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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 (way over simplified) 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.
It has problems, and may crash... The set-point (ramp) shifts for some reason that is not clear, e.g. it will run at 17.7V for a while then 17V, then back at 17.7V...
To start close the 290mA switch, and press the push button.
Modified: removed current mirror, so now the error amp is a voltage controlled current source (its about the same as a TLVH431 and current mirror I had setup).
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