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Originally Posted by CrankyDoug
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Here is a follow up on the above mentioned device. First note that I haven't had the chance to test it yet.
It came well packaged but without documentation. I was about to contact the seller when an email arrived with the PDF manual. It is well written and clearly explains how to use the board. I was a little disappointed that it did not contain the technical data I was after. Since none of this data would be useful to the average buyer I can't blame the seller for omitting it.
I was able to extract enough details from the seller's website to extrapolate the rest. A PIC16F690 provides the MPPT algorithm. There is a MIC 10 blocking diode and a rather large zinc oxide varistor for spike protection. The double sided PCB is thoughtfully layed out and all components are screenprinted on the top side.
Being insensibly curious I wanted to know how this thing worked. So I started following traces and looking up component datasheets. Nothing made sense because because I really didn't know the first thing about an IGBT. It also didn't seem to have any means of monitoring current.
Re-reading the website I finally got it. This device doesn't need to monitor current to perform MPPT because the load is a simple resistance.
P=V**2/R.
The maximum power point therefore occurs at maximum voltage. There is no requirement for a buck/boost circuit. The PV voltage simply passes through the IGBT to the water tank heating element. The PIC modulates the on-time and a few electrolytics act as a buffer.
At 100% duty cycle the standard 240V, 4400W element would run at about 1100W if three PV panels in series were operating at a knee voltage of 120V and a little over 9A.
Purists might quibble over the use of the term MPPT. The end result is the same. It's just a special case made possible by the fact that the load is entirely predictable.
There is what appears to be a MOSFET next to the IGBT, both of which are heat-sinked to the aluminum mounting plate. I'm guessing it is used to drive the IGBT - a wild guess as I still don't know much about IGBT fundamentals.
Best of all, no batteries!