Circuit Analysis of the 1.8kW Induction Hotplate

Finally Complete!  Enjoy.

Cool stuff today, folks, cool stuff. As was mentioned in the first article about the Burton induction cooktop, we traced out the circuit and found some very interesting tidbits to share.  This device is amazingly simple!  It’s actually a self-oscillating resonant drive with integrated power control.

That microprocessor that we were speculating about?  It just monitors stuff, it isn’t necessary for the resonant heating.  The “PWM” output from the micro to the heart of the power stage is actually R-C filtered down to a DC voltage, and then fed to a comparator to do the power control.  Replace that with a potentiometer and you’ve got yourself a pretty sweet power control knob with no micro needed!

But we’re getting ahead of ourselves.  In the time that’s passed since the last article, we’ve traced the circuit and put it into both LTspice and eagle so we could simulate and make our own test PCB.  It works great!  You’re not going to get the PCB today (sorry) because it’s big and clunky and developed for 600V IGBT’s, whereas you need 1200V IGBT’s to really run this beast off the AC line.  But we will give you the LTspice files so you can simulate it on your computer and play around, adjusting stuff and making your own improvements.

For those that don’t know, LTspice is a circuit simulator developed by Linear Technology.  Get LTspice here. It’s free and is quite a good simulator, although the schematic interface is clunky and the waveviewer is damn near awful.  It looks like a knockoff of PSpice, where the sim engine was replaced with a turbocharged V8 and the seats and interior were replaced with wooden crates and astroturf.  That being said, it’s by far the best spice engine you’ll get for free.

A word of warning – to actually simulate this beast, you will need to lower the spice tolerances to get convergence.  More on that later.

ON TO THE CIRCUIT!


Fig 1 – Schematic of the Induction Cooktop Power Stage



Please bear in mind that this is a simplified circuit.  To get the full trace, just wait longer or trace it yourself!  :)  Some of the input filtering has been removed, along with the overcurrent and overvoltage protections.  Not a good idea to build a circuit without those (although we did), but what we’re focusing on today is the heart of the self-oscillating resonant control.

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