Circuit Analysis of the 1.8kW Induction Hotplate


No complex PWM IC in there?  Nah.

No driver?  Nope.

No microprocessor?  Nay.

No dual current/voltage sense?  Nein.


COOL THING #3 – Resonant control uses only LM339 comparator.

And to make it even better, only 2 of the 4 comparators in the LM339 are used for control.  The other two are used for overvoltage and overcurrent sensing.  Amazing!  We built our board with an LM393, a dual comparator, since we don’t need those pesky protection mechanisms getting in the way.  All our components already contain this built-in indicator called “smoke” that can easily be used to detect if something was driven just a bit too hard.  Heh.

OK, we know you’re salivating too.  So here’s how it’s done, and it’s easy as pie.  First comparator compares a divided-down copy of TOP to a divided down copy of SW ( the other side of the work coil that is driven by the IGBT’s).  We called these divided-down versions TOPREF and SWREF.

Fig 3 – Resonant Ringing Comparator

Anytime SWREF falls below TOPREF, it’s OK to start a cycle.  This takes care of the problem of varying AC input voltage because the trigger point is always relative to TOP.  It rises and falls in accordance with the rectified AC input, which makes the power in each cycle rise and fall in accordance to the AC input as well.

Now it’s certainly true that when the input voltage is still dropping through 168V or so, you don’t want to be turning on your power switch just yet.  That would be a lot of power for the switch to eat and might even kill it.  But thankfully for us the LM339 is kind of slow already, and with creative selection of a small (220p) cap across the input terminals it’s not hard to make sure the SW voltage is all the way low when we turn on the switch.

That’s worth mentioning as another cool thing – the ZVS (or here, LVS for low-voltage switching) is ensured by the fact that the residual current in the tank will drive the SW node negative (for all reasonable power levels) and therefore it’s not critical to hit the ZVS point accurately.  The SW voltage will fling down to negative and turn on the freewheeling diode in the IGBT, and then some time later the IGBT can turn on slowly without seeing high voltage/high current on it’s collector.  Yes, you need an IGBT or mosfet with a fast antiparallel diode.

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