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Why are 900V semiconductors not common? When 1200 are.
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Alti:

--- Quote from: wraper on October 05, 2021, 06:15:45 pm ---Here is an example.
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Lets define what mains rated component is. YMMV. If you take a 20V shottky diode from secondary of mains powered AC/DC 5VDC wallwart adapter then this component is not subjected to mains voltages. Similar situation happens when you RLC filter all mains crap, boost voltage and put some transistor after that. Or if you put a CLC pi filter after bridge rectifier (as in your picture) - this buck converter does not have to withstand mains transients and surges (in the sense of IEC 61000-4).

Same applies to triac. It can be placed in series with load directly across mains but also it might be protected by some RLC, snubber, varistor or other filter. If there were only 400V optotriac on this board then I suspect you did not show us all the picture and there was an input filter that plugs to this board. Or maybe this RC fiter placed in parallel with optotriac/triac/load does the job of limiting surges. RC filter (part of buck filter) is low impedance for high dV/dt so it is going to clamp transients (it has full bridge rectification so it works for both polarities).

Concluding - this 400V optotriac might not be subjected to mains conditions and might not have to be rated to withstand these since it (with triac and load) might be in parallel with chunky RC filter.

Or it might be a poor design - I cannot rule that out.



wraper:

--- Quote from: Alti on October 11, 2021, 06:11:07 pm ---Same applies to triac. It can be placed in series with load directly across mains but also it might be protected by some RLC, snubber, varistor or other filter. If there were only 400V optotriac on this board then I suspect you did not show us all the picture and there was an input filter that plugs to this board. Or maybe this RC fiter placed in parallel with optotriac/triac/load does the job of limiting surges. RC filter (part of buck filter) is low impedance for high dV/dt so it is going to clamp transients (it has full bridge rectification so it works for both polarities).

Concluding - this 400V optotriac might not be subjected to mains conditions and might not have to be rated to withstand these since it (with triac and load) might be in parallel with chunky RC filter.

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Datasheet clearly states this optotriac is for 115V mains voltage. Mains filter is  2 Y caps to the earth, 2 ferrite beads, a varistor, X cap, a common mode choke, again 2 Y caps to the earth. This thing works without too much failures only because this is a zero-cross optotriac which is more resilient against overvoltage. If it was a random-phase type, once in the field and years pass, a large number of them would be randomly activating a heater which is directly powered from 230V AC (I have seen plenty of that in other devices). Varistor sort of helps, however it's not likely to clamp everything below 400V if transients arrive. Pictures from a service manual:




Alti:
This is symmetrical CLC (C4+L1+C1/2) low pass and then S10K250. Indeed, one cannot expect S10K250 itself to clamp anything below 700V so I suspect it is this 1uF cap (low ESR?) that does most of the clamping job. Still, this is designed to comply with regulations so I am guessing it is enough and resulting waveform does not exceed Vdsm at any time. Vdsm might be/should be higher than Vdrm (400V) and for triacs is usually Vdsm=Vdrm+100V but for optotriacs Vdsm is rarely mentioned in datasheets. Also, if something is indended for specific application, it does not necessarily mean one cannot use it reliably in a different one (with serious filtering).



--- Quote from: wraper on October 11, 2021, 06:33:00 pm ---This thing works without too much failures only because this is a zero-cross optotriac which is more resilient against overvoltage.
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I have not seen any application notes or datasheets that confirm this observation. Any reference?

wraper:

--- Quote from: Alti on October 11, 2021, 08:14:14 pm ---I have not seen any application notes or datasheets that confirm this observation. Any reference?

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It's not from datasheets, it's from my experience of repairing stuff. IMHO it's because zero-cross optotriacs by design are made to not activate above about 20V. And the most common failure mode of 400V rapid-phase optotriacs I've seen is sporadically self activating when applied AC voltage reaches its peak.
Alti:

--- Quote from: wraper on October 11, 2021, 08:26:26 pm ---And the most common failure mode of 400V rapid-phase optotriacs I've seen is sporadically self activating when applied AC voltage reaches its peak.
--- End quote ---

I have never heard about reputable optotriacs and triacs sporadically turn on (=exceed parameters specified in the datasheet). This suggests some kind of gradual wear-out or abuse. I know that these turn on either by pulling a gate/illuminating by LED (typical way), or after exceeding Vdsm, or by exceeding (dV/dt)s but that is specified in the datasheet. Triggering via Vdsm is kind of not characterized but take a look at TOPtriac for which it is. Anyway, BTA16-600BW from ST has dV/dt specified as 1kV/us minimum, at 125oC. Had one tried faster edge, the triac might turn on, this is normal and not destructive.

So if you do not limit the optotriac current adequately (usually Itsm is 1A 100us pulse) hoping to trigger it in SW only at low voltages, then sporadic 500V peak might not destroy it through Vdsm but by killing the triac die (exceeding Itsm), or blowing the limiting resistor.

As for (dV/dt)s, this turn on mode applies to any type of optotriacs and triacs - you can trigger those at any voltage and at any point, even zero-crossing is going to turn on at random angle after exceeding (dV/dt)s.

I suspect that these 400V optos might get strained by turning on by exceeding Vdsm and as a consequence exceeding either Itsm or (dI/dt)c or both (by inadequate limiting impedance of a BTA16 gate pull). IMHO there is nothing wrong in turning a heater element sporadically ON during transients in such ultrasonic cleaner application, 10ms won't make any practical difference.

So 400V opto is ok, if you know what you are doing.
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