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I’ve been trying to build an induction cooktop, my inspiration was this post:

https://www.instructables.com/id/Powerful-yet-simple-induction-heater/

And after some help , I’ve built a circuit according to this schematic:

enter image description here

And it actually worked! I mean, not the way I expected but it worked. The problem is that it’s kind of weak, I did some tests and my convencional gas oven is capable of boiling 150 mL of water in about 2 minutes. With the circuit I’ve built not even after 5 minutes the water boils, it gets hot but I can still put my finger on it for a second or so. So, my question is, do you see anything I could do to improve the efficiency of my circuit? The easiest thing I could do is to change the value of the capacitors adding or taking some of them away. I’m using a stainless steel pan, I’ve tried using an iron pan but the results were the same, if not worse. You’ll also find some pictures of the actual setup below. Any more details needed I’ll try to provide you. Thanks in advance!

Flat coil L1 Complete Circuit

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Your circuit has a few problems:

  1. Inductor \$L_2\$ must be connected to the \$V_1\$ and drain of \$M_2\$
  2. The driver of both mosfets must be connected to \$V_1\$ or some controlled supply voltage, otherwise they are not going to work.

Here is a simulation with the proper connections. The circuit begins to oscillate after you apply the required control voltage to the gate of the MOSFETS.

Circuit

Note: Be careful with the voltage levels and the heat.

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  • \$\begingroup\$ Thanks for the answer, that was fast! Yes, my bad, I had drawn the wrong schematic, now it’s how it is. As for that second source you’ve drawn, is it really needed? In that inspirational post he connects R1 and R2 direct into DC voltage, in my case i’ve connected them to ground, so I suppose it would work if connect them to V1? \$\endgroup\$
    – Fábio
    Apr 23 '20 at 20:55
  • \$\begingroup\$ The second source is required if you want to have the ability of turning it on or off. You can achieve the same as well by connecting it to V1 through a switch. In his example, he uses a second regulator because the source voltage is too high to control the MOSFETS. Yes, if you connect directly to V1 it will work, as long as the voltage is not too high to destroy the MOSFET's rated voltage. Maybe if you share your actualy circuit, it is easier to assess it. \$\endgroup\$
    – vtolentino
    Apr 23 '20 at 21:06
  • \$\begingroup\$ Well, I've edited the image, now it's the actual circuit. The datasheet for the IRFB4227 says a maximum gate to source voltage of 30 V, so I think it’s ok to connect it to V1 since it’s 24 V. I’ll try this and later I’ll share the results. For now controlling the oven is not an issue, I turn it on or off directly turning the DC source on/off. \$\endgroup\$
    – Fábio
    Apr 23 '20 at 21:24
  • \$\begingroup\$ The gate connections are still in the wrong place. You must connect them to the battery. \$\endgroup\$
    – vtolentino
    Apr 23 '20 at 21:28
  • \$\begingroup\$ Exactly, that’s what I did, but now when I turned it on, after a few seconds one of the resistors started to burn, I’m using 1 W resistors, do you think it needs to be higher? \$\endgroup\$
    – Fábio
    Apr 23 '20 at 21:35
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I am working on an induction heater myself and would like to supply som hints from what I have learnt so far.

  1. Keep all wires and connections as short as possible to minimize unwanted inductance, EMI, noise and ringing.

  2. To be able to understand and optimize what is going on, you simply need to use an oscilloscope. Otherwise you are guessing and working in darkness.

  3. The power will be limited by the supply voltage applied across the MOSFETs and thereby also the voltage put across your induction coil. You will not get a high power output from a 5V 100A power supply, if it doesn’t match the impedance of your system (coil)

Additionally; my approach has been to control the gates of the MOSFETS manually to be able to tune the switching frequency to resonance or above, making the power output controllable. This is of your more complicated but a good learning experience!

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  • \$\begingroup\$ You couldn't be more right, but unfortunately for me an oscilloscope is totally out of hand now. But about your experience, apart from manually controlling the gates, is your layout a lot different from mine? I mean, you still have inductors before a capacitor bank followed by the work coil? Nonetheless, I suppose the maximum power output is achieved when the switching frequency of the mosfets matches the resonance frequency right? Witch this circuit should technically tune by itself. Finally, any ideas on how to match the impedance of the source and the system? Thanks already for the support! \$\endgroup\$
    – Fábio
    Apr 25 '20 at 17:12

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