Quick understanding about MOS tube of power supply
1. Judgment of 3 poles
G pole (gate)-the gate, needless to say, easier to recognize
S pole (source)-source, whether it is P-channel or N-channel, the intersection of the two lines is
D pole (drain)—drain, whether it is P-channel or N-channel, is the side of the separate lead

2. Discrimination of N-channel and P-channel
The arrow pointing to the G pole is the N channel
The arrow facing away from the G pole is the P channel

3. Determination of the direction of the parasitic diode
Regardless of N-channel or P-channel MOS transistors, the arrow direction of the intermediate substrate and the arrow direction of the parasitic diode are always the same:
Either S points to D, or all D points to S

4. Functions realized by MOS switch
1>Signal switching
2>Voltage on and off
5. The connection method in the circuit when the MOS tube is used as a switch
Key point: 
1> Make sure which pole is connected to the input terminal and which pole is connected to the output terminal
2>What is the control pole level? MOS tube is on at V
3>What is the control pole level? MOS tube is cut off at V
NMOS: D pole is connected to input, S pole is connected to output
PMOS: S pole is connected to input, D pole is connected to output
Reconciliation strengthens understanding
If NMOS: S is connected to input, D is connected to output
Because the parasitic diode is directly turned on, the S pole voltage can unconditionally reach the D pole, and the MOS tube loses its role as a switch.
If PMOS: D is connected to input, S is connected to output
Also lost the role of the switch
6. The switching conditions of the MOS tube
N channel-when Ug> Us, Ugs> Ugs(th) when turned on
P channel-when Ug< Us, Ugs< Ugs(th) when turned on
In short, the conduction condition: |Ugs|>|Ugs(th)|
7. Related concepts
BJT:
Bipolar Junction Transistor bipolar transistor, BJT is a current control device;
FET:
Field Effect Transistor, FET is a voltage control device.
According to the structure of the field effect tube, it is divided into: junction field effect (referred to as JFET), insulated gate field effect (referred to as MOSFET)
2 categories:
According to channel material: junction type and insulated gate type are divided into N-channel and P-channel.
According to the conduction mode: depletion type and enhanced type, junction FETs are depletion type, insulated gate type FETs have both depletion type and enhanced type

Type.
Generally speaking, field-effect transistors can be divided into junction field-effect transistors and MOS field-effect transistors, and MOS field-effect transistors
Transistors are divided into four categories: N-channel depletion type and enhancement type; P-channel depletion type and enhancement type.
8. MOS tube important parameters
①Package
②Type (NMOS, PMOS)
③Withstand voltage Vds (the maximum voltage that the drain and source of the device can withstand when the device is off)
④Saturation current Id
⑤On-resistance Rds
⑥ Gate threshold voltage Vgs(th)
9. Identify the pins from the MOS tube

Regardless of whether it is NMOS or PMOS, place it in the direction of the above figure, the middle pin is D, the left is G, and the right is S.
Or remember this: a single foot is D, turn DGS counterclockwise.
Here, by the way, let me mention the pin identification of the triode: also align it in the direction of the above figure, the middle pin is C, the left is B, and the right is E.
Pin number:

Starting from G foot, counterclockwise 123
The pin number of the transistor also starts from the B pin, counterclockwise 123
10. Use a multimeter to distinguish between NNOS and PMOS
Use parasitic diodes to distinguish. Set the multimeter position to the diode position, connect the red test lead to S, and the black test lead to D, there is a value display, and vice versa.

The numerical value indicates that it is an N-channel, if the situation is opposite, it is a P-channel.