When it comes to MOS tubes, your mind may be a mess.
Most textbooks will tell you a long paragraph:
MOS tube full name metal oxidation semiconductor field effect transistor, English name Metal-Oxide-Semiconductor Field-Effect Transistor, belongs to the insulated gate field effect transistor, with silicon wafer as the scale body, using the diffusion process to produce....... There are two types of N-channel and P-channel. Not only that, but it also has two brothers, namely the junction FET and the crystalLMP...
In the face of such a big paragraph, I don't know if you have understood, anyway, I didn't understand it at all in college, and I learned a lonely semester after studying.
So why are these textbooks so anti-human, can't they write and speak human language well?
I probably analyzed it, because the same textbook he needs to face students of different majors, so the most important thing about the textbook is rigorous. It is not so important to be easy to understand compared to the comprehensive. And the general textbook will not tell you what you have learned, which leads to the fact that you can easily get lost in these concepts and miss the point in learning.
What about this article, I would like to put aside the framework of these dogmas in the book according to my own work and learning experience, and start from the application side to introduce the most common and easiest to use MOS tube: enhanced NMOS tube, referred to as NMOS. When you are familiar with the use of this NMOS, and then look back at the content of this textbook, I believe you will have a different experience.
Usage of NMOS
First of all, let's look at such a simple figure (Figure 1), we can control the opening and closing of this switch by hand, so as to control the light on and off.
Figure 1
So if we want to use An Arduino or a microcontroller to control this bulb, we need to use a MOS tube to replace this switch. In order to be more in line with the actual usage habits of our project, we need to convert this diagram slightly, like Figure 2.
Figure 2
The two graphs are completely equivalent, and we can see that the MOS tube has three ports, that is, it has three pins, which are gate, drain, and source. It doesn't matter why they are called this, just remember that they are abbreviated as g, d, and s respectively.
Figure 3
We connect one of the IO ports of the microcontroller to the gate port of the MOS tube, and we can control the light bulb on and off. And of course don't forget to power it. When the IO port output of this microcontroller is high, NMOS is equivalent to the closed switch, and the indicator light will be turned on; when the output is low, the NMOS is equivalent to the switch is released, then the light is turned off, which is not very simple.
So if we keep switching this switch, the light will flicker. If the speed of switching is a little faster, because of the visual persistence effect of the human eye, the light will not flicker. At this time, we can also adjust the time of this switch to dim the light, which is the so-called PWM wave dimming, the above is the most classic use of the MOS tube, which realizes the IO port of the microcontroller to control a power device. Of course, you can completely replace the bulb with other devices. Devices like pumps, motors, electromagnets, and so on.
Figure 4 PWM wave dimming
How to choose NMOS
After understanding the use of NMOS, let's look at how to choose a suitable NMOS, that is, how NMOS is selected.
For a beginner, there are four more important parameters to pay attention to. The first is encapsulation, the second is vgsth, the third is on Rdson, and the fourth is Cgs.
The package is relatively simple, it refers to a MOS tube, there are many types of shapes and sizes. In general, the larger the package, the greater the current it can withstand. In order to understand the other three parameters, let's first introduce the equivalent model of NMOS.
Figure 5 NMOS equivalent model
MOS can actually be seen as a voltage-controlled resistor. This voltage refers to the voltage difference between g and s, and the resistance refers to the resistance between d and s. The size of this resistor will change with the change of g and s voltages. Of course, they are not linear correspondence, the actual relationship is almost like this, the abscissa is g, s voltage difference.
Figure 6 Diagram of Rds vs Vgs
The ordinate is the value of the resistance, and when the voltage of g and s is less than a specific value, the resistance is basically infinite. Then when this voltage value is greater than this specific value, the resistance is close to zero, as for what will happen when it is equal to this value, we don't have to care about this critical voltage value, we call it vgsth, that is, the g and s voltages needed to open the MOS tube, which is the inherent property of each MOS tube, we can find it in the data sheet of the MOS tube.
Figure 7 MOS tube data sheet
Obviously vgsth must be less than this high-level voltage value, otherwise there is no way to be turned on normally. So when you choose this MOS tube, if your high level is the corresponding 5V, then it is more appropriate to choose vgsth of about 3V. If it is too small, it will be triggered by mistake due to interference, and if it is too large, it will not open the MOS tube.
Let's look at the second important parameter of NMOS, Rdson, which was mentioned earlier when NMOS was fully turned on, and its resistance was close to zero. But no matter how small it is, it always has a resistance value, which is called Rdson. It refers to the resistance value between d and s after the NMOS is fully turned on. You can also find it in the data sheet. This resistance value is of course as small as possible. The smaller it is, the less it is divided into pressures, and the fever is relatively low. However, in fact, the smaller the Rdson, the higher the price of this NMOS, and the corresponding volume will generally be larger. So it is still necessary to do what you can and choose exactly what is right.
Finally, let's talk about Cgs, which is a relatively overlooked parameter, which refers to the parasitic capacitance between g and s. All NMOS have, which is a manufacturing process problem that cannot be avoided.
That will affect the NMOS on speed, because the voltage loaded to the gate end, first of all, the capacitor must be charged first, which leads to the g, s voltage can not reach a given value at once.
Figure 8
It has a climbing process. Of course, because Cgs is relatively small, we generally don't feel its presence. But when we zoom in on this timescale, we can see this upward process. For this high-speed PWM wave control scenario is fatal. When the period of the PWM wave is close to this climb time, the waveform is distorted. Generally speaking, Cgs size and Rdson are inversely proportional to each other. The smaller the Rdson, the larger the Cgs. So everyone should pay attention to balancing their relationship.
The above is the initial knowledge that everyone needs to grasp about NMOS.