Interesting. Normally you would think they would increase leakage inductance which would be undesirable. Transformers are usually a compromise between tighter coupling resulting in higher inter-winding capacitance but giving less leakage inductance -OR- less coupling/capacitance but higher leakage inductance. You get one or the other, but never the ideal case low leakage inductance and low inter-winding capacitance and high coupling all together. Toroidal transformers have much tighter primary-to-secondary coupling which gives you low leakage inductance at the cost of high inter-winding capacitance.

Now, on some high power designs it is sometimes necessary to put suppression ferrites on the switching devices to suppress undesirable high-frequency oscillations. Such lossy ferrites dampen the ringing.

My guess is that the ferrites you see on the leads are ferrites to suppress HF ringing. This ringing would be seen by the power devices due to the higher parasitic inter-winding capacitance of the transformer. They just happen to be located on the leads of the transformer rather than on the leads of the switching devices. In the design you are looking at the engineer probably found some more leakage inductance was OK if the ringing was suppressed. It may also be that adding the ferrites lowered the frequency of the ring and was a compromise for EMI purposes where you have more emissions margin at lower frequencies.

SnubberRoger,
Yep. I think Comcokid's answer is right on; Big noise suppression beads. hmmm....He might be one of these geniuses who just see's obvious things that many of us don't.

Darrell Hambley P.E.
SENTEK Engineering, LLC