Power loss in windings of Flyback transformer due to gap's fringing field? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 19, 2020, 03:36:08 pm
content: Hi,We wish to do a 240VAC 31W Offline PFC’d flyback LED driver with a Ferroxcube PQ2625 Ferrite core.The winding cross section is as attached. NP=26, NS = 14. (The “N4” winding was never used)Secondary is “sandwiched” between two primary halves.The PQ2625 core is centre leg gapped with gap of 0.42mm.We make no attempt to situate turns on the former so that they are away from the centre leg gap.The primary current (always DCM) is as attached. It has a peak of 2.9A and Bpk is 0.28T. The switching frequency is approximately 66kHz across the 10ms mains half sine. (its COT with Off-time of 13.4us)Are there any simulation tools we can use to calculate the power dissipation in the windings due to the fringing flux field in the vicinity of the core gap?TEX-E Triple insulated wire:https://www.furukawa.co.jp/tex-e/en/product/texe_series.htmlPQ2625 Ferroxcube datasheet:http://www.farnell.com/datasheets/1595868.pdf

author: jonpaul
author_rank: Super Contributor
timestamp: December 19, 2020, 04:08:55 pm
content: for production desing or just one off?Safety/EMI Compliance requirements?Jon

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 19, 2020, 04:54:03 pm
content: Its for production. We Think volumes of a few 100 at most.EMI/Safety as per EU standards.

author: TimNJ
author_rank: Super Contributor
timestamp: January 04, 2021, 01:06:59 am
content: For what it's worth, for a typical <100W design with sandwich construction, I've adopted the following rules of thumb:1. For primary "half" closest to the core's gap, use litz wire. Assume that fringing field losses are more important than DCR losses in the vicinity of the gap. IIRC, if you compare single strand enameled copper wire with litz wire, with the same total cross sectional area, litz wire has higher DCR. Sounds bad, yes, but in the vicinity of the gap, fringing fields are probably more important...though it does depend.2. For the primary "half" farthest from the gap, use solid wire. Assume that the gap is sufficiently far away that fringing field losses are less significant, and that it's better to optimize DCR losses in this half of the winding.3. I put "half" in quotes because to get full bobbin breadth/width utilization, it might make more sense to split the primary into 1/3 (litz, near gap) and 2/3 (solid, outside). You want to use the full bobbin window for lowest leakage inductance. Leakage inductance is probably most important thing anyway.I unfortunately don't have strong data to support this approach, but a few recent designs using this method have yielded very good efficiency thus far. Also consider adding copper foil shield winding, or wire-structure shield, or a common-mode bias/cancellation winding for EMI.Also, check this app note:https://www.ti.com/seclit/ml/slup338/slup338.pdf

author: trobbins
author_rank: Frequent Contributor
timestamp: January 06, 2021, 03:27:17 am
content: Back in the early 1980's I did a gap-proximity loss comparison for a 4C6 centre-leg gapped (0.9mm) 26/16 pot-core (alpha=100) using a nominal 10uH single layer inductor at 2.4MHz. The coil resistance was measured for 3 coil radius', being just under 7mm, centre average of about 8.4mm, and just under 10mm, where min winding radius was about 6.4mm, and max was about 10.3mm. The innermost coil had a resistance about twice the centre located coil, and the centre and outermost coils had effectively the same resistance. I don't have all the measurement details now, but the influence of padding was certainly significant for that configuration.