On Thu, 29 Nov 2012 09:31:06 -0800 (PST), put
finger to keyboard and composed:

"Wouldn't increasing the capacitance from 0.68uF to 1uF result in a 50%
increase in cooking energy? "

No, that is a common misconception in the business. Do you remember the formula for capacitive reactance ? Apply it and just guess the frequency is over an octave above the sonic range.

The fact is that those caps are not being used as reactive components like in a tuned system, they are being used as coupling caps.

In that circuit they are effectively in parallel. Ground and the power supply rail are effectively at the same AC potential, so it's not 0.68, it's aready 1.36 uF. That is almost a piece of wire at 20 Khz. Almost, but we are dealing with a quite higher frequency here.

Here is an induction cooker design where the capacitors and coil are
said to form a "resonant tank":
http://beaversource.oregonstate.edu/...ki/IGBT%2BCoil

The second IGBT's body diode allows the coil current to decay
gracefully when the first IGBT switches off. I forgot about that.

- Franc Zabkar
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