I have build the AC/DC flyback converter according to TI's reference design but ist is not working and I am wondering why. By the way: I have build the 5 V version, so everything connected to T1:9,7 I have not populated. It seems to me that the controller is not working at all. So far I have measured:

  • Rectified high voltage: 328 V
  • 5V Low voltage: 0 V
  • U1:4: 0 V (after power up)
  • U1:4: spikes to 40 V and decays (in approx. 100 ms) to 0 V (during power up)

What I plan to do yet:

  • Verify polarity of the transformer`s secondary windings. (after confirmation from TI about the pin out this is obsolete now)

Do you have any advice what else to look for?

Another question:

  • Why the reference design is using an external current source, though the controller's datasheet suggests using the internal one (have a look at the block diagram)

Another thing to mention is: In TI's reference design they seem to have messed up the pinning for the transformer (WE 750316908). The primary side is ok but on the secondary the schematic symbol (pinning) does not correspond to the footprint pinout (would be glad if you could double check that).

Meanwhile TI confirmed the erroneous pin out numbering. Their advice is to use the following pin out: Updated transformer pin out

What else to add? If some information is missing please let me know. Thanks in advance for your help.

edit: Link to transformer

edit (schematic added): enter image description here

  • \$\begingroup\$ Seeing as T1's pin 7 is connected to the 5V path, I think you need to populate all of the 24V caps and resistor. First rule of copying a design and doing slight changes: if it doesn't work, get it back to the original design and find out what isn't work. \$\endgroup\$
    – Puffafish
    Apr 17, 2020 at 7:53
  • \$\begingroup\$ For me that is only an addition of the secondary voltages.. But I may be wrong though.. \$\endgroup\$
    – stowoda
    Apr 17, 2020 at 7:55
  • \$\begingroup\$ "In TI's reference design they seem to have messed up" Better to send an E-mail / forum post to TI for clarification. \$\endgroup\$ Apr 17, 2020 at 8:54
  • \$\begingroup\$ Please show your schematic. "5V but..." is a bit hard to decipher. \$\endgroup\$
    – winny
    Apr 17, 2020 at 9:22
  • 1
    \$\begingroup\$ The secondary numbers are different and so if you followed their schematic (and not their layout) then you will be wrong. \$\endgroup\$
    – Andy aka
    Apr 17, 2020 at 9:29

1 Answer 1


Finally I found the issue. Initially I forgot R10, the feedback resistor. After soldering in between FB and GND the secondary Voltage went to approx. 5 V.

enter image description here


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