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paralleling of 3481

anupsingh chandel
Expert 1385 points
Other Parts Discussed in Thread: LM3481

Hello,

My converter needs parallel operation of two sepic converters. Below are the specification:

Vin for 3481: 15V

Vin for converter: 19V to 100V

Vout: 72V

As per design individuly they are working fine. But when I connect both the converters in parallel the problem starts. The current sharing is uneven (that is expected becuase of the component tolerance) but when the load increases one converter switch its pulses off.

My doubt is is there any way/suggestions for paralleling of LM3481, where I can get synchronized and parallel shared output current.

I am attacheing rough schematic for general idea .

Please help if anyone has any idea.

Thank you.

  • 0 in reply to anupsingh chandel
    TI__Expert 6860 points
    Hi Anup ,

    How are you ensuring the current sharing ?
    Are you connecting comp pin together ?
    (Master comp to buffer input and buffer out to Slave Comp)
    Connect FB of Slave to GND using 1k .

    Synchronize the converters by connecting 180 degree phase shifted clock .


    Ambreesh Tripathi
  • 0 in reply to Ambreesh Tripathi
    Expert 1385 points

    Hello Ambreesh,

    Thank you for your reply and suggestions.

    Currently what we have done is, we have developed a 555 timer and connected it with sync with of both the LM3481. Now when we turn on the power we get matched gate pulses with some nano second difference, but as soon as we apply load to it the it stops sharing the equal current.

    No currently we have not done anything that will ensure the current sharing, my believe was it will share naturally. But I was wrong.

    No we are not connecting comp pin together. should we..?..we have provided seprate compansations for both the converters.

    I will connect the FB to GND with 1K resister and update you.

    I am not able to understand the reason for connecting the converters with 180 degree phase shifted clock. Can you please explan. It will be helpul for further development. Actually my system requires such  4 parallel converters of 500 watt each, that is whay I am asking.

    Please share your view.

    Thank you.

  • 0 in reply to anupsingh chandel
    TI__Guru**** 191445 points
    Connecting the two clocks 180 degrees out of phase is typically recommended to reduce ripple on the input voltage rail.

    Generally speaking, there is no direct way to absolutely ensure current sharing with LM3481. However the output current is proportional to the COMP pin voltage for current mode control. Tying the two COMP pins together will make the two converters share current more or less. I have found with similar devices that while current share is possible, the layout requirements are tricky. The COMP node is a sensitive signal. Usually you want to terminate it directly near the IC. Routing COMP between more than one device will subject the signal to noise and may cause performance issues. Four parts in parallel is probably going to be even more difficult.

    Why don't you post yur actual requirement? A multi-phase solution is probably best for your application.
  • 0 in reply to JohnTucker
    Expert 1385 points

    Hello John,

    Thank you for the details. By 180 degree phase shifted you mean inter leaved solution. That will work for voltage ripple reduction but we doing that on 120kHz switching frequency, i find it tricky.

    currently by paralling the two converters we are facing a lot of noise issues, as you said they might be due to COMP pin. So shoild we tie the COMP pin together or keep them seprate for individual applications.

    Now my requirement is a dc to dc converter with below detials:

    Vin: 40V to 100V

    Vout: 72V

    Power : 2000W

    So to cater the power requirement we are making a 500W converters and connecting them in parallel. I hope I am making sense.We are working with SEPIC topology.

    Please share your views and I will also update as and when something gets done.

    Thank you.

     

  • 0 in reply to anupsingh chandel
    TI__Guru**** 191445 points
    I do not directly support LM3481. Sometimes I do make general comments though. Let me see if I can find someone to answer your concerns.
  • 0 in reply to Ambreesh Tripathi
    Expert 1385 points

    Hello Ambreesh,

    Regret for the delay in reply.

    So I did as you suggested, but I have some queries:

    1)  I have connected the FB of slave to GND using 1k , but doing that takes the complete system in ON/OFF control mode, and looses it's PWM control /output voltage regulation at 72V output. If I connect the load in this condition it draws huge current from source.

    I have a RC circuit connected with FB pin (as shown in ckt dia.), do I need to remove all the circuit connected with FB pin.

    2) Second condition is that, how should I connect both comp pins. Do I need to remove all the circuit connected with slave comp pin. Is it compulsary to use buffer between Master and Slave comp signal.

    3) Is there any IC available to generate 180 deg phase shifted clock. Actually if I go for such interleaving type of configuration, I have 4 such systems to work in parallel for that I will be requring 90 deg phase shifted signal. Please share if you have some idea for this.

    Please help.

    Regards,

    Anup.Full page photo.pdf