SB50 issue [RE-wrenches]

[Michael Welch, Home Power]

Hi gang. Bill Von Novak <billvon at qualcomm.com> watches this list & asked me to post this. If you reply, be sure to cc Bill at the same time.

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>Over the last weekend I received a call from a client reporting that an RV
>products SB50 that I installed for him had "burned up." . . . .

>It was discovered that one of the negative array leads measured -48 volts to
>ground.

I have had a similar failure on the SB50, but for a different reason.  I was running an application that used an odd cell string (84 cells in series) that resulted in ~50 volt OC voltages.  The charge controller blew out the first day of its use, resulting in a destroyed inductor, FET bank and a scorched board.  There was a lot of smoke but no fire.

I disassembled the unit and discovered that they used 55 volt FETs as the switch elements, although they rate the unit for 57 volts max VOC.  Normal engineering practice in such a converter would necessitate derating the FETS, rather than running them beyond their rated voltage specification.  When I pointed this out to RV Power Products, they said that the FET manufacturer had OKed the unusual application.

I replaced the FETs and the inductor and rewired the array for a ~40 volt max OCV (losing some capacity in the process.)  The controller worked fine after that.  It is probably not the same original problem, but be aware that running the SB50 at 48 volts is pushing some components very close to their limits; by my measurements in the lab, the FETs 55V rating is exceeded at an input of 49.5 volts.

-- Bill von Novak
x83801

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