I am designing a low noise SMPS like this http://www.ti.com/lit/df/tidrgc2/tidrgc2.pdf switching from 40-90 kHz.
I have a weird problem, on the output we are seeing a spike of noise at 5.5 kHz and multiple (decaying in a few multiples), tested with differential amplifier and correct techniques.
We have no idea where it comes from (maybe ringing of secondary transformer inductance + caps)?
We added a C and R across transformer, 1000 uF and 20 ohm and we seen a 6 dB reduction on noise. We are experimenting with different values to lower the noise. The only LC tank we are seeing is diode capacitance (and the RC snubber across it) and transformer secondary winding.
We used 2x47uF parallel across pin 6/10 + 50R and spike is slightly lower .