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UCC2818

Moha ABA
Prodigy 90 points
Part Number: UCC2818
Other Parts Discussed in Thread: UCC28050

Hello,
I designed a PFC converter which has the following features: Vin = 115V/400Hz, Vout=260V, Poutmax=100W.


The PFC converter goes into overvoltage at startup, the output voltage reaches the internal OVP threshold of the UCC2818 which I fixed at 300V.


I tried to boost the SS capacitor (tried many ceramic capacitor values 100nF, 1µF, 4µ7, 10µ) and it does not resolve the start-up issue.


I tried to clamp the Vaout pin, but the UCC2818 has the same behaviour.


Do you have some recommendations to avoid overvoltage at start-up please?


King regards,

  • 0
    TI__Mastermind 34465 points

    Hi

    Can you please attach a schematic for your design ?

    What are you startup test conditions such as input voltage, load.

    Have you tried slowly ramping up theinput?

    Does your pfc eventually reach the set point ?

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 90 points

    Hello,

    I did  a lot of start-up tests in many test conditions at Pmin, Pnom and Pmax (15W, 35W, 100W) with always Vin=115VAC

    We have always an over voltage at startup, we reach the internally OVP threshold of the UCC2818 which is fixed at 295V (for an regulated output of 260V).

    The PFC stop his decoupling during "ms" but it manages to regulate after 4 seconds at 260V.

    I attached here a picture of a start-up test @Vin=115V and Pout=35W:

    Yellow : PFC output 260V
    Pink : Input current 
    Green : +70V Output voltage of another isolated converter which is supplied by the PFC

    Do you have any ideas what can I do to have a cleaner startup ?

    As I said before I boost the SS capacitor, I used an external SS circuits and I used a clamp circuit on Vaout but it does not changed any thing.

    Thank you for your time.

    Best regards,

  • 0 in reply to Moha ABA
    TI__Mastermind 34465 points

    Hi,
    The 70V regulator seems to start up   when the pfc output reaches about 200V.
    Have you tried delaying the 70V regulator and testing the pfc on its own ?

    Or can you check the response of the pfc when you switch the 70V output from no load to full load.
    That should tell you how the pfc is responding to a step load.
    It may be that the loop response of the pfc is too slow.

    There is a Mathcad calculator tool for the UCC2818 located here
    http://www.ti.com/product/UCC2818/toolssoftware?keyMatch=UCC2818&tisearch=Search-EN-everything&usecase=part-number
    and this will help

    The other thing you can do is reduce the PFC peak current limit. (PKLMT)
    If this is larger than required then you can slow down the rise time.
    Or you can increase the pfc bus capacitance and this will reduce the overshoot.

    The UCC2818 is usually used for much higher power than 15W to 100W since it is a fixed frequency CCM controller.
    For this power level you should look at a boost pfc with a transition mode controller such as the UCC28050.

    Hope this helps

    Regards

    John