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Original question:

UCC28630: Start-up problem

UCC28630: UCC28630 startup latch-up overvoltage condition

Kamil Muza1
Prodigy 220 points
Part Number: UCC28630

Hello,

I'm having some issues making our SMPS to start up fully. It seems that it passes the 3 exploratory pulses and starts up and eventually latches up as time progresses.

Please see the schematic, layout and some waveforms. I can also provide more detailed waveforms on request if it would help debugging the problem more.

YELLOW --> Vdrv

GREEN ---> Vdd

YELLOW ---> Vaux

GREEN ---- Vdrv

Please let me know any additional info you might need to help me debug this issue. As you can see it works for a little while and then stops and starts latching again at a very weird frequency until it latches up. 

Thank you

  • 0
    TI__Expert 7080 points

    Hi Kamil,

    The first waveform your provided shows the VDD of UCC28630 already touched 17.5V VDD OVP threshold . then the controller will latch up.

    I think you have to minor decrease AUX winding turns to reduce the VDD voltage.

    Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 220 points

    Hi Jaden,

    Can you take a look at Ti spreadsheet with my design parameters in it.

    Please note the suggested vs overridden number of turns. We went with a transformer with Np=58, Ns=3, Nb=8. If I follow the ratios suggested in the sheet, I'm off by 1 turn on the bias. Do you think one less turn on the bias winding will fix this issue? Also, Please note I have soldered a 178Kohm resistor in parallel with Rb to tune it to 27.98 which is very close to the spreadsheet.

    Let me know what you thinksmps_calcs.xls

  • 0 in reply to Kamil Muza1
    TI__Expert 7080 points

    Hi Kamil,

    Another possible reason for VDD OVP is transformer leakage energy over charge VDD cap . you can in series a 10~20 ohm resistor on AUX rectifier path. (after the rectifier diode) since I have not find any resistor on this path and the VDD voltage is higher than 5* 8/3 =13.3V.

    Thanks.