SparkFun Forums

[metaforest]

Solar cells aren't built from silicon diode junction arrays! They are too inefficient.

Leon

So what do they look like? They may not be silicon diodes, but they ARE diodes of some kind.

[leon_heller]

Google "solar cell".

Leon

[flegmatoid]

this is an awesome thread, since I've came to exactly the same problem in my solar battery charger, where the body diode in N-MOSFET has made the current to pass in both directions, so I had to introduce huge Schottky which did introduce a big voltage drop in the circuit. Diode was really necessary, because once the cells are shaded or sun goes out, they can indeed consume current by converting in into the heat. The fact MOSFET can conduct in both directions was a surprise but it did explain everything, so the two MOSFETS, back-to-back is definitely the way to go if one is keen on saving half a volt.

[langwadt]

you only need back to back if you really need to block in both directions(like for an AC switch), if you only need block in one direction like replacing a diode just reverse the mosfet, if you don't turn it on it work just like normal diode, turn it on and you get the reduced voltage drop

if the solar cell is floating you can use an Nfet on the low side instead of pfet on high side

[jremington]

So what do they look like? They may not be silicon diodes, but they ARE diodes of some kind

The direction of the equivalent diode in a solar cell is such that in the dark, without a blocking diode, the battery will discharge through a forward biased diode (and some resistance). In Circuit Cellar, Ed Nisley presented a SPICE model for a single cell, which is basically a current source in parallel with a forward biased diode. In the figure below, consider V1 as the battery being charged.

solar_cell_model.jpg