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TMS320F28035: BLPFC current sensing questions.

Howard Zou
Genius 12365 points
Part Number: TMS320F28035


Hi, I see that there are two current sense transformers in the TI design.

Why don't we put the current sense transformer at the AC side? Thus we need only one sense instead of two.

What's the advantage of using 2 current senses and put them at the drain of MOSFET?

  • 0
    TI__Genius 11955 points

    This is explained in TI appnote SLUA517. Please review the related section of that appnote.

    Shamim

  • 0 in reply to Shamim Choudhury
    TI__Genius 12365 points
    I can see below words in the app note:
    "For conventional PFC, the current sense is easy to monitor by simply inserting a shunt sensing
    resistor at the return path of the inductor current. However, for a bridgeless PFC, current path does not
    share the same ground at each half-line cycle. A sensing-power MOSFET and diode current are
    needed, which makes the bridgeless PFC’s current sensing complicated and difficult to monitor"
    It just says that the shunt sensing mode is not applicable in BLPFC, but I still think we can monitor the current at the AC side with a current sense transformer.
  • 0 in reply to Shamim Choudhury
    TI__Genius 12365 points

    From the picture below, we can see that no matter it's positive half cycle or negative half cycle, the current will always flow through the AC side.

    In our BLPFC code, we will switch to sense Q1 current or Q2 current according to voltage's polarity, but if we put the current sense transformer at the AC side, the code can be simplified and we only need one sensor, right?

  • 0 in reply to Shamim Choudhury
    TI__Genius 12365 points

    And I found that in schematic, Iac is also sensed in AC side, and it's connected to ADCB0, but it's not used in the code.

  • 0 in reply to Howard Zou
    TI__Genius 11955 points

    Yes, you can do that as long as the sensor bandwidth is high enough for sensing the current which has100kHz switching frequency(for this design). The circuit above was added to measure the 60Hz current for input current monitoring. But this does not have high bandwidth to sense the switching current and, hence, it was not used for PFC current control loop.

    Shamim