# Induction Heater Project

Discussion in 'The Projects Forum' started by Pidgeydoyle, Mar 2, 2013.

1. ### Pidgeydoyle Thread Starter New Member

Dec 4, 2012
12
1
Hi everyone!

I am currently building an induction heater (fairly low power ≈200W). I am using the half bridge topology in my circuit of this general layout
http://domesticinductionheater.files.wordpress.com/2012/10/halfbr.png

my question is on driving my IGBTs, here they are:
http://docs-europe.electrocomponents.com/webdocs/10e2/0900766b810e25bf.pdf

I want to drive the igbts at 30khz but im having trouble choosing a driver, i know i need my driver to be isolated from the collector voltage but i dont know why.
If possible i would like to control my driver with a an Arduino which i have generating the switching signal already
but if someone could suggest an oscillator circuit that would be really helpful too!!

thanks!

Pidgey

Dec 4, 2012
12
1
3. ### neonjohn New Member

Mar 3, 2013
6
2
The IGBT drive is between the emitter and gate. on the upper transistor of the half H, the collector rises to the DC bus supply voltage when it turns on. The gate still requires (typ) 15 volts between the emitter and gate. That implies two things:

1) The gate voltage has to be 15 volts higher than the DC bus voltage. Typical high side gate drives do that with charge pumps that gather 15 volts on a supply capacitor when the lower transistor is on and then carries that to the high side gate driver to operate it.

2) the upper gate driver floats between near ground when the lower transistor is on and near the supply bus when the upper transistor is on. This means that something (high side gate driver, transformer, etc) has to receive the drive from the frequency source and translate it into the proper gate drive even when the high side transistor collector is near the DC bus voltage. As long as your DC bus is under 600 volts, there are literally dozens of high side gate drivers that will do the job.

Pidgey
You might want to take a look at this circuit

http://www.neon-john.com/Induction/heater.htm

This design uses the venerable TL494 switchmode regulator as a precise square wave generator. For the half-H to work properly, the square wave should be as symmetrical as possible.

The IRS21844 is a combination low and high side driver. It uses a charge pump (d3 and C3) to develop the high side gate drive voltage. it also generates the dead-time* necessary to prevent shoot-through. R8 sets the length of the dead time.

* dead time - the time between when the top transistor turns off and the bottom one turns on. If there were no dead time, there would be an instant when both transistors were on at once, presenting a short to the DC bus. This usually doesn't burn out the transistors - it is too brief - but it does cause excessive transistor heating and lots of noise on the DC bus.

One thing about the schematic. Experience has show that the gate resistor values are too high. Change the 2 27 ohm resistors to one 5 ohm 2 watt resistor.

This is a simple manually tuned heater. If you get the Q of your tank high enough by building a capacitor with a low enough ESR, (orange drops aren't the best for this. CDE makes some much better capacitors designed for the purpose), you'll find that you're always chasing the resonance peak as the tank components heat up and if you're heating steel, as it passes through the Curie temperature.

After you get tire of that you might be interested in this:

http://www.inductionheatertutorial.com/

Jonathan designed an auto-tune heater that uses an Arduino as the control element.

John DeArmond
fluxeon.com

Pidgeydoyle likes this.
4. ### Pidgeydoyle Thread Starter New Member

Dec 4, 2012
12
1
Thanks for the info neonjohn you've given my lots of food for thought!

do you think i could replace the tl494 with my Arduino and connect my digital out pin to the IN of the IRS21844 as they could potentially perform the same function (albeit the Arduino will probably be less efficient)?

Perhaps i would have to fix the operating frequency and forget about power output control?
thanks a million!

Regards
pidgeydoyle.

5. ### neonjohn New Member

Mar 3, 2013
6
2
You're welcome.

That might be a problem. You need to raise your operating frequency goal to at least 100kHz to get decent coupling to your work. I don't know if the Arduino will go that high.

My memory's foggy at just what problems Jonathan had trying to use the Arduino directly but I seem to recall it was not enough resolution. If you look at his circuit you see that he uses PWM output integrated to form a DC voltage and that is applied to a VCO.

You won't be able to get away with a fixed frequency. With any decent capacitors at all, the tank Q will be so high that it will drift almost completely out of resonance as it heats up. You have to realize that even at the modest power goal you have, there will be a hundred amps or so circulating in the tank. The copper tubing will get hot, expand, change dimensions and therefore inductance.

You will also change the resonant frequency when you introduce work (what you want to heat) into the coil. Fairly significantly, in the case of magnetic material.

On the heater described in that article I referred you to, the frequency control is a 10 turn pot and even with that resolution, tuning to compensate for heat-up and inserting work is just a fraction of a turn. The tuning is that sharp.

I strongly recommend going with the TL494. It costs only a buck or two. If you like you can replace the pot with an Arduino output, though that would simply complicate tuning.

A note on power control. Many commercial heaters vary the power by moving the driving frequency slightly off the resonant point. That obviously works but is quite touchy without closed loop feedback. If you go that route, always go to the high side of resonance. Same with tuning. Always come in from the high side. Going to the low side is very stressful on the transistors. I'm not sure exactly why but it know that it's a fact.

A much better method of power control is to vary the DC bus. That is easily done with a buck converter and that converter can be driven by the Arduino. That's the way I do it in my commercial machines. I use the Atmel AT90PWM316 that contains 3 hardware pulse width modulators. The only problem for hobbyists is that it comes in a fairly small surface mount package. If you did decide to use that processor (that could also drive the gate drivers), I can give you some code to make it work.

John DeArmond
Fluxeon.com

6. ### Pidgeydoyle Thread Starter New Member

Dec 4, 2012
12
1
Thanks Again,

My plan is to operate my half bridge at comparatively low switching frequencies. The plot below is one i did for a mains voltage but the frequencies should be the same.
http://domesticinductionheater.files.wordpress.com/2013/02/r2.png?w=584&h=343
I hope to operate at approx. 30khz for a high power situation, i had intended to vary the switching frequency for medium (45khz) and Low (65khz) situations but that my prove too difficult.

I dont think this will be as much of an issue for me, my induction heater is for domestic induction heating as opposed to smelting or similar activities.
The coil i'm using is a mass produced product for domestic induction cookers so i would say the inductance will remain fairly stable.
http://domesticinductionheater.files.wordpress.com/2013/03/img_0812.jpg
But i could of course be very wrong!

This is my plan in a best case scenario. starting off at 65khz and moving towards the resonant frequency in ZVS mode.

Regards
Pidgeydoyle

7. ### neonjohn New Member

Mar 3, 2013
6
2
I have attached a very good application note from Fairchild on induction cooking. You hopefully will find it very useful. You might want to take a look at the quasi-resonant design that they mention in the note. Like the Royer, it is inherently tuned to the resonant frequency. Last year I designed a 1kW heater using a quasi-resonant SMPS chip. It worked quite well, though the chip wasn't the best one for the application.

You might also consider getting one of these to poke around inside

http://www.amazon.com/Max-Burton-6000-1800-Watt-Induction/dp/B000MVN1M6

I have 2 of these in my kitchen and no longer use my electric range. Despite the price, this is a sophisticated design. Every time it detects a cooking utensil it sweeps across a spectrum from about 35 to 50 kHz looking for the optimum operating point. It actually makes two passes. Because it doesn't use a filter cap on the DC bus, one can hear the sub-harmonics as the oscillators starts and stops 120 times a second.

I hope you find induction heater design as fascinating as I have. That's all I do now is design induction heaters. It's like a beast that takes over your life

John DeArmond
Fluxeon.com

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8. ### geoffers Active Member

Oct 25, 2010
244
6
Hi,
How are you getting on with your induction heater? I'm looking for a way to efficiantly warm milk for my calf feeder project. I've been looking at different types of induction water heaters but think I maybe better off making my own? Do you know if stainless tube would work? I only need to warm it from 1-2 deg c up to 37c so don't need huge power. The flow rate is low too about 0.1 l/m.
Cheers Geoff