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Induction heater topologies

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Oct 28, 2010
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Hi!. It's quite common to see the LCL (series L, parallel C and L) topology after the output transistors in DIY literature. This calls for few turns on the working inductor, of high diameter water cooled copper tubing, with parallel capacitors.
Instead, I designed my 50kHz 1kW prototype heater with a simple series LC configuration, this way the inductor has more turns of thinner wire thus having less skin effect, so it has higher Q, and does away with losses on the first (matching) inductor.
Can anybody tell me why this simpler topology is almost never used?.

I haven't experimented with induction heating.

A search turns up this lengthy webpage examining various topologies. Looks like it might answer your question.

The additional coil might be needed to reduce spikes caused by switching large currents through the main coil.

Guy named Richie Burnett. He made an induction coil powerful enough to make nuts and bolts glow white hot.

High Frequency Induction Heating
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BradtheRad, sorry, I had long forgotten to thank you for your hint. Burnett's site, though I didn't find a 100% answer to my subject there, it does have very enlightening info on these circuits.
Thank you again!

The purpose of the extra components is to allow you some freedom in choosing the impedance seen by the FETs. With just a series LC circuit, you will just see the effective ESR of the coil, which may be impractically high or low for the driver.
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Chuckey and Mtwieg: you're both right!. I don't mean to be obstinate on my reasoning, I don't like to walk opposite to the rest of the world, just need to convince myself of the cons on my approach. It is: my freedom to make the transistors see an adequate load impedance is adjusting the number of turns on the work coil. What could be simpler?. It worked fine in a 50kHz 1kW prototype, no cooling necessary. Yes, N was something messy (29 turns in 3 layers, wire didn't self-support) but not impractical.
Could it be that water cooling becomes mandatory at higher powers? (pipes with small OD wouldn't exist). Could it be the additional series inductor helps comply with harmonic radiation regulations?. Maybe I'd rather read a book on induction heating basics.
Thanks all of you, and have a bright new year!.
P.D. Yes, there IS a danger on the simple LC: series resistance at resonance is very low without a workpiece, but I just shift the driver's frequency higher when this situation is detected, and no transistors blew.
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Yes, you can just change the number of turns to vary the ESR of the load. If you can arrive at a reasonable impedance that way, then it should be okay. I would try to avoid stacking multiple layers, you'll start getting proximity effect losses and your Q will degrade.

I don't think most induction heating cookers have water cooling, most probably use litz wire and some forced air. Certainly at some point water cooling becomes necessary, not sure where though. Blowing air through copper tube is pretty effective as well.

I too doubt if domestic oven induction elements are water cooled. I have seen a bit about metal hardening and tempering using VLF where the metal can become white hot, so the solenoid copper coil is normally water cooled. WILL this system will work for aluminium/copper/silver/gold as the circulating currents do not cause any heat because they are good electrical conductors?. Or do you use a higher frequency where the losses are higher?

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