Insights from an engineer on the R&D front line on switching power supply design!

　　Now that I have retired from front-line research and development, I recall the difficulties I faced in the early stages: there was so much information and tutorials, I could not finish reading them all, and I seemed to understand them only partially.

　　I'm an old hand now, and I'm considered a hardworking person. I've been doing this for five years, and I've basically tried all kinds of topologies and powers.

　　If you put down your skills for too long, you will become rusty. In order not to waste what you have learned through hard work, and to enable novices to quickly find a way to learn, get started quickly, and really step into the world of switching power supplies , I plan to start a post and write a tutorial. I will write about the commonly used topologies one by one, using the simplest and most popular language, and the most reliable theories that have been tested by engineering practice.

　　Science is understandable and not mysterious at all. We are not Chinese medicine practitioners, so we don't try to be profound.

　　I hope you will give me more support, and I also welcome senior experts to give me some comments.

　　Let's first talk about the theoretical basis needed for making switching power supplies : We power supply engineers are divided into two categories, one is research and the other is engineering.

　　The so-called researchers are those who study various new technologies, new materials, new processes, new topological structures, etc. These people need a very high theoretical foundation and of course must be highly educated in mathematics, electromagnetism, electronics, automatic control, etc., all kinds of majors, all kinds of awesome.

　　Another type is the most common power engineer, who is an electronic engineer working on projects in the company's development department.

　　This article is aimed at the second category, that is, power supply design engineers at the application level.

　　I must add that for books written by masters of the level of monsters like Tao Xianfang and Zhao Xiuke, no beginners need to work hard to read them, as it is very time-consuming and laborious. You can just read them in a hurry and understand as much as you can. Then, as you grow up, you can look back and read them again, and you will gain a lot.

　　We are engineers, and they are theoretical. It is impossible to fully understand the books written by masters at once. Those books are written in great detail in many aspects, with complete theoretical derivations and very comprehensive coverage. But I still advise novices not to get entangled in mathematical formulas.

　　Those books can be used as technical manuals. There is a process of growth in technology, and when you reach a certain level, those books are very useful.

　　The theoretical foundations that our application engineers must have include: the basics of analog electronic technology. Let's talk about the depth of learning analog electronic technology first. It is generally impossible to learn analog electronics well just after graduation. If someone really thinks that he is awesome just after graduation, there are two possibilities: either he is arrogant and ignorant; or he has actually worked on projects with his mentor and studied the theory diligently. For us power engineers, I will list the things that must be understood in analog electronics:

　　1. Resistors. Resistors are the most basic components in various electronic circuits. The main applications of resistors in switching power supplies are the voltage divider networks of various control return circuits, and then the power dissipation in the absorption loop. In our design, we must pay attention to the resistor packaging, power consumption, voltage resistance, and accuracy.

　　2 Transistors. Transistors have two uses in switching power supplies: First, as a switch tube. The switch tubes of switching power supplies are mainly MOS tubes, triodes, and IGBTs. Second: for signal processing. In the control circuit of the switching power supply, the most commonly used transistor is a simple small signal switch in the protection circuit, and then a linear voltage regulator (auxiliary power supply in the main circuit).

　　What do you need to know? At the beginning, it is OK to know how to turn on and off the transistor. Then know what is the linear working state and what is the switch state. Put aside the messy calculations in the book. You will hardly use them in normal times. When you need them, you can check them and understand them quickly. Don't dive into the theory. It will waste time and energy. When you need them, first refer to the component specifications, then ask others, and then go back to read the book.

　　3 Diode. Forward conduction, reverse cutoff. Knowing what is diode junction capacitance, diode turn-off time, reverse withstand voltage, forward conduction voltage, forward continuous current, pulse current, these concepts are OK, basically enough. When you encounter problems at work, then go back to read the book.

　　4 Operational amplifier. This thing is really important in power supply design. You must understand the virtual short and virtual disconnection that teachers taught in school. This cannot be discounted. It is the basis for the design and calculation of feedback amplifiers, current amplifiers, and various amplifiers. As for frequency characteristics and phase characteristics, those who are capable should of course learn them, which are also very useful. For the time being, leave the others alone and don't get stuck on the theory.

　　5. Comparator. It is mainly used as a fast protection circuit in the switching power supply. Of course, operational amplifiers can also be used for protection, but especially for overcurrent protection, operational amplifiers are not good because the response time is too long; then there are frequency generators, PWM comparators, hysteresis comparators, etc. Compared with op amps, comparators are simpler.

　　Then electrical and electronic technology must master the following things:

　　1 Single-phase AC power.

　　2 Single-phase AC rectification.

　　3 Three-phase alternating current.

　　4 Three-phase AC rectification.

　　5. Capacitors. Classification of capacitors: electrolytic capacitors, the most commonly used rectifier and filter capacitors. Electrolytic capacitors are divided into polarities. If the polarity is reversed, the capacitor will undergo a chemical reaction and short-circuit and explode. The main parameters of electrolytic capacitors: withstand voltage, capacitance, equivalent resistance (ESR), operating temperature, service life, and appearance size. Ceramic chip capacitors and metal film capacitors are not divided into polarities and have small ESR. They are generally used for high-frequency bypass. Because the ESR of electrolytic capacitors is relatively large, high-frequency capacitors with small ESR are usually connected in parallel next to the electrolytic capacitors. Y capacitors are required by various standards. (Search on Baidu). Regarding capacitors, when analyzing circuits, there is one most important thing I must emphasize: when charging a capacitor, the current must flow from the positive pole of the power supply, through the capacitor, and to the negative pole of the power supply; when discharging a capacitor, the current must flow from the positive end of the capacitor through the external circuit back to the negative end of the capacitor.

　　5 Inductor. There are two types of inductors in switching power supplies: one is used for energy storage, and the other is used as a filter.

　　6 Transformer, the transformer in the switching power supply refers to the transformer, which has the same working principle as the power frequency transformer. (Flyback, LLC transformer, in the strict sense, is not a transformer, these two are coupled inductors).

　　7 Magnetic core. You must know: what is the core cross-sectional area Ae, what is the window area Aw, what is the BH curve, this must be understood.

　　8 Copper wire. You must know what penetration depth is, what current density is. Why use triple-insulated wire, and why you need to build a retaining wall.

　　The above should be basically enough. If there are any omissions, please give me some advice and feel free to add them. Don't mislead others.

　　If a power supply engineer who has just entered the industry can fully understand some of the basic concepts listed above, then you can be a qualified assistant and are welcome to join the power supply industry.

　　What I want to emphasize is that we are engineers, not theoretical people. The most effective way to learn is to find problems in work and solve them in learning and experimentation. Purely sticking to theories and purely trying things out are not advisable.