A post on linkedin was posted by a member regarding building a 24W battery charger using an adjustable regulator to do a quick proof of concept PV system for a mini projector in a rural setting in Africa. They were looking for postings that focused on linear design instead of PWM and resonant switching power supply design. So…why do we not use linear regulators?
First of all there are so many simple solar converter circuits out there based on a simple switcher chip such as the TPS54561. Even though solar panels have gotten cheaper it makes no sense to throw away power on a linear regulator in 2015. Here is a link to a product that would easily work: http://www.ti.com/product/tps54561
The efficiency of linear regulators goes down with the difference between input and output voltage. Efficiency for typical linear regulators is usually no better than 70%. Would you want to buy a solar panel that is 30% larger just because you wanted to use a linear regulator?
The most simple charge controllers are based on something we call PWM charging, These units control charging by connecting the battery to the solar panel when the battery needs it and then disconnect it when the battery is charged. As the battery voltage increases, the time connected vs. the time disconnected is reduced until the battery is fully charged.
A better way is to go with MPPT, but that is a topic for later.