Two inputs power sources

Your solution needs two blocks

1) Power selection between 28V and batteries

This can be done easily but wasting power, or with more components, and power friendly.

It is done easily with two Schottky OR-ing diodes. The problems is the power wasted on them, specially when using the batteries.

So a more power friendly solution is to use two "ideal" OR-ing diodes based on LM5050-2

Power prioritization is done automatically since in the OR-ing diode scheme the higher voltage wins automatically

2) The second stage is a buck converter with input from around 6V to 30V input to 5V@2A output. For this stage there is a huge offer of posibilities from TI, you can make the power with a controller or take an integrated module (complete solutions). I counted 49 offers on the power section:

http://www.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=751&uiTemplateId=NODE_STRY_PGE_T#p32=1&p238min=3;7&p238max=30;75&p451max=2;5

What I didn't understand from your questions is if you also need a battery cuttoff.

Best regards

Albert

Thanks Albert,

I think you've resolved my issue regarding the power selection, the OR-ing seems to do the job. Thanks!

Because I'm new to Li-Poly power, from what I've managed to understand, I need some protections to my circuit when connecting them.

Protections from overdrawing current and cut-off voltage, to make sure the batteries and the following circuit will not be damaged. 

Maybe only a capacitors will do the work, but still I'm not an expert in this... 

Also,  when using DC/DC like the buck converter, it pumps the current in peaks from the batteries, so I assuming it can damage them, 

maybe I need a special DC/DC only for the batteries. If so, it will complicate the circuit because I'll be needing now two DC/DC, one for each source and

also to make sure each DC/DC will be turned off when the other source is selected, for saving power.

Another problem is the efficiency, if I will use only one DC/DC converter, because the big difference between the two sources voltage, the effeciency

will not be optimal for both them, and efficiency in my system (10W max) is critical.  

Best Regards,

Michael

Dear Michael,

I have two questions:

1)You said: Input 2: 2/3 Li-Po Cells,....

I need to know, What is the voltage of this cells?

2)You said: Also,  when using DC/DC like the buck converter, it pumps the current in peaks from the batteries, so I assuming it can damage them,
My answer: I suppose these batteries are new, not discharged. Am I right? Other point if you design the first DC/DC converter with output at Voltage float point of these batteries all is ok.,...the picks probably do not damage the batteries.

So, you need to make a converter to arribe to float point of the battery pack first,... after you need a second DC/DC converter with that output that you need.

The conclusion will be that you give the required efficience at every stage of your power supply. That's all!

Please, search at www.national.com the Battery charger and solar charger,...it is very interresting to read.

Hi Axel,

Each cell has a nominal 3.7v and when it's fully charged about 4.2v. The batteries are new and fully charged. I use an external charger and don't want 

to integrate one on my circuit. 

1. I didn't understand why the current peaks won't damage the batteries?

2. What is a voltage float point of the battery pack? and why should I care about it?

3. Didn't understand the DC/DCs arrangement you suggesting and how do I solve the switching problem which I mentioned in my previous 

message (when using more than one DC/DC converter)?

Thanks,

Michael. 

Thanks Albert , it really makes sense now. 

I think I will use the Buck-Boost converter LM25118MH it gives me the best results: 86% to 88% efficiency,

not ideal but I don't have a better alternative, also it has the under voltage you mentioned. 

Thanks!