What is the best way to connect the SUBSTRATE pin on the EPC2010C or should we connect it at all? Is the SUBSTRATE pin connected to the D, G, or S in any way?
The substrate pin must always be connected to the source pin on the PCB regardless of the voltage potential of the source pin sits at.
Example: half bridge high side FET. The substrate pin would be connected to the switch node same as the source pin.
The best way to think about it is to forget it is a substrate pin and treat it like it is a source pin and connect accordingly.