4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 1/7

Oscilloscope Tracings III

Search

I have to share some bad waveforms I got one day. I hadn't used my heater all summer and wanted to try it outbefore giving it to a friend. Below are the voltage/current waveform. Underneath is a tracing of the gate drive signal

and the inverter voltage from another run. Notice how the current is no longer a nice sinusoid. The negative currentprematurely starts to rise and then go back down before resuming its normal cycle.

Here is another image. The waveform is different from above, but still bizarre and not a good sinusoid.

4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 2/7

This is the inverter voltage (yellow) and gate drive (blue). Notice how the voltage heavily sags and the gate signal is

no longer a clean square wave.

4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 3/7

Here is another gate wave that is abnormal taken at a different time.

As you can see, I was getting strange waveforms and I did not know why.

4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 4/7

At first I thought it was the mosfets so I swapped them out. When that failed to fix the problem I redid the gate

resistors and shielding. Then, I pulled out the inverter capacitors and replaced them. Still no good. Frustrated, I tookout the board and replaced the gate drive capacitors. When this failed I redid the entire circuit board thinking I wasgetting some type of cross-talk or a failed component. I saved myself from buying another tank capacitor by

connecting the coupling transformer to another LC tank. Again, I had the same problem.

I thought I checked everything and I couldn't understand how the waveform had deteriorated. Sometimes, thecurrent appeared to go at twice the frequency of the inverter voltage. Then, I had a final thought. I started looking at

my high voltage DC supply. I must have reconnected the HV wires to the inverter early in the summer. Notice in thepicture how they are not together.

They should be together to cancel out any stray inductance as shown below.

4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 5/7

Amazingly, after days of racking my brain, this simple solution was all that was needed. I twisted the HV wires close(as I had done in the past) and made sure they were close on my inverter board before splitting to each of the HVrails. At the frequency I am driving my coil, stray inductance and capacitance on the HV lines is significant and

clearly affected my waveforms. Not only did this affect the LC tank, but it affected the gate signal and the voltagesupply signal to the circuitry, making things even worse. Hopefully, my experience will make someone's life easier ifthese symptoms appear.

Below are the waveforms for the inverter voltage and current immediately after this repair. I have the frequency

deliberately higher than resonance to prevent reverse currents.

4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 6/7

Below is the gate signal after this repair.

4/5/2015 Induction Heater

http://inductionheatertutorial.com/inductionheater/induction7a.html 7/7

Next Page