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UCC28180: About restart time

Tomohiro Yoshimura
Prodigy 100 points
Part Number: UCC28180

Hello.

I’m designing power supply system using UCC28180.

I want to know the time to automatically restart in the input voltage instantaneous stop test.

The VCC voltage is fixed at 15.0V.

 

I have attached a calculation sheet.

 UCC28180_design sheet.xls

 

Regards.

  • 0
    TI__Mastermind 19455 points

    Hi Tomoshiro-san,

    Thank you for the query on UCC28180.

    During instantaneous stop, the UCC28180 does not have brown in/out functionality implemented with this is and it has to implemented externally. 

    EPGI Discrete brownout function of UCC28180 (1).pdf

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Mastermind 19455 points

    Also, the calculation look fine to me.

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 100 points

    Hello,

     

    Thank you for your answer.

    Because the output voltage stops by overcurrent protection at an input voltage drop, I thought that brown in/out functionality is not necessary, but is this dangerous?

    After this overcurrent protection movement, is there the method to identify the time lag between the input voltage restart and the output voltage restart?

    Regards.

  • 0 in reply to Tomohiro Yoshimura
    TI__Mastermind 19455 points

    Hi Tomohiro-San,

    Thank you for the reply.

    There could be a possibility that, normally a controller that measures input voltage will go into brownout when the high voltage input drops below a set level.
    In brownout, the soft start capacitor is discharged and once the input voltage is re-applied the soft start cap starts to charge again.
    This causes a gradual increase in gate drive pulse width. But with the UCC28180 if the high voltage input drops out the gate drive goes to max duty cycle because the input voltage is not being monitored and all the UCC28180 see is that VOUT is low.
    Once the input voltage comes back online (meaning when you turn on), the converter will go to an over voltage condition because of the slow loop response of the pfc.This is could potentially damage the MOSFET when a heavy load step is applied. So it would be better to have a brownout detection circuit.

    On your other query, the time between input start and output high is decided by soft start capacitor. In this case it will be the capacitor on VCOMP_P pin (connected in parallel to the Rvcomp & Cvcomp) will actually act as the pseudo softstart cap and decides this time.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 100 points

    Hello,

     

    Thank you for your answer.

     

    I add brown in/out functionality.

    Is there the calculation formula about soft start time?

    Is the RC time constant of the VCOMP pin all right?

     

    (From UCC28180 detasheet and UCC28180EVM-573)

    Regards.

  • +1 in reply to Tomohiro Yoshimura
    TI__Mastermind 19455 points

    Hi Tomohiro-San,

    I think you are correct with your calculations. The following is the screenshot of EVM board startup.

    So with the waveform above it seems like the soft start time dt = C(dv)/ i = 5.17uF x (4.25-2) / 40uA = 290ms roughly coinciding with what we have on the figure. 2V being the value of Vsense pin. This is just rough value, but the capacitors can be adjusted to get the required SS without affecting the bandwidth during normal operation.

    Regards,

    Harish