Need some help in flyback transformer design - Page 1
Forum section: electronics / projects

author: Bgdn
author_rank: Contributor
timestamp: January 22, 2023, 10:38:43 am
content: Hello everyone,i am thinking to start a little project, for an auxiliary power supply.I took a look over some docs listed bellowhttps://www.fujielectric.com/products/semiconductor/model/power_supply/technical/box/pdf/Fly-Back_transformer_Design_Rev_1_0_E.pdfhttps://www.infineon.com/dgdl/an-1024.pdf?fileId=5546d462533600a401535591115e0f6dAnd used the excel tool from the link bellow to do the mathhttps://www.infineon.com/dgdl/Infineon-CalculationTool_fixed-frequency_5th_generation_CoolSET_ICE5xRxxxxxZx-DevelopmentTools-v01_10-EN.xlsm?fileId=5546d46261d5e6820161e7487a3b34d7In my case i have the followings requirements :90-265Vac input15Vdc - 0.5Amps outputEfficienty 80%Switch frequency 100khzFor the transformer i plan to use E20/10/6 - N87 core from epcos.Using the excel tool from above, and my parameters, i get for the primary a inductance of 646uh at ~82 turns.Since for this type of transformer i need a gapped core, i was thinking to use the 0.25mm gap variant which has AL=171nh/N^2, but here is where the confusion startsIf i compute 82 turns at 171nh, it yelds me about 1149 uH. ( L = N^2 x AL)So i am wondering how to deal with this, should i lower the number of turns to comply with the initial calculated inductance?What about the B cuvres since i know that lowering the number of turns would yeld a higher Bflux?Or do i need a larger gapped core to lower the AL value, therefore the resulting inductance would aproach my initial Lp for the given turns?

author: jonpaul
author_rank: Super Contributor
timestamp: January 22, 2023, 05:17:58 pm
content: Get the ancient but still working free Intusoft Magnetics Designer.http://www.intusoft.com/demos.htm#magShould solve it easily.Jon

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: January 22, 2023, 06:02:18 pm
content: Hi Bgdn,You may check out thisdocumentwritten by Lloyd H. Dixon.However, some Excel work will be necessary before being really useable.The flyback transformer design strategy § starts at page 5-7 (51 in the PDF).

author: Bgdn
author_rank: Contributor
timestamp: January 22, 2023, 08:16:04 pm
content: Hello,i took a look over the document , and found the following statement marked in yellow.Given this, it seems that i need to calculate the required AL to fulfill the desired calculated inductance.AL_required = 1000 x (Lp/Np) -> for 646uh & 82 turns it gives me AL should be 96 nh/N^2My initial chosen core with 0.25mm gap has AL =171nh .I see there is a standard 0.5mm gapped core with AL = 103To achive AL of 96 it seems i need a gap of ~0.54mm thats pretty close to the standard value. Maybe i will need to grind a bit of the materialWhat do you think?

author: moffy
author_rank: Super Contributor
timestamp: January 22, 2023, 11:04:59 pm
content: If you look at the TDK Ferrites and Accessories 2013 edition(https://www.tdk-electronics.tdk.com/download/519704/069c210d0363d7b4682d9ff22c2ba503/ferrites-and-accessories-db-130501.pdf), page 82, there are a set of curves for the N87 material, the one you are using I believe. Since H = NI you can read off the amount of current for a given number of turns and gap/Ue. Page 422 of the same book for the E20 core quotes the Ue as 118 for a gap of 0.5mm. So going back to page 82 and using the Ue=100 curve you have a maximum NI of about 1000 A.Turns @ 100 C. Adjust for safety margins, but that will tell you the maximum current you can put through your primary before saturation.