Creepage means offline flyback bobbin is not able to pass isolation requirement - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: July 31, 2019, 07:23:09 pm
content: Hi,The E13/7/4 Horizontal Bobbin by Epcos is not suitable for isolated offline Flybacks. This is because the secondary pins are less than 5mm away from the coils of wire as they will lie on the bobbin (horizontal bobbin on page 4 of datasheet) . Some of these coils will obviously be ”primary side”, and so thus the spacing between primary and secondary is <5mm. This means it cannot pass the common EU isolation requirements.Do you know of any small bobbins like this which can allow a pass of isolation requirements? The Horizontal bobbin is dimensioned on page 4 of the E13/7/4 datasheet.A contractor gave us an offline Flyback design and we had to reject it on this point. He had used a primary of enamelled copper wire. A primary_side bias coil of enamelled copper wire. Also, he had used a secondary coil of Triple insulated wire. (TEX-E)E13/7/4 datasheethttps://www.tdk-electronics.tdk.com/inf/80/db/fer/e_13_7_4.pdf

author: dmills
author_rank: Super Contributor
timestamp: July 31, 2019, 08:17:45 pm
content: FFS! Just provide a layer of reinforced insulation (Say a couple of layers of suitably rated tape and maybe keep the winding away from that edge)...5mm is an air gap requirement between uninsulated conductors, kind of difficult to see that a transformer fits that description!I have used those bobbins, but not in that particular application, they are just fine for the right job.

author: bjbb
author_rank: Contributor
timestamp: July 31, 2019, 09:47:08 pm
content: Ok people. Let us be most careful about on-the-fly responses to compliance engineering engineering questions; that is, do not answer these question unless you have direct experience in product safety evaluation and testing, and writing reports for formal submittal to notified bodies. And even Dave himself, whom usually gets this stuff correct, has emitted a poor response to this type of question.Creepage and clearance - the former is spacing along a non-conductive surface, and the latter is spacing through air vary much depending on circuit design, the intended end-use environment, and the directives and standards scoped for your type of equipment. There are not the same and are tabulated differently depending on the standard that is scoped for the equipment.'Reinforced' insulation is a system of rated components/materials. When using sheet or tape material, if the minimum thickness is met, the 'typical' construction requirement is met by using three layers of rated materials, where any two layers will meet scoped standard's requirements for dielectric withstand per the OV category, and per the measured working voltage. Or you can use mag wire with multiple layers that are rated to meet the requirements for double or reinforced insulation (P. Leo, Rubadue, etc). Where rated mag wire is used, there is no internal creepage spacing requirement, but you obviously need to meet creepage and clearance where the wire is stripped and soldered to the bobbin pins. This type of wire can change other design stuff such as leakage L and winding C.The 'typical' working voltage for a flyback transformer using 230 mains is typically 350V. Depending on the end-use standard and the intended end-use, creepage from primary to an exposed secondary circuit will be between 6.2mm and 6.8mm.Compliance engineering is an unholy combination of physics, chemistry, and law. Welcome to the Dark Side.

author: Yansi
author_rank: Super Contributor
timestamp: July 31, 2019, 10:29:20 pm
content: You either use a TEX wire and terminate its ends further on the board (not on the bobin pins) to get enough distances ... or ... you get a correct bobin type, where one side of pins is offset to meet the criteria.Remember also, that even the ferrite itself is considered conductive material.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2019, 10:33:11 pm
content: They make bobbins with extended secondary pins. They look like this guy,Tim

author: TimNJ
author_rank: Super Contributor
timestamp: October 24, 2019, 12:18:00 am
content: May not be acceptable (mechanically), but you can remove all the secondary side pins and go with flying wires. Might need to use some sort of bottom side glue/adhesive on the core.

author: Neilm
author_rank: Super Contributor
timestamp: October 24, 2019, 06:27:43 pm
content: Quote from: TimNJ on October 24, 2019, 12:18:00 amIf flying leads are acceptable then the pins could be left in with suitable creepages around them on the PCB.