TRANSIENT SUPPRESSION SYSTEM

Abstract: A transient suppression system comprising at least one transient suppressor for clamping short duration pulse voltages and a further device comprising at least one positive temperature coefficient thermistor (12) for con­ trolling the electrical power input to said at least one transient suppressor (11) should be improved. At least one transient suppressor (11) is placed in intimate thermal con­ tact with at least one of the positive temperature coef­ ficient thermistors (12).

Description: This invention relates to a transient suppression system comprising at least one transient suppressor for clamping short duration pulse voltages and a further device com­ prising at least one positive temperature coefficient thermistor for controlling the electrical power input to said at least one transient suppressor thereby preventing longer duration fault conditions damaging said transient suppressor. A transient absorption semiconductor device (transient suppressor) presents a relatively high impedance over a given range of applied voltage and a low impedance in response to transient voltage excursions from the given range, the device having particular application as a pro­ tection device in an electrical circuit, for protecting the circuit by selectively conducting transient surges in current flowing in the circuit. It is known that a PN junction in a semiconductor body can be used to protect an electrical circuit from tran­ sient voltage surges. The PN junction is connected across a voltage supply to the circuit such as to be reverse biased. The breakdown voltage of the reverse biased junc­ tion is selected to be somewhat greater than the voltage normally developed by the supply. Thus in normal use, the junction does not conduct substantially but in the event of a transient voltage surge which results in the supply voltage exceeding the breakdown voltage, the reverse biased junction breaks down and conducts the current surge associated with the voltage surge, so as to protect the circuit from the surge. The breakdown voltage is typ­ ically in the range 2 - 1000 V the value thereof being selected for the junction concerned by control of the doping levels utilised in manufacture of the junction. For higher voltages in the aforementioned range of break­ down voltages the breakdown occurs by virtue of the Ava­ lanche effect whereas at lower voltages the Zener effect may predominate. Typically the current flowing in the reverse biased junc­ tion prior to breakdown is of the order of 1 - 10 µA whereas after breakdown, heavy current surges with peak currents of 25 amps or more may flow. Transient suppressors are frequently used in connection with fuses in surge protection circuits such as that shown in Figure 1. The transient suppressor 1 is used to clamp short duration pulse voltages with time constants in the range 10 nsec to 10 msec. These voltages can reach 5000 V and the associated currents 5000 amps. The fuse 2 is used to prevent longer duration fault conditions damaging the transient protector 1 and thereafter subse­ quent electronic devices in the system. The fuse 2 controls the electrical power input to the transient suppressor 1. The transient suppression system of Figure 1 has the input 7 and the output 8. Figure 2 shows two such circuits as shown in Figure 1 in a configuration often used in telecommunications applica­ tions. The transient suppression system of Figure 2 com­ prises two transient suppressors 3, 5 and two fuses 4, 6, the two transient suppressors 3, 5 having one common ter­ minal which is set on a reference potential. Instead of fuses positive temperature coefficient thermi­ stors have been used for this application as they are un­ affected by short time constant current and voltage tran­ sients. Such a transient suppression circuit offers a high degree of protection to subsequent components and operates well under a wide range of fault conditions. There is however a range of long time constant surges which are too low in current to affect the thermistor resistance but large enough to cause excessive dissipa­ tion in the transient suppressor device. These can lead to degradation of the transient suppressor device and the circuitry it is protecting. The present invention provides an improved transient sup­ pression system in which the aforementioned disadvantages are overcome. At least one transient suppressor is placed in intimate thermal contact with at least one of the pos­ itive temperature coefficient thermistors. The transient suppressor device is placed in intimate thermal contact with the positive temperature coefficient thermistor. This allows heating of the transient suppres­ sor device over the temperature at which the thermistor resistance exhibits large changes (∼100 °C) to result in current limiting. This current limiting effect will pro­ tect both the transient suppressor device and the subse­ quent circuitry from damage. Those skilled in the art will readily be able to deter­ mine from the following explanations and by routine trial and experiment suitable junction configurations and doping levels which produce this decrease in voltage upon breakdown of the reverse biased junction. Whilst, as will hereinafter be discussed, variation of one or more of the manufacturing parameters of the tran­ sient suppression system according to the invention enables systems to be manufactured consistently and reliably with breakdown voltages within a relatively wide range and up to relatively high values of the order of 150 to 200 V for example. For applications in the higher voltage areas the invention further proposes, rather than the provision of a single transient suppres­ sor, to couple together in series two or more transient suppressors each of a lower breakdown voltage rating. The resultant composite system has a breakdown voltage sub­ stantially corresponding to the sum of the breakdown voltages of the individual transient suppressors of the composite multi-device structure. In order that the invention may be more fully understand embodiments thereof and their operating characteristics will won be described by way of illustrative example and by way of contrast with prior art systems, reference being had to the accompanying drawings wherein: Figures 1 and 2 show prior art transient suppression systems; Figure 3 shows a transient suppression system ac­ cording to the invention; Figure 4 shows the electrical circuit elements of the transient suppression system accord­ ing to Figure 3. Figure 3 shows one application of the invention. A tran­ sient suppressor device 11 type L5C 150 CD manufactured by Semitron Cricklade Ltd, Cricklade Swindon, Wiltshire SN6 6HQ, GB, is soldered to a positive temperature coef­ ficient thermistor 12 type manufactured by STC Com­ ponents, GB. The transient suppressor device L5C 150 CD is of a type known as a foldback device (European Patent Application 82 304 718.8 = EP 0 088 179 A and US-Patent application Ser. No. 418 357). Such a foldback device comprises a body of semiconductor material including a region of a first conductivity type disposed between regions of opposite conductivity type so as to define first and second spaced apart semiconductor rectifying junctions, first and second electrodes so ar­ ranged that when a potential difference is applied there­ to one of said junctions is forward biased and the other of the junctions is reverse biased, the junctions being disposed to interact in such a manner that upon breakdown of the reverse biased junction from a relatively low con­ ductivity state to a relatively high conductivity state and with an increase in the magnitude of the current flowing through the device, the magnitude of the poten­ tial difference developed across the electrodes decreases from that which causes said breakdown but does not de­ crease to a value less than a given non-zero value. For an applied voltage V up to a given breakdown volt­ age V b , the current flowing through the foldback device is the foldback device leakage current which is very small, for example 5 µA, and this has a significant ef­ fect on the operation of the circuit being protected. In this situation, one PN junction is forward biased, and the other PN junction is reverse biased but not such as to exceed its breakdown voltage. When a voltage transient occurs in the supply voltage V, such as to apply to the device a voltage exceeding V b , the reverse biased junction breaks down and conducts by means of the Avalanche effect. Consequently, the foldback device then provides a conductive path for surge currents produced by the voltage transient, and the surge currents thus by-pass the circuit to be protected to prevent it being damaged. As the surge current increases, the volt­ age developed across the device rapidly reduces from the foldback device breakdown voltage V b to a predetermined non-zero value V m and as the current increases further, the voltage remains at the value V m at least over a con­ siderable range of surge current. The voltage V m may be selected by means of the doping process used to make the device and principally by con­ trol of the proximity to one another of the two PN junc­ tions by control of the dopant diffusion depths and will be selected in dependance upon the characteristics of the circuit to be protected to be in excess of the minimum safe supply voltage for the circuit being protected. Such a foldback device 11 is designed to breakdown at approximately 160 V. The voltage across the device 11 is then reduced to approximately 100 V due to the "foldback" action of the device 11. The thermistor 12 is designed to change from a low re­ sistance state (20 ohms) to a high resistance state (20 kohm) by heating from 100 °C to 150 °C. The assembly has three terminals 13, 14, 15 soldered to it with solder 16, 17, 18, and is coated with silicone rubber 19 to pas­ sivate the semiconductor junctions and to prevent flash­ over. Metal electrodes 20 are placed between the solder 16, 17, 18 and the respective surfaces of the devices 11, 12 in order to improve the electrical contact between the terminals 13, 14, 15 and the devices 11,12. The resultant composite device has the circuit elements shown in Figure 4. One such device is required for the circuit shown in Figure 1 and two such devices are re­ quired for the circuit shown in Figure 2. The terminals 13, 14 of the device of Figure 3, 4 are supplied with the electrical input and the termi­ nals 14, 15 yield the corresponding electrical output. An assembly of the type depicted in Figure 3 has shown excellent powers of circuit protection. The thermistor 12 protects against continuous excessive current. The tran­ sient suppressor 11 protects against short time constant surges. The thermistor 12 also reacts to excessive tem­ perature due to power dissipation in the transient sup­ pressor device 11. The combined unit therefore acts as a protector in intermediate fault conditions. Such behav­ iour was observed when subjecting the device to a 200 mA/240 V AC fault current for a duration of 5 min­ utes. If the thermistor 12 and transient suppressor 11 were not placed in thermal contact, the transient sup­ pressor 11 would be degraded by this condition. The new innovative device limited the current to prevent exces­ sive temperature rise in the transient suppressor de­ vice 11. The transient suppression system does not only apply to Zener diode based Semiconductor devices, but can apply to other transient suppressor devices such as gas discharge tubes and thyristor based protectors.

Claims: 1. Transient suppression system comprising at least one transient suppressor for clamping short duration pulse voltages and a further device comprising at least one positive temperature coefficient thermistor for con­ trolling the electrical power input to said at least one transient suppressor thereby preventing longer dura­ tion fault conditions damaging said transient suppres­ sor characterized in that at least one transient suppressor is placed in intimate thermal contact with at least one of the at least one positive temperature coefficient thermistors. 2. Transient suppression system according to claim 1, characterized in that at least one transient suppressor is soldered to at least one of the at least one positive temperature coefficient thermistors. 3. Transient suppression system according to claim 1 or 2, characterized in that at least one positive temperature coefficient thermistor is designed to change from a low resistance state to a high resistance state by heating from 100 °C to 150 °C. 4. Transient suppression system according to one of the claims 1 to 3, characterized in that at least one transient suppressor is designed to break­ down at approximately 160 V. 5. Transient suppression system according to claim 4, characterized in that after the breakdown of the at least one transient sup­ pressor, the voltage across the at least one transient suppressor is reduced to approximately 100 V due to a foldback action of the transient suppressor. 6. Transient suppression system according to one of the claims 1 to 5, characterized by three terminals soldered to it. 7. Transient suppression system according to one of the claims 1 to 6, characterized by a coating of silicone rubber. 8. Transient suppression system according to one of the claims 1 to 7, characterized by at least one Zener diode based Semiconductor device as a transient suppressor. 9. Transient suppression system according to one of the claim 1 to 8, characterized by at least one gas discharge tube as a transient suppres­ sor. 10. Transient suppression system according to one of the claims 1 to 9, characterized by at least one thyristor based protector as a transient suppressor.

Inventors: HOWARD, CHRISTOPHER GEORGE, WOODS, IAN
Applicants: SEMITRON CRICKLADE LIMITED
Application year: 1985
Main classification: H02H9
IPC codes: H02H9