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UCC28634: No output on the secondary

JAGADISH K
Prodigy 30 points
Part Number: UCC28634

From my preliminary examination, I found that the VDD is fluctuating from 12 to 5v, My inference from the datasheet is that the High voltage current source is helping till it reaches VDD(start) and turns off to let the bias voltage take over, but that's not happening, so it resets, and reinitiates the startup sequence and is in a loop. I'm getting roughly around 300V at the bulk caps after Full bridge rectifier and zero volts at the secondary. Kindly note this is an Altium file of TI We bench generated schematic. The PWM signals will drive the power Mosfet only when VDD Is at 14.7v. The symbol used in Altium's library for that transformer shows the dot placed wrongly. The transformer used was constructed according to Ti's design report. If someone could help me out on this one it would be greatly appreciated.

  • 0
    TI__Mastermind 40580 points

    Hello Jagadish,

    Thank you for your interest in the UCC28634 flyback controller.

    For a flyback topology, the transformer symbol in the Altium schematic that you posted has the CORRECT polarities shown for each winding connection. In other words, the schematic is connected properly and the polarity dots are in the correct positions.

    Now, the polarity dots are somewhat different in the transformer design report, however, that report (as shown) does not assign pin numbers, so this drawing can be equivalent to the Altium symbol if the pin numbers are assigned according to the schematic diagram.

    I think this may be the source of the problem. Please examine and make sure that your physical transformer windings are connected to the circuits with the polarities shown in the schematic, regardless of the pin numbers. 

    Since the transformer report AUX and SEC windings appear to match the schematic symbol, but the PRI winding appears to be inverted, I suspect that you may have your primary winding connected backwards.  If this is the case, you will see positive voltages at the AUX and SEC dots during the MOSFET on-time.  Correct operation of the flyback should have negative voltages at these dots when the MOSFET is on.
    Please check it.

    If this is not the problem, then follow this start-up debug procedure, many parts of which also apply to the UCC2863X controllers:
    http://www.ti.com/lit/an/slua783/slua783.pdf 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 30 points

    Hi Ulrich,

    Thanks for your help. After going through the documents with regards to your suggestion, I have a doubt, when the MOSFET is ON the Diode of secondary is in reverse bias, but the auxiliary winding's diode is forward biased. I'm assuming the auxiliary winding's diode should also be in reverse bias when the Mosfet is ON if this is not the case, please correct me.

    Regards,

    JAGADISH K

  • 0 in reply to JAGADISH K
    TI__Mastermind 40580 points

    Hello Jagadish,

    Your assumption is correct: both the Auxiliary winding and the Secondary windings' diodes should both be in reverse bias when the MOSFET is ON.  They will both go into forward bias when the MOSFET turns off. 

    Your finding suggests that either the prototype board was constructed incorrectly or the auxiliary winding polarity is backward. 
    Either way, this condition is preventing start-up because the controller is not seeing the correct level at the VS input.  It expects to see a current coming out of VS (due to negative voltage on AUX) when the MOSFET is ON, and expects a positive voltage during flyback interval, when the MOSFET turns off. 

    If you swap the AUX winding pins you may achieve the start-up, if there are not any other faults also present.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 30 points

    Hi Ulrich,

    I swapped just the auxiliary winding pins and even tried swapping exactly to the schematic convention, but nothing worked out. So I downloaded webench schematic and compared, and found that the resistor used in Vsense pin is different, i.e., R2 in webench report and R8 in my schematic. The webench report uses 35.7K, but I have used 30.9K in mine. Can this difference be substantial for the device to stop working?.2625.WBDesign10.pdf

  • 0 in reply to JAGADISH K
    TI__Mastermind 40580 points

    Hi Jagadish,

    Yes, the value of R8 in your schematic is important, in that it influences both the threshold voltage for AC start-up and OVP for the output.

    Making this value too low will raise both levels.  This may be preventing your start-up because the controller thinks the bulk voltage is too low.
    Please correct this value. 

    Regards,
    Ulrich