Battery power back-uped circuit

[flotulopex]

Hi All,

I'm now (finally...) building my project and its about time to order the missing components to have something to play with (without any reference to naughty thoughts ).

So, here's the circuit I'm building:


Reading the (very interesting) MicroChip's AN1149 about battery charge and power backup, I can see that a particular attention is to be paid to the choice of the Q1's (the P-Mosfet) where a "proper" Vgs(th) has to be chosen.

Unfortunately, I'm not an electronician, just an electrician creating himself (lots of) problems with this PIC stuff = I don't know how to calculate or determine with exactitude how to dimension the needed P-chanel Mosfet

Can anyone help me out and suggest a component (according to the DS, a combined S-diode and Mosfet is nice) or the Vgs(th) value I will have to find to make my circuit work? Instead, I would appreciate someone explain me how to "calculate" this Vgs(th) I'll need....

[flotulopex]

BTW, I don't think the above schema will work.

Is this really correct?




Shouldn't it be something more like this?

[pedja089]

First schematic can work. But MOSFET have no purpose. There is no need for one.
Second schematic doesn't allow battery discharge when charging.
Don't know where you found that schematic, but look at
http://ww1.microchip.com/downloads/e...tes/01149c.pdf
Page 2.

[flotulopex]

Thanks pedja,

The schemas come from this exact document you refer to in your last post, adapted to DC supply.

But MOSFET have no purpose. There is no need for one.

Can you please explain?

Without any "switch" (MOSFET in this cas, how I understand it), how would hte load be fed either by the main supply or the battery in case of main supply failure?

[pedja089]

You shorted gate to GND in first schematic. So MOSFET are allways on.
As you see in document anodes of LEDs are connected to positive input supply via diode. Not to GND.
I overlooked D1 in second schematic. So my explanation for not powering load while charging is not correct.
But you must be careful with that schematic. As you can have 5.1V on output while charging. Then voltage will drop to 3.7V. You can't power modem from that. Not directly.
I'll suggest that you power modem directly from battery, and rest of circuit via 3V LDO, or 2.8V. You can get it witht Iq less than 1uA.
Then just build 1 or 2A 4.2V CC/CV charger for battery. I'll suggest that you use switch(step down), not linear charger.
If you want to stick to microchip AN1149, then you need really good SEPIC power supply for modem(to get recommended 3.8V). Also modem must have low ESR capacitors near it.
So not that trivial circuit to design.
EDIT:
SEPIC or some isolated topology is must, because you have higher input voltage when powering from AC, then output voltage(stepdown). But when you run it from battery you will have lower voltage on input then output.
You could try some hot swap PSU with battery while modem running, but i think modem will restart...

[flotulopex]

Thank you pedja,

I'll give it a try with what I have in my drawer up to now.

I still don't know what MOSFET I should choose....

[pedja089]

Mosfet is easy. Chose one with low Vgsth and as low as possible Rdson. Current more than 4A for modem only.
I usually use AO3401. To be safe I put 2 parallel. Also it is good idea to PIC have control over them. SIM900D had sometimes issue where it would stop responding, and continue to work for months after power off on.
So from then I always make modem PSU controled by PIC, and add discharge resistor. Sometimes I also make resistor PIC controlled, if it battery powered device.