I found this device here on the site.
I need something to switch to a 9 V backup battery when there is a 48 V phantom power outage.
How can I change the FET to switch OFF the BAT1 (9 V) backup battery when the main V1 (48 V) is on?
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If you cannot afford a diode drop on the 9 V side this will do:
D1 and D2 will lift gate and source to the same voltage if V1 is active. In this case M1 is off. TSM680P06 is an overkill here, but at least, it is available..
If V1 is offline the body diode of M1 will conduct and initially deliver 8.5V at the load. Via R1 the MOSFET can turn on and finally 9 V is delivered.
The original circuit will work well with one major modification.
The voltage from the 48V supply must be reduced (typically using a two resistor divider) so that the FET reverse gate voltage applied when the 48V supply is live is substantially less than Vgs_max for the FET.