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When to use MOV, TVS or GDT?

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MikeEagle95:
Hi

I realized that in most cases when I design input protection, I use MOVs, sometimes TVS diodes and almost never GDTs.
It's well known that TVS diodes reacts much faster than the MOVs, but MOVs have higher voltage rating.
But what should anyone consider choosing input protection method and why?
For example, when to use GDT instead of MOV?

T3sl4co1l:
Is it?

MOVs are basically thicker, looser TVSs on legs.  They are both semiconductor devices.  They have the same speed and the same limitations: C and ESL.

MOV leads and bodies do tend to be longer, so that's a thing.  They might be used at lower impedances as well, exacerbating ESL.

Thyristor TVSs and GDTs have a definite delay or rate limitation, however.

Low capacitance TVSs also have a slight delay, though to a lesser extent.  The trick is, they get the low capacitance by using a PN diode in series with a biased TVS (usually the TVS is connected to VCC).  The PN diode exhibits forward recovery, which looks like ESL but is actually due to voltage drop across the junction.

Just FYI :)

So, that said, when to use GDTs (and similarly, thyristor TVSs)?  Whenever you need to shunt a lot of surge current, and when there is not a continuous power source connected.

Classic use case: telephone lines.  These are subject to induced lightning surge, but supply little power (under a watt) and limited current -- not enough to keep a GDT conducting.  The line also has pretty high resistance, so the surge current is fairly modest, and all the surge voltage gets dropped across that resistance, instead of across your circuit.

MOVs are used where high energy capacity is required.  Ever checked the price on mains-capable TVSs?  Wheee-hew!  GDTs must NOT be used on mains, because of the power supply -- once it sparks over, it's going to keep going until destroyed.  This also means the MOV must drop a large voltage (about 2-3 times nominal mains voltage) during the surge, making it all the more critical that it be a big dumb lump of cheap semiconductor material that can absorb a shitload of energy, quickly.

There is, actually, one trick where GDTs can be used near the mains.  It is for line-to-ground protection.  We do not normally expect neutral-to-ground voltage, so a GDT could be used here...BUT, we must always be weary of the case where the wiring is cross-wired, or supplied from commercial three-phase (e.g., 208V in the US, where each is 120V to GND).  The trick is this: you put a MOV in series with the GDT, so the mains voltage is dropped, preventing the GDT from exploding.  What this saves you is ground leakage current, which can be significant for an MOV already, but worsens with age.  The GDT remains off until breakdown voltage is seen, and then it switches in the MOV.

Some other examples:
- General purpose, low voltage facility wiring, e.g., furnace or doorbell wiring, alarm system wiring, industrial Modbus, etc.
- High frequency signals (the GDT has very low capacitance), e.g. Ethernet (which can be run at great enough lengths -- between facilities, say -- that it can become a surge hazard)

Note that it takes length (actually loop area, but, assuming a worst-case condition of shitty grounding...) to get significant induced surge voltages and currents.  You wouldn't bother with this on short runs, unless you're REALLY worried about EMP (fast EMP, like bombs and nukes).

(And even then, nuke EMP is probably too fast for a GDT, you'll really need a combination of shielding, filtering and TVSs I think?)

Tim

David Hess:
I have the Motorola version of this application note, A Review of Transients and Their Means of Suppression, on my desk at the moment which covers zeners (TVSes), MOVs, GDTs, and some other less common devices.

The thing which always surprises me how the peak power capability of zener diodes and TVSes.

T3sl4co1l:

--- Quote from: David Hess on June 20, 2018, 06:29:45 pm ---I have the Motorola version of this application note, A Review of Transients and Their Means of Suppression, on my desk at the moment which covers zeners (TVSes), MOVs, GDTs, and some other less common devices.

The thing which always surprises me how the peak power capability of zener diodes and TVSes.

--- End quote ---

Oh yes, for short pulses.  It's, not quite proportional to time, actually, more like sqrt(t), thermal diffusion or something.  Even for planar chips I guess.

The peak power goes way up at short times, of course.  Not way way up because of this, but still, lots of kW for a little SMT! :D

Related:

I built a soft-starting and current-limiting module, where the lost power is dissipated in a stack of TVSs.  Nominal rating 30V 20A, and using three SMDJ ("3kW") type TVS diodes.  It current-limits for 150ms, burning up to 600W * 0.15s = 90J.  The diodes are perfectly happy with this, and their surface temperature step-changes (such that it can step, as measured on the surface -- within a second or two of the event) by 10-20°C.

So you can hit the "start" button repeatedly, and crank up the diode temperature quite a lot.  They start to smoke a little (that's my filthy fingerprints evaporating, not the magic smoke!), then the thermal cutout stops it (which is delayed by a few seconds, due to thermal conduction over the PCB).

I was quite satisfied with the performance.  I want to build an even bigger one, say for industrial application (400V 30A?).  Unsure if I want to just use a bigger stack of diodes for that (it's only about $30 worth -- not all that much considering!), or a couple of MOVs in parallel (which would be well within their ratings then, and have a reasonable clamping voltage).  Biggest problem is simply dissipating the heat once it's there: even at one pulse per minute, that's an average of 30W! ;D

Tim

David Hess:
Avalanche rated rectifiers and TVSes but not necessarily zener diodes are fabricated to produce a uniform junction to prevent hot spots increasing their avalanche capability.  Any exposed surface of the junction is also passivated to prevent surface effects.

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