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Original question:
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
I done some further investigation, and yes seems really to be more a Switching noise instead of AC Ripple:
Measurement has been made with the tip-ring to shorten as soon as possible the ground path.
Trace #1 (Yellow): Ouput , coupling in AC
Trace#2 (Green): Pin N° 1 LM5002 (coupled in DC).
Hi David,
I think the behavior is mainly caused by the probe setup as show in the image
