Passive input overvoltage protection with resettable fuse?

[mtwieg]

For a small board I've laid out, I wanted to implement an effective scheme for protection against input overvoltage (Vin nominal is 12V, want to limit it to under 13) and reverse voltage. Reverse voltage works fine via the standard PMOS circuit, but for overvoltage I wanted something simple and passive, as opposed to a crowbar+fuse circuit. So I tried just using a couple zeners (two 3SMAJ5920B-TP in series) and a PTC resettable fuse (1206L016WR). Circuit is shown below:



The OVP doesn't work as I'd hoped. When I load the 12V rail to 120mA (which is the max I expect the circuit to draw at steady state), and let it sit for a bit, and then bump the input voltage up to 15V, it works fine and the PTC kicks in before the zeners get very hot. But if I just plug 15V or so into the circuit (after everything's cooled down), the PTC takes much longer (like a couple seconds) to work and the zeners get extremely hot by then, hot enough that they're probably close to failing. In fact the only thing that really saves them is probably my bench supply current limiting at around 500mA upon plugging it in. Obviously the latter scenario is the one that's most likely to occur, so I have to address this issue, since I'm not counting on this thing being supplied from a current limited source.

So my PTC isn't fast enough when the circuit hasn't been loaded for a while. But I need a PTC that can at least conduct 120mA without tripping, so I can't go for a much smaller fuse. Is this just a bad design, or will a different PTC work better? Are there any other simple OVP circuits which don't involve special supervisory ICs and SCRs and such?

Thanks.

 

[chuckey]

If you direct the current through the base- emitter junction of a power junction transistor and connect the collector to one line and the emitter to another, you will have the current gain and the thermal dissipation of the power transistor to your advantage. For instance, change a 6.2V zener to 5.6V, break the earth connection and connect the zener to the base of a 2N3055, its emitter to the 0V and its collector to the +12V. Now when the zeners conduct 10mA the 2N3055 conducts .5A and "blows" the PTC. Also 12 V X .5A = 6W which should be OK for .5 secs or so for the power transistor.
Frank

 

[dick_freebird]

One key question is, is there a reasonable upper limit to the
overvoltage-to-be-withstood. If yes then an "off" condition
of a transistor may be preferable (if its "on" is suitably low
resistance). But semiconductor devices often fail shorted
and may not be suitable for all safety apps.

Now since it is overvoltage, you don't need a passive
scheme - you will have the energy needed to actuate.

What about a zener triggering an NPN, to pull on the
coil of a latching relay? NC contacts for normal operation,
kick it off and it stays off until you pulse the reset.