All about ultrafast diodes app note from IXYS. Link here (PDF)
During the last 10 years, power supply topology has undergone a basic change. Power supplies of all kinds are now constructed so that heavy and bulky 50/60 Hz mains transformers are no longer necessary. These transformers represented the major part of volume and weight of a traditional power supply. Today they have been replaced with smaller and lighter transfomers, whose core materials now consist of sintered ferrites instead of iron laminations and which can operate up to 250 kHz. For the same power rating, high frequency operation significantly reduces the weight and volume of the transformer. This development has been significantly influenced by new, fast switching power transistors, such as MOSFETS or IGBTs, working at high blocking voltages (VCES > 600 V).
Apart from the characteristics of the transitor switches, the on-state and dynamic characteristics of the free wheeling diodes have a significant impact on the power loss, the efficiency and the degree of safety in operation of the whole equipment. They also play a decisive role when it comes to increasing the efficiency of a SMPS and to reduce the losses of an inverter, which clearly mandates that ultrafast diodes be used. The ultrafast diodes described here embrace all characteristics of modern epitaxial diodes, such as soft recovery, low reverse recovery current IRM with short reverse recovery times.