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Offline matthuszagh

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Comparison of overvoltage protection methods
« on: July 10, 2022, 11:03:44 pm »
I recently asked a question (https://www.eevblog.com/forum/projects/lt3045-crowbar/) about a crowbar for an expensive load, which elicited several comments suggesting I use a comparator + MOSFET approach. What are the advantages and disadvantages of each of these approaches? When might you want to use one over the other?

Let's start with the obvious: the crowbar senses an overvoltage and shorts the output, which is intended to blow an upstream fuse. The comparator + MOSFET, on the other hand, disconnects the output from the input by controlling the gate of a series pass MOSFET.

A few thoughts in no particular order:

- Crowbars are slower. The comparator + MOSFET approach is capable of very high speeds (into the ns range I believe).
- Crowbars place more stress on the power supply since they short the output. Depending on the power supply and other protective measures taken, this can very easily destroy the power supply before the fuse is blown. A related point is that fuses tend to be slow and aren't really precision components.
- Crowbars only work for overvoltage, whereas the comparator + MOSFET circuit can work for overvoltage or undervoltage (and many disconnect ICs provide both capabilities).
- The comparator + MOSFET circuit can reenable the output when the triggering condition disappears (and ICs often allow various forms of delays).
- Both can work off the supply voltage and draw very little current when not triggered.
- A comparator + MOSFET circuit is subject to an overvoltage that can damage (and probably short) the MOSFET, destroying the downstream load. However, a fuse would probably act pretty quickly in such an extreme scenario. Also, other devices such as TVS diodes (again with a fuse) could presumably be used here to mitigate damage to the load.
- There are many comparator + MOSFET-based ICs, making the job for the designer quite easy using this approach.

Crowbars are an older technique, whereas there are a lot of modern ICs using the comparator + MOSFET approach, which probably indicates manufacturers think this is a good way to go.

The comparator + MOSFET approach seems to be a pretty clear winner. Are my points unfair? What am I missing?

What about for very low-noise supplies? Would either of these approaches be favored for a very low noise scenario (eg LT3045 supplying a high-quality OCXO)? I can't think of any reason either circuit would have an appreciable noise effect. Just speculating: I guess noise on the MOSFET gate could cause RDS(on) to fluctuate, potentially creating supply noise, but if this is operated in a region where RDS(on) is minimized, then RDS(on) should change very little with small changes to gate voltage and I expect this would be essentially a negligible effect. Thoughts?

Other thoughts? Any other protection methods we should consider?
« Last Edit: July 10, 2022, 11:41:22 pm by matthuszagh »
 
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Offline mag_therm

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Re: Comparison of overvoltage protection methods
« Reply #1 on: July 11, 2022, 12:23:44 am »
Some of the topologies I worked with ( some published I think)  used both the series switch and the shunt crowbar together.
Also when in DC link to resonant inverter for example, it is better for the crowbar to fire  a pulse into a resistive  or LC loop, rather than a short circuit.
Some circuits are meant for fast restart to maintain production, so they use a "3 strikes and out" auto sequence with restarts.
That is similar to auto reclosure breakers that the utilities use, for the purpose of significant savings in unnecessary shutdown.

I don't recall ever seeing a crowbar that intentionally caused fuse failure. Even the 1963 vintage HP6253A power supplies on my hobby bench have fast crowbars, but also fast current limit so the fuses stay intact.

A lot of this stuff was developed in 1970's when control electronics noise was a big problem also semiconductors were costly, and marginally rated. So they masked it with clever crowbars and stuff,
some of that protection on separate boards to separate power semiconductors from the main control.

The early 3 phase AC drives and UPS inverters that I worked on in early 1970's were very dreadful.
They had no protection against mis-fire or load perturbation etc except maybe a circuit breaker, so main semiconductors failed....
And then the guys trying to find the fault would fail 20 more.....
Power electronics companies went bankrupt by it.

 

Offline liteyear

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Re: Comparison of overvoltage protection methods
« Reply #2 on: July 11, 2022, 03:04:03 am »
Some additional crowbar pros:

  • No (additional) series impedance. Useful for high efficiency or failure mode sensitive designs.
  • Lower residual voltage across a short than an open. Useful for voltage sensitive, high source impedance designs such as hazardous area electronics.
  • Redundancy and grading is easy, since their impact is quite isolated.
  • Latch and recovery behaviour is different, which can used as an advantage in some designs.
 

Offline ejeffrey

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Re: Comparison of overvoltage protection methods
« Reply #3 on: July 11, 2022, 03:22:12 am »
A lot of modern AC/DC power supplies have tight current limits or fast overcurrent cutoff.  In many cases a power supply won't be able to blow an appropriately rated fuse even under short circuit conditions unless the PSU is way oversized.  In this case a crowbar is more of a liability than a benefit.

 

Offline David Hess

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Re: Comparison of overvoltage protection methods
« Reply #4 on: July 11, 2022, 04:44:10 am »
- Crowbars are slower. The comparator + MOSFET approach is capable of very high speeds (into the ns range I believe).

Nothing prevents using a MOSFET in shunt mode in place of a thyristor to short the output and blow the fuse.

Quote
- Crowbars place more stress on the power supply since they short the output. Depending on the power supply and other protective measures taken, this can very easily destroy the power supply before the fuse is blown. A related point is that fuses tend to be slow and aren't really precision components.

That should never be a problem.  One common requirement of a crowbar plus fuse is that the bulk capacitance be able to immediately blow the fuse.  The power supply's current limit must not prevent the fuse from blowing.

Sometimes the crowbar is placed in front of the regulator but this requires special attention to the regulator design.

The crowbar may also be placed after the regulator with the intention of current limiting not blowing the fuse, so the protection circuit becomes resetable by removing power, assuming that the regulator is designed to survive a short circuit which is not difficult.

Quote
Crowbars are an older technique, whereas there are a lot of modern ICs using the comparator + MOSFET approach, which probably indicates manufacturers think this is a good way to go.

They do not have a choice because IC processes are not suited to high current crowbar circuits.

Quote
What about for very low-noise supplies? Would either of these approaches be favored for a very low noise scenario (eg LT3045 supplying a high-quality OCXO)? I can't think of any reason either circuit would have an appreciable noise effect. Just speculating: I guess noise on the MOSFET gate could cause RDS(on) to fluctuate, potentially creating supply noise, but if this is operated in a region where RDS(on) is minimized, then RDS(on) should change very little with small changes to gate voltage and I expect this would be essentially a negligible effect. Thoughts?

Neither series nor shunt protection makes any difference for noise.

Quote
Other thoughts? Any other protection methods we should consider?

If the protection circuit is fast enough, this creates the possibility of transmission line effects which cause a voltage spike destroying the load.  Switched power supply outputs which drive logic or any sensitive load must be designed to take this into account.  Integrated load switches usually take this into account.

 
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Offline Siwastaja

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Re: Comparison of overvoltage protection methods
« Reply #5 on: July 11, 2022, 06:09:25 am »
- Crowbars are slower. The comparator + MOSFET approach is capable of very high speeds (into the ns range I believe).

Speed is irrelevant metric. DC supply and load always have significant capacitance. Both crowbar and series switch, and both thyristors and MOSFETs easily prevent voltage rise above the trigger equally well, and it all boils down to the accuracy of that trigger.

Quote
- Crowbars place more stress on the power supply since they short the output. Depending on the power supply and other protective measures taken, this can very easily destroy the power supply before the fuse is blown. A related point is that fuses tend to be slow and aren't really precision components.

Makes no sense. You should not have a non-short circuit protected PSU in 2022. Nearly all DC/DC ICs have protection. All linear regulators, even from 1970's, have the protection. Modules have it. I don't even know if you can get one that doesn't! Maybe if you design your own switcher with discrete parts instead of IC?

Fuse is only to protect from fire anyway.

You really should use a power supply with latching overcurrent limit if possible, otherwise if the overvoltage fault is permanent, a hickup-type PSU is going to reapply voltage approaching your trigger voltage once a second or so. This behavior is same for shunt and series type protector.

Quote
- A comparator + MOSFET circuit is subject to an overvoltage that can damage (and probably short) the MOSFET, destroying the downstream load. However, a fuse would probably act pretty quickly in such an extreme scenario. Also, other devices such as TVS diodes (again with a fuse) could presumably be used here to mitigate damage to the load.

Series switch HAS to be designed with enough margin it does not blow short, otherwise it can't protect. Fuse is too slow to protect the precious load. Besides, overvoltage can damage the load without causing immediate excessive current draw. TVS is not accurate and steep enough. Or if it is, why not just use TVS-only solution to begin with?

Quote
- There are many comparator + MOSFET-based ICs, making the job for the designer quite easy using this approach.

Don't know about that, the simple TL431 thyristor crowbar circuit is already very simple (see https://axotron.se/index_en.php?page=26 )

Both series and shunt solutions are quite simple, I see no big difference here.

Quote
Crowbars are an older technique, whereas there are a lot of modern ICs using the comparator + MOSFET approach, which probably indicates manufacturers think this is a good way to go.

Sounds like some sort of emotional coping mechanism, makes no engineering sense. Both are ages old and viable solutions, besides age doesn't matter. Thristors are older than MOSFETs, but you can also use MOSFET in the crowbar (shunt) topology if you so wish. The reason to use thyristor today is the simplicity of control as it provides the latching feature internally.

Quote
The comparator + MOSFET approach seems to be a pretty clear winner. Are my points unfair? What am I missing?

You are missing your psychology. As an approved kitchen sink psychologist, it is obvious to me you have already chosen to use series topology with a MOSFET, without realizing you have made a subconscious choice, and are just rationalizing your choice. I know because I catch myself with the same behavioral pattern quite often.

But it's OK. Series switch is probably fine in your application. Another plus you did not mention is you can use the same circuit as generic on/off (enable) control.

Quote
What about for very low-noise supplies? Would either of these approaches be favored for a very low noise scenario (eg LT3045 supplying a high-quality OCXO)? I can't think of any reason either circuit would have an appreciable noise effect. Just speculating: I guess noise on the MOSFET gate could cause RDS(on) to fluctuate, potentially creating supply noise, but if this is operated in a region where RDS(on) is minimized, then RDS(on) should change very little with small changes to gate voltage and I expect this would be essentially a negligible effect.

Sounds far-fetched to me. The regulator output voltage is going to drift more than the dU = Iload * dRdson drop of any sensibly chosen MOSFET.
« Last Edit: July 11, 2022, 06:12:50 am by Siwastaja »
 

Offline pqass

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Re: Comparison of overvoltage protection methods
« Reply #6 on: July 11, 2022, 07:13:13 am »
Quote from: matthuszagh
- The comparator + MOSFET circuit can reenable the output when the triggering condition disappears (and ICs often allow various forms of delays).

I'd prefer a manual reset rather than automatically re-enabling the output. ie. like a virtual fuse.  I didn't see this mentioned in your other thread.

This is what I came up with (see below). A comparator+flip-flop arrangement using just a dual comparator (OC outputs), LT431 and resistors.  An LM393 is kind of poky at hundreds of ns.  My best Digikey search lead to MCP6567 with 80ns but then it maxes at 5.5V so you'd need a regulator.  There are more expensive comparators but they're not open collector/drain.

How fast does it need to be? Surely the power supply's output isn't going to rise that fast.

See simulator here: http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKABZIJCORsEUQGfr2EQ8AHQDOYBFKazp8eFLBtopUhiWZsRBXmgo2YIQWOmjYFUqqQpaazaXgpkFgHcQ3KtmyEKhD5+UCwATgFB-giB4Bj+VARwYRG8wdFUxvHI8MnevJBsXjwmWWBKHkU+BZW8hHgh4Xm4hUICzVDZSZ6t+S3CfAJu4T3tPShGHWVJYIQCPSWCwuOFAgAmdABmAIYArgA2AC4Me3Sr4B22sKye6bH+eQtung9xiwKPLABKKQu3YCj1WwgYpIIEwBAsADmKUyMN0FwqCH+vRq7SeFGRviiMSxIU8bDY9XaBKJwTc0JJqX8lOweEKthYkmBhJRSLa1SoEAOoR2dEZzNJUUxZPO3N57E4VJAmAElJl5wk0gU8kcSns0EoQkICAQ2GICGIerw3HVvjN5otfFVcCg9jsiic8Bckjc32Wd3AMUeHR4amIFygGuS7oW7pQ0UmUyG0om4f87t0QKj5JjhUTqZEgzxUrjGdz0faAKJ1W9UYqhbwgullezbNZyPl6Lr8rlGEGiK9r3aH26-WEeBmmZCAA9BADeHhsCBiGhmRlpYUAPIAYSkAHsdgcAA6b8TiAB2BzXUgA9ABRABy69CUgvAAUrwet1tJJIpNytvvJNApAAjF98qOWDgOOKDEJyCD1CshQAMo7FuW57AAnu+x6SAAtmu+4AJZHjeB5SFIAAU3LYVub6SAAAlICDQNgABqACULCjtgba8IObDBNgbDUguICERRMF0Ace77p8dCSCJLBbsCxD+H4UHycCg4QAyslNHUNSwmp2ZsMpikpLi6J5LCtywk2MSwvp1KDm4xhUPK6a5jpIAAH4ACoADJcWpUg8CgKiMP8rjQDgtJtpwzDgaohquNK9jBYFkgMDASiBJwRDjLgdJ4JAGDxfaVgpWl8AZf8swoDlhL5aF4V4JFzBgDF+nYCoHBSG5Bx7L5pwMAAjvuAD6py2pIuh5WNRg2vakW2FIvh0Zay3xiohhJXVKD-GADXRHgCAEpARjxclKDQGwG0lbA8B+BgxC8ZAPG4GQ+3FfN0hyKVcC3fdgRPZAL1Ipt227XUB0cMd0gdZIXU9dgYB9YNI2rAwACCYCGGNO3QDtuMDvjeMJZIZ1oI6TjYJdMCEBgk7GMYDVCPJtUukTagcGTSgI0wyVpTMhCBGg-Dgb4-zEIq71vVc-MC5AQuPfDYHi9A1O06oO1YAagQFVD9qw71KNI31qMk2NeXvQFQXc3VBRsFtCt0hgbBtm9RMoJTsBbfqRBwGLjvyfthUqKFR3NfwGW+zZAcumFNt2-8DtO1t7W691+sDcNo32uz5uOZbIXR1obDROBBQkIaWDa7nxOXalsBkPpdOaK1+0KBLn1137jf14aLfW0XJBwLxpBsVgyedan8OIxnZz2pNyX0EwbUF2gcSOyYeVD1xJ158ltcEHgxBlGghBlNwOCBx9V374fR2QCfd9O0vMCRav5gbyQW86+PcMIwb09o+CU2Vcub51rloPwXFwLGn1GUT+VcKZW2jkoBqj0kSThprSHiCBK5jSYMHeAKC+A7TYpOWm2D27gO4HqfAJAkQFCXoEb+ad+pbmRkAomIDd7P1QfdB+stKB0m3uNd2aAV4A2oerO6MgL54OjqI-K4iYoM0PsqbhfBeHeB1PgNgUhGEwwnr-AarDM5SDNkAnOHD3Ztj9lI+WBoBzECEW7RBtd9R3R4okEWhAiEX2Kq47BhpCRoC8T45eNib6+Hsc1Me+if5TzYUVSgpsiYL3zs-eSGAwB6i0DgKqIshEgL8RjTJ+AkR7WNDMXUPMsZyGKWUV65ST7al0HVDJWSyD5V0GaYqqgU5xL-sjNGoJrRjQttIGuMA3GVikXtcMRcn6u0umdHANMAYvTbNEPwwUFlFXVCsvKmgGpxi2YvduUyGoqLqHM3UMS9aTwGaNZKdzDGrASaqImzy+qvMeXnSW5yZlXIcS7U6wU-FpXro7RI8yDQzifhUJySx2JFmzPKZFFZARQhRLCYk8IGSji2j4Mg1Ypzs2BPxbyvkWIUDup6MQANPSFGgiAAAqvuUIdBIT7C2AcU4UhJDwUQkhFgtJpQQAAGK6UKP4JgIB6JrkOFsSEfIgA

EDIT: You likely need a cap on the reset line to initialize.
If I put one in, the simulation sometimes works or doesn't upon page render.
« Last Edit: July 11, 2022, 07:31:54 am by pqass »
 
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Offline Siwastaja

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Re: Comparison of overvoltage protection methods
« Reply #7 on: July 11, 2022, 07:54:42 am »
One of the upsides of a correctly designed and dimensioned crowbar is that it requires manual fuse replacement for reset.

Even better if you use a soldered (e.g. SMD) fuse.

Because of course, overvoltage can be only caused by broken (by design, or by meteor hitting it) PSU. Just resetting the protection circuit is not an answer, the broken PSU circuit needs to be investigated and fixed, too.

Therefore, an unmarked, non-user-replaceable fuse is the best. Whoever repairs the PSU, and verifies it is fixed, will then trivially replace the fuse, too, and the crowbar circuit did its job to protect the expensive load from the failed PSU.

It's difficult to see what is the idea of auto-resetting series switch. Are you expecting a PSU to malfunction every now and then, outputting overvoltages, and planning to use such broken PSU to power an expensive load, and are relying on a single layer of "protection" in-between?
 

Offline T3sl4co1l

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Re: Comparison of overvoltage protection methods
« Reply #8 on: July 11, 2022, 08:31:42 am »
Well, obviously a crowbar can't ride through something and self-recover, that's a pretty big disadvantage I'd say.

Crowbars were effective back in the day, because thyristors were the only available active devices with adequate ratings; in some ways, they still are today: while comparable MOSFETs are available, they may be prohibitive in cost or size.  Along with diodes, they hold the impressive distinction of being the only semiconductors with a fusing rating.  Enough even to short out a DC filter capacitor, mostly dissipating its contents through its ESR.  So they were a common sight on power supply outputs (or internal common rails, as the case may be).

Which was especially important in the early days of SMPS, when controls were weak, if present at all: a 2-4 transistor self-oscillating circuit might be all you get.  Saturable reactor / magamp designs weren't uncommon, either (and still weren't, for certain applications, all the way until very recently*).

*ATX PC PSUs need 12, 5 and 3.3V outputs, with pretty reasonable regulation on each, prioritizing 5 and especially 3.3.  You can't just throw all those onto a common power transformer, rectify and filter, and have it.  Instead, they get 5 and 12 from taps, regulate the 5 and 12V as a weighted sum (making their combined response quite stable, but leaving them a little squishy in cross-regulation), and for 3.3V, they take one 5V tap (making 2.5V once filtered), and half of the other, but a variable amount -- using a magamp, which is pulled into saturation using a TL431 controlling a PNP (to clamp a variable amount of below-GND flyback from the magamp choke).  Thus, the 3.3V rail is 2.5V plus some pulse width of 0-2.5V, giving it adequate control range, and the TL431 means it's more accurate than anything else on the supply!

I forget if they still do that today (magamp regulation of 3.3V); most now are 2-switch forward converters -- making poorer use of the transformer (it's half wave), but the improvement in Fsw and EMI is considerable.  You see same-size transformers doing 1kW today, that struggled to do 200W back then.

There's an old Unitrode appnote or two, discussing this (magamp) technique -- basically you can have an open loop (give or take current limiting/fault protection) full-wave forward converter at the primary side, transform it, and let each secondary manage how much it wants to draw from that.

Magamps by the way, act, very, very roughly, like a magnetic thyristor.  In that, you basically use them for phase control; they're just programmed kind of weird.  When "pulsed DC" is applied (such as from a diode from a bipolar square wave), the choke (this is nothing but a single winding on a "square loop" core material) absorbs the flux up to some point, then it saturates fairly suddenly (can be fractional ┬Ás!), and starts conducting with low impedance (essentially the inductance of the winding as if it were now air-cored).  When the pulse ends, the stray inductance can generate a little flyback, but the core mostly stays in place -- it generates very little flyback, much less flux than the initial gulp in any case -- so remains magnetized, and on the next pulse, it takes very little time to saturate again: delivering full power.  So it's full-on by default, and can be turned off by forcing some reset flux.  If a negative bias current is applied during this reset phase, a flyback pulse can be coaxed out of it -- how much, depends on the applied current, thus making the single-winding magamp a transresistance amplifier (current in, voltage out).  Or if applied voltage (through a clamp diode, to avoid drawing current from the AC supply), then the amount reset is (reset time) * (applied voltage), which will exactly match the positive output (once averaged by a filter), thus making it an inverting unity-gain voltage amplifier.  By arranging feedback (in shunt, as in aiding/opposing the reset voltage; or in series, as aiding/opposing the reset current), you can get other characteristics, like useful voltage gain, or hysteresis.

(Aha, once again the diversion grows longer than the original comment ;) )

Tim
« Last Edit: July 11, 2022, 08:34:49 am by T3sl4co1l »
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Offline langwadt

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Re: Comparison of overvoltage protection methods
« Reply #9 on: July 11, 2022, 10:29:43 am »
One of the upsides of a correctly designed and dimensioned crowbar is that it requires manual fuse replacement for reset.

Even better if you use a soldered (e.g. SMD) fuse.

Because of course, overvoltage can be only caused by broken (by design, or by meteor hitting it) PSU. Just resetting the protection circuit is not an answer, the broken PSU circuit needs to be investigated and fixed, too.

Therefore, an unmarked, non-user-replaceable fuse is the best. Whoever repairs the PSU, and verifies it is fixed, will then trivially replace the fuse, too, and the crowbar circuit did its job to protect the expensive load from the failed PSU.

It's difficult to see what is the idea of auto-resetting series switch. Are you expecting a PSU to malfunction every now and then, outputting overvoltages, and planning to use such broken PSU to power an expensive load, and are relying on a single layer of "protection" in-between?

dirty power, in a car for example.  Generic power connector like a 5.5mm barrel jack so someone might plug in one with the wrong voltage

 

Offline Siwastaja

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Re: Comparison of overvoltage protection methods
« Reply #10 on: July 11, 2022, 10:34:15 am »
One of the upsides of a correctly designed and dimensioned crowbar is that it requires manual fuse replacement for reset.

Even better if you use a soldered (e.g. SMD) fuse.

Because of course, overvoltage can be only caused by broken (by design, or by meteor hitting it) PSU. Just resetting the protection circuit is not an answer, the broken PSU circuit needs to be investigated and fixed, too.

Therefore, an unmarked, non-user-replaceable fuse is the best. Whoever repairs the PSU, and verifies it is fixed, will then trivially replace the fuse, too, and the crowbar circuit did its job to protect the expensive load from the failed PSU.

It's difficult to see what is the idea of auto-resetting series switch. Are you expecting a PSU to malfunction every now and then, outputting overvoltages, and planning to use such broken PSU to power an expensive load, and are relying on a single layer of "protection" in-between?

dirty power, in a car for example.  Generic power connector like a 5.5mm barrel jack so someone might plug in one with the wrong voltage

Oh yes, that would be input protection for the PSU. I totally agree that series type is better in this case, and preferably with self-resetting characteristics.

Output protection protects against design failures of the PSU, plus severe abuse conditions where the normal specifications are exceeded and as a result, destruction of the PSU can be allowed, but destruction of the expensive load is to be prevented.
 

Offline T3sl4co1l

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Re: Comparison of overvoltage protection methods
« Reply #11 on: July 11, 2022, 11:48:05 am »
Oh also, note that resettable fuses can perhaps be used with crowbars, repeatably as the name suggests.  So, circuit breakers and polyfuses.  But they're not especially reliable, polyfuses in particular.  If it's something like preventing damage from plugging into unsuitable power (say 28V instead of 12) or rare faults/transients (like load dump), it's not a bad idea.

The main downside is, both types have HUGE I^2t, even for fairly small values.  There's simply a lot of material to heat up, while still being at a fairly low voltage drop (in the still-closed state).  And polyfuses aren't practical for large values (SMTs rated for say 30V go up to around, oh, 5 or 8A I think? and THTs continue to the teens or so A).  So they're mostly only suitable for the lower range, where the crowbar itself isn't too prohibitively large (to handle all that I^2t).

Tim
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