I am in the process of designing my very first SMPS flyback transformer. My knowledge on the subject has gone from almost zero to the point where I am confident enough to go ahead with the project (that's what I love about electronics). Now there is one subject that has been bugging myself and giving me sleepless nights for the past couple of weeks (one of the downsides to electronics!), and that is the safety requirements
. I am a "young player" in the Electronics industry, as Dave would say, (not long out of university) so please bare with me.
First, a very brief summary of the project:
* The transformer is for an offline flyback (90Vac to 265Vac), constant current LED driver (<9.1W). This is a primary side regulated (PSR) design.
* Due to space constraints I am forced to use a relatively small core (EFD is suited due to low profile).
* At the moment it looks as if an EFD20 will do the trick (however low Ae and small winding width means I will need multiple primary layers which is fairly unavoidable with this design). I have another question on interleaving windings but I don't want to bombard everyone with lots of questions at the moment...
* The transformer must meet the safety requirements of EN61347-2-13
* The LED driver is a class 2 (double-insulated) safety product.
So here goes the question(s)...
1. I want to use a margin-wound transformer construction to meet the appropriate creepage/clearance requirements. However I have seen many different methods of meeting the requirements. Some application notes/designs use margins on both sides of the bobbin whilst on the other hand some use margin tape purely on the secondary side?
2. If anyone is familiar with EN61347 then I would love to hear from you. I am confused about the actual distances required for the design? Being a class 2 product, I believe I require supplementary insulation? (According to a table I read in EN60598-1)
3. Am I right in saying that you should account for the inherent clearances of the bobbin itself?
4. Where exactly are the clearances/creepage distances measured to and from (in regards to transformers)? Is it purely from primary to secondary windings?
5. Can all of the above be avoided by simply using triple insulated wire (TIW) throughout?
I apologise in advance for all the questions. I would not normally ask in the forums but I think I have exhausted all other options.
P.S. I am sure some sort of beer payment can be made for any helpful answers!