GB2302624A - Surge protector - Google Patents

Abstract

A surge protector 22 is arranged to operate as an isolator for at least one signal line between an input 10 and an output 12 and comprises a solid state switch 14 connected in series between input and output which allows the surge current to flow to a voltage reference point 18 for its duration. Secondary shunt protection devices 26 may be connected on each side of the switch 14. Preferably, the protector includes a voltage reference section, thereby to enable clamping levels to be programmable.

Description

SURGE PROTECTION This invention relates to surge protectors and to methods of surge protection.

Surge protectors are used in various operating circumstances. Conventionally, surge protectors can be classified as being either surge diverters or surge isolators. Surge diverters interrupt the signal flow during the surge, but they require a co-ordinated and purposeful surge path to earth. Surge isolators do not interrupt the signal, but they tend to be more expensive as they require more electronic components to implement the isolation. In broad principle, surge divert technology is current limited and surge isolation technology is voltage limited. A surge diverter will ultimately fail on current and a surge isolator will have a voltage break-down limit prior to its destruction.

A somewhat different approach would be to quasiisolate the surge. This has been done using chopper amplifiers and bucket isolators. Here the interference is sensed and the circuit is switched into an open state for the duration of the surge. Lightning protectors have been developed based upon this principle. In one form of lightning protector, a small reed relay is used to open the surge from secondary protection diodes. The lightning protector is designed to operate within the window which exists between the firing of a primary protector gas arrestor and the destruction characteristics of the secondary protection diodes. However, this lightning protector is still essentially using surge divert technology since the surge is diverted to earth either via the primary gas arrestor or, at a lower current, through the secondary protection diodes, outside the operating window.

It is an object of the present invention to produce a quasi-isolator without recourse to using surge divert components. It is a further object of the invention to achieve this using solid-state technology, and in particular FET technology.

In accordance with the present invention there is provided a surge protector arranged to operate as a quasi-isolator for a signal line or lines between at least one input and at least one output, the protector comprising switching means connected in series between said at least one input and said at least one output which opens the signal path for the duration of a surge measured with reference to an earth reference point.

With such a surge protector, primary protection is not mandatory and no special earthing arrangements with their inherent inductance problems are needed.

In order to cover a wider range of applications and hazard levels the inclusion of a primary protector may be desirable. This would increase the specification of the surge protector. However, any heavy duty earth wiring would not suffer the same constraints as with conventional devices with regard to inductive voltage drops, and this would therefore simplify the installation of such devices.

In a preferred embodiment of the invention the switching means comprises at least one pair of FETs connected back-to-back. It is envisaged that the circuit of the surge protector could be fabricated on a semiconductor wafer, in which case it would be relatively simple to duplicate the sense and drive circuits in order to achieve more i/o per chip.

According to another preferred feature of the invention, the surge protector includes a voltage reference section or an equivalent, so that the actual clamping levels of the chip are programmable by the user. This can be implemented using a simple external programming resistor.

In order that the invention may be more fully understood, more details are now given of one presently preferred embodiment of surge protector in accordance with the invention. This is given by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a block schematic diagram to illustrate the general function of the surge protector; Fig. 2 is a block schematic diagram showing a typical cell construction for the surge protector; and Fig. 3 illustrates an application of the surge protector.

Fig. 1 is a functional diagram to illustrate the invention. This shows a plurality of inputs 10 connected by signal lines to an equal number of outputs 12 by way of the surge protector of the invention. If the circuit has no primary protection then the maximum input voltage would be of the order of 1.5 kV. If primary protection is provided, then the maximum input current would be of the order of 5 kA. A switching device indicated at 14 is connected in series in the signal lines between the inputs 10 and the outputs 12 and is connected to a sense circuit 16 which is connected to an earth reference point 18 and which is connected through an amplifier 19 to an input 20 which receives a clamping voltage which is programmable. The surge protector of the invention is capable of dealing with a surge of infinite duration.

Fig. 2 shows a typical solid state CHIP (commoned hybrid isolating protector) cell construction 22. Here the switching device 14 is shown connected in series between an input 10 and an output 12, with a fuse 24 between the input and the switching device. The sense circuit 16 is connected to the series device 14 and to an earth reference pin 18. The sense circuit 16 also receives an input from a programmable clamping voltage.

The series-connected switching device 14 is based upon the use of FETs, and preferably MOSFETs. In a preferred embodiment, by way of example, one could use a pair of MOSFETs connected back-to-back, or a number of such pairs.

The sense circuit 16 is essentially a comparator which compares the incoming signal with a reference level, and the output of which drives the gates of the MOSFETs to switch them on and off.

Shunt elements, shown as bipolar Zener diodes 26 for example, are connected as secondary protectors on each side of the switching device 14 between each side of the switching device and the earth reference pin 18.

The shunt protector on the input side could alternatively be a VDR.

Fig. 3 illustrates a printed circuit board which incorporates the surge protector of the invention. A programmable resistor 28 is shown connected to the wafer 30. The circuitry is provided with an additional gas-discharge tube (gdt) 32 to improve the rating.

A benefit of the surge protector of the invention is its small size, thereby giving high density protection, possibly on a semi-conductor wafer. It can also be fabricated at low cost. It can give protection on actual printed circuit boards, and therefore would be a desirable product for circuit designers to use on a pcb whenever inputs need better protection than simple esd.

It is also envisaged that the surge protector of the invention will withstand intermittent mains power faults. The device would then have application for telecom and datacom markets where many terminations are compressed into a small area. Existing products either catch fire when mains power is inadvertently applied, or else are very expensive.

A unique and highly desirable feature of the surge protector is the provision of a programming terminal which can be used to set clamping levels.

Claims (12)

CLAIMS:

1. A surge protector arranged to operate as a quasi-isolator for a signal line or lines between at least one input and at least one output, the protector comprising switching means connected in series between said at least one input and said at least one output which opens the signal path for the duration of a surge measured with reference to an earth reference point.

2. A surge protector according to claim 1, which includes a primary protector.

3. A surge protector according to claim 1 or 2, which includes secondary shunt protection means on each side of said switching means.

4. A surge protector according to claim 3, in which at least one of said secondary shunt protection means comprises a Zener diode.

5. A surge protector according to any preceding claim, in which said switching means comprises at least one pair of FETs connected back-to-back.

6. A surge protector according to claim 5, in which said FETs are MOSFETs.

7. A surge protector according to any preceding claim, which includes a sensing circuit arranged to compare an incoming signal with a reference level and to actuate the switching means in accordance with the comparison.

8. A surge protector according to any preceding claim, which includes a voltage reference section, thereby to enable clamping levels to be programmable.

9. A surge protector according to claim 8, in which the voltage reference section includes an external programming resistor.

10. A surge protector according to any of claims 1 to 7, which includes a programming terminal which can be used to set clamping levels.

11. A surge protector according to any preceding claim, mounted on a printed circuit board.

12. A surge protector substantially as hereinbefore described with reference to the accompanying drawings.