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I have a problem I need to solve.

I have a power filter circuit that looks like this: power filter circuit

The input voltage is 18V to 32V and I need my circuit to be protected during overvoltage situations. I have TVS diodes to protect from transient surges, but I also need to protect my circuit from 5 minutes of 60VDC.

How can I accomplish this? The 60VDC blows away most, if not all, TVS diodes that are in the proper specification range.

I was thinking of using some sort of high side switch that shuts off if my voltage exceeds a certain level, but how would I approach that?

To make things worse, this circuit needs to work up to 125C.

Any insight or ideas would be greatly appreciated! Thank you.

edit: 200mA of current is the current draw of the circuit.

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    \$\begingroup\$ Crowbar circuit that blows a fuse? \$\endgroup\$ – JRE Jun 6 '17 at 20:30
  • \$\begingroup\$ Or possibly replace the fuse with a PTC resettable fuse? Depends on what your normal operating draw will be. \$\endgroup\$ – Chris M. Jun 6 '17 at 20:50
  • \$\begingroup\$ PTC probably not going to work at 125C ambient. \$\endgroup\$ – John D Jun 6 '17 at 21:02
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    \$\begingroup\$ @Trevor Oh, yes. You can get parts rated to work at over 175°C, 200°C, even 225°-250°C (especially for geophysical downhole work) but bring a progressively thicker wallet as the temperatures increase and don't expect many hours of life. \$\endgroup\$ – Spehro Pefhany Jun 7 '17 at 14:49
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    \$\begingroup\$ @SpehroPefhany yes, thanks for the info. I regret the brevity of that comment. I was rather eluding to does anything in the OP's circuit actually work as intended at the defined temperature range. Designing to work at high temps and designing to work at wide temperature range are two different, and as u mention, expensive things. \$\endgroup\$ – Trevor_G Jun 7 '17 at 14:56
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Something like this that shuts off the power when it goes over the zener voltage should work. Parts need to be rated for greater that 60V of course.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ @DaveTweed good point.. I'll add another Z \$\endgroup\$ – Trevor_G Jun 6 '17 at 21:42
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    \$\begingroup\$ You can also use a variant of this circuit to limit the voltage at the maximum level of the next power supply part. If it must operate at 60V. Like a discrete linear regulator. But M1 will take all the heat, might be hard at 125C ambient. \$\endgroup\$ – Jeroen3 Jun 7 '17 at 6:48
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    \$\begingroup\$ You need a series resistor for D1 and it might have to be rated for some power dissipation. Putting ~25V on the base of Q1 will do something rather dramatic to D1/Q1. A B-E resistor for Q2 is essential to keep Q1 leakage from being multiplied by Q2 beta- this is especially important at high temperatures as specifically required here. I did something like this to meet the an aviation requirement and used a Schmitt trigger single gate (with a supply regulator, to set the threshold) to do the switching so there was no chance of the pass transistor being left partly on and burning up. \$\endgroup\$ – Spehro Pefhany Jun 7 '17 at 14:58
  • \$\begingroup\$ @SpehroPefhany good points.. Schematic updated appropriately. \$\endgroup\$ – Trevor_G Jun 7 '17 at 15:07
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A 30 or 35 V zener diode in serie with a n-mosfet gate. The n-mosfet should turn off when voltage rise 2 or 3 volts above the Zener voltage.

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  • \$\begingroup\$ You're going to have to add a schematic to show how that's going to work. \$\endgroup\$ – John D Jun 6 '17 at 23:34

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