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UCC28600 startup problems

Jonas Laridon
Prodigy 50 points
Other Parts Discussed in Thread: UCC28600

Hello everyone,

I'm a student Industrial engineering from Belgium and i'm trying to build a power supply with the UCC28600. The company i'm doing this for already has a 350V DC supply, so I don't need the AC/DC conversion. From this 350V I have to maken something between 30 and 40V at 100W. My available lab equipement only supplies me 150V, so I recalculated everything for that input and only the Rsu made a critical change so I raplaced it. In the 2 pictures below you can find my PCB layout and my schematic

The problem I'm facing is that for some reason when passing the startup threshhold (13V) at the VDD pin, it immediatly goes back to the switch off threshhold (9V). Nothing else happens exept a smile spike at the feedback pin. (see pictures below) 

VDD:

feedback:

Is there ayone who can help me? Please do, cause I've been searching for weeks.

Kind regards

Jonas Laridon

jonas.laridon@student.kuleuven.be

  • 0
    TI__Genius 15910 points
    It looks like you need to increase the bulk capcitor on VDD (swap out that 10uF for a 22uF). Did you use the design calculator to verify the component selection? the UCC28600 requires very specific compnent values for Rpl, Rcs, and the Rovp resistors and the transformer design is critical for operation.
    C3 should be closer to the UCC28600...it isn't doing you any good that far away from the device pins. Cbp on VDD should be as close as possible to the UCC28600 pins with short traces...layout is key when it comes to PWM controllers...anybody's, not just TI. If a pin has a capacitor connected to it, put that capacitor close to the IC pins with short traces, every time. but increase the bulk cap on VDD first because it looks like there isn't enough energy for the bias windings to take over because there isn't enough holdup.
  • 0 in reply to Lisa Dinwoodie
    Prodigy 50 points
    Dear Lisa,

    First of all thank you for your answer! To increase the Cvdd capacitor was also my first idea. I already put a second one on top of it to make it 20µF, but that gives the same result. It only takes longer for the VDD pin to charge to the startup voltage. Should I increase the capacitor even more?

    In my next design I'll also take the layout remarks in account.

    Jonas
  • 0 in reply to Jonas Laridon
    TI__Genius 15910 points

    try a larger cap, 47uF, bigger if needed.  If you still can't start up, you need to go through the design calculator and verify component values, a transformer redesign might be in your future...

  • 0 in reply to Lisa Dinwoodie
    Prodigy 50 points

    thank you, I finally got it working!