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Topic: Ferrites with USB (Read 1534 times) previous topic - next topic

Ferrites with USB

I am currently in design of boards for a wireless power project.  The goal, eventually, is to achieve EnergyStar certification (maybe IPC).  Currently I am powering the device from either a DC wall adapter or the micro-B USB connector (no signals are used).
I have not used ferrite beads on either, but I suppose I will need them on at least the USB, as insurance.  The capacitance component of the LC filter derives from the fact that the power and ground rails are the floods of the top and bottom planes, so the board substrate is a nice big fat capacitor.  The question is, is that good enough for most emission testing, or will I need to redesign for a purpose-made bypass cap as well?  Board real-estate is very critical right now, which is why I have initially skipped the ferrite beads.

Re: Ferrites with USB

Reply #1
nice big fat capacitor

Hm.... A 10x10 cm FR4 board of 1.6 mm thickness has about 250 pf capacitance.
So big in size, yes. Big in capacitance, not really.

Re: Ferrites with USB

Reply #2
The signal and primary analog traces are expected to handle 1.5A, so I was anticipating using 2oz copper on both sides.  I take your point, however, after noodles and googles, that the capacitance is not expected to exceed 67pF, while the capacitance of USB power circuits tends to 1uF or thereabouts.  So in addition to one (or two) ferrite beads I need to add another cap to an already overcrowded board.  Urf.