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Original question:

LM5160: LM5160 Load Regulation Simulation Difficulties

LM5160: LM5160 Eval Board (LM5160DNTFBKEVM) Difficulties

Matthew Anauo
Prodigy 110 points
Part Number: LM5160

Has anyone had any issues with LM5160 evaluation board LM5160DNTFBKEVM?

Using the LM5160 calculator spreadsheet, I modified the following components to model my intended application:

R4 -> 170kΩ (to set ripple voltage feedback).

R7 -> 160kΩ (to set switching frequency to 300kHz).

R8 -> 2.7kΩ (to set primary output voltage to ~4.7V).

T1 -> 1:1, 100μH coupled inductor (744877101).

The issue is that the load regulation on the secondary/isolated output is extremely poor:

Load Resistor Targeted Load @ 4.3V Measured Vprim Measured Vsec
1MΩ 0mA 4.73V 5.2V
30Ω 145mA 4.73V 2.12V
16Ω 270mA 4.74V 1.41V
9Ω 480mA 4.73V 0.82V
6Ω 700mA 4.72V 0.66V

Can anyone advise?? Since the primary side is well-regulated, I'm suspicious of the transformer, but I don't see anything about it that violates the design guidelines.

  • 0
    TI__Mastermind 25590 points

    Matthew,

    My first thought is leakage inductance. See figure 4 in this article. I couldn't find the leakage inductance for your coupled inductor but let me know if you have this info. In the meantime I'll look deeper to see if I can find other possible issues.

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 110 points

    LM5160.TSC

    Thanks for the response Sam. I read the article you linked, which was helpful, but given that the article focuses on the challenges faced at high duty cycles, and my application would produce a duty cycle in the range of 15-20%, is it applicable in this case?

    I tried measuring the leakage of the transformer by soldering a short and measuring inductance, my LCR meter read 61uH, which seems very high to me on a 100uH coupled inductor (?).

    I have also ran some simulations with that leakage value, and it does produce results fairly similar to experimental measurements. My concern though is that when I run the simulation with no leakage inductance at all, I am still not getting good load regulation on the secondary side (TINA file is attached for reference).

    -Matthew

  • +1 in reply to Matthew Anauo
    TI__Mastermind 25590 points

    Matthew,

    You're correct about the lower duty cycle but that's a lot of leakage. How good was the short on the secondary?

    Summary to help keep track:

    EVM --> Matthew

    VOUT_PRI: 8V --> 4.7V

    FSW: 300kHz --> 300kHz

    Turns ratio: 1.5:1 --> 1:1

    R4 ripple: 51k --> 170k (might be a bit too much. Maybe more like 80k)

    L_Leakage: 0.6uH --> 61uH (double check. If true, this is probably the problem)

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25590 points

    Matthew,

    I have not heard back. I'll close this thread before it locks. Feel free to post again if you are still having issues.

    -Sam