protecting an LDO with a Schottky

[nomar123]

My issue is related to overvoltage protection. I have a board that needs to be protected from a 100V overvoltage short. I designed a PNP protection circuit to protect most of the circuitry but I still have a vulnerable LDO. I want to simply use a schottky diode with low Vf at the output of my LDO so that no current goes back into my supply. i can really only afford to lose .3V so my electronics on the 3.3V supply still work at around 3V when taking vf of the diode into account. Cant really find a 100V diode with that low Vf, so wondering if I should put it on the input of the LDO where I can afford to lose more than .3V. Would this still sufficiently protect the LDO?

[thm_w]

Its not entirely clear what you mean.
You are saying 100V overvoltage may appear on your 3.3V rail? Or 100V may appear on the input power supply to the circuit?

Do you have a schematic to explain?

[nomar123]

my 3.3V rail is at risk due to a 80V short.

Due to a boost circuit the electronics ground is 80V above earth. Poor design in my opinion but it is what I have to deal with. I have a switch in my system that is connected to the electronics via a 3 foot wire, and is mounted to an isolated portion of the system. So this switch is sitting at a potential of +100V above earth ground, but in the normal condition there will be no short and thjere are no issues. But there is the possibility of the switch coming loose due to vibration and shorting digital ground (80V) to earth ground (0V) or mains (160V). This should never happen, but if it does i my electronics need to survive long enough till the fuse pops. I have deigned a PNP circuit that properly protectt most of the electronics but not the output of the LDO. So the only failure failure mode that needs to be addressed is 80V present on the output of the LDO, current goes the other way and blows the LDO and damages other electronics before the fuse can pop. So I want to put a forward biased diode on the output of the 3.3V to block any current from going back into the LDO if there is a 80V short. but I cant use a diode with more than a .3V Vf drop because anything more than that will make the rest of the circuit not work during normal operation. So Im wondering if I just put the diode on the input to the LDO will that provide the protection I am looking for?

I want to spec the diode to protect from 100V short to have margin

[rs20]

A. If the electronics ground is 80V above earth, wouldn't your 3V3 rail be exposed to –80V, not +80V?
B. Can you please provide a diagram; what you're describing is presumably really simple but we're 4 messages in and still debating semantics when a simple hand-drawn diagram would have been crystal clear from the start.

[nomar123]

see attached.

S1 has the potential to short to 160V. since ground is at 80V above earth, the 160V supply will look like an 80V short.

I want to protect the output of my LDO with a 100V diode so current cannot flow into my 3.3V and 5V supplies from S1.

My Schottky implementation at the bottom does not work because the forward voltage drop is too great, and it will cause my 3.3V supply to droop to 2.9V, which is unacceptable. I cannot find a 100V diode with a Vf of <300mV @ 100mA. So that implementation does not work.

I then think to put the Schottky on the input of the LDO, because if my 5V droops to 4.2 V or so, that wont matter casue it can still regulate to 3.3V.

But the whole point is to protect the LDO, and if the schottky is on the input I worry the chip will get fried still even though it would protect everything else on the 5V rail.

[Terry Bites]

So you have two different GNDs? One on the switch and one on the regulator?

[m98]

Could it be that what you really want is just an isolated converter?

[langwadt]

use a variable LDO and put the voltage divider setting the voltage after the diode

[magic]

This could work, perhaps with some additional feedback input protection components.
Note that schottkys have more reverse leakage than ordinary silicon diodes, so the latter may be preferable if the LDO can tolerate the voltage drop.

[Kleinstein]

Even a schottky diode drop would be too much for most electronics. The diode may protect the LDO, but the circuit itself would still suffer / blow up.

The more logic protection would be to protect the lines that go to the switch, e.g. by using some isolation there, up to the point of using an extra DCDC converter and opto-coupler, so that the switch part would be fully floating. Even a connection of one switch sides to mains would than not even interfere with normal operation.  A minimum protection would be fusible resistors in all lines and than robust clamping (e.g. TVS diodes) of the signals from the switch.