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Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
We decided to use one of those new 1.5F super-capacitors to stabilize a high-current low-voltage supply, but it seems to be running rather hot for the setup (<50kHz switching frequency).
Note that this has not exceeded the rated 80'C limits, but I am concerned its operational lifespan is going to be rather short. It was operating normally while not under load, and therefore unlikely its the cap itself that's damaged.
Anyone have experience with how well these new caps (EDLNF155B5R5C) tolerate ripple stress?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I would think those supercaps would have higher ESR than proper low-ESR SMPS caps, even of much lower capacitances. The output capacitors heat up because of ripple current. Assuming you use a forward converter, the ripple current is directly proportional to the output inductor value, at a given frequency. So you can try increasing the output inductance and make sure it does not saturate, or increase the frequency.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
They're designed for backup applications, not high current discharge. Either connect an ordinary low-ESR electrolytic across it, or fix your control loop so the converter is stable with a normal-sized output capacitor.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
According to the datasheet your cap is rated for 1mA DC current and has a large series resistance: it's just not suited for a situation with large ripple current. I seem to recall that capacitor series resistance usually helps stability in SMPS's so that may be what's stabilizing your power supply rather than the capacitance.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
I am rather disappointed a negligible 0.35v ripple caused these caps to cook. Even with 50 ohm Z at 1kHz specs, the other normal caps on the bus should have at least reduced part stress within SOA. ...package type "F" indeed...
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
0.7*0.35*1.5 ... if it could actually smooth out the voltage (which it can't, with it's internal resistance) you'd be running at 367 times over maximum rating, you should be elated you're not showered in exploded ultracap :)
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Super caps are not designed for smoothing applications. They are used for energy storage to back up real time clocks and data held in static ram.
Use proper low ESR caps these usually have a tall thin in diameter ratio to aid cooling and just like modern high power supplies use several in parallel
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SMPS capacitor overheating from ripple
It was operating normally while not under load, and therefore unlikely its the cap itself that's damaged. 
...package type "F" indeed... 
