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

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.