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UCC28950: DCM CCM MODE CONVERSION ISSUE

Venkatesh B
Intellectual 627 points
Part Number: UCC28950

Dear TI team,

I have developed 3.3KW EV charger, i am using Front end Interleaved PFC(230V AC to 400V DC)

then DC DC stage PSFB converter using UCC28950 IC

I have tested full load with Rheostat loading of both PFC stage and PSFB stage separately

But now when i integrate PFC+PSFB there is a mismatch at operating modes

PSFB operated at DCM mode at all range of load

Kindly let me the root cause of this issue

Thank you

venkatesh B

  • 0
    TI__Guru 73375 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 73375 points

    Hello,

    If you phase shifted full bridge worked and supported the load.  Then most likely it is design correctly.

    If it is not working once you connected the PFC pre-regulator.  Most likely it is having issues regulating the load produced by the phase shifted full bridge converter.  The PSFB converter does not have input under voltage and maybe producing to much load current when it powers up.

    I would suggest powering up your system with 5 to 10% load.  When the PFC is in regulation then vary the load on the output from 5% to full load to see if the design will work to full power.  If this is the case you might want to design and  input under voltage lockout that will keep the PSFB off until the PFC is up an regulated.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 627 points

    Hello mike,

    Thanks,i have started with Enabling PFC and get 400V DC output and then enabled PSFB ,gradually increasing the current till 40% of load still same response

    Thank you

  • 0 in reply to Venkatesh B
    TI__Guru 73375 points

    Hello,

    You need to check the PFC output to see what is happen.  If I was to guess the PFC output is dropping out.  When this happens the PSFB cannot regulate.

    You had mentioned that the PFC can handle the full load.  I wonder if the power limit of the PFC is set to low and does not account for the losses of the phase shifted full bridge.  You might want to revisit that.

    I generally design both power stages to handle a 125 of the regulated output power.   If you do this I believe that you will not have an issue.

    Regards,

  • +1 in reply to Mike O'
    Intellectual 627 points

    Hi mike,

    Thank you for suggestion,

    I did rechecked separately again when i did,PFC output is stabled but input current is not sinusoidal,and then i touch up all the components(resolder a bit) now its working fine

    But initially it takes some seconds to regulate 

    Thank you

    venkatesh B

  • 0 in reply to Venkatesh B
    TI__Guru 73375 points

    Hello,

    Thanks for letting me know.

    Regards,