Again with a power supply input protection

A circuit will have 24V @ 1.5A (max) input from a wall wart. I'd like to design in reverse polarity and maybe over voltage input protection. (There are lots of threads about this.) I don't like the Zener/ polyfuse idea, cause there is going to be a lot of power in the Zener... I'm afraid it will smoke before the polyfuse lets go. So some FET circuit. Here's Jim T's offering. (from a past thread.)

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The gate source voltage on most of the P-fets I've looked at is

+/- 20V. So the 24V is a bit of a concern. I can just put in some resistors to make a voltage divider.. so not a big deal.

But do I need two P-fets? (Oh snap, never mind.. I had to draw the circuit.)

Well I'll make it a question anyway... are then any P-fets that can handle more G-S voltage... (well more in absolute value.)

TIA George H.

Reply to
George Herold
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Den fredag den 27. maj 2016 kl. 18.16.55 UTC+2 skrev George Herold:

just add a ~12V zener to gate

-Lasse

Reply to
Lasse Langwadt Christensen

Yes, as you said. Two needed, one for reverse polarity and the other for over-volts but they need not be p-channel. If your input supply is floating using n-fets will save money per ohm rdson.

piglet

Reply to
piglet

Polyfuse+transzorb+cap works fine. A bunch of semiconductors and passives (9 in Jim's case) will have hazards of their own. Like ESD damage, for example.

Most fets blow out around 75 volts Vgs. There are lots of fets that have internal g-s ESD zeners, typically +-40 volts turf, and they can be protected with just a series current-limit resistor.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

It might be worth considering the Polyzen and any equivalents. It is a series polyfuse followed by a shunt Zener, the trick being that the Zener is thermally coupled to the polyfuse so that when the Zener starts cooking, it heats the polyfuse which makes it go high-impedance even if it was not conducting anywhere near its trip current. In that way the Zener is thermally protected. Availability might be a problem.

Reply to
Chris Jones

24 V is quite high voltage so a series diode 0.6 V voltage drop doesn't too much harm. The diode solves the reverse polarity problem. Select a diode with a large reverse voltage rating, say 1 kV.

If the source is floating, use a bridge rectifier, so the device works regardless of polarity and even works on AC as a bonus.

For large positive peaks, use some series inductance to limit the peak current, helping any parallel protection component to do their job.

However, if you expect constant over voltages, then some fusing component in series would be required.

Reply to
upsidedown

you can often use that when things aren't floating too, If they're both grounded the ground connections just short out the 0v line diodes.

NT

Reply to
tabbypurr

Thanks, I worry that there won't be enough current to blow the poly fuse, and then the transorb/ Zener is sitting there dissipating ~10's of watts.. and won't last long.

We're just going to hard wire the power supply in place, not as pretty but no fuss.

I don't know FET's that well, but a quick sampling of spec sheets seems like they all list Vgs max as +/- 20V. I was wondering if this is mostly a "lazy" spec*. In that few people really care, and it would be too costly to spec it and measure. It seems like a 200V (Vsd) FET should have Vgs somewhere near 200 V.

George H. Like the reverse bias voltage on LED's, all listed as 5V.

Reply to
George Herold
+1 for Polyzens--they're magic. A 16V PolyZen with a 10V zener in series with the ground lead will do a great job. A series Schottky from there to the load will help protect againdt supply reversal and input shorts.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

We use polyfuses with gigantic (SMB package) unipolar transzorb zeners. Seems to work.

The surface-mount polyfuses are pretty bad, so we generally use the radials.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

SMB isn't gigantic. I have a DO218 (or equivalent) TVS diode on almost every board (not on daughterboards that get power from the motherboard).

Surface mount fuses aren't much better, though I use them for prototypes.

Reply to
krw

Huh, OK split load. I'll have to think more...

Testing the last poly fuse thing, I found that somewhere in V-I space, the poly/zener has to operate as a (crapy) voltage regulator... And happily waste whatever power you ask.

It'd be nice if poly fuses had a sharper knee... Oh.. polyzens... I haven't tried those. Do they make 24- 30 Volt ones?

George H.

Reply to
George Herold

somewhere in V-I space, the poly/zener has to

And happily waste whatever power you ask.

Do they make 24- 30 Volt ones?

They've been widening the range, but last time I checked they only went up to 16V or so (hence the auxiliary zener). Still, way over half of the zener dissipation is in the Polyzen, so it really helps sharpen up the knee.

Cheers

Reply to
Phil Hobbs

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