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LM5070 - Primary side current problem
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?
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.
Youhao Xi (email@example.com)
Applications Engineering, Texas Instruments SVA
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.
OK posted the schematics but they didn't post :( I'll try again
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.
Youhao Xi, Applications Engineering, Texas Instruments SVA
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