Thank you for the information, looking at the circuit of the IRS2186 it seems like the high side output will switch between Vb and Vs, which as you say shouldn't exceed 18V difference, however doesn't Vb have to be at a voltage level higher than than the H-bridge supply, as it needs to drive the high side mosfet?
The reason the schematic doesn't show the high side source connected to the low side drain is because the chip is for one half of a Hbridge, not a half bridge, and both the high and low sides are designed to be turned on at the same time, unlike a half bridge where they are turned on opposite to one another.
For a regular half bridge how is the high side gate driven to a higher voltage than the supply, or does it even need to be using a similar IC?