Transition loss

Transition loss - a component of the total power loss in a transformer, caused during an interval when the currents are commutated from one winding to another.

In flyback transformers the transition loss occurs over a very short interval, during which the magnetic flux is relatively constant. But the current sustaining the field moves from the primary to the secondary winding. Such losses might be ten times greater than other eddy-current mechanisms. This can be alleviated by designing the flyback transformer such that the current ripple is reduced.

Animation of an FEM simulation
The clip shows an animation of a 2D FEM simulation of transition loss in a 1:1 flyback transformer. The primary winding (black contour) is closer to the central limb of the core. The current is ramped to its peak value, at which point the secondary winding (blue contour) takes over. At the instant of take over the distribution of magnetic field strength changes significantly, even though the ampere-turns remain the same.

The primary winding is engulfed with the fast-changing strong field, which increases the eddy current loss in the conductor of the winding. http://www.youtube.com/watch?v=RZxx3OdwPsc