Charging supercapacitors with a DC-DC converter - Page 1
Forum section: electronics / projects

author: depot
author_rank: Contributor
timestamp: May 10, 2021, 02:38:49 am
content: I have a design with some problems using LTC3130 -https://www.analog.com/media/en/technical-documentation/data-sheets/3130f.pdfI laid out a straightforward circuit that I think follows the datasheet. On page 30 there's an example circuit that's basically the same as mine, but 5 supercaps and wider range PV input. The IC has features for MPPT and the input and output voltage ranges are wide. So I expect it's simple (haha, I'm awful). These parts are getting more sophisticated and I feel the design is more hands-off than before.The plan is to connect a small solar battery, like a square foot size, and use <1A to charge up some supercapacitors. It's enough to keep a small load running overnight. Even on a cloudy/rainy day it charges at a trickle with the RUN/MPPT doing something useful even down to 4V. On a sunny day it gives something like 12V and everything is peachy. I've noticed before that SCs in parallel don't store power quite as satisfyingly as series, so I set it to output 15V and buck that later.When I build a bunch of these pcbs, I get a couple that work as expected, but 5 or so of them output really low voltage, never really charging the supercapacitors. A couple others have been destructively failed with magic smoke and all. It's frustrating. Am I designing this thing poorly, against the spec? Maybe I should somehow limit the output current going into the supercapacitors. I also feel wary about a wide range at both input and output. Breaking the parts in assembly? My soldering skills are pretty average I feel, I usually do better than 50% yield though.I don't really know how to troubleshoot this.

author: depot
author_rank: Contributor
timestamp: May 10, 2021, 03:37:44 am
content: I found out something today. I guess I'll share instead of deleting this.When I change my design to output only 8v, it can achieve this. So the thing I overlooked is that the converter is limited by something like "minimum on time" or just an incredible output power limit. In this case a quick test for this was to relax the requirement for the output voltage and check to see if it works then.At 8v output it can give quite a bit of current and charge up the supercaps.edit, later:Lowering the output is not enough.When I ask friends about this, they say that the supercapacitors act as a short to ground at startup and ruin compensation. One suggestion I like is to control the output by current instead of voltage. Another idea is to use a resistor between the switcher output and the supercapacitors.I hope those measures will fix it. I feel dumb. I saw some pretty strange things and some of the other problems were also my soldering.

author: rfclown
author_rank: Frequent Contributor
timestamp: May 25, 2021, 02:41:34 am
content: For charging super-caps you need to use a converter with current limiting. The datasheet indicates that it has this, but you need to ensure that this is being used. Like others said, the cap looks like a short when it is empty. Some converters are designed specifically for changing super-caps and can handle this (they use current limiting).