LT3825 Flyback Transformer Inductance - Eval Schematic

Hello Julian, There are multiple solutions for a Flyback. The best advise is on page 15 of the the LT3825 Data Sheet which suggests "A simple spreadsheet program is useful for looking at trade-offs." For a transformer, N * A * B = L* I. In this case, 170uH would require 5 to 6 times more turns (N) or core area (A). Everyone likes (their own) rules of thumb. For a Flyback, I check the ratio of the peak secondary current to the output current. In this case, with 20uH, this is about 15A / 5A ( = 3:1) which isn't too high. Good Luck.

Hello Julian, There are multiple solutions for a Flyback. The best advise is on page 15 of the the LT3825 Data Sheet which suggests "A simple spreadsheet program is useful for looking at trade-offs." For a transformer, N * A * B = L* I. In this case, 170uH would require 5 to 6 times more turns (N) or core area (A). Everyone likes (their own) rules of thumb. For a Flyback, I check the ratio of the peak secondary current to the output current. In this case, with 20uH, this is about 15A / 5A ( = 3:1) which isn't too high. Good Luck.

Hi Kurk,

Thanks for your reply. On your advice I have started to build a spreadsheet for all the calculations and it does shed light on things. I have also downloaded a complete LTSPICE simulation and played around with that looking at transformer currents etc. Really useful for understanding these things and remarkably accurate.

For me it's a new area, I'm very happy with boost and buck converters, I just need to get more experience and as you say get some rules of thumb for the isolated converters.

Thanks

Julian