I have not followed along for a while.
Have you thought about ORing the input voltage and the battery voltage together then having one PWM in SEPIC mode.
The PWM will get power from the input or the battery, which ever is larger. You will need to choose a diode that has low forward voltage.
Yes, someone here already had the idea to make the ORing circuit at the input, the diode model is not a problem, I can find a suitable schottky easily.
The problem is when the battery is powering the circuit, because the current would be very high and could damage them. To solve this problem, somebody here gave a suggestion to use 4 Li-ion cells connected in series to reduce this current, but I don't know a single charger IC capable of charging 4-cell connected in series.
The original idea of this SMPS was a 12V output capable of 6A, but now I reduced to 4,5A and maybe I will reduce to 3A, by switching-off some devices that doens't need to be connected to the output when the circuit is running from the batteries. I am studying that. And also studying the possibility to reduce the output voltage to 10V, I need to check this yet.
Following the previous posts, the main ideas are:
1) Connect 4 Li-ion cells in series, use a ORing circuit at the input, and use just a Buck converter capable to output 12V @ 4,5 to 6A. I also think this is the most correct solution.
2) Connect just 2 Li-ion cells connected in series, and use a Buck-Boost / SEPIC topology converter capable to output 12V @ 4,5 to 6A. Using this method, a Buck-Boost converter suitable I found is the
LM25118 from Texas. There are Dev. Boards for this IC also.
3) I am also open to new ideas.
LM25118 is a Buck-Boost topology IC and has the required (and also extra) features, like:
- Input voltage range of 3V to 42V
- ENABLE input
- Programmable switching frequency from 50 to 500 KHz
- Undervoltage lockout
- Soft-start function
- Adjustable output with 1.5% feedback reference accuracy
- Integrated high and low-side gate drivers
- Only available in SMD packge TSSOP-20 (OK)