LNK623 design and Transformer insulation issue

Posted by: josefrypar on 04/18/2013

Dear PI sipport,

I'm working on simple Flyback design using PI Expert software. The design requirements are universal AC input and 10V@0.2A output. The PI Expert suggested to use Linkswitch-CV family, LNK623DG controller.

This controller utilizes feedback winding that is connected directly to the LNK623 without any optocoupler. I consider this winding to be primary-side winding that does not need double or reinforced insulation to the Primary winding, however it shall be double insulated from Secondary (output) winding. Is it correct?

The transformer construction generated by PI Expert however shows 3 layers of insulating tape between Primary and Feedback windings and only 2 layers of tape between Feedback and Secondary winding. The level of isolation is therefore lower between Feedback and Secondary.

Electrical diagram generated by PI Expert as well as in datasheet of LNK623 indicates that the feedback winding is on the secondary side. The Primary is connected to pins 1&2 and Secondary and Feedback is connected to pins 3,4,5,6 - this is also indication that the Feedback is secondary-side winding.

Please clarify this discrepancy. We have already ordered transformer samples according to the PI Expert report and I'm afraid we will fail during HiPot testing. A screenshots of that report is attached.

Thank you in advance for timely response.

Best Regards,

Josef Rypar

Files

EP4030_10V@200mA_LinkSwitch-CV_NoShield_var2_complete.pdf (103.51 KB)

Comments

There are 2 things that you should be wary of

1) Isolation between primary and secondary (which is what your post is about) - 2 layers of tape is the minimum you need, You may add 1 more layer if desired.

2) Safety of the bobbin itself - Need to provide 6 mm creepage between primary winding and secondary side pins (and also secondary winding and primary side pins). Usually 3 mm is provided by Margin tape so you need to verify that you have 3 mm separation between the bobbin wall and the pins... Usually bobbins have extended creepage to address this issue

3) Can you use triple insulated wire for secondary?

Thank you for answer

ad1)

If 2 layers of tape is enough than it is fine, one more layer can be easily added as you said. Confusion comes from the fact that the feedback winding is primary-referenced so I'd expect better isolation between Feedback and Secondary (e.g.3 layers) and weaker between Primary and Feedback (e.g. 2 layers). Also the schematic symbol shows the Feedback to the right of the core which implies the Feedback is secondary-side winding. Would not be better to show Feedback winding to the left of the core?

Construction of the transformer in AN-45 shows Primary, Feedback and Bias winding separated with 3 layers of tape from the three Secondary windings. Also AN-45's schematic in Figure 16 shows the transformer symbol in the correct way = primary referenced windings on the left, secondary windings on the right.

ad2) The pin assignment on the bobbin is ok and creepage shall be met.

ad3) The TIW is possible however will imply cost, although I don't know how much.

In general the transformer design made by PI Expert sw is correct (even marginally) but still slightly misleading. You may think to update the schematic symbol and unambiguous level of isolation clearly defining isolation boundary that separates all primary and secondary windings in next version of PI Expert sw.

Thank you.

Josef Rypar

Agreed. Thanks for your feedback.