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Noisy output voltage from DCM flyback

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Newbie level 1
Nov 4, 2009
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(This is my first post here. )

I have built a DCM Flyback for 400 VDC to 750 VDC input voltage.
With a output of 15V/2A.

Actually i followed this application note ( when i built it.

One of the bigger problems i had was that the output was quite noisy so i putted
a extra LC filter (30µH/ 1000µF) on the output. That did allot!

I also placed (two 2,2nF serial connected to handle the voltage) Y1 Capacitor
between the output ground and the primary ground, to couple back capacitive coupled noise currents. That also did some to help.

But still there is some high frequency noise at the output that seem to come from
the primary side oscillations between the leakage inductance and C_cs, and also a ringing frequency that seem to come from when the transistor turns on and discharge the C_cs [ESBT].

Anyone have any tips? Perhaps i would need a screen in my transformer to stop
capacitive coupling.. i am quite frustrated so any tips would be real nice.
I am thankful for any help. :D

maybe you can just put snubber over secondary diode or across secondary?

this is a pretty common problem actually. often its because designer got a quick "off the shelf" transformer only to find out later the device is for communication - not power - as most transformers you find easily are for communication field.

maybe put a series R-C across terminals of secondary.

good luck, noise is a tough problem to solve. you can bandage it a bit with caps and such, but really the only solution is a good understanding of both mechanical and electrical design to get noise down. sadly, you don't learn what to do until AFTER you do it wrong a couple of times.

ps. DCM is supposed to be noisy, that's the down side.

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