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battery charger with load share problem

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Sharagim

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Hi,
In this schematic I use LEDs as load. When I unplug the power supply from 220V I have a fast blink in LEDs(a delay to switch over battery) but when I disconnect the 5V(VCC) there is no problem and battery switch immediately. With Scope I see voltage decreasing over the gate of the mosfet and maybe the curve making this problem. Is there any way to solve it?

Schematic:

Scope:
 

is the 220v power supply is wired by you or readymade ?
 

220V power supply is a readymade and 0-30v adjustable one, even I changed it and try other model also but same problem.
 

it is due to the charge retained in the supply filter after switch off and slow decay.

Connect an additional load to something that consumes more than 100to 200ma in parallel with LED and notice the changeover..
 

I tried different loads, and there isn't noticeable changes.
 

You may need to use a comparator to get a clean switch-over like in the circuit below. As mentioned, the filter caps in the 5 V supply hold the gate high for some time.
 

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  • batt_sw.png
    batt_sw.png
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Thx, the schematic helps me and I am getting clean switch now.
Is it possible to know what is R2 doing in this schematic!?
BR
 

R2 provides a bit of hysteresis feedback so that any noise into the circuit won't cause it to oscillate around the switching point. If you leave it out, you may not notice anything unless you look with a scope on VF2 for any signs of multiple transitions at the time of switching. The circuit will still function without this.
 

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