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LM5180-Q1: The output DC voltage of LM5180 is 0 !

Tom Tom
Intellectual 530 points

Part Number: LM5180-Q1

Attached is the figure. The parameters I used are as follows:
Vin=12V, Ruv1=147K, Ruv2=34K, Rfb=130K, Rtc=360K, Rset=12K, SS pin to ground capacitance 47n, transformer ratio 1:1.33:0.5, primary winding L=26u. After the transformer are the normal rectifier circuit and filter circuit, but the test results show that the two secondary voltage outputs are 0. What's going on? Where should I start investigating?

Thanks !

  • 0
    TI__Intellectual 1800 points

    Hello Tom,

    What are your output requirements? Could you please share the design calculator used for your design requirements.

    TI Design calculator: LM5180DESIGN-CALC Calculation tool | TI.com

    When you refer test results, do you mean hardware results? If yes, could you please share your schematic and layout for review.

    Thanks,

    Hareesh.

  • 0 in reply to Hareesh Babu Takasi
    Intellectual 530 points

    Upon inspection, it was determined to be caused by poor soldering. However, there is now a problem: the actual measurement of this parameter is Vout2=6.5V, and I want to obtain an output of 7.5V. I changed Rfb to 160K, which should theoretically result in an output of 7.5V, but the actual test result shows that Vout2 output is 0. Is the feedback resistor affecting something?

  • 0 in reply to Tom Tom
    TI__Intellectual 1800 points

    Hello Tom,

    Tom Tom said:
    actual test result shows that Vout2 output is 0

    Is this happening when you change RFB? This should not happen, RFB helps in voltage regulation. Changing RFB changes Vout proportionally when RSET is kept constant. It is recommended to use RSET value as 12.1Kohm. I think you are using 12.1Kohm because it is standard value although you mentioned 12Kohm above.

    Tom Tom said:
    I changed Rfb to 160K,

    How did you come up with this value? Can you cross check with the TI design calculator.

    How is Vout1 varying when you vary RFB?

    Thanks,

    Hareesh.

  • 0 in reply to Hareesh Babu Takasi
    Intellectual 530 points

    It was calculated based on the second secondary voltage of 7.5V, with a ratio of 2. The formula is (7.5+0.3)*2*10K=156K. But now, strangely, when the actual Rfb is 130K, the output is already 7.5V.

  • 0 in reply to Tom Tom
    TI__Intellectual 1800 points

    Hello Tom,

    In your case, decreasing RFB is increasing your output voltage, is this happening proportionally? Generally, Vout must be proportionally increasing with increase in RFB.

    Can you provide switching frequency, turn ON time and SW node waveforms at these 2 RFB values? Apart from RFB, did you change anything else?

    What is the leakage of transformer you used in your design?

    Thanks,