Shield winding on offline Flyback SMPS is "gappy"....but still OK? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 06, 2021, 03:34:44 pm
content: Hi,Just doing an Offline Flyback transformer for 13.7V and 3A output. VAC input is 240VAC.We are doing it sandwiched [Pri1 – Bias - Shield1 – SEC – Shield2 -Pri2]I found it impossible to wind the shield without “gaps” (as attached) between the coils, due to the uneven winding surface. (due to my club-handed winding) Do you think that it will still be OK? The shields are connected to Primary side (post rectifier bridge) GND.You can see the blue tape between the coil turns due to the gapsAll coils are using Enamelled copper wire as we cant source TEX-ELZ (TIW) cheaply and at short lead time.Shield winding as on page 8...https://ac-dc.power.com/sites/default/files/product-docs/an18.pdf

author: ocset
author_rank: Super Contributor
timestamp: March 07, 2021, 09:52:01 pm
content: I take it thats a big NO to the gappy shield?Though its still a decent bit of metal between pri and sec, and also it reduced capacitive coupling between pri and sec?In AN-18, they actually say that in an offline flyback, the bias winding often acts as a shield winding...and they confess that the bias often doesnt completely fill the former length, and so has gaps......so i guess they are saying that even a gappy shield is good....just not as good as a full shield.

author: Gyro
author_rank: Super Contributor
timestamp: March 07, 2021, 10:00:31 pm
content: I think it was a big no in terms of being able to see anything in a 150x86 pixel image (with most of it background)!Edit: Your bobbin takes up 24 x 25 pixels!

author: f4eru
author_rank: Super Contributor
timestamp: March 08, 2021, 07:37:03 am
content: My eyes get old.Or is it the phone cameras finally getting back to reasonable pixel numbers?

author: penfold
author_rank: Frequent Contributor
timestamp: March 08, 2021, 06:33:05 pm
content: A gappy shield isn't the end of the world, its a progressive failure in that 100% shield is good, small gaps less good, big gaps less good again, more gaps than wire even less good, no shield no good

author: ocset
author_rank: Super Contributor
timestamp: March 09, 2021, 09:23:05 am
content: Hi,If we have a offline flyback with shield windings as follows……Pri1---Bias-----shield1---sec----shield2------pri2Do you agree that Shield1 dot should be at same end of bobbin as pri1 dot, also Shield2 dot should be at same end of bobbin as pri2 dot?This will reduce the voltage between the primary and shield windings and reduce dv/dt

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2021, 11:06:52 am
content: Where the wires are taken out on the bobbin, makes very little difference...Tim

author: ocset
author_rank: Super Contributor
timestamp: March 09, 2021, 08:18:29 pm
content: Thanks,The thing is, as you know, the shield coil will have a voltage which alters polarity every switching cycle.Shield1 also lies virtually alongside Primary1 (first half of primary coil). As such, we would like to have adjacent ends of shield1 and primary1 coils both positive and both negative at the same time. The thing is, if you receive a consignment of wound transformers from a winding company, then how, (without powering it up in a flyback), can you tell that they have correctly phased the shield windings?A contractor has done a offline flyback transformer where secondary dot is at the same end of the bobbin as primary no-dot and bias no-dot.Surely there could be no reason for this? It just makes dv/dt and EMC worse?Surely this must be a mistake?