I have a circuit I'm working on that has two power sources, one varies between 40-60V, and the other is 72V. The 72V power source will typically always be on, and the 40-60V source may be on or off.

I am trying to make a circuit that will prioritize the 40-60V source as the main source, if it is switched off, the 72V source takes over automatically.

I've found chips like the LTC4355 but can't handle the higher voltages. I've looked into something simple using a couple op-amp comparators like in the model below but pretty sure I need to use p-mosfet's since the switch needs to be on the high side. If I have to use p-mosfet's then the opamp voltages need to get much higher to drive the mosfet which I'm not sure how to do (mosfet gate driver IC?)

Looking for advice, direction, or anything to help me figure this out. I'm not an EE, obviously. Thanks.

EDIT: Operating range for the load right now is 30-60V. I cannot have a loss of power. The load can be up to 12A at 48V.

ANOTHER EDIT: Redone the whole circuit with the below circuit.. Still can't quite get it to switch but I think this circuit makes a little more sense. Trying to run spice sims on it but having trouble with spice models of the parts right now..

EDIT3: So after trying various circuits, I came across this and High Voltage MOSFET Switching Circuits for H-Bridge Motor Controls and used the same through process for this circuit. Found parts that seem to work and ran it through Pspice. Seems like it works. Anyone see any issue with this circuit (other than some decent cooling on the mosfet)? I attached the results from Pspice as well Circuit Schematic PSpice results basic diagram

  • \$\begingroup\$ How much current are you trying to switch? What sort of load is it? What is the operating voltage range for the load, particularly the minimum operating voltage? Can you tolerate a brief loss of power while switching? \$\endgroup\$ – Elliot Alderson Dec 28 '18 at 13:30
  • \$\begingroup\$ Operating range for the load right now is 30-60V. I cannot have a loss of power. The load can be up to 12A at 48V. \$\endgroup\$ – Dan Dec 28 '18 at 13:43
  • \$\begingroup\$ Any other specs ? like losses or efficiency ?, voltage regulation? , Voltage threshold range to force a transfer or Vmin ( 40V? ) Any other problems? Iout max? Otherwise Ic=C*dV/dt e.g. 10uF / 0.4mA=25mV/s is pretty slow, expected interval between dropouts? \$\endgroup\$ – Sunnyskyguy EE75 Dec 28 '18 at 13:55
  • \$\begingroup\$ @TonyEErocketscientist no spec on efficiency.. The 40-60V source is either on or off (it would be nice to switch to the 72V if below 30V but not necessary at this time). 12A @ 48V max through mosfet. Ignore the cap and load resistor, those were just put in when I was messing with the circuit. \$\endgroup\$ – Dan Dec 28 '18 at 14:04
  • \$\begingroup\$ is this in case of battery failure as in short circuit? or weak charge? because all you need is a 10A charger \$\endgroup\$ – Sunnyskyguy EE75 Dec 28 '18 at 14:21

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