Electronic Barrier Device for Intrinsically Safe Systems

Abstract

The present invention provides for an electronic barrier device, which can form part of a further device such as an isolating barrier or a zener barrier, and comprising a voltage limiter such as at least one zener device for voltage limitation in a circuit during a fault condition, the barrier device including a crowbar device arranged to latch across the at least one zener device to reduce power dissipation in the at least one zener device in the circuit fault condition, wherein the crowbar device is arranged to latch responsive to a change in a current sensed in the barrier device.

Claims

1. An electronic barrier comprising a voltage limiter device for voltage limitation in a circuit during a fault condition, the barrier device including a crowbar device arranged to latch across the voltage limiter device to reduce power dissipation in the voltage limiter device in the circuit fault condition, wherein the crowbar device is arranged to latch responsive to a change in a current sensed in the barrier device.

2. A barrier device as claimed in claim 1, and including a fuse and wherein the said sensed current comprises a current through the fuse.

3. A barrier device as claimed in claim 1 or 2, and including a limiting resistor in an output line thereof.

4. A barrier device as claimed in claim 1, 2 or 3, and comprising an intrinsic safety barrier arrangement.

5. A barrier device as claimed in claim 1, 2, 3 or 4, wherein the crowbar device is arranged, when latched across the voltage limiter device, to remove all but negligible power dissipation within the voltage limiter device.

6. A barrier device as claimed in claim 5 and arranged such that almost all of the power dissipation during the circuit fault condition occurs across the crowbar device.

7. A barrier device as claimed in any one or more of the preceding claims wherein the voltage limiter device comprises at least one zener device.

8. A barrier device as claimed in any one or more of the preceding claims, and arranged to reduce power dissipation in non-voltage limiter devices within the barrier device.

9. A barrier device as claimed in claim 8, and arranged to reduce power dissipation in at least one limit resistor of the barrier device.

10. An electronic circuit including a zener barrier device as claimed in any one or more of the preceding claims,

11. An electronic circuit as claimed in claim 8 and comprising an isolation barrier.

12. A crowbar device arranged to latch across a voltage limiter device of a barrier device, to reduce power dissipation in the voltage limiter device, wherein the crowbar device is arranged to latch responsive to a change in a current sensed in the barrier device.

13. A crowbar device as claimed in claim 12 and arranged, when latched across the voltage limiter device, to stop power dissipation in the voltage limiter device.

14. A crowbar device as claimed in claim 12 or 13, wherein the voltage limiter device comprises at least one zener device.

15. A crowbar device as claimed in any one or more of claim 12, 13 or 14, and arranged to reduce power dissipation in non-voltage limiter devices within the barrier device.

16. A crowbar device as claimed in claim 15, and arranged to reduce power dissipation in at least one limit resistor of the barrier device.

17. An electronic circuit including a crowbar device as claimed in any one or more of claims 12 to 16.

18. An electronic circuit as claimed in claim 17 and comprising an isolation barrier.

19. A method of reducing power dissipation in a voltage-limiter of a barrier device during a circuit fault condition, including the step of latching a crowbar device across the voltage limiter to reduce the said power dissipation, and wherein the step of latching the crowbar device occurs responsive to the sensing of a change in current in the barrier device.

20. A method as claimed in claim 19, and including the step of sensing a change in current through a fuse device.

21. A method as claimed in claim 19 or 20, wherein the crowbar device, when latched across the voltage limiter device, removes all but negligible power dissipation within the voltage limiter device.

22. A method as claimed in claim 21, wherein almost all of the power dissipation in the barrier device during the circuit fault condition occurs across the crowbar device.

23. A method as claimed in any one or more of claims 19 to 22, wherein the voltage limiter device comprises at least one zener device.

24. A method as claimed in any one or more of claims 19 to 23, and including reducing power dissipation in non-voltage limiter devices within the barrier device.

25. A method as claimed in claim 24, and including reducing power dissipation in at least one limit resistor of the barrier device.

Description

[0061] The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which:

[0062] FIG. 4 the circuit diagram of a barrier device employing a voltage crowbar device according to an embodiment of the present mention; and

[0063] FIG. 5 is a voltage diagram illustrating the operational tolerances of a voltage crowbar device such as that illustrated in FIG. 4.

[0064] As will therefore be appreciated, the invention involves triggering the shunt on a sensed current rather than an input voltage.

[0065] Turning to FIG. 4, which for ease of comparison has a similar configuration as the prior art arrangement of FIG. 2, an undefined voltage with undefined current capability is present at an input 404 and some or all of its power is available on an output 405. The voltage is subjected to voltage limiting which, in this illustrated example, comprises zener clamping 401 to guarantee a maximum value ≤Vz. This now protected voltage, while passing current through a limit resistor 403 restricts the maximum current available on the output 405 to ≤Vz÷RI.

[0066] In addition, the input current is sensed through a resistor 440 and evaluated by a comparison element 441. If the input current is higher than a safe value, the comparison element will close the shunt element 420. This shunt element is advantageously of the latching type and will remain closed until its current is removed. Additionally, because of the circuit, closing the shunt consequently increases the current through the sense element 440 and it is therefore self-latching.

[0067] The maximum power dissipated is now across the shunt and is now only (1.7×the fuse rated current)×(closed shunt voltage). The power has thus been reduced, not by sensing the voltage, but rather by sensing the current. That is, the both power has been reduced and also the trigger for activation of the crowbar is different.

[0068] Normally the user is not operating the output in short-circuit but in some useful mode, which will be well below the maximum current available through the fuse. This means that the current clamp can advantageously operate at a much lower value than the nominal fuse value×1.7.

[0069] Examples of practical operating ranges of such a shunt are illustrated in FIG. 5.

[0070] That is, the zeners have a voltage tolerance 501, 502. Additionally the nominal zener value decreases with the temperature giving Vzh, the maximum zener voltage 511 and Vzl, the minimum zener voltage 512. Vzh is the safety voltage 505 used to describe the intrinsically safe output 405.

[0071] The incoming voltage will be subject to noise 541. Practically, the noise range must be situated underneath the zeners to prevent them conducting repetitively. The useful voltage will be Vzu, the minimum noise level 542.

[0072] Since the shunt circuitry is not operating in the voltage range, this means that the useful voltage Vsu is the minimum noise level 542.

[0073] Practically it is desired to operate the output up to In, the maximum operating current 570. In current mode, the noise level is much lower than in voltage mode. Allowing for a small current noise 591, and the small tolerances 561 on the current trigger, this permit to place Isl, the minimum current trigger level 571 very near the noise. One can set Is, the nominal current trigger level 581 just after Ish, the maximum current trigger level 572.

[0074] The following are examples of possible values for illustrative purposes, when possible using the same values as our previous example, for comparison:

[0075] Fuse 402: 1 A

[0076] Zeners value 401: 10V total

[0077] Zener tolerance 501, 502: ±5% (full temperature)

[0078] Limit resistor 403: 20Ω

[0079] Voltage noise level 541: 1V

[0080] Current noise level 541: 0.1V

[0081] Shunt trigger point tolerance 521: ±2%

[0082] Vzh=10V+5%=10.5V Vzl=10V−5%=9.5V

[0083] With a safety margin of 0.5V: Vzu=Vsu=9.5V−0.5V−1V=8.0V

[0084] The safety voltage max 505 is Vzh=10.5V

[0085] The useful voltage max 542 is Vsu=8.0V (76% of Vzh)

[0086] Maximum zener power (no shunt): Pz=(10V+5%)×(1 A×1.7)=17.8 W

[0087] Maximum shunt-on power: Ps=1V×(1 A×1.7)=1.7 W

[0088] Maximum current available: In=10.5V÷20Ω=0.53 A

[0089] With a safety margin of 0.1 A and the current noise:

[0090] Isl=0.53 A+0.1 A+0.1 A=0.73 A Ish=0.73 A+4%=0.76 A

[0091] With a safety margin of 0.1 A: Is=0.76 A+0.1 A=0.86 A

[0092] We can see in this example that the useful voltage available has increased from 58% to 76%, a substantial gain.

[0093] Various advantages arise from the use of an over-current triggered shunt as compared with the present art. In particular, more voltage is available at the output of the barrier device. Also, there is less distance between the maximum declared safe voltage and the practical voltage available

[0094] Operating with reference to an input current value, and because the maximum current is defined by a short on the limit resistor, it is easy to place the trigger level just outside that input current. As the circuit must be triplicated, and there is no need for a reference, the overall cost is not severely impacted by such requirement for triplication.

[0095] Also, if a fault develops, which takes any substantial current, the current after the fuse is likely to increase. This means that the trigger circuit will activate. In consequence every shunt element must not be required to take the full power available with a 1.7 times the nominal fuse current. If, in one chain of zeners, one fails with a short, or low value, the current will increase to the shunt trigger value and therefore will automatically be safe.

[0096] By setting the shunt trigger current very near the practical maximum output current, it will be very near the fuse rating or lower without being affected by the factor 1.7.

[0097] Various potential fail-scenarios are outlined below, with discussion of the respective reactions of a voltage sensed shunt as in the current art, and a current sensed shunt according to an embodiment of the invention.

[0098] First, it is envisaged that one zener might fails, either with a lower value or short. Then, for a voltage sensed shunt, the shunt may never trigger as the voltage is likely to decrease, and also any other zener(s) will see the full power available. However, with a current sensed shunt, it remains fail-safe since the shunt will trigger early as the current will rise.

[0099] Next it is envisaged that one shunt element fails, it can fail in any mode. If it fails open, a voltage sensed shunt will remain fail safe since the shunt is triplicated. In a similar way, for a current sensed shunt, fail safe operation will be retained as again the shunt is triplicated

[0100] If one shunt element fails, it can fail in any mode. If it fails resistive or short then for a voltage sensed shunt, the element may have to take all of the available power. The triplication will not help as the voltage is likely to decrease, therefore no trigger action is guaranteed. However, for a current sensed shunt, it will remain fail safe and, as the element will take more current, the rest of the triplicated shunt can or will trigger.

[0101] Further, if one comparison element fails, it can fail in any mode and with a voltage sensed shunt, it will be in fail safe mode as the shunt is triplicated, or else it will be triggered. For a current sensed shunt, again, it will remain in fail safe mode as the shunt is triplicated, or it will be triggered.

[0102] If one reference fails, it can fail in any mode and, as above, the voltage shunt will be in fail safe mode and; as the shunt is triplicated or it will be triggered. As there is no reference, in such a fail mode a current sensed shunt is not applicable.

[0103] As a final example, if the sense resistor fails, it can fail with a higher value or open and in such case a voltage sensed shunt will not be applicable. However, a current sensed shunt will be in fail safe and the shunt will trigger early as the current will be sensed at a higher level.

[0104] It should of course be appreciated that the invention is not restricted to the details of the foregoing embodiments. For example, the invention need not be embodied solely in a zener barrier device per se, but can also related to associated apparatus using a barrier arrangement as a possibly integral part of the circuitry, such as for example isolating barrier devices.

[0105] Further, the crowbar could additionally be arranged to reduce power in other components of the barrier, such as for example the limit resistor.