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Royer induction heater

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Gabriel35

Wed Feb 29 2012, 01:01PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Lowering the input voltage, you will consequently lower the power output.

I think it won't cease the process, but voltages lower than 10v can make the gate drive signals too weak.

When I was imagining how a Series ressonant circuit would be, I was imagining something like this:

Crazy idea?
Impossible?

Davebmx

Mon Mar 05 2012, 04:17PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40

Gabriel if the voltage was below 10v would there be the chance that this might cause the mosfets to not switch? could this possibly result in the mosfets getting stupidly hot eventhough they are well within tolerance?

m4ge123

Mon Mar 05 2012, 04:39PM

Registered Member #4118 Joined: Mon Oct 03 2011, 04:50PM
Location: MD
Posts: 140

Gabriel, you could do that, but I think you have to leave in the resonant cap on the the primary. Try simulating it.

Gabriel35

Mon Mar 05 2012, 06:18PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

m4ge123 nice idea! I'll try to simulate that, any suggestions on a program to do that?

What does the others think about series ressonant Royer IH?

m4ge123

Mon Mar 05 2012, 06:37PM

Registered Member #4118 Joined: Mon Oct 03 2011, 04:50PM
Location: MD
Posts: 140

Gabriel35

Tue Mar 13 2012, 01:11PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Marko, is there any difference between soldering the two inductor's legs directly to the mosfet's drains or connecting them before the capacitor bank on work coil side?

Isn't there a difference between this two points in waveforms?

I'm asking that because I'm using the two inductor's legs soldered almost directly to the mosfets's drains, but at schematic it shows them connected as the tank side...

I got a misterious cool mosfet failure here, one of them shorted and I don't know why.

My circuit was working properly, and it happened... all the other components were good, just one mosfet failed. (Took my 50A Bridge with it too)

I was using 32vDC (dont sag under this) and it was draining 33Amps from two transformers that I Have here...

Everything was cold...

Marko

Thu Mar 15 2012, 08:39PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys

Well, I think the latch-up failures of Mazzili circuits are pretty common, just not so many people tell about them! As the circuit can enter a "stable" state with one mosfet permanently on it's no surprise at all. I was actually impressed that my circuit never failed in normal use (until I miswired it's power supply and blew one mosfet up)

Over last months I experimented a lot with various modifications of this circuit, attempting to make an oscillator that starts up from linear region so the mosfets don't blow up instantly on turn-on if something goes wrong, but just conduct some current and produce little heat instead.

To stay honest, it was all pretty much a complete fail.

A severe hurdle in acheiving this with mosfets and IGBT's is their severe negative tempco of treshold voltage. I may set the gate bias so that mosfets start oscillating fine but they'll still blow up after they get hot. Trying to compensate this with source resistors failed, they ust seemed to kill the gain too much for reliable oscillation.

Alternative would be measuring the temperature of the mosfets by PTC or NTC thermistor and using feedback from this to control the gate bias, in hopes of preventing a catasthrophic runaway.

Regarding the experiment with BJT's, I'm not even sure why is it failing. I ideated using a big ferrite toroid stepdown transformer feeding a tank circuit on secondary side, and I think the parasitics of the transformer are hampering the circuit. The concept works great with small mazzili's and ATX transformers, though.

Or, the gain of the BJT's may just be too low for reliable oscillations to start up. They are pretty ancient technology anyway!

Marko

Gabriel35

Fri Mar 16 2012, 03:19AM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Interesting, I'm keeping up the progress with Royer too.
Experiencing with diffrent types os tank capacitors, form 1uF to 6uF, to see what happens.

Now I'm using a supply that sags on 38v. and its pretty nice to play with the Induction Heater now.

What bridge rectifier are you using Marko?

I already got two KBPC3510s (35A) Killed (due to high current i think)

And when I was running 6uF on the tank, with a KBPC5010 (50A), The bridge and One mosfet fried, (I Think it's because the high capacitance...)

It really drops in current consumption when the capacitance is a bit high.

If we are going to try a different design what would it be?
Current fed Oscillator like Kim Ladha's one or Voltage Fed like Uzzor's one?

Or let's keep with royer? I think a current control circuit for It would be simply awesome!

Nice to see the topic alive.

Linas

Tue Mar 20 2012, 05:34PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Is it good idea to use these capacitors for high power induction heater ?
i need minimum power of 10kw and i planing to use 4 of them so(20KW max)
it should be around 350Kvar for 4 of them, but does they can hold 50KHz ?

Gabriel35

Tue Mar 20 2012, 06:20PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

I think They will work fine if they are Mica or Polypropilene types...

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