For my OpenRB-150 projects, I never let any 12 V wires “touch” the OpenRB-150 so I don’t even use that PS Terminal Block on the OpenRB-150. So I would power the MX-28 DXLs at 12 V as normal via their own PS and using the regular 3P Molex cables. From the OpenRB-150’s end I put the Jumper on 5V and from the X3P Hub on the OpenRB-150, I used a “modified” X3P Convertible Cable to connect the 150 to the “last” MX-28 of the daisy chain. See this link if you do not have these X3P compatible cables yet.
Robot Cable-X3P 180mm (10pcs) - ROBOTIS
This cable is "modified" by cutting out the VDD line (which carries the 12 V from the MX-28 daisy chain) >> this way the 12 V circuit is isolated from the OpenRB-150 completely, but the MX-28s still share their GND line with the OpenRB-150, and also their DATA lines (5V TTL) with OpenRB-150 >> therefore you still can use your Arduino code to control the MX-28s.
You can check out my newest application of this approach at this posting for OMX:
If you are interested in more Arduino Applications with OpenRB-150, you can check this free Amazon Kindle sample at this link
https://www.amazon.com/Using-ARDUINO-ROBOTIS-Systems-Ngoc-ebook/dp/B0BPXGQ6YX
However using SIX MX-28s in ONE daisy chain has an additional and different kind of Power & Control Issue also. Please read these two posts also
You can try first with only 1 12V PS and ONE daisy chain. But you most likely will need 2 separate PS at 12 V, each for 1 daisy chain of 3 MX-28s, and two “special” X3P compatible cables coming from the OpenRB-150 to control each of the two MX-28 daisy chains.