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Our compliance agency believes that it is all too likely that a 50 V power adapter is used instead of our 5 V power adapter. Regulation wants a fuse which can protect from 50+ V, and not allow 15 W dissipation in any case.

I am aiming for an IEC/UL approved 500mA fuse, but for overvoltage protection the zener diodes are a bit iffy. The idea is to short circuit any overvoltage to blow the fuse. However the zener diodes will be subjected to a very high power say 5.6 V*1 amp=5.6 W for at least some milliseconds. The power rating of these SOD323 is 300-400mW. One datasheet showed 40W for 100us.

Does anybody have experience with this kind of passive protection? The equipment is solely expected to break and not be repaired and needs to meet IEC/UL standards. I have no room for crowbar solutions.

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  • \$\begingroup\$ Have you thought about using a crowbar circuit as a safety mechanism? \$\endgroup\$ – vtolentino May 14 '20 at 9:20
  • \$\begingroup\$ Have you looked into crowbar circuits? They are designed to blow the fuse in the cery case you describe. What makes them too large for you? How much space do you have? \$\endgroup\$ – winny May 14 '20 at 9:20
  • \$\begingroup\$ @vtolentino Damn it, you beat me by 16 seconds! \$\endgroup\$ – winny May 14 '20 at 9:21
  • \$\begingroup\$ @winny internet connection seems stable and fast today :). \$\endgroup\$ – vtolentino May 14 '20 at 9:22
  • \$\begingroup\$ @vtolentino Low ping bastard! Probably wall jumper and hacker too! :-D \$\endgroup\$ – winny May 14 '20 at 9:23
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For this. It would be a crowbar or an e-fuse. The crowbar is cheaper in general. But the e-fuse is much faster and non destructive

If your requirements are only 50V and 500mA it could make sense to just fit a buck converter on the input with a low drop bypass mode. When he circuit starts up. If the voltage is too low. Then the buck converter is in bypass. If the voltage rises. It leaves the bypass mode. And the buck converter fires up and keeps your circuit in spec.

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  • \$\begingroup\$ It is a pretty wide input voltage requirement, especially for something which is a corner case fire protection. Also I cannot know the failure mode of the buck, maybe it just continues to conduct 50 V to the "safe" net? \$\endgroup\$ – Bonnevie May 14 '20 at 13:07
  • \$\begingroup\$ I checked out TI's e-fuse selection, and will do the job, but they are always 10+ pin chips, unfortunately. \$\endgroup\$ – Bonnevie May 14 '20 at 13:47
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I have successfully tested a 1 A fuse along with a single 5.2 Volt "normal" Zener diode in a 300mW SOD323 package. I have a "deadzone" between 5.2 and 6.5 V where the zener could potentially melt before the fuse. At 6.6 V the fuse melts after 26 seconds (1 test).

At higher voltages we are talking milliseconds to microseconds. So I am quite confident in using a >fuse + zener + zener< for cheap OVP/OCP .

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