S&F Ref: 957376 AUSTRALIA PATENTS ACT 1990 INNOVATION PATENT SPECIFICATION Name and Address Tactical Technologies (IP) Pty Limited, of Applicant: an Australian company, ACN 066 744 661, of 5 Butterfield Street, Blacktown, New South Wales, 2148, Australia Actual Inventor(s): Bradley Xinxiang Yan Address for Service: Spruson & Ferguson St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: An electronic fault protection circuit The following statement is a full description of this invention, including the best method of performing it known to me/us: 5843c(2817940_ I) An Electronic Fault Protection Circuit Field of the Invention The present invention relates to fault protection and more particularly to an electronic circuit for short-circuit fault and overload fault protections. 5 Background Output overload fault and short-circuit fault protection is essential for electrical safety standards and user requirements. Depending on the application, various forms of current limiting may be used. However, such arrangements suffer from the disadvantages of being affected by ambient temperature fluctuations or 10 allowing the short-circuit current to exceed the maximum allowable fault current. An object of the present invention is to overcome or at least substantially ameliorate these and other disadvantages of existing arrangement. Summary According to one aspect there is provided an electronic fault protection circuit. 15 The circuit comprises a first thermistor having a negative temperature coefficient and a resistor wherein the first thermistor is connected in parallel with the resistor. The resistance and the temperature coefficient of the first thermistor and the resistance of the resistor are chosen according to a characteristic of a desired short-circuit current. Preferably the circuit further comprises a second thermistor having a positive 20 temperature coefficient wherein the second thermistor is connected in series with the parallel-connected first thermistor and resistor. Preferably the characteristic is the peak value of the short-circuit current or the shape of the short-circuit current with respect to time. Preferably a fuse is used instead of the second thermistor in the circuit. 2790396 I.DOC - 1 - According to another aspect there is provided a discrete electronic component comprising the any of the above circuits. Drawings At least one embodiment of the present invention will now be described with 5 reference to the drawings, in which: Figure 1 shows an individually fused multiple-output power supply system; Figure 2 shows empirical data of a short-circuit current spark on a polyswitch under short-circuit fault condition; Figure 3 shows typical Curves of Tripping Time vs. Current; 10 Figure 4 shows a circuit for fault protection; Figure 5 shows an individually fused multiple-output power supply system; Figure 6 shows empirical data of the short-circuit current through a relatively cold polyswitch; and Figure 7 shows empirical data of the short-circuit current through a relatively 15 hot polyswitch. Detailed description DC power supplies such as switched-mode power supplies (SMPS), low dropout (LDO) power supplies, and switched-mode AC power supplies use electronic control to limit the output current in the event of overload and to shutdown the power 20 supply under output short-circuit faults. Line-frequency AC power supplies and line frequency transformers typically use a non-resettable fuse having a suitable current rating for overload or short-circuit fault protection. DC power supplies and AC power supplies can be classified as a power supply of limited output current. As such, throughout this specification, a power supply of 25 limited output current will be referred to simply as a power supply module. 2790396 l.DOC -2- For a power supply module, it is necessary that overload fault and short-circuit fault protection is fast and accurate and that the protection threshold is tightly programmed or selected according to the electrical safety requirements. Therefore, during a fault protection event, loss of power to the load equipment is expected. 5 Figure 1 shows an individually fused multiple-output power supply system 100. In practice, the individually fused multiple-output power supply system 100 should not only meet electrical safety requirements, but also the normal operation of the remaining output channels should not be affected, under a fault condition of one or more output channels. Typically, the individually fused multiple-output power supply 10 system 100 has two-stage fault current protection, comprising the first protection stage 105 and the second protection stage 110. The first protection stage 105 protects the power supply module 115. Each output channel of the multiple-output power supply system is individually fused in the second protection stage 110 so that any single channel overload or short-circuit fault will not trigger the operation of first 15 protection stage 105 of the power supply module 115 so as not to affect the normal operation of the rest of output channels. The individually fused multiple-output power supply system 100 can be used for powering multiple security surveillance devices and IT-related devices simultaneously. Preferably, a resettable type of fuse is used in the second protection stage 110 20 for the reason that it is relatively maintenance-free and therefore economical. A polyswitch is a natural candidate as a resettable fuse. A polyswitch is a positive temperature-coefficient (PTC) thermal resistor (thermistor) made from a polycrystalline ceramic material that is normally highly resistive but is semiconductive due to doping. A polyswitch exhibits resistance-temperature 25 characteristics having a very small negative temperature coefficient until the 2790396 IDOC -3 polyswitch reaches a critical temperature that is referred to as the "Curie", switch or transition temperature. As the transition temperature is approached, the polyswitch begins to exhibit a rising positive temperature coefficient of resistance and a increase in resistance. The increase in resistance can be as much as several orders of magnitude 5 within a temperature span of a few degrees. The dramatic rise in resistance of a polyswitch at and above the transition temperature makes a polyswitch ideal for overload protection where the polyswitch is placed in series with the component that requires overload protection. Under normal conditions, the polyswitch has a relatively low resistance value. The current flowing 10 through the polyswitch does not provide enough energy to self-heat the polyswitch to its transition temperature. However, if overload condition occurs, the polyswitch will self-heat beyond the transition temperature and the resistance of the polyswitch rises dramatically so as to limit the current to the load. Removing the fault condition decreases the current flow and allows the polyswitch to cool to its normal resistance 15 mode. A polyswitch can be switched repeatedly and will return to the initial resistance value without being subject to hysteresis. While polyswitches are reliable and effective for overload protections their implementation has certain disadvantages for short-circuit faults because the short circuit current may be narrow but large enough to trigger the first protection stage 105 20 before the polyswitch in the second protection stage 1 10 trips. Figure 2 shows an example of short-circuit current spark 205 on a 30V/1.1A polyswitch which is connected to output of a 12VDC/lOA SMPS in an individually fused multiple-output power supply system. As can be seen, the initial short-circuit 205 spark is 76A. In this example, the current spark 205 did not trip the polyswitch 25 but triggered the SMPS module to shutdown. Also shown are current spikes 210 as a 2790396 I.DOC -4result of the SMPS module attempting to automatically restart but failing due to the presence of the short-circuit fault. The short-circuit current exceeding the maximum fault current of the polyswitch such that the short-circuit current triggerings the first protection stage 105 5 before the polyswitch causes protection malfunctions in the power supply module system 10015 and presents an electrical safety issue. To overcome this problem, the individually fused multiple-output power supply system 100 requires that in the event of a short circuit of an individual channel, only the second protection stage 110 trips and the first protection stage 105 does not. 10 The magnitude of the short-circuit current magnitude may depend upon: 1) The total resistance in the output path including the source resistance, cable resistance, PCB track resistance, contact resistance, and cold resistance of the polyswitch; 2) The ambient temperature; 15 3) The output voltage of the central Power Supply Module; 4) The output current limit of the central Power Supply Module; and 5) The output capacitor storage of the central Power Supply Module. As such, in practice it is difficult to predict the magnitude of a short-circuit current spark and therefore to protect against a loss of output of all of the output 20 channels that defeats the purpose of an individually fused multiple-output power supply system 100. Furthermore, the individually fused multiple-output power supply system 100 becomes even less reliable under full loaded because even a small short circuit current spark might trigger the central power supply module 115 to shutdown. In order to overcome the above problems, some manufacturers of PTC 25 thermal resistors have attempted to improve the design to achieve a faster switching. 2790396 I.DOC -5- However, commercially available polyswitches are generally still not fast enough. For example, Figure 3 shows typical Curves of Tripping Time vs. Current (RMS) at 250 for a commercial series of polyswitches rated at 30V with a holding current ranging from 0.9A to 9A. Taking the polyswitch 30V1 10 of 30V and 1.1A (curve number 2) 5 as an example, at 10A RMS current, it takes about 0.3 seconds for the polyswitch to trip, which is demonstrated by the current-time area 305. Referring to the example in Figure 2, current-time area 305 is much larger than that of the short-circuit current spark 205 of the polyswitch at the faulty output channel before the central power supply module 115 shuts down. 10 Even if non-resettable fuses are used for second protection stage 110, the above-mentioned problem during short-circuit faults may still exist depending on the fuse type and current rating and the capacity and current-limit characteristics of the central power supply module. In order to overcome this problem, one method is to limit the magnitude of 15 short-circuit current to a level which will not trigger the power supply module 115 but ensure that the polyswitch in second protection stage 110 trips reliably. For example, a linear power resistor of relatively small resistance value can be inserted into each output path to limit the short circuit current. While this method may work for some applications, it has the following drawbacks: 20 1) During normal operation, voltage drop in the inserted power resistor is significant, especially at high load current, resulting in very poor output voltage regulation; 2) Too much power dissipation and heat generated by the inserted power resistor; 2790396l.DOC -6- 3) It is difficult to choose the resistance value for a good design compromise. Figure 4 shows an electric circuit 400 being operable as a resettable fuse that requires less maintenance when compared to a non-resettable fuse. The circuit 400 5 trips at a relatively small short-circuit current and is relatively less sensitive to ambient temperature fluctuations when compared to a polyswitch. The circuit 400 is suited for short-circuit fault and overload fault protections and comprises a PTC thermal resistor (polyswitch) 415 connected in series with a parallel branch of a negative-temperature-coefficient (NTC) thermal resistor 10 (thermistor) 405 and a linear power resistor (linear resistor) 410. The circuit 400 is suited for placement in each output channel of a multiple-output DC or AC power supply, in series with each load. NTC thermistors exhibit high resistance when cold and small resistance when hot. As such the current-time characteristics and resistance-temperature characteristics 15 of the NTC thermistor 405 generally do not work well with those of the polyswitch 415 to achieve the needed combined current-time characteristics and resistance temperature characteristics for short-circuit fault protection. Further as NTC thermistors exhibits relatively large resistance when cold, the NTC thermistor 405 cannot limit the current effectively in repetitive faulty situations or at high ambient 20 temperatures. Further still it is difficult to choose a suitable NTC thermistor to work well together with a polyswitch for a good design compromise. However, the combination of the NTC thermistor 405 and the linear power resistor 410 in parallel exhibit current-time characteristics and resistance-temperature characteristics which differ to those of the linear resistor 410 or the NTC thermistor 25 405 alone. As such, the resistance and the temperature coefficient of the NTC 2790396 I.DOC -7thermistor 405 and the resistance of the linear resistor 410 are chosen according to the characteristics of a desired short-circuit current. Figure 5 shows an individually fused multiple-output power supply system 100 comprising one or more circuits 400 in the second protection stage 110. 5 Overload Fault Protection The circuit 400 provides overload fault protection in the same manner as a standalone polyswitch because the polyswitch 415 in the circuit 400 is in series with the load. Output Voltage Drop AT Normal Load Current 10 The voltage drop on the NTC thermistor 405 and linear power resistor 410 at normal load current is less than the instance where only the linear power resistor 410 is inserted. This is because a partial load current will heat the NTC thermistor 405, thereby lowing the resistance of the NTC thermistor 405, and, consequently, reducing the voltage drop across the parallel NTC thermistor 405 and linear power resistor 410. 15 At a higher load current, the proportion of load current flowing the NTC thermistor 405 increases, thereby limiting the voltage drop across the NTC thermistor 405 and linear power resistor 410, resulting in improved output voltage regulation compared to the situation where only the linear power 410 resistor alone is used. Output Short-Circuit Current Limiting at low ambient temperature 20 At a low ambient temperature or first time short-circuit fault, the NTC thermistor 405 exhibits a high resistance value. As such, resistance of the parallel NTC thermistor 405 and linear power resistor 410 is relatively high, thereby limiting the magnitude of the short-circuit current spark. 2790396 .DOC -8- Figure 6 shows an exemplary waveform of the short-circuit current on a cold 30V/1. I A polyswitch in a single channel within the circuit 400. The magnitude of the single short-circuit current 605 spark has been reduced to 12A, compared to 76A shown in Figure 2. More importantly, during the short-circuit fault, the polyswitch 5 415 in the faulty channel tripped reliably and the central power supply module 115 continued normal operation so that the remaining the output channels remained in operation. Output Short-Circuit Current Limiting at high ambient temperature At higher ambient temperature or in the case of repetitive short-circuit faults, 10 the resistance of the NTC thermistor 405 decrease, which tends to increase the short circuit current. However, at the same time, the resistance of the polyswitch 415 is high due to the higher temperature, thereby reducing the short circuit current. The positive temperature-coefficient effect of the polyswitch 415 compensates the negative temperature-coefficient effect of the NTC thermistor 405, which, combined with the 15 effect of the linear resistor as described below, keeps the magnitude of short-circuit current spark within a relatively small range. Figure 7 shows the waveform of short circuit current on a hot 30V/1.lA polyswitch within the circuit 400. As can be seen, Figure 7 is very similar to Figure 6. Near-Linear Resistance-Temperature Characteristics 20 Although a polyswitch (PTC thermistor) and a NTC thermistor have opposite temperature coefficients, their connection in series does not provide the desired resistance-temperature and current-time characteristics for the purpose of limiting the short-circuit current. Further, the use of the available standard values of polyswitches and NTC thermistors makes it impractical to achieve the desired characteristics. 2790396 1.DOC -9- As such, in the circuit 400, the linear resistor 410 is used to alter the current time characteristics, voltage-current characteristics and resistance-temperature characteristics of the parallel branch of the circuit 400, and consequently shape and limits the short-circuit current spark. Because a portion of the short-circuit current 5 flows through the linear resistor 410, the initial current flowing the NTC thermistor 405 is smaller than where no linear resistor is used, thereby slowing the heat generation on the NTC thermistor 405 resulting in a slower decrease of the resistance of the NTC thermistor 405. During the course of short-circuit fault, the temperature and resistance of the 10 polyswitch 415 increase rapidly. From the beginning of a short-circuit fault to the moment the polyswitch 415 trips, the resistance-temperature characteristics of the circuit 400 remain constant if the circuit 400 is designed properly. This can be verified by the plateaus of the short-circuit current sparks as shown in Figure 6 and Figure 7. As such, the circuit 400 is less sensitive to ambient temperature fluctuations, being an 15 important advantage over polyswitches alone which are quite sensitive to ambient temperatures. Possibility of a New Integrated or Hybrid Device Using the Circuit Topology It is possible that the parallel NTC thermistor 405 and the linear power resistor 410 may be manufactured in a single package using integration or hybrid technologies 20 such that installation of the circuit 400 will only involve two components, being a polyswitch and a hybrid NTC-Linear resistor. Wide Application Range of the Circuit As the circuit 400 comprises only passive components, it is suited for both DC and AC power supplies and is advantageous over other electronic types of overload 2790396 lI.DOC - 10fault and short-circuit fault protection circuits that may be suited only for DC output, require a DC bias voltage, have a limited voltage range or are complex and costly. There is no voltage polarity issue with the circuit 400. The circuit 400 topology is also suited for non-resettable fuses. That is, the 5 polyswitch 415 can be replaced by a non-resettable fuse. There are occasions where short-circuit current exceeds the maximum fault current (interrupting rating) of a fuse used for overload fault and short-circuit fault protections. For electrical safety compliance, the short-circuit current needs to be limited to a level which is within the interrupting capacity of the fuse. The circuit 400 therefore can be used to limit the 10 short-circuit current without affecting the overload fault protection and short-circuit fault protection functions of the fuse. The circuit 400 can be added to the output of any commercial power supply module to form an individually fused multiple-output power supply system, as illustrated in Figure 5. 1 5 Design guidelines for the circuit Analytical design for the circuit 400 is complex due to the non-linear thermal and electrical characteristics of the circuit 400. While computer modelling and an iterative program can be used for accurate and optimum design, a practical and simpler design method can be used. A reasonably good first approximation can be 20 made for circuit element values of the circuit 400 based on the heat capacity of the devices, the dissipation constant of the devices, the source voltage, the source resistance and the resistance of the devices at a specified ambient temperature. This first approximation may be adjusted using experimental testing and evaluation until an optimum circuit is achieved. Usually, only a few iterative steps are 27903961 .DOC - I I required to obtain a circuit that is suited to satisfy specific applications. The following steps are the design guidelines for the circuit 400: 1) Choose a polyswitch having a hold current greater than the rated current of the output channel and a voltage rating greater than the 5 maximum steady state voltage across the polyswitch; 2) Choose the resistance value of the linear resistor according to the desired maximum short-circuit current at cold (25"C) which should be about five times of the hold current of the polyswitch chosen above. As the parasitic resistance value in the shorted circuit is generally very 10 small, the inserted linear resistor together with the NTC thermistor will dominate the total resistance of the shorted circuit when cold. 3) Select an NTC thermistor such that the initial short-circuit current determined by the equivalent resistance of the parallelled NTC thermistor (at 25"C) and the linear resistor is more than six times the 15 hold current of the polyswitch. This will usually ensure that the polyswitch will trip during short-circuit faults and overload faults in the desired operational temperature range. 4) Iteratively evaluate the output voltage at open load and full load, overload fault protection, short-circuit fault protection and component 20 power ratings across the entire operational temperature range and adjust the component values iteratively if required. The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. 2790396 I.DOC - 12 - In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of". Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings. 2790396 .DOC - 13 -