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Recommendations for isolated topology

Marc Haeberlin
Prodigy 30 points

Hi,

I am relatively new to power electronics and am designing a 200W step up isolated dc to dc converter. Input voltage is 20-50V supplied by a solar panel and output is anywhere from 280-410V. 

I am considering a few topologies. I have a few requirements. Efficiency is my number one priority. Second is the amount of input filtering. Since I am dealing with a solar panel input source, I would like a topology that requires low input filtering. Output filtering is not as necessary since my load is a battery. 

I am considering:

Active Clamp Forward (good effiiciency)

Half Bridge?

Push Pull (may require too much input filtering)

I am also interested in doing MPPT, but I am not sure if there are any IC's out there that have MPPT as well as a converter controller. 

If anyone has any recommendation on a topology to use, it would be appreciated. Also, if there are any IC's along with your topology recommendation that would be very helpful.

Thanks!

  • 0
    TI__Expert 3095 points

    Hi Marc Haeberlin,

    My advice would be to use a Boost converter followed by an LLC converter. The LLC converter will provide fixed voltage translation (by transformer turns ratio) and isolation. The LLC should operate, at fixed frequency, slightly below its resonant frequency giving a voltage of 0.5*n, where n is the transformer turns ratio.

    The Boost converter should be used to raise the solar cell voltage up to the level required to support the current battery voltage. The transformer turns ratio should be adjusted so that the minimum battery voltage corresponds to a Boost voltage slightly above the maximum solar cell voltage.

    This may sound complicated, but you are trying to do as much of the voltage translation as possible with the fixed LLC transformer turns-ratio. Voltage translation by the transformer is almost free (in terms of efficiency) while voltage translation by the Boost converter costs efficiency.

    To cope with low power operation I would suggest that the LLC converter is operated in 'Burst Mode' so that it does not consume power switching when there is little or no energy to deliver to the battery.

    I hope this helps with your design.

    Joe Leisten

  • 0 in reply to Joe Leisten
    Prodigy 30 points

    Thanks for the advice. What would be the advantage for having a pre-boost stage? If I recall, LLC's have a pretty wide input voltage range. 

    Also, I have never built an LLC converter before. How difficult is it to implement a custom control loop for it?

  • 0 in reply to Marc Haeberlin
    TI__Expert 3095 points

    Marc Haeberlin,

    Correct the LLC can achieve a wide range of voltage control, but only at the expense of full power efficiency.

    My suggestion is that you run the LLC converter at fixed frequency, in this mode it behaves rather like a DC/DC transformer.

    You can use the Boost converter to perform all the control function that you need.

     I hope this helps.

    Joe Leisten

  • 0 in reply to Joe Leisten
    Prodigy 30 points

    Ok that makes sense. That is a good option. I think I will plan on designing the preboost to LLC.

    What do you think about a current fed push pull? This seems like an easier solution to implement. I realize efficiency may not be as high, but for the initial design phase it may be quicker to design than an LLC.

    Thanks

  • 0 in reply to Marc Haeberlin
    TI__Expert 3095 points
    Hi Marc Haeberlin,
    The LLC has many advantages. ZVS switching ensures good efficiency at high switching frequency. I would encourage you to get to grips with this topology. The following link may help with your LLC design and understanding, although the part (UCC 29950) is not suitable for your use because it cannot address DC input operation:
    www.ti.com/.../slua733
    Thanks
    Joe Leisten