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Switching transistor replacement using the BQ24650 charge controller

Victor Vargas
Intellectual 265 points

Hello Forum,

I have posted several questions regarding the PCB layout and specific circuit functionality before. 

In this opportunity I want to discuss a possible replacement of the switching transistor used in the EVM. 

Due the fact that I'm using a a charge current of less than 1 amp, a 20 amp rated ( as the Si7288 used in the EVM) would be over sized for the application and it will cost more. 

I have came up with a spreadsheet in which I have listed my best approximations regarding cost / system integrity. MOSFET options.xlsx

I would be highly aprecitated if you could help select the correct one. At the time I have chosed the DMN4031SSD-13 from Diodes Incorpowww.diodes.com/.../DMN4031SSD.pdfrated. as a preliminary candidate. 

Thanks again for you rich answers Geniuses 

Kindly 

-Alb 

DMN4031SSD-13
  • TI__Expert 5565 points
    Alb,

    You want to pick a transistor that can handle the peak current through your inductor, and leaves some room for margin. With your input/output conditions along with your inductor value, your peak current won't be more than ~1.5A. Did you follow the equations 16 through 22 to help you pick a MOSFET? These equations will tell you which variables are important for the performance of the MOSFETs.

    As far as size/cost goes, you will have to decide for yourself, but the equations 16 through 22 should help you in deciding your tradeoffs.