Hi,
Thank you for posting on Infineon community and I would like to answer your query point wise.
1) Since, you are using the IC at higher switching frequency, there will be higher switching loss.
2) The best way to handle the unused pins is to connect them to the ground rather then keep them floating. Yes, you can connect them all together and connect to common ground but make sure there is no or minimum ground loop in order to avoid any ground noise (if any, normally there wont be any noise due to fixed ground connection) . Another way is that you connect the unused pins to the ground separately which is basic and normally used.
3) Yes, all the pins of the IC are connected to die in order to have same thermal resistance through out the surface which tends to uniform lateral heat transfer across the body.
4) If the junction temperature is crossing the limit given in the data sheet then, you can use copper pads(it depends upon what copper grade is used like TG150 etc and what is PCB material like FR4 or any other) on the PCB to make the heat dissipation more efficiently. This methods is generally used for SMD components and you can use it for your application as well.
As per the power loss and heat dissipation, you can choose what copper area is needed even if after providing the copper area, if temperature increase is more, you can provide some air vents in the copper area of the PCB for better heat convection.
5) There are some other options like thermal pads available which can be used for the application but they act as thermal insulators as well. So, if you opt for this better check the compatibility properly.
If you need any further design help please let me know.
BR,
AZIZ HASSAN
