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Forum Index : Solar : Solar buck converter

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nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 170
Posted: 10:25pm 02 Nov 2019
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Hello,

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:
https://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

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
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 3267
Posted: 03:05am 03 Nov 2019
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That is a truly great effort there Nick.
Its now just a case of tuning up the software

Belgium is going to be just a little bit different to Australia!  
Probably the biggest issue will be how many cloudy days in a row you are likely going to strike in winter. There is no substitute for local climate experience.

That is going to be the killer, and when the sun stays away for several days in a row, more panels or a larger battery are just not going to help. I believe a battery charging generator of some type is going to be a necessity at some times, even if its not very often.

Until you have experienced at least one winter, its not going to be really possible to estimate your typical consumption or solar input with shorter days and longer nights.
Cheers, Tony.
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 170
Posted: 08:54pm 03 Nov 2019
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Since I want to keep everything safe for myself and others it will need an enclosure. So I thought why not using an old grid tie inverter for that.

It looks like the old casing will house 3 converters just fine.
So I will have inputs for 3 strings, and the outputs of the converters will be paralleled after a series schottky diode. Maybe I put in a fuse on the output, the solarstrings are allready fused.

The heatsink is about 32*54*6 cm so there will be no active ventilation needed, de  pcb I made has solderpads to connect 2 Ntc's for tracking the heatsink and inductor temperature. The µc controller can act when there are things out of range.








The inductors will be placed on top of the capacitors. I hate this part of a project, spent hours to drill an tap those holes.
It's verry hard to finish projects, I hope I could get this job done.
Edited 2019-11-04 06:59 by nickskethisniks
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 170
Posted: 09:11am 07 Nov 2019
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Well, I got sick the other day. My body is empty, pushed it to far I geuess. It was a hard year, so I need to throtle back somehow the coming weeks/months. This week I took a few days of and next week my parental leave (2w) starts. I will become a dad for the first time, my wife was calculated for the the second of november...

So back to the project, the breakers are just for reference, I need to buy the right values. Thinking about it now, it would have been better to place one after each converter.

Saved by the sun today, we had 6 days of really clouded days in a row. So the lifepo4 were as close to empty @ 3V/cell. Can't estimate the level of the lead acid ones @2V/cell. I think we know more after this month. Today we will do break even, better days to come they say.

Maybe next week I will have a play with a rpi to see if I can monitor my system on a easy way.


Edited 2019-11-07 23:20 by nickskethisniks
 
mackoffgrid

Guru

Joined: 13/03/2017
Location: Australia
Posts: 460
Posted: 10:19pm 07 Nov 2019
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Hi Nick,  

Very nice.  You explanation covers it pretty well.  A schematic is probably not important but would be nice if you've done one.

I'm playing with charger design atm (higher voltage though) and was debating with myself whether to use a uProc or dedicated controller.  Seeing this post I'm going to try the uProc approach first.


BTW, Do I spot a 3d printed former  

Cheers
Andrew
 
rogerdw
Regular Member

Joined: 22/10/2019
Location: Australia
Posts: 78
Posted: 12:06pm 08 Nov 2019
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  nickskethisniks said  

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 have to agree with the others, a great looking project and looks like you've done lots of homework. Awesome.

Most of this is new to me  ...  so what sort of money do you have to pay for a mainstream 48V @ 40A controller?

Is that Litz wire in the inductor  ...  or something you combined yourself.

I kept a heap of old Panasonic Inverter Microwave transformers for the Litz wire on them  ...  though don't think I've ever unwound one to find out just how much is on each one.

I'll be following this thread with interest.  


Cheers,  Roger
Cheers,  Roger
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 170
Posted: 12:10pm 12 Nov 2019
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Hi there,

Yes that's a 3D printed former, a colleague of mine has one. I bought those big ferrite cores, but couldn't find the formers. I'm going to build a new series inductor for my ozinverter.

I combine the wire myself, I used 0.56mm wire, so @ 4A/mm² that is about 1A per wire. I then put them in a vise and twist the wires togheter with a drill.
A few years ago bought a small litz wire bobbin, think it was 200gr for 30€!
I'm not doing that anymore! There is a small company (Bobimat) that's selling all sorts of wire and spools for making transformers nearby. There I payed about 140€ for 12kg enamelled wire rated for 180°C.
For my inverter I did this too but I had to buy a spool of 42kg... that was 10mm² wire.





I don't know if you could build this cheaper then a commercial controller.

I made a schematic, normally I just do it on paper, it's faster that way, but I need to do it more in the future. For more complex pcb's there's no other way...
I didn't used parallel diodes on my gate resistors, and no snubbers on diodes and mosfets. I don't think they are needed, mosfets and diodes are overrated for my use (48V 30A).
With some inductors I experimented with, there were some really nasty spikes on my input and output capacitors. But ones the inductor was well designed (overdimensioned maybe) they were as close to zero.

This one for example worked verry good, but gave a lot of high spikes:
https://www.tme.eu/be/nl/details/dtmss-47_0.10_30-v/ringvormige-smoorspoelen/feryster/

I had it for a half year in a test rig, maybe it would be better with higher frequencies.

Not sure about all the values, the filter on the ouput of the current measuring is not tested.

All measured values are going in a differential amplifier before they are going in the uC. There was something about the outputcurrent of a µC, about sinking and sourcing capability.  It could be important when driving an optocoupler @ high speed.


Edited 2019-11-12 22:34 by nickskethisniks
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 170
Posted: 01:33pm 17 Feb 2020
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The prototype version is still going strong, but I still need to draw the controller pcb.

Today I saw 84A peak, good for more then 4600Watts in to the battery!
The unit is fused with a 63A breaker so it allows this kind of peaks.

I must say the inductor is cooled actively, at this kind of power it gets hot, +/-90°C.
And important, the tracks are short but the board is only 1oz thick, so the pcb is getting hot as well.

The unit is now running allmost 2,5 months without problems. There is 5600W panels connected. The final install will be verry conservative @ 40A max per unit.
Edited 2020-02-17 23:40 by nickskethisniks
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 3267
Posted: 08:23pm 17 Feb 2020
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That is an excellent result.

Using multiple controllers is a very good solution to spreading the heat, and it also provides some redundancy if you ever need to work on part of the system.

It may not end up being all that much cheaper than a Chinese magic mystery box, but at least you will be able to repair it, and fine tune the software to do exactly what you want. Plus there is the satisfaction of doing it all yourself.
Cheers, Tony.
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 170
Posted: 10:32am 19 Apr 2020
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Thank you Tony,

Well it's a cloudy day so why not spamming on this forum.

Picture of the 3 new inductors, 40 wires parallel for 40A. It's now 1,8mH but the airgap needs to be adjusted for higher saturation point and the right inductance.


 
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