I understand the basics of how they work but this is of very little help when designing them and I am having difficulty finding clear simple schematics or design ideas online. It seems very common and fairly simple to build DC to DC switch mode power supplies just not mains AC to DC.

My thought was to build one as a battery charger to see if my idea would work. I was thinking 40.5v output with adjustable current limiting of say 1 amp to 3 amps. That would be to float charge 3 12v lead acid AGM batteries in series each at 13.5v.

How easy would it be to construct a SMPS such as the one stated above and approximately how would I go about doing it?

Thank you