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PMP21516: Blowing the sensing resistor... does the reference design need an isolation transformer? Does it need to operate under a load?

Part Number: PMP21516

Hello,

I am using the PMP21516 reference design as the basis for a power supply I have created for an audio amplifier. I have so far popped 3 of the 0.075 ohm sensing resistors on the board as soon as I turn it on. As exciting as that is, I would rather it not go up in flames whenever I turn on my amp. Can anyone help me understand why?

I designed it in KiCAD and had the boards made by JLCPCB. Is there something about this design that I should know?

I added a voltage regulator coming off of the 24V rail down to 5V, but even after unsoldering that part the resistor still pops. I have attached my schematics below, just above the official schematic.

Any thoughts would be great!

Thanks,

Peter

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  • Hi Peter,

    Can you try to attach your files again? They are not showing up in the previous post.

    What size current sense resistor are you using?

    Ben

  • Hello Ben,

    Oh, I am sorry. I'll try to attach them again.

    I am using a 0.075ohm resistor. A109790CT-ND specifically.

    Thanks for the reply!

    Peter

  • Hi Peter,

    A109790CT-ND package size is 0805 and rated for 0.25W. My initial thought is that it cannot handle the current during startup.

    The reference design schematic shows a 1206 resistor, but if you look at the image in the test report, R8 looks closer to a 2512 package size. I recommend using a larger package resistor. 0805 should be too small to handle the current at 100W output also.

    Ben

  • Thanks for the reply!

    I have replaced the above resistor with this resistor 696-1679-1-ND, a 2512 size package with 3W power handling.

    I also put two 2A fuses in parallel (to total 4A, right?) at the beginning of the system and both of those failed, but the resistor did not. The reference design seems to have a 2A fuse on it.

    Without a load on the system do you know how I might determine how much startup current that switching circuit will experience?

    Thank you so much for the help!!

    Peter

  • Hi Peter,

    I would not trust that you are getting a 4A rating by placing the fuses in parallel. Variations in resistance could unbalance the current between the two, causing them to fail in quick succession. The fuse used in the reference design has a time delay and can handle >2A for short periods of time.

    Please let me get back to you with some calculations.

    One thing you can look at is increasing the startup time. You can increase your soft-start cap (C203) or startup resistor (R101). You can read more about this in the LM5021 datasheet.

    Ben

  • Okay, here's my latest setup:

    Sensing resistor: 0.075Ohm 3W 2512

    Soft startup cap: 470nF 0805, changed from 100nF 0805, the data sheet used a 220nF cap as default.

    Circuit fuse: single 2A 

    Result: the sensing resistor popped, but the circuit fuse did not.

  • Peter,

    What is your input voltage? Do you see the controller switching during startup? Is there an output voltage before the failure?

    Ben