MOS tube (Ⅱ)

2019-10-31 08:53:10

From the above content of detailed description of MOS tube (I), you have preliminarily understood what is MOS tube, and the basic structure of MOS. Today, the editor will continue to lead you to have a deep understanding of the construction principle and application field of MOS tube. J today, we will continue to deepen the construction of MOS tube, based on detailed description of MOS tube I. first of all, we can see the structure diagram of MOS tube:
 
 MOS tube structure diagram
MOS tube structure diagram
 
Combined with the above figure and the previous knowledge points, we divide the MOS transistor knowledge into nine problem classification points for in-depth understanding (if you haven't seen the detailed description of MOS tube I, you can click the link entry "detailed description of MOS tube I"):
 
The first classification of MOS transistor problem: how to divide MOS transistor P / N type?
 
From the structure diagram of MOS tube, we can see that a narrow strip in the middle of the p-type is the channel, which makes the left and right p-type poles connected together. After the MOS tube is turned on, it is the resistance characteristic. Therefore, an important parameter of it is the on resistance. When selecting MOS tube, it must be clear whether this parameter meets the requirements. Similarly, the n-type MOS can also be understood as n-type.
 
The second MOS transistor problem classification: how to distinguish the source and drain of MOS transistor?
 
In the structure diagram of MOS transistor, we can see that the left and right are symmetrical. It is inevitable that someone will ask how to distinguish the source and drain. In fact, in principle, the source and drain are indeed symmetrical and indistinguishable. However, in practical application, manufacturers usually connect a diode between the source and drain to protect them. It is this diode that determines the source and drain. In this way, the package is also fixed and convenient for practical use.
 
The third MOS transistor problem classification: what is enhanced MOS transistor?
 
The enhanced type is conducted by "thickening" the thickness of the conducting channel. As can be seen from the figure above, the lower the grid voltage is, the closer the positive ions of the p-type source and drain are to the middle, and the farther the negative ions of the n-substrate are to the grid. When the grid voltage reaches a value, called threshold or threshold, the positive ions separated from the p-type are connected together to form a channel, which is the graphic effect. Therefore, it is easy to understand that the gate voltage must be low to a certain extent in order to turn on. The lower the voltage is, the thicker the channel is, and the smaller the on resistance is. Because the strength of the electric field is proportional to the square of the distance, the channel thickening caused by the voltage drop is not obvious when the electric field is strong to a certain extent, and it is also because the "retreat" of n-type negative ions is more and more difficult. The depletion type is to make a conduction layer in advance, and use the grid to thicken or thin to control the conduction of the source and drain. However, this kind of pipe is generally not produced, and can hardly be seen on the market. Therefore, we usually say that MOS transistor is enhanced by default.
 
The fourth classification of MOS tube problems: what is the metal oxide film in MOS tube?
 
The metal oxide film marked in the structure diagram of MOS tube is located at the upper part. This film is insulated, which is used for electrical isolation, so that the grid can only form an electric field, not through DC, so it is controlled by voltage. On DC electrical, the grid and the source drain are open circuited. It's not hard to understand that the thinner the film is: the better the electric field, the smaller the threshold voltage, and the stronger the conductivity at the same gate voltage. The disadvantages are: the easier to break down, the more difficult to make and the more expensive the price. For example, if the on resistance is ohmic, buy one at about 10 yuan, and if the 2402 is 10 milliohm, buy more than 2 yuan (in batch). The retail price is about 4 yuan.
 
The fifth MOS transistor problem classification: what is the parasitic capacitance of MOS transistor?
 
The gate in the structure diagram of MOS transistor forms a capacitance between the metal oxide and the substrate. The thinner the film is, the larger the parasitic capacitance is. Often, the parasitic capacitance of MOS transistor reaches NF level. This parameter is one of the most important parameters in MOS transistor selection, which must be considered clearly. MOS transistor is used to control the on-off of large current, and it is often required to switch the frequency of tens of K or even tens of M. in this application, the gate signal has the AC characteristics. The higher the frequency is, the larger the AC component is, and the parasitic capacitance can pass through the current in the form of AC current to form the gate current. The consumed electric energy and the generated heat can not be ignored, even become the main problem. In order to pursue high-speed, we need a strong grid drive, which is also the truth. Imagine that the weak drive signal suddenly becomes high level, but in order to "fill" the parasitic capacitance takes time, the rising edge will slow down, which will pose a major threat to the switching frequency until it can not work.
 
The sixth MOS transistor problem classification: how does MOS transistor work in the amplification area?
 
MOS tube can also work in the amplification area, and it is very common. For image current source, Op Amp, feedback control, etc., MOS transistor is used to work in the amplification area. Because of the characteristics of MOS transistor, when the channel is on or off, the grid voltage directly affects the conductivity of the channel, showing a certain linear relationship. As the grid is isolated from the source and drain, its input impedance can be regarded as infinite. Of course, with the increase of frequency, the impedance becomes smaller and smaller. When a certain frequency, it can not be ignored. This high impedance characteristic is widely used in operational amplifiers. The two important principles of operational amplifier analysis, virtual connection and virtual disconnection, are based on this characteristic. This is unparalleled in triode.
 
The seventh classification of MOS tube problems: what is the cause of MOS tube heating?
 
One of the main reasons for the heating of MOS tube is that when the parasitic capacitance is opened and closed frequently, it shows the AC characteristics and has the impedance to form the current. If there is electric current, there will be heat. If there is no electric field, there will be no current. Another reason is that when the grid voltage climbs slowly, the conduction state should "pass" a critical point from close to conduction. At this time, the conduction resistance is very large and the heating is relatively severe. The third reason is that after the conduction, there is resistance in the channel, which is over the main current, forming heat. The main consideration of heating is points 1 and 3. Many MOS tubes have a high junction temperature protection. The so-called junction temperature is the channel temperature under the metal oxide film, generally 150 ℃. Beyond this temperature, it is impossible for MOS tube to turn on. When the temperature drops, it recovers. Pay attention to the consequences of this state of protection.
 
The eighth classification of MOS management problem: what is the difference between MOS management theory diagram and real object?
 
The structure diagram of MOS transistor is only principle, and the actual element adds a protection diode which is connected between source and drain, so as to distinguish the source and drain. The actual element, p-type, substrate is connected to the positive power supply, which makes the grid become a relative negative voltage in advance, so the p-type tube and grid do not need to add a negative voltage, and the grounding can ensure the continuity. It's equivalent to forming a channel that can't be conducted in advance. Strictly speaking, it should be depletion type. The advantage is obvious, the application of the negative voltage aside.
 
The ninth classification of MOS transistor problems: what are the applications of MOS transistor?
 
Application of 1: P MOS transistor
 
Generally used to manage the on-off of power supply, it belongs to contactless switch. The low level of grid is fully on, and the high level is completely off. Moreover, the gate can increase the voltage of over power supply, which means that 5V signal can be used to manage the switch of 3V power supply. This principle is also used for level conversion.
 
Application of 2: n type MOS transistor
 
Generally, it is used to manage whether a circuit is grounded. It belongs to contactless switch. If the grid is high, it will turn on and lead to grounding. If the grid is low, it will turn off. Of course, the gate can also be cut off with a negative voltage, but this benefit is of no significance. Its high level can be higher than the power supply of the controlled part, because the gate is isolated. Therefore, 5V signal can be used to control the grounding of 3V system. This principle is also used for level conversion.
 
3: application of MOS tube amplification area
 
Working in amplification area, it is generally used to design feedback circuit, which requires a lot of professional knowledge, similar to op amp, which cannot be explained in detail here. It is often used as mirror current source, current feedback, voltage feedback, etc. As for the integrated application of op amp, we don't need to pay attention to it. People have done it well. Just watch the datasheet. You don't need to consider the on resistance and parasitic capacitance according to the MOS mode.
 
Classification of the 10th MOS tube problem: what products are MOS tubes basically used in?
 
In addition to the PFC technology used in the switching power supply of HD, LCD and plasma TV, MOS transistor with excellent performance is used to replace the high-power transistor on the components, which greatly reduces the efficiency, reliability and failure rate of the whole machine. Because there are essential differences in structure and characteristics between MOS transistor and high-power crystal triode, in application; the driving circuit is also more complex than crystal triode, which makes it difficult for maintenance personnel to analyze the circuit and fault. In this paper, MOS transistor and its application circuit are briefly introduced to meet the needs of maintenance personnel.
 
Through the above ten knowledge answers of MOS management, I believe you have a certain understanding of MOS management. For more details of MOS management, please pay attention to hecotec MOS encyclopedia. Some of the above contents are sorted out through the network. If there is any infringement, please contact the administrator in time to delete.