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I was searching for a TRIAC to be used as part of a pre-charge circuit for large DC link capacitors, and found one (T435T-600FP) that is described as "snubberless". An on-line search did not find anything definitive, although it appears that they may be limited to three quadrant operation. For my application I will just be using the TRIAC with a zero-switching opto-TRIAC on the AC side of a FWB to precharge a 560 uF 350 VDC capacitor through a 100 ohm resistor (56 mSec TC), before closing a relay for operation.

It appears that the TRIAC has a minimum of 5.3 A/ms rating without snubber, and critical rate of rise of ON current of 50 A/us. I thought the snubber was more for dV/dt protection, which is listed as minimum 750 V/us (apparently without snubber). These specs probably are not important for my present application, but I'd like to know more in case I need to use a TRIAC where it may matter - perhaps for inductive loads like solenoids or motors.

Some relevant references:

https://www.st.com/en/thyristors-scr-and-ac-switches/standard-and-snubberless-triacs.html#documentation

https://www.st.com/resource/en/application_note/an4363-how-to-select-the-triac-acs-or-acst-that-fits-your-application-stmicroelectronics.pdf

(edit) A discussion about this, but with two differing opinions:

https://www.edaboard.com/threads/what-does-it-mean-if-a-triac-is-said-to-be-snubberless-in-its-datasheet.256415/

and

https://www.eevblog.com/forum/beginners/snubberless-triac/

Bottom line seems to be that a snubber is not needed for three quadrant operation, but term may be marketing hype.

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    \$\begingroup\$ I'm pretty sure you mean A/ms, amps per millisecond, not A/mS, amps per millisiemens (which would just be a weird way of writing kV). \$\endgroup\$
    – Hearth
    Sep 21, 2022 at 4:14

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It's many years since I've used a triac in a design, but I recall some triacs suffer from "latch on", caused by the fact that they are both current and voltage mode devices.

They turn on and then never turn off if there is a large enough phase shift between the current and voltage across the load terminals and the current and voltage on the gate terminals, e.g. caused by a largely inductive load. An RC (or RLC) snubber network is commonly used to correct for this.

Another issue is the large voltage transients caused by inductive loads disconnecting suddenly (e.g. through commutation) the R in the snubber helps consume part of that energy.

See ST Application Note AN437 for a better explanation.

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  • \$\begingroup\$ That application note was very helpful. I'd accept your answer, but it does not definitively provide the definition of a "snubberless" triac. The datasheet for the ST Z01 triac mentioned in the article has a minimum dV/dt of 10/20/50/100 V/us depending on part number suffix (Z0103 is 10 V/us), whereas the snubberless triac I found has 750 V/uSec, so I think it means that the snubber is usually not required. \$\endgroup\$
    – PStechPaul
    Sep 22, 2022 at 5:13
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    \$\begingroup\$ I did not want to define snubberless as I think it's a marketing ploy. Surely it depends on the application over all operating conditions? \$\endgroup\$
    – Jay M
    Sep 22, 2022 at 12:50

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