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UCC3895 is not stable and sometimes it doesn't have output.

user5139383
Prodigy 20 points
Other Parts Discussed in Thread: UCC3895

My  UCC3895 sometimes does not works normally.  Until now，I haven't found  any problem in the peripheral circuit and no matter if I change the chip, the 3895 is always not stable. When the UCC3895 doesn't  output  square wave, I find that the pin 19 is low level but some other  pins such as pin 12 and pin 4 are all normal. Then I link the pin 4 and pin19 in order to give the pin 19 a high level, however, I find that the pin 4 also becomes low level and there are't any square waves in the pin 13,14,17,18. I don't know why. Could someone help me? Thank you very much.

  • 0
    TI__Mastermind 25570 points
    Hi

    It could be that the controller is going into and out of UVLO on its VDD pin. This in turn may be a result of noise on the VDD pin. I'd check the decoupling capacitor between VDD and PGND and the connection between PGND and GND. It needs to be as close as possible to the pins -

    During this event, is the oscillator still running? - you can probe the CT pin for this - although the probe capacitance may shift the frequency a few percent.

    Is there any particular operating condition that triggers this behaviour ?

    Please let me know how you get on.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 20 points
    Thank you for your reply.I have also considered this issue. But when UCC3895 doesn't have output ,I also measure the level of VDD .The voltage is more than 14V and the Vref is more than 4V, that is to say, the problem is not relevant to the UVLO.Besides, I have analysed the block diagram of UCC3895 but I haven't any problem in principle until now.
  • 0 in reply to user5139383
    TI__Mastermind 25570 points
    Hi
    The Vref should be 5V not 4V - can you double check that.

    It is possible that the device is being 'upset' by noise. One potential source is common mode currents from the gate driver circuit. Or noise coupled from OUTX lines tracked under the device.
    Can you post your schematic, your PCB layout (gerber files are preferred) and some screenshots of the voltage across the transformer. If you can't post them here then send them to me at colingillmor@ti.com

    Regards
    Colin