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LM5160A: Flybuck practical Circuit not working as simulation results

Maqsood Cassiem
Prodigy 40 points
Part Number: LM5160A

Hi 

I'm designing a Flybuck DC-DC converter using the LM5160A. The design has an input voltage in the range of 25 V to 45 V. I have designed for an output of 12.5 V on the primary side and 14 V on the secondary side. I simulated using LTspice. In the simulation, with no load, I get a primary side output voltage of 12.58 V and a secondary voltage of 16.25 V, which for me was fine. When I built the circuit, the primary output voltage rises to 8.5 V and the secondary output rise to a voltage close to the input voltage (Input voltage at 28 V). The secondary side voltage starts dropping, starting off fast and as it gets close to only a few volts starts dropping slowly. The secondary side eventually increases again, sometimes above the input voltage, before it starts dropping again. The primary side voltage also starts dropping at some point, this happens slowly.

While looking for what the problem could be I noticed that the SW pin is not switching, what could possibly cause this to happen? Please see my attached schematic. Any help will be appreciated.

Thank you In Advanced

Regards

Maqsood Cassiem

  • 0
    TI__Mastermind 23845 points
    Hi,

    Do you have correct VCC voltage?
    Also, i dont think you are using the correct transformer because you need a flyback transformer or a coupled inductor because it can handle more current before it saturates.

    Try using a coupled inductor and see if the part switches.

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 40 points
    Hi Arief,

    Thank you for your quick response.

    The Vcc voltage pin measured as 7.3 V which is close to the typical 7.5 V stated in the Datasheet.
    (Note that the VCC label net on the secondary output and the Vcc pin is not connect in the design, primary out is only labelled as VDD and Secondary output as VCC in the schematic)

    I am only designing for a 200mA output voltage with a low ripple current, therefore to my understanding the transformer chosen should work. I am not certain that it would make a huge difference, but in the simulation I ran, I had the transformer model which was downloaded from the manufacturers website.

    Regards
    Maqsood
  • 0 in reply to Maqsood Cassiem
    TI__Mastermind 23845 points
    Yes, but the transformer on the schematic did not have a specification at which current that the transformer saturates. Therefore i suspect that it might be a lower saturation current transformer
    When i compare with the transformer on the EVM guide, i can see that normally the transformer that they use have a higher saturation current as shown on this BOM
    www.ti.com/.../snvu408a.pdf

    Thanks and i hope that helps
    -Arief