Input problem with OpAmp-controlled ZVS Induction Heater
I'm trying to build a circuit which uses an OpAmp to drive 2 mosfets, which power the LC tank of an induction heater.
The idea is to detect when the voltage over the LC tank crosses zero, and at that voltage I would make the 2 outputs of the OpAmp change states from fully on, to fully off.
The two OpAmps (both inside one chip) have their positive and negative inputs connected to eachother, but with reversed polarity. This would make sure that one output is high, and the other one is low.
Why use an OpAmp? I wanted the MOSFET gate switching to go faster as usually, because in the mazzilli circuit, the gate voltage actually slews at the same rate as the LC tank's voltage slews when it crosses zero volts.
In the mazzilli circuit, it actually doesn't switch when the voltage is 0v, but when the tank voltage drops below the gate threshold voltage.
This would mean that you are always switching current at 5v (for example) instead of at 0V.
So for these two reasons, I wanted to try switching them with an OpAmp.
Probe I on drain 1, probe II on drain 2, and GND on the circuit's ground, gives me an expected result:
when switching states, at 0 voltage of the LC tank, the voltage on one side stays low (since it's pulled to ground) and the
voltage on the other side goes from 0v up to 50v, back down to 0v, like a sine-wave. Then the OpAmps switch again, and the one side now goes up to 50v as a sine wave, and the other one stays low at 0v.
All good, this is working just fine.
Probe I on the one differential input line, probe II on the other differential input line.
Since this is nothing more than just a 1/10 voltage division of the previous measurement, I'm also expecting the signal to be exactly the same, but 11x smaller.
However, this does not happen. Because of some strange reason, as you can see in the scope images: Both channels go high, Twice per cycle, instead of going high only once per cycle, and staying low for the next half of the cycle.
This really isn't good!
Do the inputs affect the waveform in some way?
Remarks about scope images:
Image: 2 gates
Blue gate voltage seems 'quite' fine. Turning on looks good, turning off is not really good becuse if tends to turn on for a short time once again, before it fully turns off.
Yellow gate voltage is terrible. Turning on doesn't happen as expected. Voltage drops back to 0 for a long while which is very bad for the circuit.
Frequency seems fine; 50kHz is as expected with the 14µH and 6µF.
Image: 2 drain voltages
These voltages were measured with a 1-10 voltage divider, and thus show only 1/11th of the actual voltage.
This is getting close to what I want the LC tank to do. The voltages seem quite like sine waves. I suspect that if the gate voltages would be as they should be, these drain voltages would also be perfect sine waves.
The regular sine amplitude of 50V is as expected, with a 24V supply voltage, but
at the moments when the drain voltages strangely drop down to 0v, as you can see in image: '2 gates', at these moments the drain voltage seems to spike over 250V!!
Image: 2 differential input lines
This is the image which I don't understand. I expect the same wave as in the previous picture, but only 11 times smaller because of the voltage divider. However, the voltage does NOT reach 0v while the drain voltage does, and its shape is also completely different.
In this image, both channels are doing one (half) sine wave, twice per switching period. They should be LOW for half a period, as the drain voltages do in the previous image.
Better quality images:
2 drain voltages
2 differential input lines
Coils and capacitors:
Line inductor value: I'm not sure if this value is correct.
Center-tapped main coil inductance: This value should be pretty correct, I calculated it by measuring it's size and windings, and the operating frequency is also nearly the same as the calculated one.
tank capacitance: 6 times a 1µf MKP capacitor
- Why these strange large spikes?
- Why is the waveform suddenly different when reading it near the differential inputs?
For the first time ever, I can provide you with scope images! I finally bought a (quite cheap) oscilloscope. I hope it helps a lot.
Oh, and one more thing: The induction heater does actually work already. I'm getting huge currents in the LC tank, since the 6mm copper tubing gets hot after a minute. Water cooling has been added, and it works like a charm! The MOSFETS do get quite hot after 15 seconds of heating an object, or after 40 seconds of heating nothing. This, probably because the gate voltage isn't what it should be.