Friday, November 21, 2014

v0.0.1a of hardware design released

Today I edited this BLOG to put in the resource links above, and also posted the final release hardware design for the initial build v0.0.1a:

See 'Schematic' link above for larger image .pdf file

And here are some 3D renderings:

Top view

Bottom view

The module is approx 3x4" (actually, 80mm x 100mm), utilized 100v FETs and caps (I am suggesting keeping solar panel voltage below 85v), and can support up to 25A battery current.  These give it the following range of charging support per module:
  • 12v battery:         345W capacity
  • 24v battery:         690W
  • 32/36v battery: 1,000W
  • 48v battery:      1,380W
 To each the above capacities one or more solar panel can be used.  Example, for a 12v system one could use a single large 280W panel per controller.  For a 48v system, 4 such panels could be deployed in a combination of serial/parallel connection.

The costed BOM come to $73.30 + price of PCBs, which could be found for under $10 using overseas supplier.  I expect parts to arrive some time in December and will start assembling one unit then, and when I return to Viking Star will be able to start playing with trial runs on our solar panels as well as start developing the more advanced firmware.

For those interested there is a LTSPICE model of the buck converter using in this design under the CAD resource tab above - kind of fun to play with.

Saturday, November 8, 2014

Final 'Rough-Cut' schematics #3

Tonight I posted what I am calling the last of the 'Rough Cut' schematics; last as I have completed my initial review of the switchers - and am well underway in PCB layout.  Next step will be to release v0.1 of the design, along with the CAD files.

Over the past weeks or so I have been focusing on the FETs and doing a bit of SPICE modeling, paying attention to the transitions.  As a result I swapped out the FET driver ICs for ones that are able to drive the FETs harder, removed any imposed gate resistance, and also removed the bypass diodes around the lower FETs.  This last step might seem odd, but in refining things I have been able to greatly shorten the dead-time, currently looking at using 35nS as a very safe starting point.  This short time seems contrary to some folks thinking for such large FETs, but actually is in line with start-of-the-art designs - Drive the FETs Fast-n-hard, and with the right drivers (ala the LT's I am now specifying) am able to get real tight switching w/o any adverse effects.

A higher resolution PDF file:

Some other small changes include:

  • Finalization of 5V and 13V power supplies. (though may need to change out 10Ohm R3, am just trying to reduce the number of different parts used)
  • Addition of 'Service Port' which can be used instead of populating USB
  • Upgrading all FETs to 100v parts, allows use of two common panels in series for 24v/48v batteries.
  • Reduced some of the large switching  CAPs, still have plenty for the design.
  • Changed Amp Shunt resisters from 1mOhm to 2mOhm for better use of ADC ranges

With these changes, the .xls sheet is indicating 98.6% - 98.7% conversion in the core.  SPICE modeling seems to confirm this, will see with actual hardware.

I have also been revising the layout of the core switching elements to minimize current paths.  I still have some work to do, and may end up modifying the schematic some, but largely I think this is close to what the finial design will be.

Note how some parts are still not places, I need to work on the drivers a bit more - but overall am happy with how this layout is shaping up.

We are about to begin our move from Minnesota back to Washington (and Viking Star) - with a stop in Portland for the Holidays.  It is likely I will not have too much time to put into this over the next few weeks, hence I wanted to get what I had posted.   But I do intend to finish this in time to order up some PCBs and parts so I can start playing with hardware after the new year, and when I am back at the boat with the solar panels!