what is the use of ultrafast diodes in wireless battery charger ckt
If you used "normal" diodes you would find that the oscillator would be a very poor performer - frequency would be massively lower and efficiency would be terrible. It comes down to the parameter of a diode known as "reverse recovery time" (\$t_{rr}\$).

If you looked at a "normal" diode like the 1N400x series you would find that it's \$t_{rr}\$ is in the order of 30 us.

\$t_{rr}\$ basically tells you that for 30 us after the polarity of the voltage across the device has changed from forward conducting to reverse biased, the diode will not instantly block but takes 30 us. LINK: -


  When switching from the conducting to the blocking state, a diode or
  rectifier has stored charge that must first be discharged before the
  diode blocks reverse current. This discharge takes a finite amount of
  time known as the Reverse Recovery Time, or trr. During this time,
  diode current may flow in the reverse direction