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Help with a Simple MOSFET Voltage Selector Circuit

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Thane of Cawdor:
Hi,

I'm working on a circuit which has potentially two power inputs (two battery packs regulated at 5V - in the diagram the primary supply is 5V and secondary supply is labelled 5V1) and was looking at using this power selector circuit attached.

My first question: will this work in all cases where 5V and 5V1 inputs are connected/disconnected? The main thing throwing me off is the presense of the body diode on Q1 - doesn't this make the P-MOS unnecesary in the circuit since, whether we switch it or not, it will always conduct when the 5V battery is connected (if the voltage on the output line is low enough to allow current to flow and overcome the body diode Vf)?
 
If so, could I just replace this with another Schottky diode or is it still better to use the MOSFET due to the lower forward voltage?


My second question is a simple one: is D1 and R1 (polyfuse) required for reverse current protection or does Q1 already protect the output if the 5V rail is inverted?

Thanks :)

Zero999:
Yes, the body diode makes the MOSFET unnecessary.

If you do away with the MOSFET and use two Schottky diodes, then there's no need for a reverse paralell diode for reverse polarity protection.

Schottky diodes do have a significant voltage drop and reverse leakage current. It is possible to use MOSFETs to solve this problem, resulting in a very low voltage drop, but the answer is to use an ideal diode controller IC, rather than attempting to make your own.
https://www.google.com/search?client=firefox-b-d&q=ideal+diode+controller+IC

Thane of Cawdor:
Thanks for that.

I keep seeing a similar method of voltage selection using a MOSFET where the body diode would render it unnecessary in a number of designs (e.g. https://cdn.sparkfun.com/datasheets/Wireless/WiFi/SparkFun_ESP8266_Thing.pdf in the power supply section and https://www.arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf in "+5V Selector" section). So I'm just confused as to why this seems to be the standard rather than use two Schottky diodes as you suggest?

The ideal diode IC looks interesting - I'll keep that in mind too.


Zero999:

--- Quote from: Thane of Cawdor on December 11, 2019, 04:07:07 am ---Thanks for that.

I keep seeing a similar method of voltage selection using a MOSFET where the body diode would render it unnecessary in a number of designs (e.g. https://cdn.sparkfun.com/datasheets/Wireless/WiFi/SparkFun_ESP8266_Thing.pdf in the power supply section and https://www.arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf in "+5V Selector" section). So I'm just confused as to why this seems to be the standard rather than use two Schottky diodes as you suggest?

The ideal diode IC looks interesting - I'll keep that in mind too.

--- End quote ---
Those are P-channel MOSFETs, so they turn on when the gate is below the source. The diodes still block voltage, when they're reverse biased, so those circuits will work.

Thane of Cawdor:
Right, is that not the same configuration as the initial circuit I provided (Q1 is also a P-MOS)?

In the case of the SparkFun design, the Vbat voltage would always be present on the VIN line even when VUSB is turned on due to the body diode being forward biased when Vbat is powered. I assume they're not isolating the voltage on VIN rather just current sharing in the case when both sources are connected - so wouldn't they be better off using ideal/schottky diodes too? 


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