the circuit of fig3 is a derivative of the circuit of fig2 however the single fet in each input power leg is replaced with a series connection of two back to back fets 33 34 35 36 connected to a common output 37 the use of two back to back fets 33 34 35 36 in each voltage source leg in lieu of single fets allows one or both of the voltage sources 31 32 to be completely disconnected from the common connection in the event of an unacceptably high over voltage it should be noted that the use of a single pet in each voltage source leg instead of two back to back fets with the voltage source connected to the drain and a common source connection would be unsatisfactory this would allow conduction of the body drain diode to conduct from the output in the event that one of the voltage sources was shorted referring to fig3 voltage source a 31 is compared to a fixed reference voltage by a comparator 40 the voltage reference and comparator comprise over voltage sense shutdown block 41 when the voltage level of source a 31 exceeds a preset value a signal from the over voltage sense shutdown block 41 is fed to voltage comparator 40 and causes the gate drive voltage to be removed from fet switches 33 34 similarly.