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MOS tube type, structure and conduction characteristics

date: 2019-08-28
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MOS call English full name of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor), the type of Metal Oxide Semiconductor Field Effect tube, MOS tubes have excellent switch performance, a triode faster switching speed, so the main role in the power or drive.At present, there are not too many MOS tubes on the motherboard or graphics card. Generally, there are about 10 MOS tubes. The main reason is that most of them are integrated into IC chips.Now let's take a look at the types, structures and conducting characteristics of MOS tubes.


1. MOS tube types and structures


MOSFET tube is one kind of FET (the other is JFET), which can be manufactured into enhanced or exhausted type. There are four types of P channel or N channel, but only enhanced N channel MOS tube and enhanced P channel MOS tube are applied in practice. Therefore, NMOS or PMOS are usually referred to as these two types.


As to why do not use exhausted MOS tube, do not recommend the root cause.


For these two kinds of enhanced MOS tube, the more commonly used is NMOS.The reason is that the on-resistance is small and easy to manufacture.Therefore, NMOS is commonly used in switching power supply and motor drive applications.In the following introduction, also mostly NMOS.


There is parasitic capacitance between the three pins of MOS tube, which is not required by us, but due to the manufacturing process constraints.The existence of parasitic capacitance makes it difficult to design or select the drive circuit, but there is no way to avoid it, which will be described in detail later.


In the MOS tube schematic diagram, it can be seen that there is a parasitic diode between the drain pole and the source pole.This is called a volume diode, which is important in driving inductive loads such as motors.By the way, bulk diodes are found only in individual MOS transistors, which are not normally found in IC chips.


2. MOS on-off characteristics


Conducting means acting as a switch, which is equivalent to switching off.


NMOS features, Vgs larger than a certain value will be turned on, suitable for the source grounding case (low-end drive), as long as the grid voltage reaches 4V or 10V can be.


PMOS features, Vgs less than a certain value will be turned on, suitable for the source to connect to the VCC situation (high-end driver).However, although PMOS can be easily used as high-end drives, NMOS are usually used in high-end drives due to high on-resistance, high price, and few alternatives.


3. MOS tube application circuit


The most significant characteristic of MOS tube is that it has good switching characteristics, so it is widely used in circuits requiring electronic switches, such as switching power supply and motor drive, as well as lighting dimming.


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