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Update : 2013.5.17 Fri 11:47
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Solar panels are gaining acceptance as a practical means of electricity generation in remote locations, but despite
improvements in the technology, solar panels remain expensive.
Much of the expense is the panel itself, where the size, and thus cost, of the panel rises with required power output. Maximizing panel performance is important in order to realize the smallest, most cost effective solution.
This Design Note presents two simple circuits that increase the energy harvested by a solar panel. In both of these cases, the panel charges a battery, which in turn, supports operation of the application circuit when the sun
isn’t shining.
Design Requirements
Optimum design of a solar battery charger requires some understanding of the characteristics of the panel. First, solar panels are leaky due to the large junction area, so the battery can discharge through the panel during dark
periods.
Also, every solar panel has a characteristic VI curve with a maximum power point, so to the extent that the load characteristics and the panel characteristics don’t match, energy extraction is reduced. Ideally the panel would be constantly loaded at the maximum power point in order to fully utilize the available solar energy and thereby minimize the panel cost.
Solar Diode and the LTC4412
The panel leakage problem is typically solved with a diode in series with the panel. Reverse leakage is reduced to a low value, but some energy is still lost in the diode forward drop. A better solution uses the LTC®4412 ideal diode controller. This device, when combined in a simple circuit with a small external PFET, functions as an ideal diode - it has only 20mV of forward drop and low reverse leakage.
This means the LTC4412 combines better charge performance and better discharge performance than a simple diode - you get more charging energy and you get to keepmore of it. The low voltage drop of the LTC4412 ideal diode also allows the solution to scale to several amps of panel current without the heat generation problems of a conventional diode.
Two problems remain, namely the control of the float voltage into a fully charged battery and the loading of the panel at the optimal point for power generation. These problems can be solved by a switch mode charger using
an efficient buck regulator.
For details, click http://cds.linear.com/docs/Design%20Note/dn1012f.pdf
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