I'm currently redesigning the inverter for driving my induction heater.
While I've sorted out most problems I encountered I'm still struggling with the actual inverter.
Picture 1 shows the waveform problems I encountered.
Yellow is the actual inverter output.
Purple is the resonator voltage.
Cyan is the driving signal delivered to the driver.
Everything works fine until I go up in power levels.
Oscillations only occur at peaks of mains voltage, so it has to be some sort of current problem.
Scoping the output of the gate of the IGBT modules reveals that the signal supplied to them represents the output fairly accurately (also ringing.)
The IGBTs used are skm100gb128d which have an integrated gate resistor of 5 ohms.
The driver consists of optocouplers, voltage regulators and gate drive ICs.
What could cause this ringing?
Why is it visible in the cyan signal which is optically isolated?
How to prevent this?
Are 5 ohms of gate resistance enough if the signal at the IGBT gate looks alright (no overshoot?)
The second picture shows the setup of the IGBTs and the drivers associated with it.
UPDATE: The ringing was caused by the oscilloscope ground loop being located too close to the actual inverter. The voltage induced in the oscilloscope probe then acted back on the signal generation which was part of the reason the distorted signal was visible even before the Optoisolator.