAN card. The unipolar solid state protection devices may be disposed between a common mode isolation device of the digital signal processing card and external connection points of the digital signal processing card. Alternatively, the unipolar solid state protection devices may be disposed between an output side of the isolation device and a digital signal processing chip of the card.

According to a further embodiment, a cable may be provided that includes one or more of the differential mode surge protection apparatuses. In that case the pair of input connection points may terminate upon a first connector of the cable, and the output connection points may terminate upon a second connector of the cable. The first connector may comprise a socket, and the second connector may comprise a plug. Alternatively, the first connector may comprise a plug, and the second connector may comprise a plug. Typically the cable comprises a LAN cable and the first and second connectors comprise LAN cable connectors.

Further preferred features of the various aspects of the invention will be apparent from the following description of preferred embodiments, which will be made with reference to a number of figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now toFIG. 5, an isolation module34according to an embodiment is depicted. Isolation module34includes a common mode isolation device42, which comprises a transformer. It is understood that common mode isolation device42could instead be some other common mode isolation device such as an opto-isolator. Isolation module34includes two identical series connected unipolar transient blocking units (TBUs)36and38connected at respective sides of primary coil40of transformer42.

A TBU is a transistor device configured to open-circuit once the current through it reaches a certain predetermined trigger level. TBU36consists of two depletion mode FETs being N-channel FET Q1and P-channel FET Q2. Q1and Q2are connected with their conduction paths in series. The gate electrode of transistor Q1is coupled to the drain electrode of transistor Q2. The source electrodes of Q1and Q2are coupled to each other and the drain electrode of transistor Q1is coupled to a first terminal44. Resistor R1extends between the drain electrode of Q1and the gate electrode of Q2. The drain of Q2is connected to a first side of primary coil40. In effect, TBU36functions as a fuse for positive current flowing from terminal44through to the primary coil. The structure of TBU38is identical to that of TBU36so that TBU38acts as a fuse for positive current flowing from terminal46towards primary coil40. TBU38and TBU36may be selected so that they have as close as possible insertion loss in order that the balance of the circuit is maintained.

TBUs36and38protect a load connected across terminals43and45from differential surge currents by triggering to an open circuit when the current through the load reaches the preset trigger level. If the differential current into terminal44attains the trigger level then TBU36would open-circuit. Alternatively if the differential current flowing out of terminal44attains the trigger level then TBU38would trigger. Of course, the TBUs could be reverse-orientated relative to the configuration depicted inFIG. 5. In that case if the differential current into terminal44attains the trigger level then TBU38would open circuit, and conversely if the differential current flowing out of terminal44attained the trigger level then TBU36would trigger.

In the event that the voltage across a triggered TBU continues to rise, then eventually a breakdown voltage would be reached at which the TBU conducts again. At present TBUs with breakdown voltages up to 800V are available. In order to provide protection from surges that exceed the breakdown voltage, surge arrestor48may be included. A gas arrestor, as depicted at item48is typically used. However, any suitable surge arrestor could be used as an alternative. Surge arrestor48is connected line-to-line across terminals44and46and is selected so that it will trigger at 90% of the TBUs' breakdown voltage. Consequently, in the event of a differential mode surge approaching the breakdown voltage of the TBUs, the surge arrestor would trigger and short the surge before it can cause breakdown of the triggered TBU and subsequently be transmitted across isolation transformer42to potentially damage any load connected between output terminals43and45.

Suitable TBU's may be purchased from Fultec Pty Ltd of Building 76A, University of Queensland Campus, St Lucia, Brisbane, Queensland, Australia. Circuits for implementing suitable TBUs are described in granted U.S. Pat. No. 5,742,463 and in International Patent Application No. PCT/AU03/00175, both by the present inventor. The descriptions of both U.S. Pat. No. 5,742,463 and PCT/AU03/00175 are hereby both incorporated in their entireties by cross-reference.

With reference toFIG. 6, according to a further embodiment of the invention a differential mode protection device43is conveniently provided in a single package having four connectors A, B, C, D as shown. The device may be provided with or without a surge arrestor as desired. A schematic diagram of the internal circuit of device43without a surge arrestor is shown inFIG. 7, whereas a schematic for a further version with surge arrestor is shown inFIG. 8. Further variations of the device are possible. For example an eight terminal version of the device might be provided including two of the circuits ofFIG. 7orFIG. 8.

A schematic diagram of a digital processing card, in the present example a LAN card, according to a preferred embodiment is shown inFIG. 9. It is noted that the LAN card ofFIG. 9incorporates two pairs of TBUs51A,51B and53A,53B placed in series with the TX+, TX−, and RX+, RX− terminals of socket18respectively. (The TBUs are each identical to TBUs36and38ofFIG. 5and might be provided in an eight terminal version of device43ofFIG. 6.) Surge arrestors19and21are connected across the TX+, TX− and RX+, RX− pins, respectively. The surge arrestors may be selected so that they will trigger at 90% of the TBUs' breakdown voltage. As previously explained, in the event of a differential mode surge approaching the breakdown voltage of the TBUs, the surge arrestors would trigger and short the surge before it can cause breakdown of the TBUs and subsequently be transmitted across isolation transformer to potentially damage logic circuitry24.

FIG. 10depicts the external appearance of a differential mode surge protector50according to a further embodiment whileFIG. 11is a corresponding schematic diagram. Surge protector50includes a J8-45 plug52for connection to a socket of a LAN card. The J8-45 plug is coupled to a differential mode protection module54which includes TBU's and surge arrestors as shown inFIG. 11. The surge protector further includes a J8-45 socket58which is coupled to circuitry in module54and which is intended to receive a LAN cable.

FIG. 12is a variation of the differential mode surge protector ofFIGS. 10 and 11comprising a cable63that incorporates differential mode protection. Cable63includes a connector52for connection to a digital processing card. In the present example it is intended that cable63be a LAN cable and so connectors52and59are J8-45 plugs although other connectors might be used in other applications. A differential mode protection module54, identical to the one inFIG. 10and containing the circuitry depicted inFIG. 11, is coupled to connector52. A relatively long run of cable61connects protection module54, which includes TBU's and surge arrestors as shown inFIG. 11, to remote connector59. In use it is intended that connector52be plugged into a digital processing card such as a LAN card whereas connector59is plugged into a remote device such as a network hub. If desired, a second module may be incorporated proximal to plug59. Variations to the cable shown inFIG. 12are possible. For example, the circuitry that is provided inside housing54might be integrated into one or the other of connectors52and59.

Although the present invention has been described in terms of preferred embodiments, it is not intended that the invention be limited to these embodiments. Equivalent methods, structures, arrangements, processes, steps, and other modifications apparent to those skilled in the art would fall within the scope of the following claims.