Highest Density Capacitors for high density design

[magudaman]

Wow there really is a forum for everything!

So I am working on putting together a 500w charger for electric bikes that is a very high power density. Currently designs on the market are lucky to get 9 watts / in³ , I am shooting for 30 w/in³ but currently it looks like I'm going to end up with around 20w/in³.

So my question goes to my boost stage capacitor requirements. The largest single part of my charger besides the switcher module, is my boost stage capacitors. The design requires around 600uf of 450V capacitors. Though they makes some nice 12.5mm diameter caps they just end up taking up too much room and I have to revert to the 18mm capacitors.

So I wanted to ask is their anything that is considered new and exciting that is pushing the energy density of capacitors? It seems that same radial caps that were used in my power supplies from 1999 are used today.

[mariushm]

Re: Highest Density Capacitors for high density design

nevermind, it's too stupid even as a joke.

[magudaman]

Re: Highest Density Capacitors for high density design

Yah I didn't quite follow it anyhow. I did look at some surface mount stuff but it seems their density wasn't that exciting since they left quite a bit of space between one other.

[999999999]

Re: Highest Density Capacitors for high density design

They are not limited in power density because they can't find a small enough capacitor, they have as high a power density as they do because they didn't try to use a smaller capacitor, because nothing that is cost or space effective exists to replace electrolytic caps with those specs.

Capacitors exhibit self-heating from their ESR. The larger the capacitor can (casing) size, the lower the ESR (all else being equal), the lower their own self-heating, the lower the cap thermal density which always exceeds the average thermal density of the entire power supply. Increase power density too much and electrolytic capacitors will have very poor lifespan.

The point is, if you want to avoid early failure of the design then using larger caps will allow higher power density (all else being equal such as high voltage rating, lowest ESR obtainable), you'll have to have more empty space and more heatsinking to keep the smaller caps from failing. That space increase is greater than the size increase of the capacitors themselves. This changes a bit if the design already uses a fan and you can increase fan RPM to get more airflow, but still the best design will use high temperature rated, lowest ESR rated capacitors from a major brand and accepting whatever size those are.

If you were building something low voltage then the story changes, along with semiconductor advances came advancements in ceramic and solid polymer caps which can withstand far higher temperatures and retain good life, but those (in more than a few uF capacitance) are for ~dozen volt circuits not 450V.