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A conceptual MOSFET self resonant oscillator
This design is a result of small scale experimentation. I originally wanted to use SCRs because of the simplicity of the design. But it turns out that low cost, readily available SCRs do not handle a frequency high enough to heat small crucibles full of metal, nothing above 10KHz. My application induction of heating requires frequencies between 200kHz and 1Mhz. So I turned to MOSFET devices and a self resonant circuit made the most sense.
Please note that this circuit is conceptual only. Please do not email me asking for part numbers and component values. They will not do you any good! I have experimented using small devices, and because the circuit is still under development, you should not take this information as how-to! This page is here only to detail my research, experiments, and observations.
Below is my understanding and explanation of the circuits operation. For those of you who know more about induction heating than I do, please send me your comments!
Waveform in MOSFET self resonant circuit
Heres what I observed in a similar concept circuit on my Phillips 35MHz oscilloscope.
The circuit was running on 12 volts. The top trace was taken at the drain lead on the MOSFET (I was using an IRF510 at this particular moment) which is also the connection between L1 and C2. Basically it represents the charge on C2.
The bottom trace was taken at the connection between C2 and L2. It represents the voltage across L2. Following is an explanation of how this circuit works.
When the circuit is powered up, a charge gradually builds in timing capacitor C1. When it exceeds the reference voltage set by VR1, the op amps output snaps up and closes Q1. This rapid change sets up a ringing in the resonant power stage. The positive half of this ring is conducted by Q1 itself, and the negative half is conducted by Q1s intrinsic diode (gotta love them MOSFETs!). In the oscillogram, note only one complete cycle occurs.
During this resonant state, some other things are happening. Full power supply voltage is drawn across inductor L1 while Q1 is closed. This builds up flux in L1, which then releases a spike of energy into C2 when Q1 opens. This spike is seen on the oscillogram on the top trace as a sloping signal.
Also, while Q1 is closed, the voltage on C1 is drained, and when it drops below the reference set by VR1, the op amps output snaps back low, opening Q1 and letting L1 yank C2 back up to about 1.5 times the power supply voltage. This rise is timed by C1, and the cycle repeats.
What we ultimately want to achieve is a high voltage, high amperage, high frequency alternating current (ringing) in L2, which is the work coil. This will induce currents in a mass of metal, heating it and ultimately melting it. The industry has this principle perfected for industrial use, and my goal is to develop a circuit applicable to home shop use.
Revised concept circuit
NEWS FLASH!! For all of you waiting with bated breath for progress on this page, I have revised my concept circuit. First I will go over the fundamental differences between this and the original concept circuit:
First, the R/C timebase gets its signal from a current sensor rather than a voltage sensor. This is the main difference. Resistor R1 is a very low resistance, in the milliohms, like an ammeter shunt. 30-50 amps across this resistor will only draw a voltage drop around 1 volt. The negative supply to the op amps will probably be -5 volts, to give them a good operating margin.
This current sensing method to charge and discharge timing capacitor C1 means that the oscillating frequency of the circuit is proportional to (a) the inductance of L1 and L2, and (b) the resistive load presented to inductor L2.
The frequency (read: power draw) can be tuned by adjusting variable resistor VR. Moving the wiper on VR to the right will cause the frequency to be higher, and power draw to be less. Moving to the left will cause Q1s on-state to be longer, making a lower frequency, and a higher power draw. R3 and R4 simply set the upper and lower limits of the "gain" of the Sensing Preamp stage of the circuit. These values are not established yet.
This circuit presents a "convenience" over the previous: Because the charging and discharging of RC Timebase capacitor C1 follows the RMS or "area under curve" value of the amperage through (and resulting voltage across) shunt resistor R1, the current draw of the circuit will be constant regardless of the inductance values of L1 and L2. The convenience is the ability to operate the circuit with a variety of inductances and frequencies, and the current regulation (and to some degree, power output) will be automatically regulated by the circuit.
No, I do not have an oscilloscope trace photo yet. Havent gotten that far yet. Thats how new this stuff is. Yes, I have tested a low-power version of the circuit with a scope. I dont know when the next update will be, so please be patient. This schematic may have to be "new" for a while.
On a further note, I have also discovered a good source of "scrounged" inductors. Old TV sets have two good sized ferrite cores. One is from the flyback transformer, and the other is the yoke inductor. My preference is probably going to be the yoke inductor because of its round shape. Toroid inductors, as they are called, are notable for keeping their magnetic fields confined. And at the high power levels and high frequencies present in these circuits, thats a good thing.
High amperage MOSFET for induction heating
A high amperage, high frequency semiconductor is needed for induction heating. This picture is a SOT-227 package, which is a common choice for induction heating power supplies. The particular device I see as the most promising for my application is the IXYS IXFN36N100. This is a 36 amp, 1000 volt MOSFET with fast intrinsic diode. It has a suitably quick rise and fall time for my design. It is availabe from Digi-Key for around $90. The IXFN44N50, rated 44 amp, 500 volt, is $33 and may suffice as a lower cost alternative.
High amperage diode module for power supply
Also needed is a good sized rectifier module to provide DC voltage to the circuit. The best possibilities come packaged in an ADD-A-Pak module, pictured.
The one I have in mind to use is the IRKC71/06, a 70-amp 600 volt standard recovery diode module, made by International Rectifier and available from Newark Electronics for about $30. This module contains two diodes in a common cathode configuration, just perfect for full wave rectification of a three wire, 240v supply. If the IRKC71/06 isnt big enough, the IRKC166/08 may have to do, but its more expensive.
Link to a great website on induction crucibles
Here is a great website that gives a good how-to on replacing the crucible in an induction melting furnace. There are lots of pictures of a small furnace being restored.
The furnace in these pictures appears to be one that would melt a hundred or so pounds of iron. My furnace would be quite a bit smaller than that, I suspect, more like 25 or 30 pounds. More...
SCR based induction heater
The following is a result of my research on the US Patent Website on the subject of induction heating. I went to considerable length studying SCR based designs, but SCR technology is a bit lacking on high frequency devices, above 20kHz. But you can definitely see the similarities between these patent circuits and my design, which uses MOSFETs.
NOTE: These schematics are actual snapshots of U.S. Patents. These are NOT my intellectual property. I simply present them here as an outline of my research of electronic induction heating. Thanks to the Clinton administration, the U.S. Patent Image database is open to the public. You can get there and search for yourself the complete patents here.
Any use of this information is subject to patent law. It is up to you to make certain that your use of any patented design is legal! (In reality, all the patents I reference on this page are from 1974, with the exception of the last one, which is 1981. So they are more than likely expired!)
The schematic above is from patent number 3,786,222. Click the image to see the full page. It is an induction heater that was designed to heat up food or other substances wrapped in foil, by heating the foil itself. It is the actual schematic that we are interested in, note its simplicity! I have scoured the internet for circuits like this, and the patents are where I have found the best information. These are nothing more than induction cooking range schematics, but I believe the principles could be applied to heat treating and melting of metals.
High speed SCR module for induction heating
For an SCR based home shop induction heating power supply, the most likely candidate I could come up with was International Rectifiers IRKHF200-12HJ. This is a 200 amp, 1200 volt, high speed MagnaPak SCR module with recovery diode.
This is an amazing unit that can actually handle its rated amperage at 10kHz! As you can see in the picture, this is a very rugged unit with a thick heatsink base and large terminals. The price is also a bit rugged. Arrow Electronics lists this baby at $180!
Its limitation is the frequency at which its rating peaks out at: 10kHz. My application (small crucible sizes) requires much higher frequencies, between 200kHz and 1 MHz. But the principles remain the same.
Another simple SCR circuit
From patent number 3,786,219. This one is practically identical to 3,786,222 but shows the power source with rectification of AC. Also note the tank circuit formed by capacitor 27 and inductor 31, whose LC characteristics no doubt sets the frequency of the induction heating signal.
Waveform in single SCR design
Heres what happens in the circuit. Note how the SCR is triggered and conducts positive, and then when the inductive components kick then the parallel diode takes the negative half.
Twin SCR induction heater, half bridge
From patent number 3,814,888. This circuit is fundamentally different from the others in that it uses SCRs in a half bridge that trigger in alternating sequence to form an ac signal.
Waveform in half bridge design
Again, heres the internal workings. Dont confuse the waveform and operation of this circuit with the dual self commutating design in the three phase design, next.
Three phase, industrial induction heater
From patent number 3,814,888. This one uses two SCRs, but operates like two back-to-back single SCR circuits in patents 3,786,222 and 3,786,219 mentioned earlier. For the waveform, click on the picture and you can see the full page.
Power circuit for induction cooking range
Another single SCR basic circuit, but when you click on the image you get a full page view that shows four individual circuits and the input filtering.
So there you have the extent of my patent research of induction heating schematics. One day I hope to build a small home shop induction furnace that can melt iron and other metals that require more heat than my resistance furnace that I use to melt aluminum.