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LM3481: Feedback method for Flyback converter

Jitendra Suthar
Intellectual 295 points
Part Number: LM3481
Other Parts Discussed in Thread: , TL431

Hello guys,

I have designed a flyback dc to dc converter using  LM3481, Specifications are mentioned below;

Input supply: 9-30V

output supply: 5V

Output current: 500mA

switching frequency: 250KHz

There are 2 pins i.e. 1.feedback and 2.compensation in LM3481 which are useful to take feedback from secondary and control PWM accordingly.

I took reference from "www.ti.com/.../snvu528.pdf" --> "Figure 17. LM3481-FlybackEVM Schematic" to build this dc-dc converter.

My question is, this application note suggest to ground feedback pin and use compensation pin for feedback purpose but I don't understand how this feedback mechanism works?

If I use feedback pin instead I am not getting regulated 5V output.

Kindly help me out to understand why compensation pin is used and how it works and why feedback pin is not used.

Thanks.

Kindly help me out

  • 0
    TI__Guru* 78190 points
    Hi Jitendra,

    Thank you for using the LM3481.

    When you do an isolated flyback, you are using an error amplifier on the secondary side, which is the TL431 in this case. You do not need a second error amplifier, and the LM3481's internal error amplifier should be disabled. Grounding the FB pin will put the COMP pin to a current source and the external opto transistor can pull it down as required by the loop. If you are using the FB pin, you will involve two error amplifiers in control, and they may conflict.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Intellectual 295 points

    Hi Youhao Xi,

    Thanks for your reply.

    I have one more query.

    As per datasheet, If Vin <6V, Vdr = Vin and if Vin>/=6V then Vdr = 6V but whenever I give 6V to Vin  I am getting 4.5V at Vdr and as I increase Vin from 6V to 8V, Vdr increase from 4.5V and get settled at 5V.

    But if I give 5.9V or less at Vin, Vdr  follows Vin. 

    Why it is not working as per datasheet?

    Thanks...

  • 0 in reply to Jitendra Suthar
    TI__Guru 58391 points
    Hi,
    The responsible engineer is out now. We will reply by 4/12.
    Regards,
    EL
  • 0 in reply to EricLee
    Intellectual 295 points
    Hi,
    I am waiting for your reply.

    Regards,
    Jitendra
  • 0 in reply to Jitendra Suthar
    TI__Guru* 78190 points
    Hi Jitendra

    It does not sounds correct. The VIN supplies to VCC through an internal LDO which has an regulation setting point at 6V typical, and the VCC supplies the driver. This setting point may vary a little from part to part but it should not be at 4.5V. When Vin> the setting point, VCC will regulate at that setting point (about 6V), and when Vin is lower, the LDO saturates and its output (VCC and also the driver) will follow Vin.

    I am afraid you have some errors on your circuit, either the external component connection or the IC itself is damaged somehow.

    Thanks,
    Youhao