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Problem with ADUM3221

Hello, I use an ADUM3221 to drive an inverter with SiC JFETs. Each JFET switch is driven with one ADUM and has its own isolated gate power supply. At 300V DC-link voltage the inverter operates well. But at 450V or higher the ADUMs of the upper switches turn off just after they turned on. (The turn off is during the voltage transient of the inverter output) After 1µs they turn on again. Actually, the dv/dt of the inverter output is not that high - it is lower than 10kV/µs, which is much below the specified 25kV/µs.

I guess, that the Icoupler detects a "switch off" signal during the voltage transient (which is a single pulse in the Icoupler coil , right?).   Anyhow, this also happens without load. So there cannot exist any magnetic field caused by large output current. On the other hand, there is an 50MHz oscillation between the JFET output capacitance and DC-link stray inductance which might cause some radiated EMI. But I think, it is a common-mode problem. There are additional stray capacitances in the isolating DCDC-converter of the gare drive (60pF) and between inverter output and the grounded heat sink (500pF) . The control ( including primary side of ADUMs ) is grounded, too.

Both primary and secondary supply of the ADUM3221 is coupled with a 100nF capacitor directly at the package pins. Thus, undervoltage detection of the ADUM should not be a problem.

I realy do not know, what the problem is, since the dv/dt is within the specifications. Any ideas?   

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