I am trying to make a circuit that will have 2 power sources. One will be be a main and one will be a battery backup. When backup or main is disconnected I want to make sure that the supplies are not being put into the other. (like a battery backup circuit) However most of those that i saw use 2 diodes. Ones on each of the positive rails.

I have wondered why not use mosfets to do this as they have a diode built in. But I can't seem to understand the datasheet. It seems that source to drain voltage is -1.5v so I won't be able to do it in this case. But I thought that power mosfets are used in some circuits this way. Here is my circuit. I have 2 irf9520 p channel mosfets I can use. I want them to also work as reverse voltage protection and in their current configuration I know they will work for that. But I'm not sure if this configuration will cause an issue with that also. I have kept away from the diodes as they cause to much of a voltage drop. The load takes 7v minimum and the battery backup does not last that long so I would prefer that method.

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I believe I'm not reading the datasheet right. Can anyone tell me if this will work ? If it will not work can you help with an explanation and possible a circuit to be able to do this. Thanks.

Load current draw is between 25ma and 200ma. Being held mainly at 170ma.

  • 1
    \$\begingroup\$ The body diode of a MOSFET should not be used to conduct significant amounts of current. \$\endgroup\$
    – Hearth
    Feb 15, 2019 at 1:49
  • \$\begingroup\$ @Hearth how would it be conducting current in this manner ? \$\endgroup\$ Feb 15, 2019 at 2:10
  • \$\begingroup\$ I am not sure what you're asking. \$\endgroup\$
    – Hearth
    Feb 15, 2019 at 2:17
  • \$\begingroup\$ What would be significant amounts of current then ? I added my current draw of my load to the question. \$\endgroup\$ Feb 15, 2019 at 2:21


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