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TPS23755: What is highest switching frequency?

Yu Ni13
Prodigy 90 points
Part Number: TPS23755
Other Parts Discussed in Thread: TPS23757,

Hi Team,

I am looking to design one 12W PoE controller flyback, size and volume are very critical and I would like to switch it as fast as I can. On the datasheet for TPS23755 or same family, it only claims 250kHz as a typical value. However, the figures show some other frequencies but there is no upper limit in the datasheet. It would be wonderful if I can have this information.

Thanks.

  • 0
    TI__Expert 5615 points
    Yu Ni13,

    An application engineer from our team will look into this and advise on Monday, US time.

    Thank you for your interest in PoE products from Texas Instruments,
  • 0 in reply to Thomas Lewis
    TI__Genius 9230 points
    Hi Yu,

    We are designing a 700kHz non-isolated TI Design with TPS23755 right now. We recommend keeping the switching frequency under 730kHz.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 90 points
    Hi Tom,

    Thank you for your reply. 730kHz is good. What is the estimated output capacitance of the MOSFET? There is no value on datasheet right now.

    In addition, how about the max switching frequency for TPS 23757? Since the TPS23755 is not ready on market, I am planning to use 23757 as alternative choice for 13W PoE dcdc.

    Meanwhile, may I require for some engineering samples for TPS23755?

    Thanks,
    Yu
  • 0 in reply to Yu Ni13
    TI__Genius 9230 points

    Hi Yu,

    Let me get back to you on the output capacitance of the drain. I need to get confirmation on this spec.

    For you design, what is your intended output voltage & current? I can recommend a fully tested reference design for you. Is there any specific reason you are using TPS23757?

    Samples can be ordered from the TI store: http://www.ti.com/product/TPS23755/samplebuy

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Prodigy 90 points
    Hi Tom,

    Thank you. The reason for using TPS23757 is only because of TPS23755 is not ready. But I found you have good amount of engineering samples in stock. Since TPS23755 has integrated transistor, I will use TPS23755. Please send me a reference design if it is avaiable. My spec is output voltage 12V, current 1.08A, total power 13W. I would like to use 500kHz switching frequency.

    Thanks,
    Yu
  • 0 in reply to Yu Ni13
    TI__Genius 9230 points
    Hi Yu,

    Our TPS23755EVM-894 design is 12V/1A at 250kHz, but the frequency can be changed at the FRS pin.

    To further optimize it for cost, you can reduce the output capacitance & choose a transformer with a small inductance. Both values can be roughly decreased by 1/2, but you would need to test this.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    TI__Genius 9230 points

    Hi Yu,

    Below are the max drain capacitance (Cds) values we expect for the internal switching FET. Since we do not spec CDS in the datasheet, we cannot guarantee the nominal values. However, hopefully the below values shows that the internal switching FET capacitance is relatively low.

    Vds = 100V, Vgs = 0V, Cds = 30pF
    Vds = 10V, Vgs = 0V, Cds = 90pF
    Vds = 1V, Vgs = 0V, Cds = 190pF

    Regards,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Prodigy 90 points

    Hi Tom,

    That's very helpful. 

    Thanks,

    Yu