LM5070 - Primary side current problem

Doug Allender

Other Parts Discussed in Thread: LM5070

Hello

I have a problem with a POE design I have which follows the application note for the National LM5070. The problem I have is that the transformer primary does not exhibit the normal flyback behavior with a ring above the input voltage, and the current waveform through the FET current sensing resistors is triangular and not zero in the off phase of the FET. Tracking is not exactly optimal but should not really cause this problem as far as i can tell. I have tested my transformer and opto coupler on the development board, and these perform correctly.

Anyone any clue as to what I have missed?

Regards Doug

over 13 years ago

0 Youhao Xi over 13 years ago

TI__Guru* 78400 points

It will help is you can show your schematic and a few scope captures. Also make sure the transformer winding polarities in all windings are used correctly.

Thanks,

Youhao Xi (youhao.xi@ti.com)

Applications Engineering, Texas Instruments SVA

0 Doug Allender over 13 years ago in reply to Youhao Xi

Prodigy 40 points

See scope plots Channel 1 is Drain Voltage, Trace 2 is Voltage across current sense resistor, and Trace 3 is Fet Gate voltage. NOTE the voltages on Trace 2 & 3 are incorrect Trace 2 should be 200mV and Trace 3 should be 5V.

0 Doug Allender over 13 years ago in reply to Doug Allender

Prodigy 40 points

OK posted the schematics but they didn't post :( I'll try again

6242.POE.pdf

and Transformer

4075.WurthPoE7491193912.pdf

0 Youhao Xi over 13 years ago in reply to Doug Allender

TI__Guru* 78400 points

Hi Doug,

Thank you for the schematic, transformer datasheet, and scope capture, in the previous two posts. I do not see any obvious problem in the schematic. However, the scope capture indeed looks incorrect. Something must be wrong. Instead of guessing, would you give me a call at 480-629-2487? After we find out the root cause, we can post the finds in E2E.

Thanks,

Youhao Xi, Applications Engineering, Texas Instruments SVA

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LM5070 - Primary side current problem