circuit to selsect between two power sources

[acerdisma]

Dear all,

what is the most power efficient way to choose between two power sources?
To be more precise, I have a solar powered circuits with a dc regulated output and a rechargable battery, and want to be able to automatically select the best of those two upon voltage level conditions (i.e.always keep a 3V output).

Any ideas/suggestions welcome!
Thanks a lot!

 

[OutToLunch]

A diode ORing circuit would be the simplest/cheapest, but that approach has zero intelligence about voltage levels, etc.

I would suggest MOSFETs as switches to connect one rail and disconnect another, but you would need some type of bootstrap supply to maintain gate-source voltage. That approach may also require two FETs per rail to counter the effect of the body diodes.

As far as logic for control goes, one approach would be to use two comparators - one for the solar array and one for the battery. The output of either comparator would be high if the voltage is above 3V. The solar array comparator would then directly control whatever switch device is in place - so if the solar array voltage is good then the voltage is supplied via that and the battery is cutoff from the circuit. The outputs of the two comparators would also go into an AND gate such that if the battery voltage is good and the solar voltage is below 3V, the AND gate output is high. The AND gate output would then control the switch device for the battery voltage.

There may need to be some extra circuitry to insure that the rail doesn't drop out when switching from solar to battery. Also, if both rails are below 3V, neither would be servicing the load.