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UCC28951: Startup under full load Issue

WB
Intellectual 360 points

Part Number: UCC28951

Hello,

The cycle-by-cycle current limiting is getting tripped (OCP) when applying >80% load to the output or when starting up the system with >80% load connected to the output. The CS pin waveform under 78% is shown below.

 

The CS pin waveform when applying >=80% load or starting up with already connected load:

It seems that the primary magnetization inductance of the transformer is high causing a higher peak current during transient. The system was able to startup with 100% load when Rs (R15) was reduced/recalculated to add 50watt margin on top of the maximum load (600W). However, the CS pin peak voltage under 100% load is only 1.4V, which will effect the duty-cycle.     

 

Any suggestions on how to reduce the transformer transient effect to prevent OCP when stepping the load from 0~100%?

Thanks

  • 0
    TI__Intellectual 2790 points

    Hello, I have assigned this thread to an expert engineer, please wait 1-3 business days for his response.

  • 0 in reply to Claire Chang
    TI__Mastermind 24375 points

    Hi,

    Thank you for the query on UCC28951. 

    Looking at the waveform, it seems like the current transformer (CS input) seems to go into saturation at full load. Couple of suggestions would be:

    1. To increase the mag. inductance with a higher turns ratio (lower mag. current) to counter the efffect of saturation at full load.

    2. Try increasing R14. Select R14=100*R15

    Please let us know your observations.

    Regards,

    Harish 

  • 0 in reply to Harish Ramakrishnan5
    Intellectual 360 points

    Hi. Thanks for your feedback.

    1) I am assuming you are talking about the the PSFB transformer. Calculated L(MAG) is 1.96mH. Used L(MAG) is 4.7mH. Can't be increased further for TR design limitations. 

    2) Increasing R14 will to 4.2K caused the OCP to trip at lower load.

    Just to clarify, increasing the load from 0 to MAX in steps doesn't cause OCP. OCP happens only if the load is connected to output and the converter is enabled to startup with full load. The transient current in the PSFB transformer is causing the OCP. 

  • 0 in reply to WB
    TI__Mastermind 24375 points

    Hi,

    Thank you for the reply.

    I get your concern that starting at full load causes this OCP event. The duty cycle will be hitting the max. limit and you might be gettingirregularities in duty cycle (like a double pulse) which can cause volt-sec imbalance and potentially lead to current waveform as you have posted. You can try the following to see if it helps:

    1. The range of the error amplifier output of 0.25 V to 4.25 V far exceeds the PWM comparator input ramp-signal range, which is from 0.8 V to 2.8 V. One option is to clamp COMP pin to less than 3.5V to see if it helps with the start up. 

    2. Or the current limit could be set at higher level to ensure it does not kick in while the converter is not regulated (operating at maximum duty cycle) as you have tried (by reducing the CT burden resistor)

    3. Try adding a 560k ohm between SS and GND.

    Please let me know if the above helps.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Intellectual 360 points

    Hi,

    1) I checked the voltage on COMP pin, which is around 2.4V at maximum load and didn't exceed 2.5V under load transient and during OCP.

    2) Setting the current limit to a higher level has two problems: 1) Adding more stress on the converter when the load exceeds the maximum load or during short circuit until the converter OCP trips. 2) The duty cycle in normal operation will be less effecting the efficiency.

    3) I already have 825K in parallel with the soft start cap (1UF). 

    Regards

  • 0 in reply to WB
    TI__Mastermind 24375 points

    Hi,

    Thank you for the observations.

    Can you please confirm your duty cycle at steady state operation at full load (slowly increasing) and at startup. If think you may be operating at maximum duty cycle all the time.  This would indicate your transformer turns ratio might not be correct and maybe a bit too high.  I would suggest double checking that as per the app note and let me know your observations.

    Regards,

    Harish