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PMP20199: Output Voltage going above desired value at Start up

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Replies: 8

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Part Number: PMP20199

Hi Expert,

I have implemented a 54V 30W power supply using the UCC28700. The supply has another secondary winding for 4V output. When observing the start up both the outputs, the 54V output seems to rise above 57V. I think this is causing a Over-Voltage Protection event to take place. Due to this reason the Secondary 2 winding output is dropping for around 100ms. I have attached the startup waveforms for both the outputs.

Can you please suggest a solution.

Thank you.

Regards,

Mikha.VPWR_VDD_Startup.jpg.zip

  • Hi Mikha,

    Thanks for the info, I am looping in the original designer to see if can figure out what exactly is happening.

    Can you give us some more info on the loading conditions, how you start up the design?

    Thanks,

    Robert

  • In reply to Robert Taylor1:

    Hi Robert,

    The power supply is started up with a no load condition. The input to the power supply is 230V 50Hz Ac.

    Thank you.

    Regards,

    Mikha.

  • In reply to Mikha Waghmare:

    Hi Mikha, 

    You are probably correct about OVP on startup.  Can you try adding some static load to the 54V output and see if the startup issue goes away?

    thanks,

    Brian

  • In reply to Brian King:

    Hi Brian,

    I did a calculation for pre-load resistor using the equations given in the datasheet. I got a value of approximately 60kohms at the Sec1. output. So I connected two 30kohm resistors in series and connected the to the output as pre-load but that did not have any effect on the startup waveform.

    The following observations were made:

    1. The output of the Secondary 2 winding is designed for 4V but it's output rises to 5.2V at start up. I tried placing a 316ohm resistor at the 4V output but it had no effect on the output.

    2. I placed a 120uF Electrolytic capacitor at the Sec.2 output. That reduced the time period of the voltage droop to around 80ms.

    3. Then I placed a 470uF cap at the Sec.2 output. After this there was no voltage drop at the Sec.2 output but the response time of the power supply was much more and Sec1 output reached only 44V instead of 54V.

    Is it a possible to increase the output capacitance and still get the desired response time or it is problem on the primary side.

    Thank you.

    Regards,

    Mikha.

  • In reply to Mikha Waghmare:

    Hi Brian,

    I have attached the start up waveform for the 54V output. It seems that after start up there are no pulses for approximately 123ms. Then the pulses start more frequently. Does this mean the IC is resetting at start-up?

    There the max. output visible is 68V which is more than 115% of 54V. Is it possible that this is causing an OVP error?

    Thank you.

    Regards,

    Mikha.

  • In reply to Mikha Waghmare:

    Hi Mikha,

    The extra output capacitance slows down the startup time, because that capacitance needs to be charged during startup.  This can help prevent the OVP issue that you initially reported, but now you may not have enough capacitance on VDD to keep the controller alive during startup.  Try increasing the VDD capacitance and see if you get a clean startup.

    I can't tell from the picture if the 68V peaks are just noise or if something else is going on.  You will need to zoom in for a short time scale on the 68V spikes to see more detail.

    thanks,

    Brian

  • In reply to Brian King:

    Hi Brian,

    Earlier you suggested starting up the power supply with a static load. Can you please specify what kind of load can be used for this purpose. What would be the power rating of the load.

    Also can the load be made using resistor bank.

    Thank you.

    Regards,
    Mikha.
  • In reply to Mikha Waghmare:

    Hi Mikha,

    Yes, just add some resistive load to the board to see if it helps with the overshoot problem.

    I would start with a 300mW or so and see if that helps.

    If so, you can start to decrease the power until the startup issue comes back.

    Then choose some value that gives you some margin, but fixes the issue.

    Thanks,

    Robert

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