I use ADUM4120 find problems. The maximum operating frequency can only be 1.5MHz. If the frequency is increased again. The output of the signal will stop. Trouble to help me
I believe you might be running into the thermal shutdown. If you have an IR camera, please check if the unit is overheating. What load is the part operating?
For the ADuM4120, I suggest the gate driver be operated at below 500 kHz in most cases. There also needs to be an external series gate resistor to help dissipate some of the switching power.