Hi guys, I did a development with PIExpert 8 for an LED driver of 78V 800mA. I selected LNK420E and transformer with EER28. I made the printed circuit according to my needs and I carefully respected all the indications. In fact the prototype works very well except for two things:
A) Operation is DM instead of CC.
B) The snubber net heats too much.Specifically R5
The first one I suppose is corrected increasing the inductance but the second one requires more work.
Please see in attached schematic and oscilloscope images.
The yellow color corresponds to Vcc (rectified voltage input, transformer pin 1) and green to the Drain of the LNK420. With great courage removed R6, that is to say that I completely eliminated the mesh. I know, a very dangerous game. Incredibly everything works well and the transient opening, although reaches almost 800V. Congratulations to PI !.I can kill that transient easily with 1nF and I have enough time to download it with 200Kohm in parallel. They agree?
Then I do not understand why they keep circulating almost 10mA per R5 throughout the toff. That energy is lost uselessly and D1 makes it look directly as current over the LEDs.
Another question: what is the mathematical expression that links the voltage in pin V, the current in FB with the current or circuit activity cycle ?.
The last one: is it absolutely necessaryD3?
Thank you very much.