Failure to pass UL clearance requirements
I designed some time ago a power supply based on your TNY275P. It worked fine (and still does), but I have found some problems trying to have the UL certification. I made the power supply with a EF16 transformer and used triple insulated wire in the secondary winding.
That problem is about the transformer and clearance distance to the secondary pins.
UL lab says that core in the transformer is considered primary and I do not meet minimum clearance distance requirement (4.8mm) between the secondary pins and the core. That is true if you consider the core as part of the primary circuit as they do.
Besides, I do not meet either the clearance distance (4.8mm) between the primary winding and the secondary pins. That is also true, because the secondary wire is triple insulated but the pin are not.
Just to be sure I was not missing something I bought and opened two 3rd part power supplies similar to my own. I was lucky and both had a transformer with a triple insulated winding. But surprisingly, in both cases tips of the secondary wires were not solder to the any pin of the transformer. Instead they were solder directly to the PCB more than 3mm away from the transformer. In that way both accomplished the UL specifications regarding to the minimum clearance distance in the transformer.
Should I follow the same solution? Or have I some allegation to defend my own design?.
Comments
Ok, as far as I understand, I need a bobbin with extended creepage distance.
In fact, it seems true always I use a tripple insulated secondary wire. Otherwise creepage distance between core and secondary pins will be always shorter than required by UL.
So I have a new problem. I am working on a new design based on your PKS607 device. Again, I am using triple insulated wire in the secondary side. But I am not able to find any bobbin with extended creepage distance on your list of recommendations for rapid transformer samples.
Does it means that I cannot use that service from Power Integrations?. Or is it possible to make an special request out of your recommended references?
Regards
Hi,
The objective of our service is to help our customers to get a working design which they can optimize.
Safety specification can change depending on application and applicable safety standard. Some times special bobbins, insulating materials, wires etc. may be necessary which we may not have in stock.
You can use our service to get a design ready for testing and optimization. It may be best to work with a transformer vendor to develop your production prototype since transformer manufacturing houses can provide you much more information regarding available choices for wires, insulation and bobbins to meet your safety requirements and also advise you on the cost impact of the decisions.
Regards
PI-Sarek
Hi,
Creepage and clearance can change depending on the type of bobbin that you use. For the same core type, you can often get a range of bobbins and some provide higher creepage and clearance.
If you are unable to find such a bobbin which will space the pins adequately to provide you the required creepage and clearance, you could solder the secondar wires directly on the board and avoid the bobbin pins as suggested.
Regards
PI-Sarek