FWIW, I don't see shortbus= as a 'complainer', he is more like one of a number of spectators, watching (unfortunately) someone flailing around in a pool. Life rings are being tossed in from every direction, but the swimmer appears to not want to grab any of them.
That's my view on this, and a number of other threads.
I think the reference to a "real induction heater" was just a bad choice of words.
You have definitely constructed real and very functional induction heaters - impressively so, from my point of view!
The debate is only about how the FETs are driven. You know the frequency it's running at & stated it earlier, about 60 KHz.
If you drove the FET gates with an external oscillator at that frequency and added proper gate drivers, as you would use for the lower half of an H-Bridge and with non-overlap timing so each FET is allowed time to turn off before the opposite one turns, the overall efficiency should be much higher and heat dissipation much lower.
That would also avoid all the problems with the choke setup, as that should then be far less critical, if it's needed at all.
As it is, each FET only turns off _after_ the opposite one starts to turn on, so they are fighting each other and producing high current spikes, which require the choke to keep under control. There is a lot of wasted power in that configuration.
In principle, you would be converting it to a push-pull switch mode PSU like this -
But without the transformer secondary or any feedback, as the heating load itself becomes the secondary of the coil/transformer when that is put in the power coil..
Edit - oops.. wrong thread, I just saw the tail end about the "real" comments...
You can get twice the overall voltage on the power coil with a centre tap and two power FETs, as you already have.
It's a lot simpler than the full H bridge and less to go wrong.
Why are you changing to IGBTs for this design?
He may be choosing IGBTs instead of MOSFETs simply due to power dissipation issues at high currents, just like DRSSTC builders choose IGBTs over MOSFETs. The power dissipation of MOSFETs increases exponentially as current increases due to its Rdson (P = I^2 * R), whereas it only increases linearly with current due to the forward voltage drop of the diode in the IGBTs (P = I * V). For this reason, above certain D-S or C-E currents, an IGBT is a more ideal solution than a MOSFET as it will waste less energy in the form of heat.
The bootstrap caps are probably too small; another data sheet I found says they must be greater than ten times the FET gate capacitance, which is 4200pF according to the IRFP460 data sheet.
That makes the minimum 42nF so probably 47nF (or bigger) using standard values.
I explained the "high impedance" supply already - it's supposed to be a 5mA current feed.
The simplest option is a 30K resistor from +170V to pin 1.
It will be dissipating around 1W; I'd use three 10K 0.5W or 0.6W in series.
Is it possible for bootstrap capacitors value to be too large? I can try 47nf how will I know if this is the perfect value? If I go bigger how will I know it the value is too large?
New circuit drawing.
View attachment 120478
Why are you using such a high gate resistor, 1K ! Some where around 50 ohms is normal. The 1K(or higher) would be normal for the Gate to Source resistors that you don't have.
If you are considering using this to drive the same coil as in the previous design, I'd advise against it. It is not suitable to drive a tuned coil.
Also remember the change in power is proportional to change in voltage squared; ten times the voltage means 100 times more power and everything needs to be built based on that, plus plenty of excess capacity in case of miscalculations.
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