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Original question:

UCC29910AEVM-730: UCC29910AEVM-730

UCC29910AEVM-730: UCC29910AEVM-730, external bias supply

Donovan_safari
Prodigy 20 points

Part Number: UCC29910AEVM-730

I want to use the UCC29910A with an external bias supply, and thus, don't need the depletion mode FET and BSCTL signal. I try to disable that part of the circuit, but the controller won't start switching.  I removed D2, and it worked fine, but when I remove R9 going to the depletion mode FET, Q1, the controller won't start switching.

BSNS and LSNS are within the recommended operating conditions, and my external bias is applied before applying line voltage., so I don't see anything on the datasheet to explain this behavior.  Any thoughts on this would be appreciated.

  • 0
    TI__Mastermind 25570 points
    Hello Donovan

    note(4) at the bottom of Page 3 explains the conditions for start-up
    Please check:
    VDD is present
    VLINESNS increases from 0 to a value between 258mV and 920mV
    VBIASSNS pin is at a voltage between 907mV and 1025mV
    VBULK = 0V (normal startup condition)

    This sequence will happen normally on the EVM as supplied. I'd suggest you replace the components you have removed and take some time to see how the start-up process operates. The controller was designed on the assumption that the initial bias energy would come from the Dmode MOSFET - if you don't want to use this part then you will need to find some other way to recreate the start-up conditions - there is a 600mV time-out on the start up process. the Dmode MOSFET is used as an alternative to more lossy resistor start-up - I don't think we have experimented with resistive startup on this part but it may be possible.

    Probably a high value resistor from line input to the 10VB node (pin 4 of U1) could charge the node so that the BSNS pin reaches the VBIASSNS threshold within 600ms.

    Please let me know how you get on.
    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 20 points
    That's the piece of information I overlooked. Thank you! Since I need to provide signal that goes up to 0.907V<VBIASSNS<1.025V then drops down to 0.463V<VBIASSNS<0.907V within that 600ms time, I think the given circuit will be the quickest solution even though I was hoping I could eliminate it.
  • 0 in reply to Donovan_safari
    TI__Mastermind 25570 points
    Hi Donovan

    I'm going to close this thread - if you have any further questions then please post them on e2e.

    Regards
    Colin