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UCC256304: IC Failing.

Brian McDonald
Prodigy 140 points
Part Number: UCC256304
Other Parts Discussed in Thread: PMP20795, , UCC256301

Hi,

Just started working with this IC and have had 5 pieces fail so far. The setup is based on PMP20795 reference design and i'm trying to power up from low voltage gradually rising to final voltage. Feeding 1.5V into BLK pin from external Dc source, applying 40Vac (rectified) to HV pin - Unit starts switching but on/off behaviour due to loss of Vcc & RVcc. Then i'm adding 12V to Vcc (through a diode, takes about 12mA) to get continuous swithcing - all ok so far. Gradually increasing Vac to about 120Vac - IC dies. Vcc and Rvcc seem to draw too much current. Layout is tight and grounding is good. Component values are similar to mentioned ref. design.

Any advice on the most likely causes for IC to fail like this?

Thanks,

  • 0
    TI__Guru 66135 points

    Hello Brian,

    The major failure for power supply controllers is electrical over stress.  This is either exceeding the current rating and voltage rating of the device.  There is a table in the data sheet that will list these ratings for each pin of the UCC256304.

    I did see in your email that you mentioned that VCC and RVCC pin seem to draw too much current.  The VCC pin is used to apply power to the UCC256304, where the RVCC pin is the output of an LDO.  Please refer to page 16 of the data sheet for the functional block diagram. The RVCC pin's output is generally FED to a resitor divider to the LL/SS pin to program burst mode. This voltage is also used to charge a boost capacitor between the HB and HS pins.  The RVCC pin should not be drawing current and should only be used as an output.

    If you have the RVCC and VCC pins tied together this could damage the IC.  VCC will start switching when it reaches the VCCReStartSelf Level of 26 V.  The VRCC pin has a maximum voltage of regulated RVCCmax + 0.3V = 12.4V +0.3 V = 12.7V.  Could you double check to make sure this is not the case?

    There is also a 120W evaluating module using the UCC256301 that you might want to order and evaluate.  This could help you in troubleshooting your design as well.

    Regards,

    Mike

  • 0 in reply to Mike O'
    Prodigy 140 points

    Hi Mike,

    Thanks for the info. I've just reviewed everything again and discovered i missed the series resistance with the hv pin. Ds says should be 5k min. I have 0r. All other pin voltages are below maximum ratings. Do you think this could be the cause? If yes, what is the mechanism?  I have some more ic's on the way and will test when they arrive.

    Thanks,

    Brian

  • 0 in reply to Brian McDonald
    TI__Guru 66135 points
    Hello Brian,

    If the data sheet recommends a minimum resistance on the HV pin and we did not use it could electrically over stress the device. Try it and see what happens.

    Regards,

    Mike
  • 0 in reply to Mike O'
    Prodigy 140 points
    Thanks mike. Added the 5k on hv + gate drive schottkys and seems ok now.
    Yhanks for your help.
  • 0 in reply to Brian McDonald
    TI__Guru 66135 points

    I am happy that this resolved your issue.

    Regards,

    Mike