General > General Technical Chat

A quick and dirty way to implement Solar switching regulator MPPT?

(1/3) > >>

depot:
I have a small solar-powered side thing where I want to use solar power and supercapacitors and something like LTC3355 to keep a device on for a long time.
When the sun is out, great. But in non-ideal sunlight it's trouble because it draws enough power to apparently drop solar panel voltage and thus power down to almost nothing.

So of course throwing more money at it for a nice MPPT and supercap management is nice, or just a bigger solar panel, fine if it's a hobby thing that I'd only build one of, but of course I'm curious how to "build a bridge that just barely stands up" i.e. a cheaper, simpler system that would still work.

A co-worker suggested that I could use some features of LTC3355 to implement something like MPPT, if only to give it a useful power point in the low-sun daytime case. And honestly, another MPPT thing LTC3130 does basically that with a pin for RUN and another called MPPC. When I try to use LTC3355's PFI input to do something similar though, it seems like the whole regulator shuts down for a longer time, very disappointing.

So what's the difference between the two parts under the hood? It looks like the MPPC and RUN pins are the "quick and dirty" features I'm looking for, is there some external way to implement these? Or, if they're connected to regulator soft start or something else that locks it up, are there things with this kind of feature? Is there anything to my co-worker's suggestion? I do like something like LTC3355 for managing the supercap.

Or maybe I'm overthinking it and a better MPPT is not so expensive and complex and I could still use it.
I was expecting to find more parts for solar + supercap designs in this kind of range for power too, this is strange and maybe I'm missing some perfect device.

gnuarm:
Rather than supercaps, what about a battery?  One advantage of a battery is that for short time periods a battery is a constant voltage source with small changes in the charging current having little impact on the voltage.  In that case if you maximize current into the battery, you maximize power, both into the battery and out of the solar cells.  So design a battery charger rather than a MPPT and you get the MPPT as a side effect.   

Any MCU can be used to implement this.  You need to sense the current out of the charger circuit and dither the duty cycle of the pass transistor feeding an inductor.  You will constantly change the pulse width slightly.  If the last pulse adjustment results in more current, adjust the next pulse in the same direction again.  If the last pulse results in less current out of the charger change the direction of the adjustment.  This will rapidly seek and track the maximum power point without really trying. 

You will need to monitor the battery voltage and stop charging when it reaches a fully charged voltage or at least change the charger to maintain a fixed current rather than the maximum.  Also, depending on the details of the battery picked, you will want to initially limit the charging current until the voltage reaches the appropriate level.  The MCU can manage all of this easily. 

depot:

--- Quote from: gnuarm on December 29, 2021, 04:11:35 am ---Rather than supercaps, what about a battery? ...
--- End quote ---

Everything you post is true, thank you, and on top of it there are many lithium battery tailored solutions, but I'm not going to for some dumb reasons:

I've spent most of my career so far doing battery-related things and I feel they're simple.
I want to try supercaps as new (ish) technology and to compare (and squeeze out) performance.
I hear good things about their future and I know some of the farad values get quite big.
Although they have a self-discharge worse than batteries, I want to find out if an overall system life might be longer. Especially, especially if a long time running supercap system will lose capacity and need replacement over time like a battery system does.
In particular I want to try energy harvesting (like solar) with supercaps.

The interesting thing about this little project is that the power reqs are a bit too high for typical tiny energy harvester things. Perhaps that means that a battery is just the most suitable, but I still want to look at it more before calling it.

Maybe is there some way to simulate the battery's natural "side effect" MPPT?

Peabody:
As I read the LTC3355 datasheet, the input regulator is buck only.  So based on the PFI pin voltage, it shuts down the input regulator, and turns on the ouput boost regulator so the supercapacitor can power the load.

But the MPPC pin on the LTC3130 just limits the current draw so the input voltage doesn't fall below the divider voltage.  I think this is only approximate MPPT, and is based on the idea that the voltage at the MPP doesn't change very much for different levels of illumination.  The MPP current changes a lot, but the MPP voltage doesn't.

It seems the LTC3130 is what you want.  I don't know how you would add the MPP function to a normal switching regulator.

gnuarm:

--- Quote from: depot on December 29, 2021, 05:52:43 am ---
--- Quote from: gnuarm on December 29, 2021, 04:11:35 am ---Rather than supercaps, what about a battery? ...
--- End quote ---

Everything you post is true, thank you, and on top of it there are many lithium battery tailored solutions, but I'm not going to for some dumb reasons:

Maybe is there some way to simulate the battery's natural "side effect" MPPT?

--- End quote ---

Ok, if the supercap is what you want, that's up to you.  I'm not speaking from experience with these circuits, just brainstorming. 

Maybe I'm over thinking this.  The voltage on the supercap will also be stable for large values of capacitance and short periods of time.  So a one stage switcher that dithers the PWM so as to maximize the output current could simply ignore the cap voltage other than to limit it for protection.  I suppose the supercap could work like the battery since even if the voltage is not long term constant, it is short term constant which is all that matters.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod