Picaxe 28X2 200v 30A Mppt PV Controller

Solar Mike

New Member
Have just completed a similar controller design using the PI Pico; however the Picaxe 28X2 cpu will also work well here, so spent a couple of hours re-arranging the pcb and schematic for the 28X2 as it maybe of interest to some members here. It is a smaller design than I would normally use and suited to lower capacity batteries.

Basic specs are max input PV voltage 250V, max output charge current 30A. I will be using it with 190 volts input and 100V battery bank, these are dangerous voltages, so caution and some experience is required to build it; can be run it at lower voltages changing the components as required. The smd power mosfets solder to the top pcb layer, heat is transferred to the heatsink through the pcb via an alloy spacer + insulator on the bottom.

Top 150 x 183mm:
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I have a few 28x2 cpu's gathering dust, so will build both versions and see how they perform.
If anyone wants the gerber files let me know and I will post them. It will be awhile before I write any software.

Schematic zipped pdf attached.

Cheers
Mike
 

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Solar Mike

New Member
Have implemented another version of this using a PicAxe 20X2 or M2, that is physically smaller foot print and suited to PV input up to 150V, with output 12-100v battery @ 10-15 amps or so. Reason I have 3 independent small battery systems in the field running off differing PV panels driving alarms, pumps etc all fitted with ad hoc chargers to keep their batteries topped up, Would prefer to standardize on a single small charger that can accept any input voltage > battery and pump out 10 amps if required.

This design has all components including the buck inductor on the pcb 150 x 125mm, the bottom of the pcb has a Led charge status module that can be snapped off and bolted behind the front panel if desired.

Here is the first cut, I don't have a schematic as its a pretty simple design, but will draw one up anyway to check I haven't done something stupid in the layout. The mosfets and Schottky rectifier sit under the pcb and clamped by it to the back alloy case panel; their power dissipation is only a few watts at most. PWM frequency is 64 Khz, buck inductor is a small HF powdered iron ring core.

Top:
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Bottom:
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Cheers
Mike
 

Janne

Senior Member
Looks pretty nice. Did you consider going full synchronous buck? I've been pondering about that with a similar project of my own (wind mppt), seems like in addition to a normal half bridge driver it might need a reverse current cutout when dealing with a battery output. At least with standard high / low side drivers which just turn on the low side switch if the high side switch is open.
 

Solar Mike

New Member
Looks pretty nice. Did you consider going full synchronous buck? I've been pondering about that with a similar project of my own (wind mppt), seems like in addition to a normal half bridge driver it might need a reverse current cutout when dealing with a battery output. At least with standard high / low side drivers which just turn on the low side switch if the high side switch is open.
For this low power version no, not worth the effort, as I wanted a physically smaller less complex controller.
My other high power designs as published on "The Back Shed" forum do use full synchronous conversion, where some sort of reverse current detection automatically turns off the rectifier mosfet during low duty cycles.

Cheers
Mike
 

Solar Mike

New Member
Schematic attached, couple very minor changes on pcb, you wouldnt notice. I will fire the gerbers off to be made soon as I have completed another project.

Any questions feel free to ask.

Cheers
Mike
 

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Solar Mike

New Member
Also decided to make a physically smaller unit for some 12 and 24V batteries, charge current 10 amps or so. Have used a 14M2 cpu.

Top: 120 x 150mm
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Will draw up a schematic.

Cheers
Mike
 

lbenson

Senior Member
Interesting, Mike. What input voltage range are you looking for with the 24V version, and what kind of amperage to you look to be able to handle?
 

Solar Mike

New Member
Interesting, Mike. What input voltage range are you looking for with the 24V version, and what kind of amperage to you look to be able to handle?
Max voltage rating determined by the track spacing, in this design its only 1.3mm, so I wouldn't go much above 100V input, for 24V battery an input range 28 to 60volts would be most efficient, as the input voltage gets higher the input ripple current increases making the input caps work harder and heat up more; mosfets and buck diode need appropriate voltage ratings, devices used here bolt under the pcb and way over spec'd but very inexpensive if purchased on AliExpress.

The charge current here limited not by the physical pcb or active devices but the minimal number of PV input capacitors used, I only require 10-15 amps charge with an input of 40 volts from a single large PV panel, so have only used two low esr can type 4700uf 63V caps on the input, their combined ripple rating is only 6 amps so at 50% PWM duty we get 10 amps or so. I tend to use more smaller caps than large devices as their combined surface area for a given paralleled capacitance is higher thus better cooled and higher ripple rating.

A single larger 35mm wide input cap can also be used on the pcb, I have some expensive ones here that have 10A ripple rating so would expect 20A output without the cap getting too warm.

Cheers
Mike
 

lbenson

Senior Member
Thanks, Mike. I ask because I bought some used 208W (new; used 170W--tested), 36.3V, 8A, power voltage: 28.7V; IPMax 7.25A panels. Would they work in parallel with this charge controller for a 24V battery? In theory, how many panels in parallel?
 

Solar Mike

New Member
Thanks, Mike. I ask because I bought some used 208W (new; used 170W--tested), 36.3V, 8A, power voltage: 28.7V; IPMax 7.25A panels. Would they work in parallel with this charge controller for a 24V battery? In theory, how many panels in parallel?
Panels of 28V mppt are a great match for a 24v battery using a much simpler PWM controller, there will be little to be gained going to full Mppt.
15 amps charge would be about the limit for this simple controller due to the limited number and thus ripple rating of the input caps, so thats only two of those panels in series. If you elected to use the same board but leave off the mppt components and just use it as a PWM only controller then it would work fine at 30 amps possibly 40 odd if you solder some heavy copper wire under the high current traces. so 5 panels in parallel, the current sensor would have to be upgraded to a high current version.

Mike
 

Solar Mike

New Member
Only few minor changes on the pcb which is good as the layout was done from memory with no actual circuit dia; so drawing that sorted out any issues.

I will post the gerbers of the little filter modules, I use them everywhere where noisy analog signals must be read by an ADC.

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Gerbers attached, drop the zip file on JLCPBC to get a quote.


Note: haven't produced any software for this yet AND the design has not been tested.

Cheers
Mike
 

Attachments

lbenson

Senior Member
Panels of 28V mppt are a great match for a 24v battery using a much simpler PWM controller, there will be little to be gained going to full Mppt.
Thank you for the benefit of your considerable experience. I actually have a cheap Chinese PWM solar charge controller. That will probably suffice for the beginning of my solar experimentation (panels on a frame on the ground).
 
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