Sunday, September 28, 2014

1st look at Buck modeling

One of the 1st steps I want to do is some efficiency modeling; there are a few alternatives for hardware configurations - and some work up front can help select, or solidify, a direction.

I am using one of the .xls spreadsheets out there to model the standard CCM buck switching power supply architecture, as that architecture is rather simple and very mature in its development.  The one tool I was able to easily download (for some reason the .xls files are kind of hidden by many venders - even though the .pdf instructions are simple to find).   But I was able to locate the one from MicroChip.  Look under the Reference tab above for "AN01471A", both the .pdf and a .zip file with the spread sheet to save yourself a bit of googling :-)

The 1st thing I did was enter the devices used in this TI 20A-MPPT reference design:
It uses two paralleled buck sections which can have some issues of balancing between the two sections.  For the modeling I ran things up to 15A, just to cover the potential for uneven distribution of currents between the two.  

FYI:  I am using the TI reference design as they have well characterized the results, but am really only interested in the power (driver) side.  The CPU side seems a mess, including using RCs to delay digital signals to set the dead-time between the upper and lower FET.  Will not be doing that...

OK, so TI claims a measured 97% efficiency in a 12v deployment.  Putting in the power side components used, the MicroChip tool comes up with this graph:

I ran two curves.  One at 200K as the reference design was designed around, and one at 50Khz - which often result sin greater efficiency due to lower FET switching loss (esp the upper FET).  In this case, it did - esp at lighter loads.  But do keep in mind the design was not optimized for 50Khz, (Notably the FET), and there may be some problems with this - as well as additional opportunities.

And here is the table behind the graph:

Efficiency Graph

Series 1 2
Input Voltage (V) 17.33 17.33
Output Voltage (V) 12.32 12.32
Switching Frequency (Hz) 200000.00 50000.00
Driver VDD (V) 10.00 10.00
HS FET CSD18532Q5B CSD18532Q5B
LS FET CSD18532Q5B CSD18532Q5B
Driver SM72295MA SM72295MA
Inductor SER2915L-103KL SER2915L-103KL

Iout Efficiency
.000 00.00% 00.00%
.500 91.06% 97.37%
1.000 95.20% 98.61%
1.500 96.65% 99.02%
2.000 97.38% 99.21%
2.500 97.81% 99.32%
3.000 98.10% 99.39%
3.500 98.29% 99.43%
4.000 98.44% 99.46%
4.500 98.54% 99.47%
5.000 98.62% 99.48%
5.500 98.69% 99.48%
6.000 98.73% 99.48%
6.500 98.77% 99.47%
7.000 98.80% 99.46%
7.500 98.82% 99.45%
8.000 98.83% 99.44%
8.500 98.84% 99.43%
9.000 98.85% 99.41%
9.500 98.85% 99.40%
10.000 98.85% 99.38%
10.500 98.84% 99.36%
11.000 98.83% 99.34%
11.500 98.83% 99.32%
12.000 98.81% 99.30%
12.500 98.80% 99.28%
13.000 98.78% 99.26%
13.500 98.76% 99.23%
14.000 98.74% 99.21%
14.500 98.72% 99.18%
15.000 98.70% 99.15%

At 1st blush I would say there is an OK correlation between the calculations and the measurements, though the tool seems to be a bit more optimistic:   98.8% efficient at 7.5A (shared, 15A total)  vs. a 96.9% measured with the sample system.  But there is one key differeance here, the .xls modeling tool does not take into account the additional losses associated with the TI's platforms additional FETs on the input and outputs.  I iwll modify the tool some and see how much changes..

But overall it is perhaps a bit encouraging.  Tools seems somewhat close, at least with a couple of % not taking into account the additional losses in the actual hardware.  It also confirms the gains associated with lower frequencies.   Over the next week or so I want to play with different FET / Inductor combinations and see what can be produced.   As well as modifying the tool to include the ability to consider losses associated with Amp shunts, protection Diodes, and perhaps on/off FETs.

Wednesday, September 24, 2014

Outlining the goals.

There are already in existence low cost solar MPPT controllers - some under $100 that were not available even two years ago.  So why do this project?

Like most of the other battery oriented projects I have done - because I want to.   But perhaps a bit more, this will be part of a larger system of battery oriented charging management projects - ones that communicate with each other to improve battery safety and performance, as well as simplify installation.  Some of the high level goals for this project can be outlined as:

  • Sized to match one common large solar panel (240-300w) to a 12v or 24v battery system
    • Current carrying speced at 25A
    • Optionally support 2 panels in series in 24v systems.
    • 48v support by ???  (current design does support 48v battery systems)
  • Modular systems approach:  
    • Multiple panel/controller pairs can be placed in parallel
    • Controllers will intelligently communicate with each other to coordinate charging
  • Simple to use DIP switches for out of the box installs.
  • Available ASCII commands to enable advanced options 
  • Integrate into CAN based battery charging / management system.
As with the other projects, this one will be open sourced for non-commercial use.  All CAD files and source code will be posted under the links above.  And as before the Arduino IDE will be used to make things a bit simpler to use and modify as people with.  (Update November 2014:  Am needing to review the Arduino IDE some - integrating the ATmegaxxM1 uC into it is not that simple..)


A bit more:   Am looking to make a Buck based solar MPPT controller.  This matches up well with common solar panel outputs, as illustrated here:

Example 240W solar panel

The optimal panel voltage in all light levels is around 30v.  This matches up well with a Buck type switching power supply for 12v systems, but could be a bit on the edge for 24v systems - care will be needed at the point where Vpanel and close to Vbat.  And some consideration should be made for a pass-though mode if appropriate.

For simple deployments, a single controller might be sufficient.  But I intend to include a CAN bus to allow communication with other controllers, as well as the SmartBMS project to gain more precise battery voltage and other needs.  This should not only improve battery safety and life, but also reduce wiring by just using CAT-5 cables to connect up all the devices.  (Plus power cables!)

Am just starting on this project, and working in parallel with the BMS project:

More come, as am just getting started....