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UCC28750: Poor SMPS Load Regulation

Part Number: UCC28750
Other Parts Discussed in Thread: , TL431

Tool/software:

We are seeking assistance with a Flyback SMPS design using the UCC28750 controller. The design was based on the UCC28750EVM-071 User Guide and the UCC28750 Datasheet.

Issue:
The SMPS is designed to output 12V, but we are experiencing load regulation issues:
At no load, the output reaches a maximum of 11.7V.
At a load of 2.54A, the output drops to 10.8V.
Please see Figure 1 for an illustration of the output voltage behavior.


This poor load regulation has been consistent across 6 different boards assembled, including 1 board that used a different transformer with approximately twice the primary inductance compared to the others.

Design Details:
The circuit parameters were chosen using the UCC28750 Spreadsheet.
The design includes an auxiliary winding to supply a 150mA load.

Attached Figures:

Figure 2
Figure 2: Feedback Section

Figure 3
Figure 3: VDD Section. D10 leads to the 15V auxiliary output for 150mA, and R70 is the third 590kΩ resistor in the HV startup path.

Figure 4
Figure 4: UCC28750 CALC Specification Input

Figure 5
Figure 5: UCC28750 CLAC Loop Gain Calculation

We would appreciate any insights or recommendations you can provide to help us resolve the load regulation issue and maintain a stable 12V output.

Thank you for your support!

  • Hi Kalyan,

    Thank you for reaching out.

    1. Please try moving the TL431 and feedback after the inductor L2 to see if there is any improvement.

    2. Also try cross checking the value of current sense resistor board.

    Please feel free to share the excel calculator to review the design.

    Regards,

    Harish

  • Hey Harish,

    Thank you for the prompt response. Sure, I'll attach the Spreadsheet right away.

    1. We tried both shorting the inductor and thereafter cutting traces and soldering the Feedback point to after the Inductor, neither approach improved the Load Regulation Significantly.

    2. As for the Current Sense Resistors, the boards were initially running on 500mOhms and we, in troubleshooting, adjusted it down to 280K but this yielded no improvement in the load regulation. 

    3. We also measured the voltage across Rfb_bot and it's coming down to 1.7V, as this is right across the TL431's Ref our expectation was that it should be close to 2.5V. Is there something we're missing ?

    4. We also tried to increase the Output Voltage to around 13V+ by adding a 10Kohms  in series with the 75K (R_fb_top) , to make 85K but this strangely did not increase the output voltage and we thus reverted to the 75K Setting

    5. We've attached waveforms of:
    A) 12V Output - No Load
    12V Output - No Load

    2.5A LoadB) 12V Output - 2.5A Load

    C)TL431 Ref Pin

    TL431- Ref

    https://ibb.co/ZYZzxZf

    UCC28750-CALC.xlsx

    Kind Regards,
    Kalyan

      

  • Hi Kalyan,

    Thank you for reaching out.

    I think there should be no slope compensation resistor needed here.as you are only running a pure DCM operation.

    Also the resistor current sense needs to be close to 480mohm (with 1k slope compensation). This will ensure CS pin voltage will be less than 900mV (max).

    R85/C47/49 needs to be tuned. C47 looks too high and needs to be checked. Please remove this. Also location of zero needs to be close to the plant pole, maybe a few hundred Hz should help this.

    Regards,

    Harish

  • Hey Harish,

    Would you be able to share any application notes, tools, or literature to help us get started with modelling a current-mode controlled flyback converter operating in DCM, as well as designing a Type II compensator using the TL431?

    We’re looking to identify our plant’s poles and zeros through modelling, but finding comprehensive literature on modelling current-mode controlled flybacks in DCM has been challenging. We've encountered some conflicting information across sources, so any guidance you could provide would be incredibly valuable.

    Thank you for your help.

    Best regards,
    Kalyan

  • Hi Kalyan,

    The following pdf file should be a good place to start.

    https://www.onsemi.com/pub/collateral/tnd381-d.pdf

    plecs_tl431 (1).pdf

    Regards,

    Harish