"Backwards Powering" a PFet High Side Driver ?

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The photo attached is from the Atmel Mega168 Explained demo board's schematic.

 

There is a Mega32U4-MU on the board that is a "Board Controller" and provides Debugging for the Mega168.

This Board Controller can turn On and Off Power to the Mega168 through a two Mosfet High Side Driver, (NFet driving a PFet).

The Board controller supplies the "Power Enable" signal.

 

VccP5V0 is 5 volts from the USB connector.

 

Vcc_Target is the switched power that powers the Mega168, the micro featured on this Xplained board.

 

IF one was to want to supply power to the Mega 168 from another power supply, by feeding one's V+ to the Vcc_Target rail, while the USB was NOT connected, would it damage the PFet in the High Side Driver?  (i.e. One is applying +5 V to the PFet's output, while its input and gate are floating, (not connected))

 

Obviously the Board Controller would not be powered up, but that isn't an issue in this instance.

 

I'm not very knowledgeable in P-Channel anythings, bi-polar transistors or PFets, and thought I might get some good guidance from those more familiar with these transistors.

 

The schematic does show J106.  Unfortunately J106 is a very, very small PCB trace, and not a Header that one can jump or un-jump with a connector shorting block.

 

The PFet's data sheet is here.

 

Any guidance would be appreciated.

 

JC

 

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If VCC_P5V0 is not connected and you apply +5 volts to VCC_TARGET then the gate of Q102 will see about 5 volts because of the body-diode in Q101.
Therefore Q102 will turn on which will turn-on Q101.
Any flowing currents would be in the sub 100 uA range so I would not expect any damage.

You could put a limiting resistor (eg 10 ohms) on your external supply and then measure the actual current to check if it is what you expect the Mega168 board to draw.

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Thanks Mike,

 

That makes good sense now that you have explained it.

 

JC

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The 'power enable' input from the Mega32U4 board controller will see the same voltage seen by Q102's gate.  Since that passes through a 100K resistor, currents would be below 50 µA.  This shouldn't be enough to properly power the Mega32U4 through the protection diode on that pin.  Since the first thing an AVR does after power-up is go into reset, and since reset current for the Mega32U4 is from 1 to 3 mA, it seems unlikely the Mega32U4 could ever make it out of reset.

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Thanks, Joey.

 

JC

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You can cut the jumper to remove the back path for your two power supply project, if you want to restore normal operation, just solder a small wire jumper across the two pads, or even a solder bridge across the pads will work as well.  

 

 

Jim

 

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Hi Jim,

 

That was certainly an option I considered as well.

 

That "jumper", (PCB trace), is very small, and the vias at each end are also very small, and all covered in solder mask.

So, although I suspect that option is technically feasible, it would be a bit of a challenge.

Soldering a wire to the pads would also be quite a challenge, although one could pick up the connections at remote sites from the actual "jumper".

 

That is why "Plan B" was to check with some P channel experts as to whether or not it would really even make a difference if I "back powered" the high side switch.

 

JC