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TPS23754: Primary side current & switching duty control

Felix Lin50
Prodigy 40 points
Part Number: TPS23754
Other Parts Discussed in Thread: PMP9563

I have been confused with the switching duty cycle control of TPS23754.

For different output load (12V, 0A ~2.5A), the Primary MOSFET almost does not change duty cycle. 

A interesting phenomenon is that minus current in primary side.(DCM)

As output load increases, the negative current decrease to zero and then system works in CCM.

Attachment was measured voltage across R15 on PMP9563.(represents Primary side current)

(PMP9563 is Forward topology. For fly back application, I also measured the same result on other boards)

I am curious about 

  1. Why does the negative current appear at R15?

  2. Why does TPS23754 NOT change switching duty cycle while output load changing.

12V/0A output: 

12V/0.5A output: 

12V/1A output:

12V/2.5A output: 

  • 0
    TI__Genius 9230 points
    Hello Felix,

    I have notified our subject matter expert & will work to resolve your inquiry.

    Thanks,
    Thomas A.
  • 0 in reply to Thomas Amlee
    TI__Mastermind 41800 points

    Hi Felix,

    1. Note current through the primary transformer is a triangle waveform. The current through primary FET is when the primary inductor is charging (positive ramp). At light and no loads you can think of removing the offset. The negative current is part of charging when the switch is still ON.

    2. The duty cycle will begin to change when there is a need for the loop to adjust. At light loads, I don't expect the duty cycle to change as long the output is regulating correctly. At mid to high currents when the output begins to droop, you will begin to see the duty cycle change to try and maintain and regulate the output.

    I don't see any issues with the concerns above.

  • 0 in reply to Darwin Fernandez
    Prodigy 40 points
    Hi Darwin,
    Of course, the EVM works well for different loading.
    I expect the duty would be less than 10% while no load.
    And, the duty will increase obviously while loading closed to full load.
    First post figures show duty from 12V0A to 12V2.5A loading
    I measured the duty was keep in 42% roughly and did not change more than 3% between light and heavy load.
    I've tested other vendor's chip, the triangle width is changed obviously correspond to 0A~2.5A output.
    You had describe the normal behavior of it and I have understood and agree.
    But, I still do not know why
    - negative current appear in both Forward & Flyback
    - duty cycle change so small from no load to heavy load
    could you explain more detail about it?