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Flyback converter with high isolation voltage

Other Parts Discussed in Thread: UCC2813-1, TL494

Hello everybody;

I need a low-power dc/dc converter with high isolation voltage between the primary and secondary-side of the switching transformer. The input voltage is 15V while the output voltage equalizes to 25V. The output power is around 5W. I decided to design a DCM Flyback dc/dc converter due to its simplicity and cost. As far as I understood, if I could make a distance between the primary and secondary windings, maybe, I can reach to 10kVrms isolation. Now, there are two questions as follows:

1- Can I use the EFD ferrite core with the below bobbin?

2- Is this type of bobbin in contrast with the Flyback operation?

Thanks.

  • Hello Alireza,

    It is difficult to tell if that bobbin is designed to accept an EFD core just by looking at it. The shape of the bobbin appears to fit an EFD core shape, but only a datasheet for the bobbin will verify if its dimensions are correct for an EFD core.

    Having said that, the split-section bobbin shape (as shown) is better suited for high-voltage isolation between primary and secondary than a single-section bobbin. However, the rating of this isolation cannot be determined just by a photograph. You need the datasheet to see what it is rated for. Besides, to ensure up to 10kVrms of isolation, you will also need isolation from each winding to the ferrite core and this bobbin does not appear to be configured for that kind of isolation. Usually some kind of interleaving plastic shells that fully enclose the windings are used for that high of voltage.

    Finally, from the number of pins on it, this bobbin and the core that fits it look to be hugely oversized for a 5-W application.
    On the other hand, if you find that the large core area allows you to use few turns, it may be simple enough to meet the 10kV target by adding a lot of insulating tape between the windings and the core. But if your design requires safety-agency approval, it may be more complicated than that.

    This 2-section bobbin can be used for a DCM flyback converter with no problem. The separated windings will have higher leakage inductance which will increase loss and reduce efficiency, but a UCC2813-1 PWM controller with 50% maximum duty cycle (or similar controller) can be configured to ensure DCM flyback operation.

    Regards,
    Ulrich
  • Hi;

    As you said before, the separate windings have higher leakage inductance. However, based on my facilities, this is a good choice. I designed a Flyback converter with TL494 controller and I have got good results. Now, I need to design appropriate clamp/snubber circuit for the primary switch and the secondary-side diode. There are several references for designing such circuits and they provide different theories that they are misleading. Please, suggest me a reference.

    Yours Sincerely;

    Alireza.

  • Hello Alireza,

    Sorry for my delayed response. This thread was marked as closed so I overlooked it, but found a reminder in my email.

    There are many ways to design a snubber, depending on one or another characteristic that you want to optimize, and depending on the preferences of the author of the design note.

    To simplify the choice, I recommend using an RCD snubber on the primary switch and an RC snubber on the output diode.
    Here are some references for the primary snubber:
    www.ti.com/.../snva744.pdf
    www.ti.com/.../slup100.pdf

    In addition, if you search on "rc snubber design" in your web browser you will find many app-notes and guides for this simple arrangement. I can't find an app-note from TI specifically for RC snubbers.

    Regards,
    Ulrich