"Ideal" Solar Panel Blocking Diode
Phil Pilgrim (PhiPi)
Posts: 23,514
Well, since solar panels seem to be a current [noparse][[/noparse]cough, cough] topic these days, I thought I'd float something I've been thinking about for years...
Solar panels all have blocking diodes to prevent reverse current flow into the PV cells. Due to the forward voltage drop of these diodes, there's some wasted power due to Vfwd * I losses in the diodes themselves. MOSFETs, on the other hand, dissipate very little power when they're fully "on"; but, in that state, they have no preferred conduction direction. My thought was to include a micropower, low offset, open-drain comparator to monitor the voltage difference between the PV and load sides and to turn on the MOSFET only when VPV > Vload. Here's a possible circuit:
In this circuit, the pMOSFET (shown with its internal diode) conducts when its gate is pulled low by the comparator. The gate is normally pulled up by the load voltage through the two resistors. The divider formed by those two resistors is calibrated to provide a very tiny bit of hysteresis to keep the comparator and MOSFET from operating in their linear regions and overheating. (It may be necessary to add a cap to limit the on/off switching rate, too.)
Anyway, the idea seems natural enough that it's probably already been done, but I've yet to see an example. As an alternative, a cheap microcontroller with two analog inputs could be used to monitor the open-circuit voltage differential at intervals and control the MOSFET from that observation. This would eliminate having to monitor a very tiny voltage differential when the MOSFET is conducting.
-Phil
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Post Edited (Phil Pilgrim (PhiPi)) : 3/16/2010 4:05:33 AM GMT
Solar panels all have blocking diodes to prevent reverse current flow into the PV cells. Due to the forward voltage drop of these diodes, there's some wasted power due to Vfwd * I losses in the diodes themselves. MOSFETs, on the other hand, dissipate very little power when they're fully "on"; but, in that state, they have no preferred conduction direction. My thought was to include a micropower, low offset, open-drain comparator to monitor the voltage difference between the PV and load sides and to turn on the MOSFET only when VPV > Vload. Here's a possible circuit:
In this circuit, the pMOSFET (shown with its internal diode) conducts when its gate is pulled low by the comparator. The gate is normally pulled up by the load voltage through the two resistors. The divider formed by those two resistors is calibrated to provide a very tiny bit of hysteresis to keep the comparator and MOSFET from operating in their linear regions and overheating. (It may be necessary to add a cap to limit the on/off switching rate, too.)
Anyway, the idea seems natural enough that it's probably already been done, but I've yet to see an example. As an alternative, a cheap microcontroller with two analog inputs could be used to monitor the open-circuit voltage differential at intervals and control the MOSFET from that observation. This would eliminate having to monitor a very tiny voltage differential when the MOSFET is conducting.
-Phil
_
Post Edited (Phil Pilgrim (PhiPi)) : 3/16/2010 4:05:33 AM GMT
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-MH
Post Edited (Shmoopy) : 3/16/2010 4:29:48 AM GMT
look at LTC4358 for example.
here's an article on them
http://cds.linear.com/docs/LT Magazine/LTMag-V18N02-14-LTC4355_57-MeilissaLum.pdf
they have a 9-26volt, 5A range, and cost $2-$3.
Mike
Thanks,
-Phil