----- Original Message ----- From: "Michael Watterson" To: "Microcontroller discussion list - Public." Sent: Sunday, May 16, 2010 12:37 PM Subject: Re: [EE] SMD power choke > >>> >From what I've seen (which is not a great deal :-)) self resonant >>> frequencies can be a lot different for the same rated inductance >>> (between >>> different inductors). I'm in the process of looking for an SMD power >>> choke, >>> around 100uH, and the rated maximum SRFs seem to vary between 5 and 12 >>> MHz, >>> a bit lower than I expected. I suppose unless you take this kind of >>> thing >>> into account, you could up with performance quite a bit different from >>> that >>> expected in an ideal situation. > Look at the current rating and DC resistance. Pick a current rating > somewhat higher than your peak current. > > In some applications a wire through a ferrite bead may be in series with > the 100uH "choke". Possibly even with a capacitor in the middle to ground. > > Is the choke to prevent ingress or egress of RF? > What is fundamental frequency? > What sort of waveforms? > > RF filtering is a complex subject. It's application dependent and system > dependent as to the significance of the SRF. > It's part of a LC filter on the output of the DC to DC converter on my isolation board. Output is 5V with an approx 100mV ripple (not sure what waveform) at around 50 KHz, 400mA peak current. I was thinking a low resistance (<0.5 ohm)100uH choke with a 10uF low ESR cap. The previous DC to DC I was looking at (with a 90KHz switching frequency, 100mV ripple) recommended a 47uH inductor with SRF >20MHz, and a 4.7uF cap, to reduce ripple to approx 1mV. The same (or very similar) circuit (dual voltage, 90KHz switching) will be used to supply the +/-5V opamps in my USB scope, so it has to be pretty quiet. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist