LM5000: LM5000 EMI Issues

Hi Alex,

there are two potential sources for ringing within a Flyback converter.

1. The snubber on the primary side is not dimensioned right

2. The diode on the secondary side (due to the parasitic capacitance).

May you can probe on both points to see if you can find the source of the 25MHz disturbance peek.

Best regards,

 Stefan

Hello Stefan,

Thanks for the fast reply.
Well I've looked now on the switching node and made some meausrements.
As you can see there is some ringing.
Do you have an Appnote where the ddimensioning of the Snuber circuit
of the primary and secondary side is described?
Because we've just an resistor between the two diodes.

Hi Alex,

please have a look at this two reports:

https://e2e.ti.com/blogs_/b/powerhouse/posts/calculate-an-r-c-snubber-in-seven-steps

http://www.ridleyengineering.com/images/phocadownload/12_%20flyback_snubber_design.pdf

Best regards,

 Stefan

Thank you for the link i've tried now the Snubber circuit and it was getting better.
But as you can see on the Picture there are still some issue at 30 MHz.

Do you have any further tips apart from the Snubber which could improve the Performance of the LM5000? 

Hi Alex,

then you might need to go for a Ferrit bead in the supply lines, so to add a PI filter set for the 30MHz

Best regards,

 Stefan