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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Hey Tonskulus, I recently made a schmitt trigger induction heater much like your design, except I successfully made it into a series resonant design instead! It works decently on 33 volts input. (I had a small tank circuit, .3 uf and a very small coil, it was an experiment)
But, it works, and it's ZCS, so very low losses in the switches! But, I had issues with grounding, as well as keeping the tank oscillating with a larger load.
Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
Hi Marko, this is a wonderful project. I have built a scaled up version of the one you have shown here with 4 IRFP260 mosfets using them in pairs connected in parallel. The tank circuit uses 24 .22uf capacitors for a total capacitance of 5.2 uf and the resonant frequency is about 40 KHZ.
So far I have taken it up to 2500 Watts input power using 70 volts and heating the pipe shown in the picture. The load depends greatly on the size of the object being heated. Running at 70 volts the capacitors get just a little bit warm.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi sir,
Well, that looks like an epic-enough scale-up. Have you had any unreliabilities/mosfet deaths upon startup and during runs? Your work coil also looks far higher inductane than mine. How much power can you push into small items like coins and such?
The obvious drawback of this method is the requirement of huge iron step-down transformer for power supply... I've been thinking about a possibility of using an arc welder for this job (the voltage would be even less stable this way though).
Hence I recommended the circuit as a newbie project where a rewound mot could serve as a cheap power supply with loads of fun to be had out of it still :)
If one was to scale this up to a mains powered design, one would have to use 1200V IGBT's along with a ferrite stepdown transformer, and think out ways to prevent the application from blowing up (some sort of overcurrent protection is mandatory). Drive could be classic mazzili style from a separate low voltage supply, or even from a feedback winding on the main transformer. I've tried this method only on low power so far.
Marko
PS. Nah - where did you get your 90nF mica caps? I'm very interested in them.
Registered Member #1034
Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154
Marko, I have burnt up quite a few mosfets. Some times too much of a load will cause one to short out and I have experimented with different mosfets and some variations to the circuit and have lost some that way.
With the work coil I am using now small objects heat up slowly and they put very little load on the system. I am planning to make another smaller work coil for smaller objects. A smaller coil will make the frequency go up so it will be interesting to see how that works out.
The transformer I am using is kind of over kill. It is so heavy it is all I can do to lift it. I got it out of an old UPS unit they scraped out where I work. It has lots of different taps so I can get about any voltage I would need.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi,
Have you thought of implementing an overcurrent protection of some sort? I'm thinking of using something as simple as a comparator + NE555 monostable. At first I thought about using a CT with mosfet sources wired through it in reverse (so it effectively sees AC). But after a while I realized this may not be the best solution for relatively slow current rise as it may occur in fault conditions of this circuit.
Hence I think it's by far most bulletproof to use a shunt, made up of a bunch of low-value resistors in parallel. This is especially practical for the high voltage igbt version of the circuit where currents aren't as high, for the IRFP260 circuit you'd have either to use a very low value shunt (pulled from a DMM or UPS's) or opt for a hall sensor (not very in spirit of this project).
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Marko wrote ...
Hi,
Have you thought of implementing an overcurrent protection of some sort? I'm thinking of using something as simple as a comparator + NE555 monostable. At first I thought about using a CT with mosfet sources wired through it in reverse (so it effectively sees AC). But after a while I realized this may not be the best solution for relatively slow current rise as it may occur in fault conditions of this circuit.
Hence I think it's by far most bulletproof to use a shunt, made up of a bunch of low-value resistors in parallel. This is especially practical for the high voltage igbt version of the circuit where currents aren't as high, for the IRFP260 circuit you'd have either to use a very low value shunt (pulled from a DMM or UPS's) or opt for a hall sensor (not very in spirit of this project).
Marko
Why not use the same way that uzzors did with his 50 watt ZVS cap charger? Just wire the comparator to pull the gates low once OC is reached.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
[/quote1319657294]
Why not use the same way that uzzors did with his 50 watt ZVS cap charger? Just wire the comparator to pull the gates low once OC is reached.
[/quote1319658060]
Hi,
Well, that basically is the idea, only with some time delay introduced to keep the whole thing of oscillating too fast (and blowing the igbt's again) in case of persistent fault condition.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hey guys,
I just tried the offline version of the circuit, with 1200V igbt's and using two ATX PSU transformers for isolation.. In short, the conclusion is that diode feedback is not going to work for this. UF 4007's blew at about 100V in and when I resorted to higher voltage and power diodes the overall voltage drop got badly over treshold voltage of the igbt's (which is, to make things worse, a badly negative-tempco parameter).
I have tried to get the circuit going from secondary side feedback, but failed miserably at it - there's no theoretical reason why shouldn't it work, provided that transformers are "ideal enough", but apparently this wasn't the case here as more low-frequency response is required.
One could use a dedicated driver circuit, but I think that would sort of defeat the idea of this topology in favor of using bridges (BTW, I think a current fed H bridge is a very attractive topology to explore for an induction heater, and other things, but I'm not sure if I'll be building something that ambitious any time soon.
One idea I'd like to test out is capacitively divided feedback. SInce mosfet gate is a capacitor basically just seriesing a small capacitor and connecting it to drain of opposing mosfet should do the job actually - ofcourse some zeners and balancing resistors would be highly advisable for HV operation.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi -
Well, my OCD circuit was actually quite sloppily made, as I hacked a "monostable" out of a comparator instead of NE555 and it turned out to be a messed up approach... a mistake not worth killing igbt's.
I'll be back with this in a month or so, and I wondered if anyone would play with this on their own.
PS. there's an error with the OCD "output pin", I hope you can figure that out... also I didn't draw an isolation transformer in any of schematics but it can be freely inserted between the inverter and the work circuit.
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Royer induction heater
