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LM5119: Odd behavoiur in interleaved mode

Andrew Dunlop
Prodigy 200 points
Part Number: LM5119

I have an LM5119 based design, with interleaved operation (single output).  The output is well regulated and stable and the switching waveforms look good.  However, I notice that that side 2 of the converter stops switching when the input voltage goes above 50V.  When the input voltage is reduced below 50V, side 2 starts switching again.  Side 1 does not exhibit this behaviour.  The EN2 pin is NC (so side 2 is always enabled).  These tests are being performed at no load.

What are possible reasons that this could occur?

I ask, because this behaviour is eventually followed by catastrophic failure of the LM5119, and I am suspicious that this unexpected behaviour is a clue as to what is going on.  Catastrophic failure always occurs at higher voltages, although I have now seen it at 50V input voltage.  The switch node ringing is well controlled and does not go anywhere near the device's 65V rating.

Kind regards,
Andrew

  • 0
    TI__Mastermind 24075 points

    Hi Andrew, 

    Can you provide schematic in pdf file?

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Prodigy 200 points

    Hi Qian,

    Thanks for responding so quickly.

    I have found the problem.  I have an extra resistor feeding the summing node at the centre of the potential divider that drives the FB node.  This is driven by a D2A and allows mer to pull the voltage.  Because it comes from a long trace, I thought it would be wise to have a capacitor to ground at the end of this line, just before the extra summing resistor.  Unfortunately the ground I chose was close to of one of the half-bridges, and therefore subject to high current, and was probably introducing switching noise into the summing node.  I suspect that this is what was causing the odd behavior.  When I removed the capacitor, the whole system became docile again and no further catastrophic events have been observed, even at maximum input voltage with large step changes from the D2A.

    It would seem to be far better to (a) not introduce noise at this rather critical point in the circuit, and (b) allow the loop filter to do its job.

    Kind regards,
    Andrew