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Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1138

Posted: 12:39am 27 Sep 2022

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What I ultimately want in a PV charge controller design is something that can be expanded ie modular; to accommodate different situations of panel array and battery bank sizing. With the shortage of available components - even some smd resistors are delivery circa 2023

, a certain mix and match capability would be helpful.

Rather than place everything on a large pcb it seems more logical to split the design into smaller modules using a max 100 x 100mm board size; this allows swapping boards out or adding extra to increase power outputs etc.

My last 3-Phase design has a huge pcb and now I cannot get some of the power devices, TO-247 mosfets and Schottky diodes are easier to obtain for DIYers, so have designed the system around them.

I have split the design into four main modules, they are linked by 15mm x 2mm thick copper bars easily obtained on AliExpress, drive signals etc connect using twisted wire pairs and pcb spring terminal blocks.

PV input board module containing the mosfets for isolation of the PV from the battery bank, 1 to 3 parallel mosfets can be used here depending on expected PV input current, they are turned on\off (slow) by an isolating voltage driver chip, the board also has some of the bulk buck caps that are shared by multiple buck converter modules.
The storage caps mount under the pcb, mosfets also mount under and pressed to an alloy right angle bar that bolts to the heatsink, or in my case the side panel of the case. Copper bars mount on the top half.

Buck converter stage module: has a single power mosfet and Schottky rectifier, 30 or 35mm main cap and some output caps, the buck inductor wound on a powdered iron toroidal core mounts off the board. This power stage is expected to be approx. 30 amps, to increase the output, additional buck boards driven from another PWM phase are bolted in place and connected by the copper bars.
Depending on the required power output - versus the battery voltage, an additional small board containing extra bulk caps can be bolted on top of each buck module.

The caps board allows various types and is cut in half with tinsnips to suit.

Final output module contains the small switch mode 12V PSU, 5V linear power and sensing for battery charge current, PV and Battery voltage, allowance for external differential battery volts measurement and temperature.
Op-Amp with some noise filtering provides a low impedance output of various voltages to the cpu controller module.
The 0v output from the terminal block is split into noisy and analog outputs for the controller.

Controller next... time for lunch.

Cheers
Mike

Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1138

Posted: 03:01am 27 Sep 2022

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Having the controller on a separate pcb allows swapping out the CPU for another, or altering the design without affecting the rest of the circuit. Using a Picaxe 28X2 here as it has just enough IO to do the job and when running at 64 Mhz, output 64 Khz PWM @1024 bit resolution.

An analog mux chip with a common low pass noise filter on its output connects to all non digital inputs. I have used optional modules from previous projects for connecting to a 1-wire data link and RS232 LCD display or a serial data led status display board.

The CPU's ADC reference voltage is supplied from a precision 4.096V reference which has enough current to also drive the various settings multi-turn pots.
To get multiple PWM phases I have used an RC buffered delay line, with outputs at 0uS, 5uS, 10uS, min pulse resolution is 0.3uS, so the PWM duty cycle range is approx 2 - 98%. Its also possible to use 3 of the CPU's PWM outputs for this by starting up the internal PWM timers in consecutive fashion, but the timing isnt as exact, so have opted for the RC delay line approach with a single PWM input.

High output voltage or charge current overloads, trip a comparator circuit that immediately shuts down PWM drive and interrupts the CPU via its external hardware interrupt pins.

To keep noise from affecting the CPU, have gone for a 4 layer PCB, the internal layers being a split plane 0V ground and the other containing 5,12,4.096V power. These 4 layer pcbs are not expensive US$7 for 5 boards and make for a better designed layout.

Top:

0V:

DC Power:

Bottom:

RC Delay Line Phase Splitter:

Hope it all works, I might draw up a schematic when I test it...

Edit Specs:
Battery voltage 12-100, charge current 30A per phase, 1 - 3 phase so 30-90 amps.
PCB track spacing will allow 250V PV input

Cheers
Mike
Edited 2022-09-27 14:08 by Solar Mike

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