• State TI Thinks Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 205 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.

TPS54308: Driving a flybuck converter

David Naviaux
Intellectual 550 points
Part Number: TPS54308
Other Parts Discussed in Thread: TPS55010

Hi,

I have an application that requires a very inexpensive isolated output to drive an LED with a regulated current.  I found TI's application note titled: "Designing a Simple and Low-Cost Flybuck Solution With the TPS54308" and it is very helpful, but the TPS54308 is not quite powerful enough for my application (I need an isolated 4A).  Is there another part that I could use?

BTW: I plan on feeding back the LED current to the controller. I expect that this would affect the compensation requirements but it doesn't look like the TPS54308 has compensation components that I can change.  I could really use some help if it is available.

Thank you,

David

  • 0
    TI__Genius 10070 points

    Hi,

    I never seen which buck can support 4A flybuck application, at lest our team has no device can support it.

    Shawn

  • 0 in reply to Shawn Nie
    Intellectual 550 points

    Hi Shawn,

    In my application, I want to have a 4A, 2V to 6.5V output voltage to regulate the current through an LED on the isolated side of the converter.

    Can I use one of your traditional "voltage mode" converters with external MOSFETs to drive the fly-buck converter?  I would use the LED current as feedback (via a sense resistor and isolated amplifier on the isolated side to the controller).  I only need +-7V of isolation so I can use a differential amplifier to pass the sensed current to the controller.

    Simulations seem to show that this will work.  In my application, the switching duty cycle would be between 10%-30%, depending on the LED forward voltage using a 1:1 transformer.  Do you have any suggestions or concerns?

    Thank you,

    David

  • 0 in reply to David Naviaux
    TI__Guru**** 191445 points

    David,

    Seems reasonable as you are providing feedback from the isolated side back to the primary side. For that much output power I definitely think it would take a controller. The energy is transferred to the secondary while the LS FET is on so the sinking current limit would need to be relatively high to get this much output power.
     
    It may be beneficial to design the transformer to operate closer to 50% duty cycle if you can.
  • 0 in reply to JohnTucker
    Intellectual 550 points

    John,

    Thank you for your help.

    I have selected a 4.7uH 1:1 transformer rated at 8.25A and Isat of 16.5A.  It has only 122nH of leakage inductance.  My simulations seem to show all is okay at the maximum 6.5V and a regulated current of 4A.

    I can roll-my-own controller, but I would like to use an adequately rated synchronous buck controller.  I understand that the flybuck switching current is not like a typical buck topology.  

    Is it possible to use a peak current mode controller to control the flybuck?

    In my case, I am not using the primary output voltage of the flybuck.  

    I appreciate any help you can give me.

    Thank you,

    David

  • 0 in reply to David Naviaux
    TI__Guru**** 191445 points

    David,

    Our group does not support controllers.  Let me see what I can find out.

  • 0 in reply to JohnTucker
    Intellectual 550 points

    John,

    What is the focus of your group.  If it is the flybuck converter I have some questions.

    Regards,

    David

  • 0 in reply to David Naviaux
    TI__Guru**** 191445 points

    David,

    We focus on buck converters with integrated switches and input voltage less than 30 V.  We do have one person in our group that has worked with one flyback previously, TPS55010.