Electronics > Projects, Designs, and Technical Stuff
Non-Zero Crossing AC SSR: Use with DC?
MagicSmoker:
--- Quote from: TimNJ on October 28, 2019, 01:00:49 am ---...
I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?
--- End quote ---
Most MOSFET SSRs switch much more quickly than any thyristor type; the possible exception is when a photovoltaic isolator is used (literally an LED shining on a PV stack that generates a feeble current at enough voltage to turn on the MOSFET... usually over a period of hundreds of microseconds).
Insufficiently snubbed thyristor SSRs may fail to turn off at the next zero crossing with very low power factor loads whereas AC/DC MOSFET SSRs retain positive control of turn-off for DC loads, so won't care about the power factor of an AC load.
That said, thyristors are far more rugged than MOSFETs for a given die area and nominal current rating. A 20A thyristor will happily switch tungsten lamps on and off all day long which can demand up to 10x nominal current during inrush, whereas a MOSFET will likely need to be much more conservatively spec'ed; perhaps needing a 100A-rated device for the same application.
Rarely is there a one-size-fits-all answer, as seems to always be the case in engineering.
SeanB:
Yes, the first DC power switch semiconductor was the SCR, though there you always had at least 2 of them, some really high value inductors, high power resistors and big capacitors to allow one SCR to be used to force commutate the other. But you could have a 200A SCR controlling a DC motor at around 100Hz PWM rate to allow variable speed control, which was a massive jump in efficiency over the previous method, which involved both multiple tappings off the battery bank and some serious power resistors to give stepped control. SCR you could use a single battery voltage, have control from near 0% to 100% with near stepless control, along with feedback so your motor speed was constant despite massive load variations. High voltage withstand when off, high current when on, and pretty much almost no switching loss in operation until you got into the audible frequency ranges, still the preferred device for ultra high power use, where IGBT devices are only now starting to get serious attention.
Circlotron:
Yep, force commutated SCRs are a discipline all their own. About as sophisticated as a steam locomotive, but also just as reliable. Huge overload capacity, as has been mentioned. Lots of interesting resonant circuits involved, and really getting to grips with basic fundamentals. Kinda sad in a way that they are gone from general use.
TimNJ:
--- Quote from: MagicSmoker on October 28, 2019, 07:26:07 pm ---
--- Quote from: TimNJ on October 28, 2019, 01:00:49 am ---...
I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?
--- End quote ---
Most MOSFET SSRs switch much more quickly than any thyristor type; the possible exception is when a photovoltaic isolator is used (literally an LED shining on a PV stack that generates a feeble current at enough voltage to turn on the MOSFET... usually over a period of hundreds of microseconds).
Insufficiently snubbed thyristor SSRs may fail to turn off at the next zero crossing with very low power factor loads whereas AC/DC MOSFET SSRs retain positive control of turn-off for DC loads, so won't care about the power factor of an AC load.
That said, thyristors are far more rugged than MOSFETs for a given die area and nominal current rating. A 20A thyristor will happily switch tungsten lamps on and off all day long which can demand up to 10x nominal current during inrush, whereas a MOSFET will likely need to be much more conservatively spec'ed; perhaps needing a 100A-rated device for the same application.
Rarely is there a one-size-fits-all answer, as seems to always be the case in engineering.
--- End quote ---
Thanks for all of that information. It's clear I'm lacking in my knowledge of thyristors, and I definitely want to learn more.
I guess my question still remains. What might be some applications where you need to turn on a big AC load in the matter of microseconds? It seems like most (industrial?) loads of that nature will probably be fine to turn on within a few 10s of milliseconds.
I'm just confused because before it seemed like people were suggesting that an SCR based solution (for the inrush tester) would NOT be as robust as a MOSFET solution. But maybe I misconstrued that idea. MagicSmoker suggested that I might exceed the dI/dt rating of an SCR SSR if used for inrush testing. Are MOSFETs any better at that?
To me, it seems like I should just use a beefy SCR at this point. Or maybe even drop the whole DIY SSR concept completley and use an off the shelf SSR as originally planned.
Circlotron:
--- Quote from: TimNJ on October 28, 2019, 10:10:16 pm ---I guess my question still remains. What might be some applications where you need to turn on a big AC load in the matter of microseconds? It seems like most (industrial?) loads of that nature will probably be fine to turn on within a few 10s of milliseconds.
--- End quote ---
It's not so much that the load needs to be turned on in microseconds but the switching device, mosfet, SCR etc needs to fully turn on quickly. When it is transitioning from off to on, depending on the nature of the load, it can be dissipating a LOT of power internally. Best to get through that transition as quick as possible.
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