Dual Power Automatic Switching Circuit (Near-Zero Drop)

A key advantage of this design is very low conduction drop, making it suitable for battery-powered and low-loss power-path applications.

This circuit uses MOSFET switching behavior and low Rds(on) characteristics to achieve automatic source selection.

Circuit and Functional Behavior

• When Vin1 = 3.3V and Vin2 is absent, Vin1 supplies Vout through the MOSFET path.
• When Vin1 is removed, the circuit automatically switches so Vin2 supplies Vout.
• Because selected MOSFETs have low Rds(on), voltage drop is typically only tens of millivolts.
• With a single source active, quiescent current is around the microamp range, suitable for low-power systems.

Principle of Operation

1. With Vin1 = 3.3V, NMOS Q1 turns on, pulling gate conditions such that PMOS Q3 conducts and PMOS Q2 is off. Output is supplied from Vin1.
2. When Vin1 is removed, Q1 turns off. Bias network drives Q2 on and Q3 off, so output is supplied from Vin2.

For practical design, choose MOSFETs with:

• low gate-threshold voltage
• very low Rds(on) at target gate drive voltage

Example device notes from the original design:

• Q2 = Q3 = PMN50XP (low Rds(on) around 3.3V gate drive)
• Q1 can use 2N7002

Final selection should be based on required current, voltage, and thermal budget.