Ferrite beads in Quasi-Resonant Switch-Mode Power Supply (QRSMPS) primary?

Question

I am using the Fairchild FSCQ series and I am shocked that nowhere is there any guide for sizing the bead on the drain. The frequency range is 24kHz min. to (looks like) 90kHz.

There is also another one on one of the outputs, but that seems to be straightforward, except, according to them, the corner frequency cannot be too low or it may affect stability. I assume that means it must be above the 24kHz.

At any rate, my intuition tells me that board layout will affect the sizing of Bead 101 in the diagram, and since they have no way of telling how the board will be designed, have left out the value.

Bead L202 on a 140v., 6A load, was sized at 5uH, 3A. Noteworthy is that is referred to as a L202, and not just "Bead" as in 101.

Anyway, the question is, does anyone have any ideas as to how to size Bead 101?

Data Sheet

Answer 1

Judging by the layout in the data sheet (p. 42), Fairchild seems to recommend just any good ol' ferrite bead with a jump wire through it.

Something like this should do the trick:

Image Source

If I were a sloppy manufacturer finding a "BEAD" on the parts list with no further specification, I wouldn't care about the ferrite material or any electrical characteristics - I would slap in the cheapest cheapo I could find (or pull from my junk bin). Some designs tolerate this way of stuffing a board, some don't. Those that don't better have a complete BOM.

The bead's job is to smooth out some sharp edges in the drain voltage, maybe above something like 50 MHz, in any case way above 90 kHz.

Answer 2

L101 along with C107 create a damping circuit which should slow down the switching edges on the primary, reducing radiated emissions and stabilizing the system (i.e. removing ringing due to overshoot). Its particular value probably isn't that important, so long as it's there. Ad for layout, well, the traces around it are inductances in series with an inductance, so they should be kept short but you don't have to go too crazy over it. (Though standard practice of minimizing switching loop area (e.g. C102-T1-L101-IC101-C102) is advised.)

The ferrite bead L202 is basically an extra filtering element (along with C202) on the 125V output. This extra filtering is likely there due to the high output voltage, which left unfiltered would radiate EMI.

Since it's not a case where you need a certain impedance at a certain frequency, the particular value isn't critical. It's kind of like an inductor analog to a decoupling capacitor in a simple digital circuit; the value is not as important as the fact that it is there.

Also, the 125 V output definitely counts as "high voltage", unlike the other outputs, and may be subject to stricter regulatory requirements.

P.S. I am amused by the bleed-power circuit feeding VCC.

P.P.S. L101 getting marked as simple "BEAD 101" is probably just an error that got past check.