• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 815 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28911: Flyback Vin=500Vdc, Vout=48Vdc/100mA

Alberto Ferrero64
Prodigy 50 points
Part Number: UCC28911
Other Parts Discussed in Thread: UCC28710, UCC28720, UCC28722

Dear All,

I need to design a flyback isolated converter with Vin=150-500Vdc and Vout=48Vdc/100mA.

Since the UCC28911 has a 700V mosfet inside, I thought: why don't use it?

Unfortunately the webench cannot help because my input data are out of its limits, so I started to design following the example in the datasheet: I fixed the pri-sec ratio to 2.5 in order to avoid Vds to exceed 700V and the sec-aux ratio to obtain about 20V.

From my calculations I found Lpri_min=3,5mH, so I made a transformer N87-EFD25 (quite over-dimensioned) with Npri=54 turns (Bmax around 200mT).

I measured Lpri with a LCR meter and I found 6mH (>3,5mH).

I put the transformer in a simple board with UCC28911 configured for a flyback converter but the results was not very encouraging: the behavior is very unstable, it starts only with particular input voltage and output loads and with Vin>180 it stops (but Vcc never reachs 25V!!)

Using a transformer Wurth with Lpri=1mH the behavior is much better, but it starts only with no load (once started I can add load) and the output voltage changes depending the load.

Someone can tell me what's wrong??

Thanks,

Alberto.

  • 0
    TI__Mastermind 31580 points
    Hi Alberto,

    I put the specification into the calculator tool for the UCC28911 and the recommended inductance for the transformer is 1.1mH. The calculator also recommends a primary to auxiliary turns ratio of 4.87.

    I thinks you may have an issue using the UCC28911 with the 500Vdc input voltage and trying to design the transformer to keep the reflected voltage and high frequency voltage spike below the 700V rating of the internal mosfet.

    You should consider the UCC28720 or UCC28722 controllers which can drive a high voltage BJT, it is possible o get devices rated to 1000V for lower cost compared to FET devices. If you prefer to use a FET power switch then the UCC28710 is a good choice.

    www.ti.com/.../toolssoftware
    www.ti.com/.../toolssoftware

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 50 points
    Hello Peter,
    thank you for your very very quick reply.
    I have a few questions about your answer:
    1) the webench calculator does not let me to set Vout>25V with UCC28911. Do you have another calculator, maybe a excel file?
    2) since the calculator doesn't let me setting Vout=48, I wrote Vout=24V and Iout=0.3 (my goal is to make a transformer with 2 secondary 24V/0.15A each connected in series). With this data I found Lpri=3.48mH. Do you think this approach is wrong?
    3) following the approach above the calculator suggests me to use a transformer with a core B66307G000X187: it is a E16 N87 ungapped core. Its Al is 1000nH, while the calculator says Al=113nH. Am I supposed to gap manually the core in order to obtain such Al?
    4) I checked UCC28720 with webench (setting Vout = 24V/0.3A because Vout is limited to 25V) but it tells me that it can't be done (duty cycle exceeds...). Do you confirm that UCC287xx are suitable for may voltages and power (Pin around 7W)?


    I look forward to your feedback.
    Thanks and Regards,
    Alberto.
  • 0 in reply to Alberto Ferrero64
    TI__Mastermind 31580 points

    Hi Alberto,

    Aplogies for nor responding to your reply from March! I am not sure what happened I normally get a system driven reminder that I have not replied.

    The Excel calculator for the UCC28911 is at http://www.ti.com/product/UCC28911/toolssoftware

    What is the current rating of the 48V output, 0.3A or 0.15A? If you design for two 24V secondaries in series, each one needs to carry the total output current. When the secondaries are in series only the voltages add.

    Please also take care of the maximum power capability of the UCC28911 compared to the power that you are trying to achieve.

    To build prototype transformers yourself you will have to gap the cores yourself or you can request samples from manufactures like Wurth-Midcom or Renco.

    I will need to check the UCC28720 on webench. There is also an Excel calculator form the UCC28720 at http://www.ti.com/product/UCC28720/toolssoftware

    Regards


    Peter