I am trying to build an 120 AC to 15 DC @ 200 mA converter using a Power Integrations LNK3206P. Using the full bridge rectifier setup on Table 10 from application note AN-70, Cin1 and Cin2 should each be >2uF/Wout * 3W = 6uF rated at 400V.
It appears that at least Cin2 should be of a low ESR to reduce the ripple voltage on the input of the converter and reduce the ripple current on the electrolytic capacitor Cin1.
Using ceramic capacitors such as this 2220 250V 2.2uF X7T, after de-rating due to the ~170 DC bias of the rectified 120 AC as seen in the right hand side middle graph of its details page from TDK, it becomes approximately 0.8uF.
This would mean that I would need at least 8 if these capacitors in parallel, for a total cost of about $30 USD.
Looking into other options such as ceramic capacitors with higher voltage ratings still results an an unreasonably high total cost. Similar thing with film capacitors.
With AC DC SMPS being so common and cheap, this does not make sense. What am I not thinking about or considering.
Best Answer
Note the symbols on that schematic: both Cin1 and Cin2 are electrolytic caps. So, even for high voltage ones, you can get them at reasonable price (10uF-100uF, 400V electrolytic caps are fairly common in SMPS).
Note also that in that application note there is a photo where it appears that they actually use electrolytic caps: pag.10, figure 3, the caps on the right appear to be 4.7uF,400V electrolytic caps.