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Non-Zero Crossing AC SSR: Use with DC?
Circlotron:
--- Quote from: TimNJ on October 28, 2019, 10:10:16 pm ---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?
--- End quote ---
With an SCR you can simply put a small value inductor in series with the SCR to limit the di/dt. Doesn't need to be much. I have even seen several turns of cable looped in a circle on a big inverter. And there is no problem with inductive kick from the inductor because the SCR turns off at zero current so there is no energy left in the inductor to cause problems. SCRs used correctly are great!
https://electricalbaba.com/di-dt-protection-scr-explained/
MagicSmoker:
--- 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.
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If you are doing phase angle control of power to an AC load (think "light dimmer") then you need to be able to control turn-on within a matter of microseconds. Also, don't confuse response time to a control signal with the time it takes to transition states (ie - switching time). Faster switching time is always better... up until it excites resonances that cause destructive ringing, anyway.
--- Quote from: TimNJ on October 28, 2019, 10:10:16 pm ---
--- Quote from: MagicSmoker on October 28, 2019, 07:26:07 pm ---...
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.
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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?
--- End quote ---
The answer was already given: very low power factor loads are more difficult for thyristors to switch, and intentionally provoking high inrush current into a capacitive input filter definitely qualifies as such (the current greatly leads the voltage). Thyristors naturally turn off when the current drops below the holding current value, or the voltage drops below the forward threshold, but in a low power factor load the current waveform is displaced so far from the voltage that voltage crosses zero when current is still high or vice versa. The thyristor tries to naturally commutate off, but instead just goes through a high dissipation quasi-off state every half-cycle, often ending in its failure within a few tens of cycles.
MOSFETs are far more robust in this respect - as long as gate drive is present they will be on regardless of any phase angle displacement between voltage and current - but they have a far smaller overload capability compared to thyristors, and you kinda need both here. However, it is easier to solve the overload capability problem with brute force, hence MOSFETs are the overall better choice.
EDIT - agree with Circlotron completely.
TimNJ:
Thank you to both of you. I think I understand the issue more clearly now. And, as you both suggested, I think my misconception was thinking the issue was related to signal propagation instead of an actual switching delay, and the associated period of high power dissipation. Though, now that I think of it, why would a discrete semiconductor device have a propagation delay on the order of milliseconds?? :o
I am sorry if you already answered these questions before; As you can tell, my understanding of thyristors is not fully developed. (I'm pretty good with MOSFETS though!)
Now the Crydom spec sheet makes a lot more sense. "Minimum Power Factor (at Full Rated Load) = 0.5". That was a meaningless spec to me before, but understood now.
Thanks again. I'll keep plugging along on this MOSFET SSR. So far pretty straightforward, just will need to make sure the ZCS circuit works reliably.
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