PI Expert bug and design doubts
Good morning,
I would like to design a two-output AC / DC converter (Universal input 85-265VAC, out 24V - 1.2A and 12.7V - 0.5A) using your PI Expert design tool. In particular, I made a flyback converter with your LNK6769V device using the wizard guide. However, I noticed some inconsistencies between "PI Expert online" and "PI Expert 9 for PC" by making the same identical project.
I attach a pdf where I show a different value for bias resistance (R9 in the online version and R10 in the PC version). Moreover, in page 3 you can see that the tool calculates 11.6V as feedback voltage while in the LNK6769V datasheet 9V max are shown.
Can you help me to understand if I do something wrong? I remember that I performed the automatic optimization and I did not change any components manually.
Thanks, I'm waiting for your answer.
Comments
Thank you very much. I have a doubt about the feedback because the tool probably fails the number of turns in the feedback side. In fact I have on the primary 44 turns and on the feedback 7 turns. In this way I have a ratio of 6.28. Considering 100V input voltage I obtain 15.9V in the feedback, different of 11.6V as expected. Now, I noticed there was an update to the PI expert online 2.0 version. Compared to the previous project, now the number of turns on the feedback is 5, so I get a ratio of 44/5 = 8.8. In this way, with 100V input I obtain 11.4V on the feedback. However, I have already made the transformer with 7 turns. Regarding the feedback to the LNK6769V I can solve with the partition in order to obtain always 2V, but regard the polarization pin (BP) do you have some advice? There are many bugs in the tool !. Thanks so much.
I am assuming you are referring to the BP pin series resistor, in which case, I would use the recommendation provided by the online tool (2.2k)
Hi Kenobi, yes, I'm referring to the polarization circuit section attached on the feedback size of the transformer. In particular, because I have 7 turns instead that 5, the voltage is about 15V, major of 10V recommended by PI. In this case, do you think that I can use the same suggested value by the tool, i.e. 2.2K?Thanks a lot
Yes, please use 2.2k.
baldos,
The inconsistency in the value of the bias resistance seems to be a bug in the tool. At this point, I would recommend you use PI Expert Online. The issue with the standalone tool will be resolved soon.
The 'feedback pin' voltage is different from 'feedback voltage'. 'Feedback voltage' is the voltage generated at the bias winding and then scaled down to the 'feedback pin' voltage. In your case, the 'feedback pin' voltage = 11.6V*(10.2k)/(10.2k+46.4k) = 2V, which is exactly what the 'feedback pin' voltage should be to regulate in steady state. Hope this helps.