Ignorance is (was) bliss: capacitance over frequency
I´m currently investigating various types of capacitors for their use at the high ripple current inputs and/or outputs of switchers. The input to a buck is the classic example. I´ve been comparing banks of pure MLCC (multi-layer ceramic caps) to mixtures of MLCC together with standard aluminum electrolytic capacitors and mixtures of MLCCs with polymer aluminum capacitors. I assumed (and yes, I know what happens when you assume…) that the electrolytics would have fairly low SRF (self-resonant frequencies) and that they´d be pretty much useless above around 200 kHz. And that´s OK – I mostly use them to damp sub-harmonic oscillations due to power supply interaction, and those subharmonic frequencies are almost always below 100 kHz.
What surprised me was that polymer aluminum devices also appear to lose their capacitance at frequencies that rarely exceed 100 kHz. I´ve measured devices at the high end of the voltage range (25V) and the low end (2V), capacitances ranging over a 33 to 470 uF range, and I´ve measured both the can-type packages (indistinguishable from SMT aluminum electrolytics unless you look very closely and see that there’s no vents) and the rectangular SMT types (D-case, E-case, etc.) I really expected the rectangular cases to have much lower ESL and therefore higher SRFs, but not a single one has an SRF higher than 200 kHz.
This is making me question the value of polymer aluminums. Yes, the ESR is very low, so they will tolerate a lot of ripple current. But they are low voltage devices, so in general you´d find them at the outputs of DC-DCs. No modern, point-of-load DC-DC regulators run at 100 kHz. In fact, most of them are touting their 1 MHz+ operation. But when I look at the plot below of this 470 uF, 2V device, all I can think is “it wouldn´t do anything at 800 kHz. Or 400 kHz. Or even 200 kHz.”
So what are these caps good for?