Full Bridge LLC of 4kW not possible with offtheshelf  gapped cores - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: May 21, 2022, 10:37:51 am
content: HiHi,Do you agree that a 4kW Full Bridge LLC converter, with vout = +/-180Vdc and Vin = 390Vdc (PFC output) cannot be done with any offtheshelf gapped ferrite core sizes? (That is, cores that can be bought ready_gapped)AYK, the biggest gapped ferrite core available offtheshelf is the ETD59 which is offtheshelf gapped to 2mm (AL = 311). However, if that is used as resonant inductor or (external) magnetising inductor of such an LLC, then its core losses will be too high (approx. 10W).Would you agree with this? Or can you design one which doesn’t suffer such high core losses when only offtheshelf gapped cores are used.The attached shows the design document for such an LLC. Done at 70kHz….the core losses come out at 10W for the resonant inductor, and 10W for the magnetising inductor, which in both cases uses the ETD59 TDK core with N87 , ready gapped with 2mm gap. These core losses are too much, would you agree?

author: jonpaul
author_rank: Super Contributor
timestamp: May 22, 2022, 11:10:52 am
content: No, Farrington.To summarize your misconceptionsa 4 kW inverters loss budget easily accommodats 20 W Pfe. Pcu may be similar.Gap calcution depends on the overall topology, frequency, inductance and BmaxStock gapped cores are in several gap sizes,expect 1..3 yrs delivery and MQ 10K setsWe use un gap cores and place a fish papers or G10 gap.Our inverters had no gap in the transformers.bon courageJon

author: Faringdon
author_rank: Super Contributor
timestamp: May 28, 2022, 11:30:35 am
content: Thanks jonpaul,I like the idea of sheet gapping, but fear of its inaccuracy, and wide tolerance.Also, thats now two of us that spell my name wrong!....its supposed to be "Farringdon", after my favourite area of London...will see if i can get it changed.Also, i still declare that a 4kW LLC converter cannot be done off PFC output (~390V) with offtheshelf gapped ferrite cores.....so the ETD59 is the biggest offtheshelf gapped ferrite core with 2mm gap to AL=201.It is simply not possible...would be too much core loss even with fan cooling on the ferrite and vented enclosure in max 35degc external ambient.Would you agree?

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 29, 2022, 09:15:56 pm
content: No, I disagree, and I have personally made an LLC converter processing more than 10 kW with an off-the-shelf gapped PQ65 core set.

author: Faringdon
author_rank: Super Contributor
timestamp: May 29, 2022, 09:42:45 pm
content: Thanks, but PQ65_with_gap is not "offtheshelf"......at least not with major vendors........and not universally gapped amongst the different vendors.Whereas ETD59 is standard gapped to up to 2mm.You must have had that gapped?...or sheet gapped it?TDK PQ65https://product.tdk.com/system/files/dam/doc/product/ferrite/ferrite/ferrite-core/data_sheet/80/db/fer/pq_65_60.pdf[EDIT.]...My apologies...it is actually available"distributed gapped" off the shelf..(if you can find it in stock like that)......but £46.44 a piece on digikey...12 in stockhttps://product.tdk.com/system/files/dam/doc/product/ferrite/ferrite/ferrite-core/data_sheet/80/db/fer/pq_65_60_dg.pdf....that to me is just as pricey as getting it gapped yourself....so it woudlnt count for us....though my apologies , because i forgot to add that "if available off the shelf, then mustnt be of a price equivalent to getting it gapped yourself"Price falls with volume of course..but so do the others............................._____-----____For comparison, if you will, please see offthe shelf gapped ETD59......£3 a piece......https://www.digikey.co.uk/en/products/detail/ferroxcube/ETD59-31-22-3C94-G200/7651633As such, ETD59 is the biggest of the 'realistically offtheshelf gapped ferrite' available....but it woudlnt be possible to do a 4kW LLC off 390Vdc with a ETD59....thats the postulation....unless others may differ?

author: uer166
author_rank: Super Contributor
timestamp: May 30, 2022, 12:48:18 am
content: Quote from: Faringdon on May 29, 2022, 09:42:45 pmI postulate that you can do a 10MW power supply using nothing but ETD59 cores. You didn't qualify how many cores do you allow being used, and as such this entire thread is a waste of server space.

author: Faringdon
author_rank: Super Contributor
timestamp: May 30, 2022, 01:56:48 am
content: Thanks, i meant no more than one ETD59 for each of .....*...LLC transformer (doesnt need gap)*...External magnetising inductor (needs gap)*....External resonant inductor (needs gap)

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 30, 2022, 09:59:17 am
content: Are you actually trying to make a 4 kW LLC? Or are you just trying to prove that it's impossible under some very convoluted and ever-changing set of circumstances?There are off-the-shelf gapped PQ65 cores for less than 10 euros a set in moderate quantities, available from stock in Europe.

author: jonpaul
author_rank: Super Contributor
timestamp: May 30, 2022, 12:33:26 pm
content: Rebonjour Cher monsieurSheet gaps are commonly used and materials available at any thickness and accuracyCore Al is normally 20...50% tolérance +/-Finally spcialty custom fabrication of core gaps in stock ungappd cores is offered by many core importers and distribution.In USA I can mention Fair-Rite, AllMag, Elna Ferrite, etcAs a surface grinder with diamond tools and slurry is used, expect a set-up fee and high per item costsFor these reasons for decades we used easily changeable sheeet gaps of G-10 fiberglass available in 32, 64, 128 100 250 mil thickness from electrical insulation suppliers like EIS and Essex BrownellBon courageJon

author: Faringdon
author_rank: Super Contributor
timestamp: May 30, 2022, 03:06:51 pm
content: Thanks, thats interesting with the sheet fibreglass......like FR-4 PCB material.....i wonder on the tolerance of thickness, since AYK, external resonant inductor and external magnetising inductor of LLC needs to be fairly tight tolerance in its inductance.....within +/-10% and prefereably +/-5%.Yes i agree ungapped core tolerances are around 20-50% tolerance.Also, thanks for the info on slurry and diamond used in gapping...sounds a nightmare for cost