Open MPPT

After having been testing the initial unit's performance for the past few days on and off, I have found a few notes to efficiency.

Using 19V input to provide 3S(12.3V)@4A output, the unit achieves extremely good ~95% efficiency of power out/power in. my bench power supply measures 51.3W output, and 12.25V@4A output.

Testing at 16V, 19V and 21V provides similar figures +-0.5% so this looks excellent for the design.

However, it appears that almost all of the 2W of heat is coming out in the QFN for some reason, which causes it to heat up rather quickly. Sometimes tripping the internal over temperature lockout.

I'm currently investigating this, and looking into easier solutions to reduce the thermal loss in the QFN.

If the thermal losses in the QFN cannot be reduced, I may need to investigate heat sinking to the outside case.

After building up the first PCB for testing the design, I have found an error in my design in that my switching loop is too large. I sort-of expected this but wanted to test the design to find out for sure. For the testing PCB, I have tacked on extra filtering capacitors as well which has helped slightly as I initially completely mind-blanked on installing ceramic filter capacitors.

I will be revising the design of the PCB to amend these issues soon.

That all said, the unit does work well at 4A output current, regulates well, and charges the battery quite nicely. But just runs hotter than expected. I will be testing efficiency in the coming days when I have more time to sit down with the unit.

PCBs!

I received the PCB's back from SeeedStudio this morning and naturally had to build up one to test the design.

As I was populating the board I realised my main mistake on the design, which was to use a completely too tiny diode package, which is why there is a large wire across the PCB.

I'm also completely out of the XT30PW connectors I need to populate the board, having only been able to find one so far. So I have run a wire for the input side for the mean time.

The boards turned out perfectly otherwise, which is very nice to see after waiting for them to arrive.

Testing

The unit was setup for an MPPT voltage of approximately 18V and output voltage of 12.3V. 

This allows the use of a standard 3S lipo with the common 19V laptop power supplies, as well as most solar panels.

On the first power up the unit did not work straight away (wouldn't it be nice if it just worked).

The Schottky diode used for the bootstrap circuit needs to be a decent model, and the first one I used had too low of a breakdown voltage (I think) which allows the voltage at the REGN pin to go up to around 9V. This resulted in the main IC getting very hot quickly.

Once this was replaced with the larger diode (seen above), the main IC was still drawing a large amount of power, so it was replaced with the spare main IC. 

After replacing the main IC the unit powered up much more stable and now appears to work... Mostly.

Running the unit at 2.5A input (4A output), the main IC runs cool and stable, and the switching MOSFETs are staying nice and cool as well. However the input filter caps that I used are not rated for the pulse current the unit is drawing, and get hot after only a few minutes of runtime.  I plan to replace these with better-rated capacitors and then test the unit for a longer period of time.

The first revision of the design has been sent off to Seeed to have manufactured.

Hopefully my layout is okay for this first revision, but If not that just makes it fun to figure out why :)

I have most of the components on hand already, so mostly just waiting for the PCB's to arrive, probably 2-3 weeks away though.

The initial design for the pcb is finished, and will be sent off to fabrication once I have more designs to panelize the design with.

As far as I know the design is complete, so anyone should be able to order one and test it out if you're really eager :)