Interesting power spply IC.http://www.microsemi.com/datasheets/UC184xA.pdf
The UC184xA family of control ICs provides all the necessary features to implement off-line
fixed-frequency, current-mode switching power supplies with a minimum of external components.
Page 5 chart Figure 1, 2.2 nF 3.3K is prox 200 KHz
Formula indicates freq is inversely proportional to capacitence,
For RT > 5k:f = 1.72 / (RT CT)
so 22 nF 3.3K should give prox 20 KHz.
Formula predicts F at 23.7 KHz, but may not be correct because R < 5k.
If you change R to over 5K and adjust C accordingly, you can calculate directly from the formula.