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LM5023: Issue when power up the IC

franco carlo
Prodigy 20 points
Part Number: LM5023
Other Parts Discussed in Thread: TL431,

Tool/software:

Hi,

I realized an evaluation board like the one in the figure to generate a stable 250v starting from an unregulated 50 volt. The 15Vdc is isolated from others two and it supplies the IC.

So the features are: 30-70Vin --> 250Vout --> Pout=70W.

The problem is the following:
- when in sequence I turn on the 50 volt and then the 15 volt the converter works correctly even if I struggle to make it stable for the entire input range of the 50V.

- when I turn on the 15 volt first, pin 7 (OUT) goes to 1, saturating the power mosfet driving transformer and causing the failure of the gate transformer driving mosfet.

How can i solve this problem? Have I chosen the right IC for my requirements?

Let me know.

  • 0
    TI__Expert 7985 points

    If 50V is not biased first, there won't be any current on Q2 when you turn on the 15V rail, so no output and the IC is continue to demand current and consequently saturates the gate driver transformer.

    What do you mean that it is not stable for the entire input range when 50V is on first?

    How did you connect the gate driver transformer? One end to the Q2 gate. What about another end?

  • 0 in reply to Ning Tan
    Prodigy 20 points
    Ning Tan said:
    If 50V is not biased first, there won't be any current on Q2 when you turn on the 15V rail, so no output and the IC is continue to demand current and consequently saturates the gate driver transformer.
    So there is no way to turn on the 15V first?
I have the same circuit where the IC is lm5020-2 and I don't have this issue.

    Ning Tan said:
    What do you mean that it is not stable for the entire input range when 50V is on first?

    It means that there are points in the 30 to 70 volt range where the duty cycle is not stable but varies continuously.

    The feedback is formed by

    Rupper=200k Rlow=2.05k (Output Voltage Divider)

    Rfb=200k Cfb=10n (TL431)

    Ccomp=10n

    Ning Tan said:
    How did you connect the gate driver transformer? One end to the Q2 gate. What about another end?

    It is arranged as forward converter in this way where the Q1 mosfet gate is connected to LM5023 OUT Pin. 

  • 0 in reply to franco carlo
    TI__Expert 7985 points

    LM5020-2 has max 50% duty cycle limit. This can prevent the transformer from saturation.

    Please share your waveform when you see instability. I think your feedback loop needs to be fine tuned but we need to see the waveform first.

  • 0 in reply to Ning Tan
    Prodigy 20 points
    Ning Tan said:
    LM5020-2 has max 50% duty cycle limit. This can prevent the transformer from saturation.

    Ok. thank you.

    Ning Tan said:
    Please share your waveform when you see instability. I think your feedback loop needs to be fine tuned but we need to see the waveform first.

    let me know first what waveform you need. At what points should i take them? Under what working condition of the converter?

  • 0 in reply to franco carlo
    TI__Expert 7985 points

    Please monitor the Vin, Vout, Vcomp, FET gate voltage and Vqr, under whichever the condition and whenever you observed the instability.