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LMR38020: Inductor Selection

TI-CSC
Genius 13075 points
Part Number: LMR38020
Other Parts Discussed in Thread: LMR38025

Tool/software:

Hello E2E Experts,

Good day.

I built a circuit board based on https://www.ti.com/lit/an/snvaa82/snvaa82.pdf?ts=1723607043871&ref_url=https%253A%252F%252Fwww.google.com%252F
But my circuit configuration is below:
Vin 12Vdc (Fixed)
Primary Vo = 5.6V@0A. * I only need the isolation output of the secondary
Secondary Vo = 5.6V@0.5A.
For now, I selected a coupled from WE. https://www.we-online.com/components/products/datasheet/7448844330.pdf
After I tested the circuit, the outcome was as below:
Primary output has 5.6V (regulation good)
Secondary output can't load the current larger than 0.3A. 
So, the question is this:
Am I choosing the correct inductor?
If not, would you kindly suggest a coupled inductor that can achieve the configuration?
Regards,
TI-CSC
  • 0
    TI__Mastermind 31315 points

    Hi,

    Sorry to hear that you are running into trouble. What is happening when the output loads the current larger than 0.3A? is there part damage, or is the output falling out of regulation? Can you provide your schematic? 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Genius 13075 points

    Hello Richard,

    Good day.

    1. What is happening when the output loads the current larger than 0.3A?
    Ans: The voltage drop is unstable, and the secondary output voltage drops significantly. Switch frequeny and duty vary. 
    CH1 Pri sw node, CH2 primary inductor current.
    CH1 primary sw node, CH2 inductor current.
    2. Is there a part damage, or is the output falling out of regulation?
    Ans: I think not, since I replaced an inductor 15uH 76889440150 with Isat 1.25A, which works 5V0.5A marginally.
    3. Can you provide your schematic?
    Regards,
    TI-CSC
  • 0 in reply to TI-CSC
    TI__Mastermind 31315 points

    Hi,

    Thank you for providing the schematic and waveform. If i'm not mistaken, I believe CH2 is the output inductor current, right?

    I think the issue is related to your inductor and its lower leakage inductance. It is noted in a flybuck that if the leakage inductance is larger, than as the secondary current increases, the primary side current will also further decrease. It may get to a point where you may be hitting negative current limit, which terminates the LS pulse and causes your output to fall out of regulation.

    I think there are two solutions:

    1) Is it possible to find another inductor that has lower leakage inductance but around the same designed value?

    2) Can you try switching to the LMR38025 FPWM variant and see if that helps fix the issue since it has a lower negative current limit. I would probe the primary side inductor current. 

    Thanks,

    Richard