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Switching between Input (AC) and Battery for TP4056 CC-CV Board

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deepak4you

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Hi,

I am trying to use a TP4056 based CC-CV Li battery charger (along with battery) to make a lamp that automatically switches between AC and Battery. I choose the TP4056 CC/CV charger because its small, gives upto 850mA of current and seems to be fairly reliable one. The LEDs that I will use will be 0.5W ones which don't need heatsink. I plan to use 3-5 LEDs that would draw about 500mA of current max.

I would want the LEDs to draw power from the INPUT side (coming from a simple 5V/2A USB charger) while the AC power is ON and switch to drawing power from the battery when AC power goes off. I want the ckt to be as simple as possible, might not be as efficient.

The circuit that I came up with was as attached.

http://obrazki.elektroda.pl/8430513900_1428411323.gif

The ckt is not final, but it is a basic functional layout on which I will add some other components later.

I see and read that I could only use two diodes (as are also seen in my layout), but i don't want to go that route because the Li battery charging side would be a little sensitive to any imbalances in the voltage. And hence I thought of going the MOSFET route.

I see this circuit on the web which seems beautifully simple:

http://www.insidegadgets.com/projects/automatic-voltage-switcher/

but I am not sure if it will work in my case because this circuit asks for two separate grounds (which I think I don't have in TP4056).

I also know that when the LEDs would be running on AC power (through the USB charger), there will be less current available for the battery for charging, but that should be taken care of it I use a charger higher than 1A (hence mentioned 5V/2A).

Any inputs/modifications/corrections would be greatly appreciated.

Regards
Deepak
 

SunnySkyguy

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It won't regulate 500 mA current in the LEDs.

try again.

1) design a 500 mA CC sink that works with <0.1V drop to max of 2V with power loss calculations using a 50mV shunt current sense R and MOSFET regulator.

2) then carefully choose two Power Schottky diodes (1V@5A) as "exclusive or" or Diode OR logic from each power source so that you know exactly the Vf @ 500mA

The LED's must be matched to run in parallel, otherwise 100mΩ series adjustments to each may be required to match them.

LED characteristics are ~ 1Watt*Ω ( my Rule of Thumb )
e.g. 2W has ESR of 0.5Ω, and 0.5W has ESR of 2Ω and 100mW has ESR of 10Ω etc..
This is approximately uniform constant of power * ESR Ω when saturated.

P= 3.3V (avg)* 500mA = 1.65 W thus LED ESR ~600 mΩ @ 3.3V ( if shared by 3 LEDs each is 1.8Ω //1.8Ω//1.8Ω ) approx.

You need to know the threshold voltage and ESR of every diode carrying power to do a more accurate calculation. ( similarly with Schottky or Silicon diodes. )
 

deepak4you

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Thanks for the quick reply Skyguy. Much appreciated. Will design a simple CC ckt and see if it works.

BTW with the switching ckt, is the configuration of the P channel MOSFET good? Or will I need a pull down resistor from the gate to ground? Overall' would the ckt work with the switching the way I want? Suggest modifications if any. Thanks!
 

BradtheRad

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but I am not sure if it will work in my case because this circuit asks for two separate grounds (which I think I don't have in TP4056).
The schematic shows positive supply as the common. However it should be possible to invert the entire circuit, so that negative is the common.

The way to do this is to substitute P-devices where there are N-devices (and vice-versa), reverse diode orientations, and reverse power supply leads. It may get more complicated than this, but that's the basic concept.
 

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