I am planning to power a 5V microcontroller through a solar panel. Solar panel's Vmax = 17.78 VDC and V(open circuit) = 21 VDC.
While selecting the appropriate buck converter, should it be rated at max input 18 V or 21 V? basically, I would like to know how does open circuit voltage affect my power circuit design.
Best Answer
The open circuit voltage is the voltage you must expect from the solar panel under no load (zero current draw).
(image source)
All DC-DC converters (even high efficiency ones) have a minimum input current consumption. But in most cases it'll be low enough (compared to the current capability of the solar panel) to be considered considered almost zero.
Thus, your DC-DC converter must be able to accept the full open circuit voltage at its input, just in case it doesn't have a load connected at the output. The DC-DC converter should be able to deal with the line voltage variation imposed by the panel (most of them are able to) as well.
Also, as suggested by Rohat Kılıç in the comments to your post, you should consider a DC-DC converter with an input range wider than the nominal one required, in order to avoid nasty surprises. The open circuit voltage depends on temperature and other environmental and operating factors, so taking adequate design margins will definitively be a good idea in this case.