Builiding of a complete 6kW PV inverter with MPPT chargers


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rogerdw
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Joined: 22/10/2019
Location: Australia
Posts: 823
Posted: 11:29am 30 Apr 2024      

  wiseguy said  Roger, the specs on the ST STPS61150CW diode are pretty good. ┬áMaybe Dex should replace his diodes with those based on your experience and if there are still issues copy your (Poidas) chokes.


I reckon someone recommended them somewhere in that epic original mppt build thread by Poida, so that's why I used them. And I didn't fancy trying to reinvent the wheel and experiment with chokes, so I just copied Poida there again  ...  and it's worked for me so far. I should disconnect one and see what the inductance is and what difference there is between the 16T and 19T ones.

It's hard to know just how many have been built and are working away in the background. There always seemed to be people putting their hands up for boards every time there was a batch ordered.


  Quote  I killed a diode and FET in my unit bench testing a while ago, so I got the sense that maybe MPPT failures might not be rare events,


Oh okay. Perhaps I'd better tread a bit more carefully with mine. I do have a heap of spare parts and could/should build up another one as a spare perhaps.


  nickskethisniks said  The higher the input/output ratio the more stress for the diodes.


I did wonder what the limits were, so maybe I was a bit ambitious running mine 3s4p  ...  maybe I should have stuck to 2s6p  ...  but so much more wiring!!!

Not going to change it though  ...  too much work to rewire.


  Quote  I've run the mppt at 100A charging current limit for years with irfp4110/30cpq100 combo, 2 each, and forced cooling.
This was only with 60-62V nominal in and 52-54V out, so high dutcycles.


Haha, I thought I was game going for 60 amps.


  Quote  I did had a MOSFET blowup but I suspect actually a PCB fault because of moist and dust build up close to the gate. The board was exposed to my basement environment, allthoug not that bad, but you could see a lot of coroded spots on the PCB.
The board transferred 15000kWh of energy before 1 MOSFET failed.


That's a serious amount of energy collected there, impressive. It's always helpful to know what causes any failures so we can implement improvements  ...  but even if we don't know for sure  ...  Poida's initial aim was for us to be able to fix them if they did blow up  ...  and he has definitely succeeded there.


  KeepIS said  Most mppt regulators have a recommended input voltage as a ratio to output, go above that and no guarantee it won't fail under high load. I'm not near the manuals at the moment but from memory 1.5 to a max of twice the output for the input voltage.


My Voc is 111v but the highest I ever recall seeing as input is 92 volts  ...  with a 48-60v output

Any further panels will come in via ac-coupling so I can have a couple of much higher voltage strings and a lot less wiring. The 6.6kW I put up recently comes in in two strings and are way under the inverter voltage limits.