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Original question:

UCC28750: The feedback design of UCC28750

UCC28750: UCC287506

Nguyen Duc Khanh
Prodigy 40 points
Part Number: UCC28750

Tool/software:

I am using UCC287506 for 36W Flyback power supply,
2 outputs are: 12V power output for fan and 14V output for light load peripheral. with an auxiliary coil of 21V to power the UCC28750 IC.

When I experimented by running with 155V power supply for the power circuit and 24V auxiliary power supply for the IC using adapter. My 12V coil is always stable. However, when I look at the switching frequency (Green channel), the switching frequency is only about 1.3kHz. I am not sure where the problem is. Please help me.

- Yellow channel: Feedback voltage
- Green channel: DRV voltage
- Blue channel: 14V voltage
- Red channel: VDD voltage (IC not connected)

I attach schematic below

  • 0
    TI__Genius 12765 points

    The first plot doesn't make sense to me. Why would the 14V and VDD voltage rise? The second plot has no DRV signal but it shows burst mode operation.

    I am not sure what the issue is. Please elaborate.

  • 0 in reply to Ning Tan
    TI__Genius 12765 points

    Hi,

    After a second look, I think you mean that the red channel is not connected to anything, but it is supposed to be connected to VDD, right?

    The IC is operating at burst mode, likely due to the light load. Your 12V has larger output cap than your 14V rail. That's why 12V appears to be more stable than 14V.

    Best,

    Ning