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Offline 20W Flyback SMPS failing QP radiated emissions at peaks 57MHz to 184MHz

treez

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Hi,
Its an offline 20w flyback SMPS in an earthed metal enclosure with its output coming out of the enclosure on a 2 metre cable...(going to a DCDC buck onverter)
What is the likely culprit of these frequencies?
--- Updated ---

Please also confirm that cable ferrites are no use for these frequencies?
 

Easy peasy

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no snubber on the o/p diode the most likely cause ...
 
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treez

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Thankyou. I placed RC snubbers on Aux and Sec of 220pF,47R (all i had in the lab) and it didnt improve the "el cheapo" radiated emissions scan that i did after fitting these RC snubbers.
(This el cheapo scan simply involved torroid wrapping the cable from the flyback through a near field probe loop (18 turns), and then looking at the probe output with a spctrum analyser (SA1002A)
 

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try to look at output diode and power MOSFET source-drain and measure the freq of oscillations at this points, is any correlations with peaks
 
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treez

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try to look at output diode and power MOSFET source-drain and measure the freq of oscillations at this points, is any correlations with peaks
Thankyou yes i was doing this, but the frequency of the oscillations there was nowhere near like the failure frequencies....mind you, i only had a 100MHz scope to look there. But then again, maybe the scope capacitance was making it seem lower in frequency than it really is.
 

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