Non-isolated 240vac flyback...transformer isolation voltage? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: February 18, 2023, 05:36:37 pm
content: Hi,we are doing 10W, 240VAC, 12vout flyback.Its non isolated....pri and sec grounds joined...so what isolation voltage for the transformer test do you think we can use and why?BTW, it must withstand 265VAC mains with "loss of neutral"....so thats [265*sqrt(3)] * sqrt(2) peak = 649Vpk[EDIT.....neutral is carried through to the secondary...it needs to be so....so its not isolated...there is half wave mains bridge to allow this]

author: dmills
author_rank: Super Contributor
timestamp: February 24, 2023, 12:14:32 pm
content: That is NOT the usual meaning of 'Non isolated' in this contextIn fact you CANNOT have a NON ISOLATED supply with primary side ground joined to secondary side negative (Which I guess is 'secondary ground') because BANG!What standard are you designing to? That should (probably by reference) give you the required isolation level.

author: jonpaul
author_rank: Super Contributor
timestamp: February 24, 2023, 12:24:16 pm
content: FTTS: question is nonsense as always.Your device will be a lethal shock hazard.Seekng advice form a forum is not ethical or acceptable.Consult your regulatory compliance and legal council before proceeding.Book liability insurancej

author: temperance
author_rank: Super Contributor
timestamp: February 24, 2023, 02:15:32 pm
content: Hi fern-covered hill, this is open AI automatic repply script setup to answer questions posted by Faringdon on EEVblog.For a non-isolated flyback converter, the transformer is not required to provide isolation between the primary and secondary side circuits. However, it is still important to ensure that the transformer can withstand the voltage stress during operation and testing.Considering that the circuit must withstand a voltage of 649Vpk due to loss of neutral, a suitable isolation voltage for the transformer test would be at least twice the maximum expected voltage stress. In this case, the maximum voltage stress is 649Vpk, so the minimum isolation voltage for the transformer should be 2 x 649Vpk = 1298Vpk.However, it is worth noting that the isolation voltage for the transformer should be chosen based on the transformer's voltage rating, taking into account factors such as the winding insulation, core material, and physical dimensions. Therefore, it is recommended to refer to the datasheet of the specific transformer being used to determine the appropriate isolation voltage for testing

author: Faringdon
author_rank: Super Contributor
timestamp: February 25, 2023, 08:13:39 pm
content: Thanks, the transformer is as attached.Do you agree that having the 0.15mm enamel copper wire in the primary right next to the 0.35mm ECW in the secondary is very bad? (there's no margin tape and no tubing)......the insulation between pri and sec can only be considered to be some 200V or so.........there is no regulatory allowable voltage insulation spec for ECW.Please see the following Fig1, page 42..https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_53/Philips_Tech_Review/PTechReview-03-1938-039.pdf..it shows that ECW has a varying coating thickness along its length, and in some places, 0.15mm ECW and 0.35mm ECW may only have an insulation of some 200V between them.So would you agree, the attached transformer must be scrapped?I mean, we havent even touched on the dv/dt between pri and sec and how that will cause dielectric breakdown over time (say a few years). Would you agree?

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 04:18:03 am
content: modern magnet wire is rated for insulation temperature and voltage" enamel" is an ancient generic term and not specifiedWe use Belden ,Electrosola ,etc Class,155 SS, HS, HPTZ, brands like soldereaze nylease.The H désignation is heavy or double coating.The Pearl Philip's papers may not apply.As non isolated supplies violate all safety compliance, regardless of insulation, this,would be a bad job.J

author: temperance
author_rank: Super Contributor
timestamp: February 26, 2023, 03:24:28 pm
content: How come you know so little about all this?QuoteWhich other things did you consider. Not to many it seems.-You didn't state which Safety Standard this piece of equipment falls into and all forthcoming requirements.-You concentrate on the primary to secondary winding insulation. There isHIGH VOLTAGEacross the primary winding too....Why do you always expectusto doyourwork?A step by step guide:-Find out which safety standers you need to comply with.-Buy those safety standards?-Read them carefully.-Make a list with things you have to comply with. Consult a notified body if in doubt. It costs you some but it will save you substantial amounts later on.-Work out solutions to for each aspect. Ask your supplier how to deal with some of those. They usually know.After doing the above, you might be able to ask more meaningful questions and get more meaningful answers. But of course, you will not reply to this topic and open a new one with more nonsensical rubbish within 24h.Besides, do you realize the an app note from Philips dates back to 1906? I would consult more recent data.