Can I use a normal Ceramic cap instead of this cap - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 24, 2019, 01:09:38 pm
content: HiI want to replace this new 1nF 1Kv 0805 cap for several reasons instead of the Y capacitor in a 10W fly-back designExactly replacing the CY1I'm currently using this part from TDKhttps://lcsc.com/product-detail/Others_TDK-CK45-B3AD152KYVNA_C342400.htmlBut I prefer to use a 0805 version like this onehttps://lcsc.com/product-detail/Multilayer-Ceramic-Capacitors-MLCC-SMD-SMT_Walsin-Tech-Corp-0805B102K102CT_C303890.htmlCan I do that?Any reason not to use this SMD version?

author: JackJones
author_rank: Regular Contributor
timestamp: July 24, 2019, 01:16:28 pm
content: Y-capacitors are designed to fail safely. They fail open, not short. With a standard ceramic capacitor you don't have such guaranteed failure modes. That is why you shouldn't replace Y-capacitors with any other type.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2019, 01:40:24 pm
content: In addition to the fail safe behavior of Y capacitors (which is the primary issue in this case) the smaller surface mount package likely doesn't meet clearance or creepage requirements for mains voltage.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 24, 2019, 01:55:33 pm
content: Please stop before you kill someone. You are not up to this task. This isreallya life-critical component. People have beenactuallykilled by this exact design inability.Look for expert help in designing this.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 24, 2019, 02:14:14 pm
content: Thanks, My problem with this cap is it's height! I want low profile version, what about two series cap? or do we have a low profile version, something under 6mm!

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 24, 2019, 02:16:29 pm
content: Ditto. Do not use a non-Y capacitor here. There's little chance you could find a 0805 package that would fit the safety requirements.You may take a look at this:http://www.vishay.com/docs/49344/_mlcc_vj_safety_certified_vmn-pt0369-1709.pdf

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2019, 03:51:28 pm
content: Yes a project like this you should be getting expert help on, ideally in person not relying on random internet posters. You will need to provide more details about the project and design such as the working voltage, pollution environment, isolation class, etc. Even assuming this is a personal project you should be designing it to the relevant safety standards even if you won't actually have it certified. And for a first design you should be following the standard recommendation rather than trying to push the limit especially on something as safety critical as a y class capacitor. Finally for mains operated equipment the layout and assembly are as important as the design and component selection. Make sure to get careful review of the layout and final assembly before powering on, and follow their advice, don't try to cut corners.As a practical note: that capacitor is for EMI suppression. It sounds like you probably don't realise it's purpose in this circuit which is concerning. If this is a personal/ one off project you would be better off just omitting it than compromising safety. If course then it will be noisy as hell. If you have to get emc certified, then you are in a commercial production environment and you should already know the relevant safety and emc standards and certifications before even evaluating potential designs.Not that I don't actually recommend designing a power supply without EMI suppression even as a learning experience. I am just saying it would be better than building it with a 1 mm clearance and a standard MLCC across the isolation barrier.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2019, 04:08:16 pm
content: There are actually ceramic capacitors available with Y ratings. They are on the long side, for obvious reasons, typically 1806 or something like that. I would recommend using at least two of them in series giving the same total value, possibly with board routs underneath them to increase creepage.Note that the degree of insulation required, decreases if you can ensure the output is always grounded (basic vs. reinforced).Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 24, 2019, 04:15:12 pm
content: Quote from: T3sl4co1l on July 24, 2019, 04:08:16 pmYes. The Vishay ones I suggested are 2008 or larger.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 25, 2019, 07:09:01 am
content: QuoteThanks

author: TimNJ
author_rank: Super Contributor
timestamp: July 29, 2019, 05:17:51 pm
content: You can use surface mount...but you should still be aware of the potential issues caused by using large surface mount capacitor packages. Cracking during soldering, board flex, etc. I'm not sure if these things affect the fail-safe nature of X/Y-rated caps...but I'd expect that theymight. About 10 years ago, the company I work for issued a directive to stop using surface mount Y-caps after having a number of issues with them. Your mileage may vary, but just keep it in mind.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 29, 2019, 05:30:25 pm
content: Quote from: TimNJ on July 29, 2019, 05:17:51 pmGood point. Y-rated SMD caps would "pass" has a valid solution regulatory-wise, but I agree they can still be less reliable.Now those caps are supposed to be built as not to fail as shorts in case of cracking, but I guess that could still affect their characteristics and have an impact on safety.

author: tooki
author_rank: Super Contributor
timestamp: July 29, 2019, 11:03:10 pm
content: Quote from: Siwastaja on July 24, 2019, 01:55:33 pmAgreed. This falls squarely into the “if you have to ask, you’re not ready to be doing this task” area.