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PLL Induction Heater Driver

glowing yellow bolt

Having played with a manually tuned induction heater a few times before I knew how tedious it could be to operate. For one thing the resonant frequency isn't constant, but varies with the work piece size and temperature of the tank components. If the operating frequency is used to regulate power, more time is spent watching a scope then the work piece. What I wanted was a driver that would automatically find the resonant frequency or regulate power on the fly. It would be nothing new actually, as this type of driver has been done before only with discretes (see links below). Tim William's IH is complicated as hell though, and I wasn't going for something that elaborate. So after many different versions I eventually got something which worked and was reasonably simple.

PLL IH Schematic

The frequency control voltage appears at pin 9 which is the VCO input. An analog OR gate (the diodes) is used to allow the strongest signal to take control of the frequency. When the circuit starts up the "soft-start" circuit is in control, consisting of just a capacitor/resistor voltage delay. This ensures that the frequency starts at maximum where power draw is minimal, it also makes sure the PLL starts on the right side of the tank's resonant frequency.

As Richie has detailed on his site the LCLR arrangement presents a capacitive load below resonance and inductive above resonance, with inductive reactance being the most forgiving for a square wave inverter. Therefor an increase in supply current or tank voltage brings the VCO frequency up. On either side of resonance the impedance of the LCLR circuit increases, lowering current draw and resonant rise. With low supply voltages or proper loads neither the tank voltage nor the inverter current will need regulation, in which case the driver must lock onto the exact resonant frequency of the LCLR circuit for max efficiency and power. The PLL takes care of this by adjusting the VCO frequency to the point where the inverter output and tank voltage are 90 degrees out of phase (inverter leading the tank voltage), because this phase difference characterizes resonance in a LCLR circuit. See Richie Burnett's excellent article on the LCLR topology for more. The 4046 will lock two phases at 90 degrees difference by default, which is practical for this usage. The tank and inverter voltage is sampled directly with a 393 comparator thanks to the circuit's common ground. The single discrete transistor inverter adds an additional 180 degree shift to the tank voltage signal. Without the additional shift the internal XOR gate in the 4046 was unable to detect the phase difference properly and would lock somewhere below resonance.

Note: I haven't tried series resonant topology with this circuit, but it should work too. Drop me an email if you try it and it works so this can be confirmed!

scope shot of tank and inverter voltage

If the load is too light allowing for greater tank Q, the voltage or current will rise uncontrollably. Since both the voltage and current sensors are similar I'll describe them as one. The desired signal is detected and sent through a low pass filter giving a more or less stable DC control signal. This signal is compared to the variable reference created by the voltage divider and if too great triggers an error. The error signal passes through the OR gate and takes control of the VCO, regulating the voltage or current to an acceptable value. To ease adjusting the potentiometers for different ranges I've made and Excel spreadsheet with various calculators available for download.

overview of IH setup

The actual power section of the circuit consists of doubled up IRFP450s, powered from fullwave rectified mains. The size of my matching inductor is 45μH, with a 1.7μF tank capacitor and 2.50μH work coil.

To test my new driver I had a large work coil and tank capacitor already built for a previous induction heater project. The tank capacitor was made up of 50x 22nF and 50x 12nF mini capacitors I purchased cheaply off ebay, giving a total of 1.7μF at 600V. So far I've had 3 capacitor failures, all of them with the small 12nF ones. Other than that the bank has held up well and doesn't seem very lossy. The most difficult component to construct was the matching inductor, which dissipates surprising amounts of power due to the large current flow. After a few failed attempts I had to use 32 strands of insulated 0.3mm magnet wire, wrapped together as litz wire. Even with just 8.6 milli-Ohms of DC resistance I still had to use a fan to keep the inductor temperature low enough. The reason litz wire is used over a solid conductor is due to the Skin effect, which has to due with current flow at high frequencies. As the frequency of the current increases, more of the electrons will travel in the outer layer of a conductor, increasing the apparent resistance of the conductor, because the current flowing portion of the conductor becomes smaller. By checking the skin depth of copper at various frequencies one can see that at 100kHz, single strand copper wire with a radius over 0.2mm is just excessive. (So by rights, I should have used 0.4mm wire instead for optimal conduction)

matching inductor

Despite doubled up IRFP450s I couldn't seem to push more than 20A through the inverter without the mosfets failing. The low inverter current limits the power my induction heater can supply which is a bit sad, since it can run nearly indefinitely as is meaning I'm no where near pushing the limits of anything but the IRFP450s (even they run cool when the current is kept around 18A) yet. At least I was able to heat objects quicker and to higher temperatures than my previous heater could.

Some more pictures and a project thread here.

Here's a list of the few induction heater projects I've found on the net:

Richie Burnett's Induction heater
Tim William's IH - very much like mine, but built with tons of discretes and even more functions. Another great contributor to this project.
Savel's IH based on my design
Neon John's Induction Heater Project
Steve Conner's IH
Karol's IH
Penguin Lab's IH

And finally thanks to Richie Burnett and Tim Williams for guiding advice on this project!

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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.

Creative Commons License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.