Electronic circuit series-MOS tube
Time：2021.09.02
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Electronic circuit series-MOS tube
Explanation 1: Channel
Explanation 8: How to work in the zoom area
The scientific name of the MOS tube is a field effect transistor, which is a metal-oxide-semiconductor field effect transistor, English: MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which belongs to the insulated gate type. This article briefly describes the structure, features, and practical circuits in the words of engineers.
Schematic diagram of its structure:
Explanation 1: Channel
In the above figure, a narrow strip in the middle of the lower p-type is the channel, which makes the two P-type poles connected together. Therefore, after the MOS tube is turned on, it is the resistance characteristic. Therefore, an important parameter of it is the on-resistance. The mos tube must know whether this parameter meets the requirements.
Explanation 2: n-type
The picture above shows the p-type MOS tube. Readers can understand the n-type based on this picture, and the reverse is all that matters. Therefore, it is not difficult to understand that applying a positive voltage to the gate of the n-type as shown in the figure will cause conduction, while the p-type is the opposite.
Explanation 3: Enhanced
Compared with the depletion type, the enhanced type is turned on by "thickening" the thickness of the conductive channel, as shown in the figure. The lower the gate voltage, the closer the positive ions of the p-type source and drain are to the middle, and the farther away the negative ions of the n substrate are from the gate. The free positive ions are connected together to form a channel, which is the effect of the illustration. Therefore, it is easy to understand that the gate voltage must be low to a certain level in order to be turned on. The lower the voltage, the thicker the channel and the smaller the on-resistance. Since the strength of the electric field is proportional to the square of the distance, the channel thickening caused by the voltage drop will not be obvious after the electric field is strong to a certain extent. It is also because it is more and more difficult for the n-type negative ions to "give back". The depletion type is to make a conductive layer in advance, and use the gate to thicken or thin to control the conduction of the source and drain. However, this kind of pipe is generally not produced and is basically not seen in the market. Therefore, when everyone talks about mos tube, it is enhanced by default.
Explanation 4: Left and right symmetry
The left and right diagrams are symmetrical. It is inevitable that some people will ask how to distinguish the source and the drain? In fact, the source and the drain are indeed symmetrical in principle, and they are indistinguishable. However, in practical applications, manufacturers generally connect a diode between the source and drain for protection. It is this diode that determines the source and drain. In this way, the package is also fixed, which is convenient for practical use. My teacher used mos tubes without diodes when he was young. It is very easy to be broken down by static electricity. It is usually placed in an iron can, and its source and drain are connected casually.
Explanation 5: Metal oxide film
There is an indication in the figure that this film is insulated and used for electrical isolation, so that the grid can only form an electric field and cannot pass direct current, so it is controlled by voltage. In DC electricity, the gate and source and drain are open circuits. It is not difficult to understand that the thinner the film: the better the electric field effect, the smaller the bump, the stronger the conduction ability at the same gate voltage. The disadvantages are: the easier it is to breakdown, the more difficult it is to make, and the more expensive it is. For example, if the on-resistance is at the ohm level, you can buy one at about 1 dime, while the 2402 at the ten milliohm level costs more than 2 yuan (buy in bulk. Retail is about 4 yuan).
Explanation 6: Difference from the real thing
The above figure is only a principle, the actual component adds a protective diode across the source-drain, thus distinguishing the source and drain. The actual element is a p-type, and the substrate is connected to a positive power supply, so that the gate has a relatively negative voltage in advance. Therefore, for a p-type tube, the gate does not need to be applied with a negative voltage, and grounding can ensure conduction. It is equivalent to forming a channel that cannot be conducted in advance, and strictly speaking, it should be a depletion type. The advantage is obvious, and the negative voltage is aside from the application.
Explanation 7: Parasitic capacitance
The gate in the above figure forms a capacitor through the metal oxide and the substrate. The more high-quality MOS, the thinner the film, the larger the parasitic capacitance, and the parasitic capacitance of the MOS tube often reaches the nF level. This parameter is one of the most important parameters in the selection of MOS tube and must be considered clearly. The Mos tube is used to control the on and off of large currents, often requiring a switching frequency of tens of K or even a few M. In this application, the gate signal has AC characteristics. The higher the frequency, the greater the AC component, and the parasitic capacitance can pass the AC The form of current passes through the current to form a gate current. The energy consumed and the heat generated cannot be ignored and even become a major problem. In order to pursue high speed, a powerful gate drive is required, which is the same reason. Imagine that a weak drive signal instantly changes to a high level, but it takes time to "fill" the parasitic capacitance, and the rising edge will slow down, posing a major threat to the switching frequency until it fails to work.
Explanation 8: How to work in the zoom area
Mos tube can also work in the magnification area, and it is very common. Mirror current source, operational amplifier, feedback control, etc., all use the mos tube to work in the amplifying area. Due to the characteristics of the mos tube, when the channel is in a non-on state, the gate voltage directly affects the conductivity of the channel, showing A certain linear relationship. Since the gate is isolated from the source and drain, its input impedance can be regarded as infinite. Of course, as the frequency increases, the impedance becomes smaller and smaller, and at a certain frequency, it becomes not negligible. This high-impedance feature is widely used in operational amplifiers, and the two important principles of virtual connection and virtual disconnection in operational amplifier analysis are based on this feature. This is incomparable to the triode.
Explanation 9: Causes of fever
Mos tube heats up, one of the main reasons is that parasitic capacitance exhibits AC characteristics when it is frequently turned on and off, and it has impedance and forms current. If there is current, there will be heat, and if there is no electric field, there will be no current. Another reason is that when the gate voltage climbs slowly, the on-state has to "pass" a critical point from off to on. At this time, the on-resistance is very large and the heat is more severe. The third reason is that after the turn-on, the channel has resistance, and the main current flows through it, resulting in heat generation. The main considerations for heat generation are the first and third points. Many MOS tubes have high junction temperature protection. The so-called junction temperature is the temperature of the channel region under the metal oxide film, which is generally 150 degrees Celsius. Above this temperature, the MOS tube cannot be turned on. When the temperature drops, it will recover. Pay attention to the consequences of this state of protection.
I hope that the above description can understand the MOS tube in a popular way. Here are a few conventional circuits:
1: pmos application
is generally used to manage the on and off of the power supply. It is a non-contact switch. The gate is completely turned on at low level and completely turned off at high level. Moreover, the grid can increase the voltage higher than the power supply, which means that a 5v signal can be used to manage the switching of the 3v power supply. This principle is also used for level conversion.
2: nmos tube application
is generally used to manage whether a circuit is grounded or not. It is a non-contact switch. The gate is turned on when the high level is high, and it is grounded, and the low level is cut off. Of course, the gate can also be cut off with a negative voltage, but this benefit is meaningless. Its high level can be higher than the power supply of the controlled part because the gate is isolated. Therefore, a 5v signal can be used to control whether a 3v system is grounded or not. This principle is also used for level conversion.
3: Application in the enlarged area
works in the amplification area, and is generally used to design feedback circuits. It requires a lot of professional knowledge. It is similar to an op amp, so I can’t go into details here. Commonly used as mirror current source, current feedback, voltage feedback, etc. As for the integrated application of operational amplifiers, we don't actually need to pay attention. After everyone has done it, just be optimistic about the datasheet. There is no need to consider the on-resistance and parasitic capacitance according to the MOS tube method.
