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Original question:

LM5002: LM5002 SEPIC with coupled inductors

LM5002: LM5002 SEPIC with non-coupled inductors - Ripple /Switching Noise issue

DAVIDE DAL FRA
Prodigy 50 points
Part Number: LM5002

Hi E2E forum!

We've used LM5002 in SEPIC configuration as per example below:

We have just changed the Feedback resistors to have 15V as output.
Our specification is Vin=14V to 20V, Vout=15V at 50mA

From the operation side the SEPIC is working quite good: sweeping from 14V to 20 the output is always stable at 15V. However something strange on the output is happening, it seems ripple but in fact seems to be more a switching noise issue.
At the beginning we was using as switching Diode (D2) a Vishay V8PAL45 but seems to have a high junction capacitance(1400pF)  so we changed with a SS16 (less than 100pF).

Changing diode and output capacitor (from electrolitic to ceramic) have improved the ripple as below image to 200mV.

Before:

After:

Zoom on the peak-pulse:

All the test has been performed with a 50mA load.

We're basically looking for suggestion on hot to improve the AC-Ripple/Switching noise at the output: We tried to add a RC snubber in parallel to D2 but unfortunately this have not helped.
Anyone has got similar issues?

Thanks in advance

Davide