Visiting contactless charger again.

I've investigated a toothbrush job and it seems straight forward... two coils linked via a ferrite slug. Interestingly, the secondary via a CT coil giving 100V pk - pk at 20 kHz. Then it's rectified and fed into a DC/DC buck regulator. What I can't figure out ( because it's heavily potted) is the primary side...... It would appear that it is similar to one of those cheap mains derived 5V supplies for driving micros on light dimmers etc( Via an X rated cap). But how is the 20 kHz produced?