Common Drain P and N channel MOSFETs configuration

[GrandDiego]

Hello,

I have a question regarding the use of MOSFETs.

I have a circuit with two 3V3 power supplies : 3V3AUX which is always ON, and 3V3 which is not always ON.
I want the output of my circuit to be connected to the not-always-ON 3V3 power supply when it is available, if not, I want it to be connected to 3V3AUX. I hope that's make sense, English is not my primary language...

So I came up with the circuit attached. Its an Operational Amplifier configured as a voltage comparator (I'm using the MCP6001 rail-to-rail OpAmp). Its output then drives two MOSFETs, one is N channel and the other one is P channel.
The idea is that when the not-always-ON 3V3 power supply is available the comparator outputs a '1' which turn off the PMOS and activate the NMOS. And when it's not available then the comparator outputs a '0' which turn off the NMOS and activate the PMOS.
The two MOSFETs that I'm using are assembled in a single package : the AO4604, I had it on hands.
As you can see in the attached shematic, I have connected the two Drains together, same for the two Gates. And each Sources is connected to one of the two power supplies.

The problem that I have is that when the not-always-ON 3V3 power supply is OFF (so NMOS OFF and PMOS ON) I have 3.21V at the ouput (which is fine I guess for a 3.3V power supply minus the Rdson drop), but when it is ON (so NMOS ON and PMOS OFF) I only have 2.7V. Why this difference ?
Also, do you think that my schematic is the best solution to the problem ?

Thank you

[Audioguru]

Your dual Mosfets have a maximum threshold gate-source voltage of 3V when they are barely turned on. They need a gate-source voltage of 4.5V to fairly turn on. Then the gate voltage must be 3.3V + 4.5V= 7.8V minimum. But you might find some of these dual Mosfets that turn on with a gate voltage of 5V in your circuit. 

[GrandDiego]

Your dual Mosfets have a maximum threshold gate-source voltage of 3V when they are barely turned on. They need a gate-source voltage of 4.5V to fairly turn on.

Oh ok thank you.
I also have these MOSFETs :
- Si2301 (PMOS) min gate voltage -0.45V (max not specified)
- Si2302 (NMOS) min gate voltage 0.40V and max 0.85V
They both have quite low RDS(on).
Do you think they would be better for the job ?

[Wimberleytech]

The problem that I have is that when the not-always-ON 3V3 power supply is OFF (so NMOS OFF and PMOS ON) I have 3.21V at the ouput (which is fine I guess for a 3.3V power supply minus the Rdson drop), but when it is ON (so NMOS ON and PMOS OFF) I only have 2.7V. Why this difference ?
Also, do you think that my schematic is the best solution to the problem ?

Thank you

Because when the Nch is driving, it is a source follower.  You are limited to 3.3V-Vt.

Here's an idea that might work for you if you have the 6002.

[GrandDiego]

Because when the Nch is driving, it is a source follower.  You are limited to 3.3V-Vt.
Here's an idea that might work for you if you have the 6002.

Thank you very much for this information !
I will try your circuit with two MCP6001 (I don't have the MCP6002 but I suppose it will behave the same).

[KrudyZ]

Because when the Nch is driving, it is a source follower.  You are limited to 3.3V-Vt.
Here's an idea that might work for you if you have the 6002.

Thank you very much for this information !
I will try your circuit with two MCP6001 (I don't have the MCP6002 but I suppose it will behave the same).

Also, have a look at the body diode in your original schematics. When the output is driven from either power source, the other source will be held up by the output voltage through those diodes regardless of the gate voltage on the FET.
So you need to turn those FETs around with the cathode of the body diodes going to the output.

[Mr.B]

I wonder how well this backyard/minimalist approach would work...

[Wimberleytech]

I wonder how well this backyard/minimalist approach would work...

Clever.  Flip D/S on Q1 (optimal) and it would work at the expense of the obvious couple hundred millivolts lost with the diode.

[KrudyZ]

Just to point out the obvious.
There are plenty of ICs out there made for just this application.
They are called ideal diode OR (controller).
Linear Tech makes a bunch of these.
They are often used in USB devices that are switching between the USB supplied power and an internal battery.
http://www.linear.com/products/powerpath_controllers_*_ideal_diodes