ADUM3220 Gate driver floating supply


I have a question about a testing set up that I am implementing using ADUM3220 evaluation board. I have a full bridge of MOSFETs to implement LLC Converter, the input DC bus can go up to 400 v the output after the transformer is 48 v dc and the power is 350 Watt.

I am using three different power supplies (E3630A ) form Keysight, two of them to power the two high side gate voltages and one for the low side switches.

When I start increasing the dc bus higher than 50 v , the gate driver power supply reading (E3630A ) goes up.

I connect the oscilloscope across the power supply but I still can see the constant 15 volt of the gate driver even though the digital reading on the power supply is different.

Any idea on what is happening?