Solar buck converter

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Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 258
Posted: 10:25pm 02 Nov 2019      


It's been a while since I posted something so there will be a lot of spelling mistakes

Last summer I didn't need something to maximize my solar input, but winter is coming and we don't have a grid connection (anymore). We bought half of an old farm and last 3 years we spent renovating it, we can say it's more like a new build house because except the front wall it's all new. We bought the piece without grid connection...

Getting a new grid connection is not an issue, it's "only" about 1200€. For solar owners there are uncertain guidelines on this moment, the introduction of the digital meter gives many owners a lot of stress... One thing is for sure, we will pay to use the grid as battery... and there is no one that will tell you how much...

So we will try to manage without as long as possible hobbywise... untill we have no choice or guidelines are fully understand. We are living in Belgium so it will not be easy to harvest enough energy during winter. We need to do it with 48V 400Ah SLA and 51.2V 400Ah Lifepo4 and 27 panels of 280W. Heating is done with a wood stove called tulikivi raita 18 and 12m² solar thermal panels, they all are connected to 800l buffer and the floor heating. All electric apliances are new and energy efficiënt. I guess it would be possible if we were not cooking on electricity. I probably need to buy a small generator or more panels.

So the solar, String 1 is 6 panels 280W, String 2 is 6 panels 280W and String 3 is 8 panels, wired in 2 series/parallel so optimized to be used with 48V. But we have shading until midday. The 4th string are 7 panels 280W in series attached to a grid tie inverter, that's connected to my Ozz inverter. The 48V strings are " controlled" by a big contactor.

So I wanted to make 3 "simple" controllers for 48V and max 40Amps, yes today you can buy good controllers for that but whats the fun in that.  

I used this site for guideline:

But the last year I read every post of warpspeed and solar Mike, allways verry good comments and insides!

So this year I made a few experimental buck converters, and guess what the most simple concepts worked best . Once I made a verry simple design with just one irfp4110 a diode and 2 caps with long wires to an arduino nano with a tlp250, it worked for months, I put it out of service now.

I did experiments with snubbers, cycle by cycle current measuring with current transformer (difficult with high duty cycles). All kinds of inductors, capacitors,... I learned a lot, didn't blow up things tho....

I made a pcb in july this year, finaly got it tested today!
Software is written verry poorly in arduino, but it works, it's just a litle bit slow when I disconnect the battery, so the output voltage rises to much, thats just software.

So I used 2 parallel mosfets (irfp4110), 2 dual body diodes 30cpq100 in parallel. For gatedrive I used 12ohm resistors on each gate to a tlp250. Switching is done on 31khz. The inductor is an etd59 with about 100uH, it has 1mm airgap.

I tested to about 62Amps going to the battery, @ about 40Amps there was 15% more current going in to the battery! With 30 amps there was no cooling needed, with 60 amps going in to the battery, a fan was required to cool the inductor. I only made it for 30Amps (4amps/mm²).

I used 9 elco's epcos 1000uF 300V on the input and 5 the same type on the output, I have hundreds of those laying around, NOS from ebay.... Also 4.7uF MKT on the input and output. ACS758-100 was used to measure in and output current.

The concept could be verry easy adjusted to other input/output parameters.

Code is like this:  

if(UoutPut < UsettingOut)
 pwm = pwm +1;
 pwm = constrain(pwm, 1, 254);

if (UoutPut > UsettingOut)
 pwm = pwm-1;
 pwm = constrain(pwm, 1, 254);

Simple but it works, Usetting is done with a potmeter, and read on an analog pin, I use 62V for my panels.

The pcb was drawn with EasyEDA,
I know it's a chaotic post, I will give more details like schematic, gerbers if wanted and code next time.

I'm happy with the result that's why I want to share it with you guys! Any help how I could improve it is welcome!
Edited 2019-11-03 08:41 by nickskethisniks