RDR291 Design understanding

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Hi I want to design a 50W led driver using lnk420. I was referring to RDR290. I have 2 question regarding the transformer.


1. It uses PQ core transformer. Lower half of that core is wrapped in tape before inserting it in bobbin. What is the use of this wrapping?

2. What is the difference between ETD core and PQ core? 


Very good observation, core wrapping at the bottom side of the core is used to meet safety isolation between primary and secondary section.

PQ was used becuase it has a smaller size (footprint) as compared to ETD cores.  If you have more space available then use other type of cores. The spreadsheet will help you to calculate the transformer.  You can use slightly smaller core cross sectional area for 50W design as compared to the one used in RD291.  Sometimes too big cores will reduce the efficiency because of increase in core loss.


Can I use PQ core without core wrapping for isolation of 2KV?


Yes, you can use PQ core without core wrapping by the following approach:

1. Secondary wires should not be terminated on the bobbin pin.  It will be terminated away from core by 5mm directly on the PCB.  Make sure to review your layout carefully to guarantee you meet safety standards. Note that this will take more time in assembly process. Reference design using this approach is RD-195. Or by,

2. Using a custom made PQ core that isolates the bobbin pin by 5mm from the core.