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BQ25703A: how to improve the system efficiency?

HongRen Chang
Intellectual 390 points
Part Number: BQ25703A
Other Parts Discussed in Thread: CSD25404Q3

Dear sir,
I'm now designing a 1S power bank with BQ25703A. Here is my test result under output 5V/9V/12V:

Vin         Iin         Vout       Iout       efficiency      note
3.988     1.48      5.16       1           87.42%  
3.948     2.94      5.12       2           88.22%  
3.906     4.39      5.01       3           87.65%  
3.955     2.71      8.95       1           83.50%  
3.884     5.24      8.57       2           84.22%        with fan
3.95       6.32      8.4         2.5        84.12%        with fan
3.93       7.23     7.89        3           83.30%        with fan
3.927     3.72     11.85      1           81.12%  
3.888     5.48     11.6       1.5         81.67%        with fan
3.93       7.08     10.78     2            77.49%        output is not stable

In order to get better system efficiency, I have some questions:
1. Can the loop partameter be re-optimised for 5V/9V/12V@1S application?
2. Can the switch frequency chang to below 800Khz?
3. The voltage drop on current sense resistor and BAT MOSFET is more than 0.1V. It may cause more than 3% efficiency loss for 1S application. Can the resistor be reduced and the CSD25402 be replaced by other MOSFET with lower DCR?
4. CSD25402 allows max 10A current. Is there any other MOSFET with higher current can be used?  My target is to support 30W output power for 12V.

Thanks

BR

Ren
 

  • 0
    TI__Guru 56650 points
    1. Loop parameter makes system stable. It is not related the efficiency.
    2. bq25703A cannot set frequency lower than 800kHz.
    3. The 10mohm sense resistor with 2A current gives 20mV drop. If you use 5mohm output sense resistor, please still keep 10mohm input current sense resistor. The PMON function doesn't work properly in this setup. Yes. You can use lower DCR FET.
    4. Does all 12V 30W comes from the battery? BTW, 12V 30W the current is only 5A @6V output. We only check TI FET. Please check CSD25404Q3 - –20 V P-Channel NexFET Power MOSFET
  • 0 in reply to Wang5577
    Intellectual 390 points

    Dear Wang,

    Thanks for your information.

    The power all come from battery. The target is to use 4 parallel 18650 battery to provide enough current. The output will support 12V@2.5A.

    BR

    Ren