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UCC28710: Fly-back controller Start up issue

Part Number: UCC28710

Hi Team,


We are using "UCC28710" in our design for flyback control of the transformer.We have powered up the board but we are not getting any output voltage.I have checked the HV pin of the controller if we are getting the initial start up voltage and we are getting 219V ,but the feedback voltage measured is only 8V.As per the datasheet the startup threshold voltage is 21V. Please let us know what are all the reasons that the controller goes in to low threshold voltage and does not start the mosfet switching operation?

We have verified all the components associated with the controller and all are in working condition. 

  • Vishal

    The way that UCC28710 starts up is that HV pin acts as a current source to charge VDD from the high voltage input.  When the voltage exceeds VVDD(on) (21V typical) UCC28710 will then send 3 PWM pulses.  If no fault condition is measured it will start normal operation, otherwise it will go into a fault shutdown.

    The HV pin has a current rating IHV of 250uA typical when the HV input voltage is at 100V.  Ideally there is only a capacitor on VDD to allow this current to charge VDD up to 21V.  In our recommended circuit there is a diode in series with VDD pin to prevent other potential loads from preventing this startup current from charging VDD to 21V.  I

    f the VDD voltage is not charging up to 21V, then there may be a load attached to your VDD rail that is preventing the VDD voltage from charging to 21V.  I recommend you verify that the blocking diode is connected correctly, and that you have no load that is directly connected to VDD pin.

    Best Regards,

    Eric