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UCC1806: High VC Current

Michael McCown
Prodigy 40 points
Part Number: UCC1806

We have had a couple of power supply boards (internal design) fail with the controller IC (UCC1806) drawing high VC current (10 to 80mA).  The controller still functions otherwise. The gate drive waveforms look normal, and the high current has definitely been isolated to the UCC1806 and not the gate drive loading.  Please define likely causes for this type of failure.

  • 0
    TI__Mastermind 27325 points

    Hi Michael,

    You've not provided much information to help. Since you are asking about the VC pin, it is the bias to the driver output stage. It may be possible the OUTA/B drive stage or ESD structure could experience a partial failure and yet still appear operational. I would ask, how is the VC current of the suspected failed IC measured and how does it compare to a known good IC? How does the "failure" appear in the converter if everything also seems normal? Are these failures happening in the field because some part of the system fails or are they caught during functional test at build? Small dead-time between OUTA/B might also cause a partial failure. Is the converter topology half-bridge or push-pull? Even if the dead time appears as expected during steady-state, what about start-up, shut-down, faulty recovery, transient, etc? If the VC is suspect, what about the MOSFETs being driven by OUTA/B - do they measure ok when tested out of circuit? Is there a gate drive circuit used between OUTA/B and the power stage MOSFETs? these are a few things I would discuss and possibly look into.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 40 points

    Steve

    The power supplies passed board-level tests but failed later during environmental tests.  I was able to isolate the high current to the UCC1806 VC pin by removing/ replacing a resistor in series with VC (part of an RC filter), and by removing and replacing a zero ohm resistor in series with the gate drive output.  Under normal conditions VIN+VC current is approx 7.5mA.  The power supply fails when the VC current gets high enough to exceed the boot regulator energy storage, and thus it doesnt start up.  The topology is a 2-switch forward and there is no gate drive buffer.  The failed power supply operates 'normally' with an external supply to the UCC1806 to keep it running at the higher current.

  • 0 in reply to Michael McCown
    TI__Mastermind 27325 points

    Michael,

    I understand the issue of start-up and why the supply is not starting. Either the start current has increased or the boot cap value has decreased or the time needed for the bootstrap winding to take over has increased. This is a simple I=C*dV/dt issue. I'm wondering how VC and VIN are connected and what is the capacitance on each pin, are they connected directly or is there a resistor between VIN and VC? Also why the supply works as expected during board level tests but fails after environmental; testing? What is the exposed environment when the failure occurs  (hot, cold, thermal cycle, shock, vibe, etc).

    If needed, have you considered to submit the suspect ICs for FT and FA?

    Regards,

    Steve