flyback windings and understanding the datasheets - Page 1
Forum section: electronics / beginners

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: November 28, 2023, 08:18:21 am
content: when switched power supplies were discussed at university, we went through the very basics of flyback converters, and a very simple transformer with just 2 windings was used as a teaching aid. But looking at the datasheets of most flyback transformers, i see they come with an extra winding (not counting the one or more secundaries) called shielding and sometimes a winding called auxiliary. The auxiliary is also usually accompanied by the electrical specifications, like voltage and current, but the shield is left marked as shield, and that's it. I've found the usage of the auxiliary winding to be quite straightforward, but not so the shield winding.Is the shield winding shielding what from what? should i leave this winding disconnected? should i just ignore it entirely? should i use it as an auxiliary winding? What i've found so far about the shield winding is contradictory, with some claiming it is an EMI shield, and others saying it's an ESI shield, with vague explanations for each.another thing, which i've found confusing with the transformers i've got from bourns is the description of the transformation ratio, as well as the pinout. pinout should be easiest to figure out as i can just measure, but it's still a bit confusing as they show a picture of the transformer as it would look like from the bottom, but it isn't explicitly stated if it is a bottom biew or a top view, and about the transformation ratio, in this particular model: 093830, they say 4:18.5:1:1, but which one is which? in this case, it's easy to see the 1:1 bit is refering to the secundaries, which are identical, but which is the primary and which is the shield? would they start with shield, then primary, then the secondaries? or primary, then shield...? is whatever they are using the standard way to describe transformation ratio in these transformers?https://www.bourns.com/docs/Product-Datasheets/093830.pdfI've linked the datasheet for the model i mentioned above.Thank you!Edit: from the voltage ranges, it would be intuitive to say that the primary is that 18.5 winding as 6*18.5 is 111, but, will this always be the case that the primary is written second in the transformation ratio? is there a standard, am i missing something, or are the datasheets for this kind of component intentionally not too verbose?

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: November 30, 2023, 06:54:32 pm
content: a bump!i still have not found much on the shield winding, any help understanding their purpose and usage is welcome.thank you!

author: Andy Chee
author_rank: Super Contributor
timestamp: December 01, 2023, 12:33:56 am
content: Due to the high frequency of SMPS operation, some of this energy can be coupled across the capacitance between the primary and secondary windings, and appears as noise. The shield between the primary and secondary windings can reduce this capacitance. This shield is made of thin foil and is normally connected to circuit common on the secondary side, though dependent on circuit topology.Also there can be a shield for electrical safety to prevent insulation breakdown creating a primary to secondary connection. This shield must be made of copper sheet thick enough to absorb full fault current to blow the main fuse, and is normally connected to chassis earth.In both cases, the shield is not really a winding in the normal sense of the word. The shield is simply a strip of wide foil (as wide as the pri/sec windings) wound as a single layer single turn, the entire foil is insulated from the primary & secondary windings, the ends of the foil layer are insulated from each other (to avoid becoming a shorted turn), a single wire is soldered to the middle of the foil strip and connected to a transformer leg. So unlike a conventional winding with a start & end wire, a shield "winding" layer only has one wire.

author: Konkedout
author_rank: Frequent Contributor
timestamp: December 01, 2023, 04:51:01 am
content: I have designed and built many flyback transformers and the power supplies that they went into. Most did not have inter-winding shields. I recommend that you do not bother with that at first, until you come to appreciate the need. They do tend to make the coupling worse, so I do not recommend it if you do not need it.Most production transformers are custom. If you want to try winding your own transformer, I highly recommend trying a core set with round center leg such as PQ cores. And to start with, choose a larger core than you think you will need. For example a PQ3230 can really do a 50W flyback, but try using that at 20W to 30W. This will all make your job easier at first.To buy a transformer, I recommend Coilcraft "Hexapath." Wurth makes similar, called "Wurth Flex". These definitely would not be suitable for safety isolation from mains, but they make a good transformer for low voltage in and out.