What is the purpose of connecting two PMOS in series?

普拉卡图

What is the purpose of connecting two PMOS in series? [Copy link]

 

As shown in the figure above, because the position number is too long, I relabeled Q1, Q2, R1, and C1 for easy analysis.

The problem is as follows:

1. What is the purpose of using the two PMOS gates and sources connected in series?

2.What is the function of C1?

Let me analyze it first:

When the lower NMOS is turned on,

The current flows along the green line through the Q1 body diode. In order to make the gate level have a low level, R1 is connected in series.

It can be seen that R1 is used to increase the voltage drop so that the PMOS can be turned on.

The gate becomes low and both Pmos are turned on.

So what is the role of C1 here?

After adding the capacitor, there will be a charging process when the power is turned on, so I guess it is a soft start?

Because the capacitor increases the conduction time, which will cause the power consumption of the tube to increase, only a small 103 capacitor can be used.

When the lower NMOS is turned off,

The PMOS gates are pulled high and the tubes are turned off.

Back to the original question:

1. What is the purpose of connecting the two PMOS gates and sources in series? (My guess is to increase the withstand voltage?)

Also, does this circuit have a specific name?

2.What is the function of C1?

chunyang

Note that the DS of the MOS tubes are connected in series "back to back" (or "face to face"), which is not to increase the withstand voltage (pay attention to the polarity of the parasitic diode). The original poster's circuit is incomplete. You should understand it if you mark the different power supplies and see how they are connected. Similarly, C1 is not for "soft start".

maychang

1. What is the purpose of connecting the two PMOS gates and sources in series? (My guess is to increase the withstand voltage?)

The purpose is not to increase the withstand voltage, but to be able to "turn off" in both directions. Of course, the circuit can also be "turned on" in both directions.

maychang

"Also, does this circuit have a specific name or title?"

You can call it a "two-way switch".

普拉卡图

chunyang posted on 2022-4-22 15:07 Note that the DS of the MOS tube is "back to back" (of course, it can also be said to be "face to face") in series, which is not to improve...

I kind of understand it, but what is the function of C1?

普拉卡图

maychang posted on 2022-4-22 15:19 1. What is the purpose of connecting the gates of the two PMOS above and connecting their sources in series (I guess it is to increase the withstand voltage?) ...

So that's why. It's because of the body diode of PMOS that a single PMOS can only achieve unidirectional on-off control, right? For this reason, two back-to-back MOSs are used to achieve bidirectional control.

maychang

Placatu posted on 2022-4-22 16:30 So that's why, because of the body diode of PMOS, a single PMOS can only achieve unidirectional on-off control, right? For this reason, two backplanes are used...

"It is because of the body diode of PMOS that a single PMOS can only achieve unidirectional on-off control, right?"

Yes.

Unidirectional control can only control DC, and bidirectional control can control AC. Although your circuit seems to have only DC power supply, there may be power supply on both the left and right sides, so bidirectional control is required.

chunyang

Placatto posted on 2022-4-22 16:27 I understand a little bit, so what is the function of C1?

The connection of C1 requires Q8307 to be turned on for a sufficient time before the two back-to-back MOS tubes can be turned on. This shows that it is to deal with the situation where Q8307 is transiently mis-turned on under interference. After all, Q8307 is a MOS tube with high input impedance and is more sensitive to interference. However, whether this is necessary needs to be examined at the control end. If the control end is a low-impedance output and is not a long-line connection, this capacitor is unnecessary.

chunyang

Placatu posted on 2022-4-22 16:30 So that's why, because of the body diode of PMOS, a single PMOS can only achieve unidirectional on-off control, right? For this reason, two backplanes are used...

I suggest you draw the power path as I said in my previous post, analyze the relationship between different power supplies based on the on and off conditions, and compare it with the case of using only one MOS, which will make it easier to understand.

普拉卡图

chunyang posted on 2022-4-22 17:57 The access of C1 requires Q8307 to be turned on for a sufficient time before turning on the two back-to-back MOS tubes. This shows that it is to deal with the transient misleading conduction of Q8307 under interference...

Got it, thank you senior!

普拉卡图

maychang posted on 2022-4-22 17:02 "It is because of the body diode of PMOS that a single PMOS can only achieve unidirectional on-off control, right?" Yes. Unidirectional control can only control...

Got it, thank you senior!

maychang

Prakatu posted on 2022-4-22 18:30 I understand, thank you senior!

Instead of using two MOS tubes, a rectifier bridge plus a MOS tube can also achieve the purpose of bidirectional switching. The MOS tube is connected to the DC output end of the rectifier bridge, and the two AC ends of the rectifier bridge are a bidirectional switch. However, the voltage drop of such a bidirectional switch is relatively large, which is the voltage drop of two diodes plus the voltage drop of the MOS tube.

PowerAnts

The function of C1 is to suppress the surge current when it is turned on. It is a must for DC switches. For the MOSFET selected by the original poster, 10nF is too small. I will try to use several thousand uF later. I will show you how bad it is.

PowerAnts

The duration of the Miller platform directly determines the SOA of the MOSFET

不亦心

The questions asked have their own analysis and understanding, and the doubts about the problems are clear and concise. I give a thumbs up.

C1 is slow start. Isn't EE disassembling the mobile power bank? Let them look inside. Most fast-charging mobile power banks have similar functional modules, otherwise the adapter is easily pulled dead.

Fred_1977

I think the two MOS tubes have another very important function, which is to prevent voltage and current from flowing back.

普拉卡图

PowerAnts published on 2022-4-22 21:22 The function of C1 is to suppress the surge current when it is turned on. As a DC switch, it is necessary. For the MOSFET selected by the host, 10nF is too small. There are thousands of...

So what is the general range of capacitance? If it is too large, it will not work, right? Or, if the switch is not used for fast switching, it doesn't matter if the capacitance is larger?

普拉卡图

Fred_1977 posted on 2022-4-23 09:07 I think the two MOS tubes have another very important function, which is to prevent voltage and current backflow.

Backflow prevention is a very important point! Thank you for your reminder!

普拉卡图

maychang posted on 2022-4-22 17:02 "It is because of the body diode of PMOS that a single PMOS can only achieve unidirectional on-off control, right?" Yes. Unidirectional control can only control...

Seniors, may I ask you again, is the essence of two-way control the anti-backflow function mentioned by the friend on the 16th floor?

PowerAnts

Plakatu posted on 2022-4-23 09:30 PowerAnts posted on 2022-4-22 21:22 The function of C1 is to suppress the surge current when it is turned on. As a DC switch, it is necessary for the host to choose...

This can't be explained in a few words. Even if I record a video, it will take a whole episode to explain it clearly.