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shivakumar.r
Participant

Join Date: Sep 2011
Posts: 4

Power Path Management for Sim900A GSM Module

05/26/2012 3:59 AM

Dear Friends,I am doing a project which include SIM900A (System load )GSM module,BQ24092 as battery charger.I am using Load sharing Concept as prescribed by microchip application note -AN1149.I am using P-channel mosfet -FDC642P. and Diode -MBRS540.Load sharing concept is like when 5V supply is available then system load should run from this 5v,and it should not consume any power from Battery.When 5V supply is not available Then System load should Run from Battery.The main intension of this concept is to save Battery power.
The problem what i am facing is during switch over from 5V to Battery voltage ,There is a Transient Dip of around 2V.During this transient our GSM module is Getting OFF.
Can one one help me how to solve this problem.GSM module should not shut down during switch over.
I have attched the reference circuit what i am using.
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phph001
Guru

Join Date: Oct 2009
Posts: 1460
Good Answers: 30
#1

Re: Power Path Management for Sim900A GSM Module

05/28/2012 4:52 AM

Your circuit does not entirely match the application note. In particular, your 5V supply arrives through a diode followed by a 10 nF capacitor and 15k resistor paralleled to ground. The purpose of the resistor (labelled Rpull in the application note circuit) is to pull the line to ground when the 5V supply is interrupted, thereby allowing the MOSFET to pass current. The effect of that capacitor will be to hold charge when the 5V supply is interrupted, which delays the opening of the MOSFET. If you need the capacitor to smooth your 5V source, then you need to review the source.
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shivakumar.r
Participant

Join Date: Sep 2011
Posts: 4
#2
In reply to #1

Re: Power Path Management for Sim900A GSM Module

05/28/2012 5:58 AM

Dear Friend,

Thanks for your reply.

I have added Diode just for the 0.6 to 0.8 v voltage drop.Is it this diode affecting the switching time of MOSFET ?.Also I have tried removing the 10nf capacitor ,even after that my result is same.Please tell me How can i Decrease the switching time of MOSFET.
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phph001
Guru

Join Date: Oct 2009
Posts: 1460
Good Answers: 30
#3
In reply to #2

Re: Power Path Management for Sim900A GSM Module

05/28/2012 12:42 PM

I can offer only 2 suggestions:
1. Use the exact circuit in the application note, or
2. Put a really large capacitor in parallel with the load, which will hold sufficient charge to power the unit while the changeover takes place.
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