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OMG Induction Heater

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Tonskulus

Tue Jan 29 2008, 07:17PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

I'm still having ringing problems at inverter output.
Inverter output voltage & current:
http://www.elisanet.fi/tonskulus/ssih/output.jpg
Inverter itself:
http://www.elisanet.fi/tonskulus/ssih/inverter.jpg
Gate drive signal:
http://www.elisanet.fi/tonskulus/ssih/gatedrive.jpg
There seems to be ghost signal when DC is applied to the igbt stage. it can be adjusted by pot beetween triggers output/input. It looks like there is two signals, one from feedback ct and another one is oscillator signal from trigger itself.

Edit: I made some tests with 230VAC input. Results were impressive indeed and IGBT is still ok.
video - http://www.youtube.com/watch?v=3WWR4bdRjbU

Kizmo

Wed Jan 30 2008, 09:35PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I just took one half of my SGTC 75nF @ 24kV MMC (72x CDE 942C20P15K) and i will make heavy duty copper busbars to connect them in parallel. It would have been great idea NOT to solder them together when i made it

Those caps have proven to be almost indestructible in heavy spark-gap and drsstc duty. But will they cope with some induction heating..

Bank i planned would be like this:

- 36 capacitors
- 5.4uF @ 2000V
- 486A RMS @ 100kHz @ +70C (13.5A each)
- 15552A Peak current (432A each)

Now i think that would do some serious damage to poor bolts and nuts

Tonskulus

Fri Feb 01 2008, 02:33PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

I made new video of my "OMG" induction heater :)
http://www.youtube.com/watch?v=iDg4pXpS-Xk

Experimentonomen

Sat Feb 02 2008, 11:49PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

I fired up my version today and it worked on more or less first try.

Pics of test setup:

R.I.P matching inductor, i was running 50V input and around 7-10A current draw, all of a sudden all waveforms went screwed, power draw went beyond 10A, room lights dimmed and igbt's died, then i noticed L-Match had melted. However ther work piece got to a orange/yellow color.

kim_ladha

Sun Feb 03 2008, 12:00AM

Registered Member #1217 Joined: Mon Jan 07 2008, 11:46AM
Location: Leicester, UK
Posts: 11

Hi All,

Sorry been away for a few days and questions have mounted up.

My gate drive waveforms are fine, The ir2184s get hot to touch but you can touch them. The igbt failure mode is unusual. The bottom of the igbts show melting in a patch on the device rear. This suggests this bit got hotter than the rest. I think that the thermal runaway I was speaking about occurs when part of the silicon die is hotter than the rest. This exhibits a lower on voltage and tries to take more of the current. The end result is that this bit fuses. The heasink is relatively cold when failure occurs (40 C). My igbt half bridge current is 40 amps rms at 120 khz (on last failure)- the datasheet says 20 amps at 100khz max- I think they are on the edge and hence die at relatively low temperatures.

Looks like people are getting things working though. I will post any improvements I make to improve reliability.

HFsstc-freak- Same thing happened to my matching inductor! Luckily I smelled the melted plastic fast enough! ou need to make the inductor large and flat to get rid of the heat better. Watch out for its magnetic field, it can induction heat things too!

Good luck to all... still waiting for someone to do a 10kw version :)

Karim

Experimentonomen

Sun Feb 03 2008, 12:18AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

I also have an advice: Keep the feedback coil away from the work coil!!

I ended up putting my C-core on the inverter side of the tank cap to get clean waveforms and ability to run high current in igbt's without failures.

My igbt's died becase of too dirty stuff going through them, not by overheating.

kim_ladha

Sun Feb 03 2008, 05:34PM

Registered Member #1217 Joined: Mon Jan 07 2008, 11:46AM
Location: Leicester, UK
Posts: 11

Hi, as you know I had an unexpected 'cold' igbt failure under normal opperation. I fixed this and repeated the conditions under a more clinical environment with lots of probes.... I have found the cause!

When the high side transistor switches off, the current free wheels down through the low side igbt diode. The diode is fast but has a poor transfer characteristic. It allows the output to drop below the 0v line by a staggering 4v. This gets worse when the current is triangular. The current through my boot strap diode and the high side zenner is the only thing clamping it during this transistion. They get hot and fail- this causes everything else to break.

THE FIX- put a 47 ohm resistor in series with the bootstap diode. This lets the free wheel diode do its jub and stops all the problems... for now.

Next I tried Steve Wards current divider idea using a small poly propylene cap. The feed back of the current has a high BW and so there is a little more hash on the oscillator input but IT WORKS. This greatly simplifies the design and I would recommend it for new builds.

I reatached my schematic with the changes

1202060083 1217 FT36475 Inductionheaterschematic

Kizmo

Sun Feb 03 2008, 05:50PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Looks great! I just ordered some parts and i will try out that schematic (that would be my first proper IH, i have done only few ZVS versions) ...so prepare for stupid questions

As tank capacitor there will be bunch of these mounted on tightest possible arrangement to get low inductance:

Experimentonomen

Sun Feb 03 2008, 06:57PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

And here is my latest results:

kim_ladha

Sun Feb 03 2008, 07:26PM

Registered Member #1217 Joined: Mon Jan 07 2008, 11:46AM
Location: Leicester, UK
Posts: 11

HFsstc-freak-

The magnetic field provides a constriction force pushing on the outside of the washer. If you heat it up slowly you can get really cool effects as the melted parts of the washer flow inwards.

Like this one:

How much power are you using?

Regards
Karim

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