Hi,
There is no need to couple the inductors L3 and L6.
Cooper Bussmann make a Versa-pac series of transformers that are suitable for this design.
Regards,
Jay_Diddy_B
capacitace
No, It isn't necessary, and may cause more problems than it's worth, as seen by Mjjz. The idea, however, is that the coupling largely counteracts the ripple present at the center tap on the secondary, requiring less filter capacitance to meet a given ripple spec.
For a practical USB charger, this circuit has some serious drawbacks, notably a lack of regulation. I imagine it would be more useful in a different application.
...
even some MP3 players might need isolated supply if you connect the headphone output of the player to the car audio's aux in.
That's exactly the problem that occured. Turns out the audio amp used by the Sansa E200 is single supply (i.e. headphone ground is vcc/2 above USB ground). Notably, this is not an issue in more sensibly designed portable products, such as the iPhone, which internally generates a split supply just for the audio amp. So, I suppose the charger wasn't really to blame.