Please help analyze this circuit.

灞波儿奔

Please help analyze this circuit. [Copy link]

 

See a battery switching circuit, as shown below,

1. Q1 and Q2 are NMOS tubes, Q3, Q4 and Q5 are PMOS tubes, and D1 is a diode.

2. BAT1 and BAT2 are batteries. The capacity of BAT2 is larger than that of BAT1. VIN_5V is the external power supply. VOUT is the output to power the system.

3. VOUT will draw power from the power source with the highest priority, with external power supply being the first choice, followed by the battery with large capacity, and finally the battery with smaller capacity. The priority order is: VIN_5V > BAT2 > BAT1.

Question: If no external 5V power supply is used and only two batteries BAT2 and BAT1 are used for power supply at the same time, what is the status of several MOS tubes Q1, Q2, Q3, Q4 and Q5?

Where can this circuit be used? Is it necessary to switch between two batteries?

maychang

"If there is no external 5V power supply, and only two batteries BAT2 and BAT1 are used for power supply at the same time, what is the status of several MOS tubes Q1, Q2, Q3, Q4 and Q5?"

When the external power supply 5V is not powered (5V voltage becomes close to zero), Q4 is turned on to supply power to VOUT. Since Q1 is turned on and Q2 is turned off, Q3Q5 are also turned off, and BAT1 does not supply power to VOUT.

baopodao

When VIN_5V has power, it will output to BAT2 and BAT1

When VIN_5V is out of power, BAT2 has power and will also output to BAT1

maychang

baopodao posted on 2021-12-17 11:39 When VIN_5V has power, it will output VIN_5V to BAT2 and BAT1. When there is no power, BAT2 has power and will also output to BAT1

"When VIN_5V has power, it will output to BAT2 and BAT1"

When VIN_5V is powered, Q4 is turned off, and the parasitic diode in Q4 is reverse biased, so it will not output to BAT2. At this time, Q3 and Q5 are also turned off, so they will not output to BAT1.

maychang

baopodao posted on 2021-12-17 11:39 When VIN_5V has power, it will output VIN_5V to BAT2 and BAT1. When there is no power, BAT2 has power and will also output to BAT1

"When VIN_5V has no power, BAT2 has power and will also output to BAT1"

When BAT2 has power, Q3 and Q5 are turned off, so there is no output to BAT1.

maychang

"Is it necessary to switch between two batteries?"

That depends on the needs of the users, which is not something you can decide.

灞波儿奔

Thank you for your analysis

So, what will happen in Q1 and Q2?

chunyang

灞波儿奔 posted on 2021-12-17 14:10 Thank you for your analysis. So, what will happen in Q1 and Q2?

If BAT2 has power, Q1 will be turned on, and Q1 turning on will turn off Q2.

baopodao

maychang posted on 2021-12-17 12:46 "When VIN_5V is powered, it will output to BAT2 and BAT1" When VIN_5V is powered, Q4 is turned off, and the parasitic diode in Q4 is reverse biased, and will not output to BAT2 ...

When VIN_5V has power, there is a 0.7V voltage difference at the GS end of Q4 due to D1, which will cause Q4 to turn on and there will be voltage output at the BAT2 end.

baopodao

When VIN_5V has power, BAT2 has no power and Q1 is turned off. Due to the internal diode of Q3, there is voltage at the S terminal of Q3. After the voltage is divided by R2 and R3, Q2 is turned on, thereby turning on Q5, so that there will be output at the BAT1 terminal.

baopodao

This design estimates that when VIN_5V has power, the two batteries will be charged, and when VIN_5V has no power, the two batteries will provide power.

maychang

baopodao posted on 2021-12-18 10:44 When VIN_5V is powered, there is a 0.7V voltage difference at the GS end of Q4 due to D1, which will cause Q4 to turn on and there will be voltage output at the BAT2 end.

"When VIN_5V is powered, there is a 0.7V voltage difference at the GS end of Q4 due to D1, which will cause Q4 to turn on and there will be voltage output at the BAT2 end."

0.7V voltage is generally not enough to turn on Q4. Moreover, Q4 is a P-channel tube, and when VIN_5V is powered, the gate of Q4 is positive and the source is negative, which is opposite to the direction of Q4 conduction, and is completely turned off.