Some Problems of PMOS as Anti-Reverse Connection Circuit

小太阳yy

Some Problems of PMOS as Anti-Reverse Connection Circuit [Copy link]

 

 

This circuit is a PMOS anti-reverse connection circuit. Why is R1 added? What is the function of R1? How to choose the size? When the voltage at point S is higher than the voltage at point G, the MOS will be turned on. Should this voltage be set higher than the VTH voltage or higher than the voltage when it is fully turned on? There are three voltage values below. Which one should be used as the reference?

maychang

"This circuit is a PMOS reverse connection protection circuit. Why do we need to add R1? What is the function of R1?"

R1 makes the gate of the P-channel MOS tube at ground potential. Without R1 (open circuit), the gate of the MOS tube is suspended and the potential is uncertain.

maychang

"When the voltage at point S is higher than the voltage at point G, the MOS will be turned on. Should this voltage be set higher than the VTH voltage or higher than the voltage when it is fully turned on? There are three voltage values below. Which one should be used as a reference?"

It is larger than all three voltages (absolute values), and should be larger than the voltage when the MOS tube is saturated and turned on, but smaller than the maximum allowable voltage between the gate and source (usually 20V).

小太阳yy

maychang published on 2022-4-13 17:18 "This circuit is a PMOS anti-reverse connection circuit. Why add an R1? What is the function of R1? " R1 makes the gate of the P-channel MOS tube at ground potential. Without R ...

I don't mean to leave the gate floating, I mean to replace the resistor with a wire

小太阳yy

maychang posted on 2022-4-13 17:21 "When the voltage at point S is higher than the voltage at point G, the MOS will be turned on. Should this voltage be set higher than the VTH voltage or higher than the voltage when it is fully turned on...

Why should it be greater than the voltage at saturation conduction?

maychang

小太阳yy posted on 2022-4-13 17:38 maychang posted on 2022-4-13 17:18 "This circuit is a PMOS anti-reverse connection circuit. Why do we need to add an R1? What is the function of R1? 』 R1 ...

It is theoretically possible to replace it with a wire. But please note: after replacing it with a wire, the power supply voltage on the right end directly acts between the source and gate of the MOS tube. The power supply voltage on the right end may carry environmental interference with a large peak value, and the gate of the MOS tube is relatively fragile. As mentioned earlier, the maximum voltage between the source and gate is 20V. If it exceeds this value, the MOS tube may be damaged. After adding R1, the resistor and the capacitor between the gate and source of the MOS tube form an RC low-pass filter. The power supply has spike interference, and this RC low-pass filter has a certain effect of attenuating the spike. Therefore, a resistor is usually added here.

maychang

Xiao Taiyangyy posted on 2022-4-13 17:49 Why should it be greater than the voltage at saturation conduction?

When the MOS tube in this circuit is normally turned on, we always hope that the voltage drop of the tube is as small as possible, or the on-resistance of the tube is as small as possible. The smaller the voltage drop of the tube, the less power consumed by the tube. Since we use the MOS tube as a switch, the ideal state is that there should be no voltage drop in the "on" state.

maychang

Xiao Taiyangyy posted on 2022-4-13 17:49 Why should it be greater than the voltage at saturation conduction?

The red box in the first post shows the voltage between the gate and source of the MOS tube when it is just "starting" to conduct. At this time, the resistance of the MOS tube is very large and the drain current is only 85uA. Divide 1.7V by 85uA, how big is this resistance?

In the red box below, when the gate voltage is 4.5V, the MOS tube is basically saturated and turned on. However, if the gate voltage is increased, the resistance can still be reduced.

PowerAnts

maychang posted on 2022-4-13 17:21 "When the voltage at point S is higher than the voltage at point G, the MOS will be turned on. Should this voltage be set higher than the VTH voltage or higher than the voltage when it is fully turned on...

There is no such thing as a fully saturated MOSFET. The saturated region is a linear amplification region, which belongs to a voltage-controlled current source. The "fully" on region is a variable resistance region.

maychang

PowerAnts published on 2022-4-13 18:26 There is no such thing as MOSFET being fully saturated and turned on. The saturation region is a linear amplification region and belongs to a voltage-controlled current source. "Fully" turned on is a variable resistor...

That's right. This is a name borrowed from the bipolar transistor. The official name is the variable resistance area.

PowerAnts

maychang posted on 2022-4-13 18:47 Not bad. This is borrowed from the name of the bipolar transistor. The official name is the variable resistance area.

The variable resistance area is a bit difficult to pronounce, so it is completely fine to say "conduction"

小太阳yy

PowerAnts published on 2022-4-13 18:26 There is no such thing as MOSFET being fully saturated and turned on. The saturation region is a linear amplification region and belongs to a voltage-controlled current source. "Fully" turned on is a variable resistor...

Then, within what range should I control the GS voltage of this reverse polarity protection MOS?