Visiting contactless charger again.

[TT_Man]

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?

 

[James Arthur]

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?

With a blocking oscillator, usually. Simple, and cheap. You might even
find diagrams on the web.

Cheers,
James Arthur