Looking into 3-phase motor drivers, especially ones for IGBT hex modules for motor drivers, I have noticed the use of transformer windings is arranged so that there is a separate winding for each of the 3 high side FETs/IGBTS/other electronic switch, but only one winding shared between all three of the low side FETs/IGBTs/switches.
As an example, Texas Instruments has a IGBT driver for 3-phase inverter, such as this one: http://www.ti.com/lit/ug/tidu670a/tidu670a.pdf
While I understand that an isolated supply is required as the supply rails being switched may have nothing to do with the power supply for the driver, why is there this arrangement of seperate windings for each high side, then one for all of the low side switches? In the datasheet linked in my question, the high side coils each supply 100mA, while the low side coil gives 300mA, so it isn't power consumption.
The only reason I know of is isolation, but why can the low side get away with only one coil: low side shares the same ground rail, but the high side share the same power rail. Or am I missing something here?