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UCC28780: Vref not starting up

Prodigy 20 points

Replies: 2

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Part Number: UCC28780

I have a circuit I am bringing up which is designed per the 45W EVM  reference kit with GAN devices. I am bringing it up by first applying 15V to Vcc, but see no 5V Vref under any conditions. INput current is 0.2ma. RUN_EN goes to 3V, SWS goes to 15V, HVG is 0V...Since the CKT is designed for Vin of 400V, I wanted to see of some of the basic  control signals output first. Is this even possible?

  • Hello Samuel,

    Thank you for your interest in the UCC28780 ACF controller.

    This controller requires that VCC rise to 17.5V first before it enables REF and attempts switch.  This is described in the datasheet.
    The preferred way to "slowly" power up this circuit is to apply a low voltage source to the AC input and keep increasing the voltage until some PMWL  pulses are seen.  

    There is a start-up debug guide available: http://www.ti.com/lit/an/slua982/slua982.pdf that will help you get your system running.  It provides a step-by-step procedure to make sure you understand what is supposed to happen as input voltage is increased.  There are a number of different modes you will encounter as the controller samples the conditions around it and determines what to do next.

    Regards,
    Ulrich

     

  • In reply to Ulrich Goerke:

    Thank you for the fast reply. I have studied, and found a few reasons why the pulses did not occur. I now   get one pulse  with Vin >25V, however, since the active clamp ckt is not working, I must limit Vin to 50V, since the inductive ringing of the primary (296uh) will destroy the  Q2 600V bootstrap fet . My computed Vbi fron the app note is 41.5V Vin...In copying the attached circuit, I made sure I used the same ref designations as in your 28780EVM -002 sheet. To my sheet, I also added RUN_EN to the gate of Q4(which I had omitted) , so the H bridge chips now get proper power. The scope screen shot shows PWM_L (yellow)  and CS (green), The pulse is only 600ns wide vs the 2us indicated in the slua982 document, but at CS = 280mv, the pulse ceases.  Assume I must get the 4 sequential pulses before PWM_H will assert??? The magnetics are  wound on an RM6 core, 39T Pri, 3 turn  NA aux and NS secondary. Hope the .pdf of the circuit come thru, as bitmaps are too low resolution to see...

    Thanks in advance  for any ideas.ZVS Active Clamp.pdf

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