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
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with respect to these materials. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.