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Poly caps used with SMPS
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Sylvi:
Hey
SMPS module app notes always specify ceramic caps all around, sometimes chokes, even electros, but never poly caps.
There are many polypropylene caps used in DC link applications with high-frequency impedance curves that look exactly like MLCCss, so why does no one every use them? Is it just because of cost? Size?
I know poly caps are physically much larger but they do not have the variable capacitance versus voltage that MLCCs suffer, nor does their value change with temperature - so much more like an expensive C0G-N0P ceramic. These days, the range of polyprop cap-voltage has been extended down to quite low voltages at decent capacitance values, and also to large-C at high-V. They are super rugged compared to ceramic so more likely to see them in railway duty - haha - why not pro audio and consumer electronics?
David Hess:
Plastic film capacitors are larger, more expensive, and more difficult to solder. They work fine as replacements for ceramic capacitors.
T3sl4co1l:
Film is only economical above 100V or so, and when more AC current is needed than electrolytic can handle (which is cheaper per uF).
As it happens, aluminum polymer has very much the same characteristics as lowish-Q film caps (say, self-healing MKS style), scaled to lower voltage ratings and larger values. Still kind of expensive, but well worth it when you need it.
Tim
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