A overly simple approximation of 36 solar cells (SLP05-12, Vmp=17, Imp=290mA, Voc=21.6V) and a buck converter charging a battery.
The string is sort of like a current source (.29A), that is limited by a open circuit voltage of 21.6V. If current flows in the zener then the buck converter can't process that power (it is lost). So the control loop (none shown) needs to keep the input voltage just low enough that the input capacitor does not fill up enough to cause the zener to conduct when the buck switch is off.
17 volts is the maximum power point voltage for a 36 cell string at room temperature. This is why the 15.2 Ohm is used.
the switch is used to force 10uF cap to MPP voltage.
Some of the pulses are a little smaller than others so the current looks a little to real, but getting the green LED just on the edge of turning off is the key.
my circuit board:
http://epccs.org/indexes/Board/CC01/
