Re: FET leakage question


Dave Wise
 

This is one tasty problem. There are so many avenues of trade-off. If I were doing this for fun, I could go for weeks.

Dave Wise

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: Friday, March 18, 2016 10:41 AM
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] Re: FET leakage question

I have to ask again, what is the maximum PV voltage available? Is it 2-2.2V? Is that per PV element, and how many are there? Your schematic shows 2 PVs, paralleled through MOSFET switches. It can be more efficient to put them in series and run at higher voltage. That's why I asked previously if you have full control of various aspects, meaning that you can connect up the elements as needed. Regarding the illumination, no, I didn't mean control over day and night, but whether the PV elements are near each other and exposed the same, so they put out roughly the same.

Depending on the PV voltage available, you can buck or linear regulate down, or boost up. If the voltage is only 2.2V, then the booster would work, as I described previously, without need for disconnecting the PV - it cannot charge or overcharge the battery since it can't put out enough voltage without the booster in operation. Also, if the PVs are to be paralleled, and are illuminated uniformly, there's no need for isolation, so Q3 and Q4 aren't needed. If they are illuminated uniformly, then you also have the option to run at higher voltage with a buck or linear regulator.

I presume Q2 is the synchronous rectifier. If this is a very small scale PV/converter system like a few watts or less, then I think if you're that worried about leakage, just use a smaller device with higher Rds and less leakage, or (if using boost) just skip the synchronous rectifier and go with regular. Or, just measure the actual leakage - you may be pleasantly surprised that it's much less than you expected. The other thing is that unless the system has to operate at quite a high temperature, the leakage should be quite low. Even if the MOSFET gets hot in operation, it won't matter since that's when PV power is available. The rest of the time when everything is shut down, it will be at ambient temperature.

Ed

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