In the process of learning circuits, there are various effective study methods for each individual. Personally, I believe that the best way to study electronic circuits is to actually operate them. If you are curious about the mode of operation, it is not enough to just read books; it is important to purchase and examine the characteristics yourself. However, studying all circuits and components in this way can require a tremendous amount of time, effort, and money. Therefore, in such cases, studying with ready-made products can be very helpful. By the way, good products to study while watching are old analog measuring instruments or SMPS. These products include basic passive components such as resistors, capacitors, inductors, as well as diodes, transistors, FETs, optocouplers, comparators, and various components, making them a very good comprehensive gift set for learning electronics and circuits. Up to now, on YouTube.

Through the lecture video, I have introduced many example circuits, but in the future, I would like to explain how they are applied in actual products. Electronic products are not effective as circuits alone; factors such as component selection, layout, PCB heating, etc., need to be considered based on the circuit. Therefore, using finished products for research can reduce many trial and error processes. In addition, in the case of SMPS, as the power increases, there are many situations to consider. Therefore, comparing topologies and standardized methods used in circuit design can be very helpful in understanding power electronics concepts. In the future, I will introduce various SMPS products, explaining internal circuits and PCB design methods, while relating them to the operation of electronic circuits and related concepts. If you have any questions you have been curious about while watching the video, please leave many comments. Today, as the first topic, let's first check the SMPS with relatively low output power. Well, then.

Now, let's take some time to study together about my favorite topic, SNPS. I will divide our discussion into four main parts. First, I will talk about the overall function. Lastly, we will delve a bit more into the operation of the secondary side circuit, specifically the PD circuit. So, let's start with what SNPS does. If I introduce this first, it would be great. The device I brought with me has an output power of 15W, taking in AC 220 volts as input and finally outputting DC 12 volts. It is a common SMPS DC-DC converter circuit that utilizes the principles of such circuits to transform AC 220 volts. Let's think about how it operates together. Now, firstly, about the input circuit, what components were used in the input circuit.

I'll consider the focus a bit. Let's take a look at the input circuit together. The input circuit receives AC 220 volts. So, in the end, it is supposed to output DC 12 volts. At this point, what goes in between? A circuit is needed to convert AC to DC. Since we cannot directly convert DC to DC, we first convert AC to DC and then convert it back to DC. Therefore, the circuit for converting AC to DC goes in first. Now, this is typically constructed using familiar components like rectifier diodes and filter capacitors. So, if you follow the circuit through the terminals where AC 220V enters, you can see that there are bridge diodes and filter capacitors here. Now, what components are in between? Firstly, there is a fuse, an S capacitor, and a line filter. Let me draw the circuit for you. AC 220V comes in.

Fuses are usually drawn like this, and here is an X capacitor, and a line filter is placed like this. I haven't explained in detail about the component called a line filter yet, but when it is designed like this, with complete symmetry on both sides, it suppresses common mode noise. There are two types of noise, differential mode noise, and common mode noise. By creating a line filter like this, it is effective in suppressing common mode noise. When AC 220V input is applied, there may be many peripheral devices using AC 220 volts. When these devices operate, SMPS noise can come in. Also, this SMPS repeats on/off with a high switching frequency, causing a lot of noise. These noises can travel along this line and may go out. So, what function does the line filter serve here? It prevents such noise from entering or leaving.

It plays a role in blocking to prevent it. By configuring a capacitor line filter and LC filter used here, it forms a low-pass filter, which allows low frequencies, such as 60Hz, to pass through without any issues, while blocking high-frequency components from passing through. Also, if you look at the back of the PCB here, you can see resistors connected in parallel with these X capacitors. These resistors are used to discharge the X capacitors. Here, AC 220 volts are input, and when you turn this off, the charges are still present on the X capacitors used to remove noise. Since the capacitance of these capacitors is not very high, they discharge quickly, but there may still be remaining charges. If a person touches these remaining charges by hand, there could be a risk of electric shock. Therefore, it would be good to discharge these charges. For this purpose, discharge resistors are used here.

It is necessary to reduce the resistance value according to the capacity of the x capacitor used. If there is a lot of noise in the environment, you should increase the capacity of the filter accordingly to effectively reduce the noise. By increasing the capacity of the filter to reduce noise, the amount of charge on both ends of the x capacitor increases when the power is turned off. To sufficiently eliminate this, the discharge resistance value should be reduced accordingly. So, in general, the most commonly used resistance value is 1 megaohm. As the SMPS power increases from 1 kW to 3 kW, the resistance values may need to be lowered to 680k or 470k. However, since this is a device that outputs a very low power of 10W, it may not be necessary to go to that extent. I will provide more detailed explanations on these aspects in the video later on.

There is no need for it. Now, looking at the pattern of the line filter, some people call this the pattern of alligator teeth. To express it more formally, it's called a spark gap. This may be a little difficult to understand right now, so I will introduce it again later. Then, passing through the bridge diode and capacitor, let's take a look at the primary side circuit together. Also, in this input circuit, besides the components I have explained, components like Y capacitor and varistor are often used. Since SNPS is not used, I couldn't explain it separately in this video. In the next video where I introduce SNPS, I will take the time to explain these parts in detail. So now, let's move on to examining the primary side circuit.

I will explain focusing on the operation principle of PWM flyback converters. Although it may be a bit exaggerated to say that almost 80% to 90% of low-power SMPS are designed in flyback mode, for example, the chargers you use at home to charge laptops or batteries are mostly, let's exaggerate a bit, over 95% flyback mode. Up to almost 100, flyback mode is widely used. Of course, recently, designs are also made in flyback mode combined with components like PFC. Anyway, this flyback method is a widely applied design method. If we do not understand the operation of PWM and flyback converters together, our understanding of the circuit is somewhat limited. That's why.

Let me draw a simple circuit and explain how it works while looking at the PCB. I will briefly explain the block diagram of the flyback converter. So, the transformer winds in reverse polarity here, uses an FT switch on this side, includes a diode here, and for the secondary side, diode and capacitor are used here. How does the flyback converter operate? By using a capacitor here, the DC voltage is applied. When the FT switch is turned on, the current flows in this direction. Let's look at the transformer of the flyback converter. When looking at the dot as a reference point, the positive voltage is applied in the direction without the dot.

Therefore, when the switch is turned on, the voltage reflected on the secondary side of the transformer becomes positive on this side. Since the diode is connected in this direction, current cannot flow to the secondary side. So, what happens in this situation is that the transformer operates as an inductor in a flyback converter, not as a transformer. When the switch is turned on and current flows, instead of transferring power due to this current, it charges the energy of the core. Going a bit deeper into this part, besides charging the energy of the core, when designing a flyback converter, a gap is used to store energy in that gap, to be more precise. Anyway, skipping these parts for now, the important thing here is that when the switch is turned on, energy flows according to the operating principle of this transformer.

It's not about transferring energy directly, but first, it's about charging energy through the inductor. And what happens when the switch turns off at that moment? The transformer acts. The inductor has a property of trying to maintain the flow of current flowing through itself. Therefore, when the current is flowing through itself and is suddenly cut off, in order to maintain this current flow, it becomes a voltage source itself. By becoming a voltage source, a positive voltage appears in this direction. So, when trying to maintain the direction of this current flow, when the switch turns off, the only path for the current to flow is in this direction. Therefore, due to this positive voltage, if we think in terms of a reference point, since the positive is applied to the reference point, similarly, this time, when a positive is applied to the secondary side in the reference direction, the current flows. When the switch is turned on, power does not flow, but when the switch is turned off.

The energy stored in the core is transferred to the secondary side. I briefly explained the operating principle of the flyback converter. I will provide a more detailed explanation later. Why did I suddenly explain this here? I mentioned the fly converter. However, when looking at the PCB, it is not visible. In the case of this PCB, the 7-pin or 8-pin IC embedded here. When designing PWM for SMPS using low capacity, many people embed it directly into the PWM chip to reduce the number of components and save PCB space. This can be a significant advantage. However, when heat is generated, there is no space to dissipate that heat, so it can only be used for low capacity. If we want to design with high power like 50W or 100W using a fly converter, we must definitely.

You need to design it in a form separated from PWM and F. Just by looking at it, they are not together. So, please refer to an SMPS circuit in which they are together. Additionally, I will explain a few more things that you can see here. If you follow the layout of this PCB and components, you will see that one pin of the transformer is connected to PWM and the other pin is connected to F. When switching this, the current should flow in this direction. So, you can almost always see a circuit formed from this pin to this side. We usually call this a snubber. I have explained snubber circuits in other videos as well. If you see diodes, high voltage capacitors, large capacitors, or components like TVS diodes or Zener diodes used here, you can just consider it as a snubber circuit.

I would appreciate it if you could understand. The most commonly used method in configuring a snubber circuit is by connecting a diode, capacitor, and resistor. However, sometimes people design it simply with just a diode and a TVS diode like this, depending on the power. I will delve deeper into these details in another video. So far, I have explained the operation principle of the 1st stage PWM IC and flyback converter. I will now explain the operation of the GaN PD circuit related to the operation of this PWM. Then, let's move on to the 2nd stage circuit. Aren't you all having fun? Let's study together, not only the 1st stage but also the 2nd stage. When you look at it like this, this side is the 1st stage, and this side is the 2nd stage. When AC 220V comes in, it is rectified peacefully.

The circuit goes through here, then switching is done, and finally, DC voltage is output through the transformer. Oh, and one more thing, earlier I mentioned that when we receive AC 220 volts as input, we convert it to DC, then convert it back to DC. Here, actually AC comes in. The transformer reacts to AC input. In the first stage, through PWM control, switching occurs, turning it into DC. So, using this AC transformer, we first pass AC, then use the diode and capacitor shown here to create DC again. If you look closely, you can see that the transformer, diode, and capacitor are placed very close together. Placing them close like this helps reduce noise. Why does reducing noise help? If I explain this, there are many interesting parts, which I will explain a bit more specifically in the next video.

I will explain. For those who are not very familiar, you may wonder why a component looks like this and why it's called a diode. In the case of this component, it is in a TO-220 package with three legs. Inside, you will find the diode like this, two of them. So, this part and this part are the anode and cathode, making it a common cathode. In the circuit, after passing through a transformer here, there are diodes and capacitors. Depending on the case, there can be multiple capacitors as it is a noisy area, so often an LC filter is used additionally to filter it out. Ultimately, through here, a 12V output is obtained. Since this is a simple SMPS, you can see two filter capacitors used here and one inductor here.

It seems and what LED is and there is a variable resistor here that can adjust the output voltage. So, from now on, what part will I explain? In the last video, I once covered this TL431, so I will briefly explain the operating principle of this PD circuit, which can be considered a key point in understanding this SNPS circuit. Therefore, I will draw a circuit and explain. Since all circuits are the same, it is inevitable that they will be the same without analyzing each pattern one by one. That's how it works. So, I suggest that you remember the circuit I draw once, and if you want to study, try analyzing the circuit directly. Drawing one circuit and explaining the operating principle of this PD circuit and a few more related parts together, I will provide some more explanations. Let's draw the circuit.

What does the circuit look like? Well, let me redraw it from the second stage. Here, there is a diode, a capacitor, and the output goes this way. And there can be some other parts added in between, but I will skip those for now. So, how does it look? There is a port coupler here. Yes, there is a port coupler here. I will briefly skip explaining what kind of component it is. If I explain everything at once, it might be a bit difficult. I will focus on explaining the overall operation principle and will explain the specific operation principles of each part later. The structure is set up like this in the feed. Photon, if you want to understand it easily, it's like a transistor. How about a transistor? It has a base, a collector, and an emitter. The current flows from the base to the emitter.

When it flows, the current and the hfe it has amplify the current flowing in the direction of the collector's emitter. This friend is called an LED emitter direction, and the current flowing in this direction can be thought of as the base current in a transistor, and it can also be thought of as HF. This friend, called current transfer ratio, when the current flows into the base, amplifies the current due to what it has, allowing the current to flow in this light-receiving part. And as I introduced in the previous video, the TS31 SS was used a lot. Here, I don't see a T431. Where is it? If you look at the back, there are components with three legs like this. Even if you don't take a picture, it's definitely a T43. There are packages that look like this, and there are friends with three legs like TO92, with a variety of components.

I will explain how it works. There are two things you need to know. One is the operating principle of T41, and the other is that this photocoupler is used in the linear region, not the switch region. For those of you who study digital circuits or are a bit distant from SMPS, you may feel a bit awkward about this part because when it comes to photocouplers, they are mostly used to send digital signals in circuit relationships related to switching operations. So, some may find it a bit awkward. In the case of SMPS, the photocoupler is used in the linear connection, not for switching purposes. Understanding the operation of TL431 is essential to understand the operation of the PD circuit. When AC 220V comes in, it should ultimately output DC 12V. If we talk about a stable and ideal power supply, this DC voltage should never fluctuate at any moment.

It is a fact that 12V cannot be done. First, it would be nice if the output voltage remains the same even if the input voltage changes. Second, it is the output current. It would be nice if it stays constant at 12V regardless of the current supplied to the 12V load, whether it is 0, 1, 2, or 3 amps. Therefore, we call these two characteristics line regulation and load regulation. Having these characteristics is what makes an ideal power device. There are many factors involved in making this work. For an SMPS that needs to deliver a lot of current, the impedance of the device should be low to allow a large current flow. Therefore, various components used here should be thick, and when using diodes, the vf should be low. There are relationships like these. Let's skip all these complex factors and talk about one simple thing.

When providing power, it is important to maintain a 12V voltage when the load is connected. So, here the load is connected, meaning that current flows. This indicates that the charge stored in the capacitor flows to the load. So, what happens in this case? As the charge stored in the capacitor flows to the load, what will happen to the voltage at both ends of the capacitor? It will drop, right? So, we need to compensate for this. Since it has dropped, we need to refill it. How can we refill it? Current needs to flow in this direction. However, should the current that flows at this time always be constant? No, if it increases too much, the voltage will rise a bit, so it needs to be reduced. And when this voltage drops, it needs to be refilled again, so it will always be different. So, with what kind of information can we monitor this and provide feedback?

It would be nice if it exists. Then, one idea came to mind here. We are going to apply the concept of negative feedback. What is negative feedback? It means that if something tries to go up, we push it down, and if it tries to go down, we lift it up. In this way, the phase is reversed, and we stabilize the system. By using this circuit, we make it operate in a negative manner. Our friend T4-3 ensures that the voltage here remains at 2.5V and controls the current flowing here. I have explained the operation of this friend in the last video. It is a transistor configured for current amplification. Please review the video again. If this circuit operates normally, the voltage at this pin must always be maintained at 2.5V. Macroscopically, it should always be at 2.5V. Microscopically, if you look closely, a little.

The voltage fluctuates little by little. Let's assume a situation where the DC voltage drops once. The fact that this voltage drops means that the charged phone is discharged as a load, which means a lot of current flows. So, we need to supply the required power from here at the primary stage. How does this operation take place? When the output voltage drops, the voltage ultimately applied here also drops. Although the voltage is said to be constant macroscopically, it was mentioned that it fluctuates microscopically because current flows and causes the voltage to drop slightly. Then, due to the operation of 431, the voltage of the source will change. How does it change? I will talk about this part separately in another video. When the voltage changes here, the current flowing in the LED direction will change. Similarly, the current flowing in the photo reception part here.

It will change. So, how is this circuit usually configured? It is usually configured in two ways. I will explain that part. What we are doing now is this. In the case of a photocopler LED, the current flows in this direction. So, we usually call it a phototransistor or a light receiving part because it receives light. The circuit is configured in two ways. First, we connect this to the ground and connect this pin to a specific side of the PWM. As the output current changes, the current flowing to the light emitting part changes, and the current flowing to the light receiving part will change. Since the current flowing from the PWM changes, we adjust the PWM duty to turn it on and off based on this information. Another way to configure the circuit is to connect a resistor to the emitter here and connect the voltage here.

Let's input a signal to the PWM. When the current changes here, the voltage across this resistor changes. By reading this voltage, we can adjust PWM 2 in the same way. Therefore, by using this principle, we can ultimately maintain a stable DC 12 volts. Here, you can see there is a variable resistor to adjust the output voltage. This variable resistor can be installed on either side. Since the voltage here must always be maintained at 2.5V, by adjusting the resistance value of this feedback circuit, we can adjust the output voltage. Of course, when determining the output voltage, it is not only related to this feedback but also affected by the transformer's turns ratio. So, there are some more complex factors influencing it, but understanding this circuit conceptually would be beneficial. So, if you see here, you can adjust the output voltage by adjusting the resistance value of this feedback circuit.

I've introduced one part that I want to share. One more thing to mention is, here is a blue-colored capacitor. This connects the first side and the second side. It's called a Y capacitor in terms of component names. It is used to address noise issues. By using a spectrum analyzer, you can compare the noise characteristics when this friend is present or absent. When using this friend, the noise characteristics are greatly improved. Of course, depending on how you arrange the parts and connect the patterns when designing the PCB, there will be differences. It's good to understand that there is quite a difference in noise characteristics depending on whether this component is present or not. So, I briefly explained about this Y capacitor. When using this Y capacitor, how you arrange it and connect the patterns.

Depending on how you do it, there are many interesting parts, so when I explain more detailed and specific contents, I will take the time to explain it again. Thank you for watching the video. When you look at this SMPS, even with just this piece of PCB, there are countless things we can learn. So, every time I study this SMPS, I feel that there are always new and interesting things to learn, and there is always something new to learn. Therefore, I will do my best to share what I know with you in those aspects, so if you have any questions you have been curious about, please leave a comment, and I will prepare and shoot a video. Thank you for watching the video.

SMPS 플라이백 컨버터 동작과 회로 이해하기

회로 공부를 시작하며 전원회로에 많은 관심이 생겼습니다. 콘덴서와 인덕터, 트랜스포머와 같이 큰 부품들을 보고 있으면 기분이 좋았고 재미로 시작했던 일에 전문성을 가지고 싶다는 욕심이 생기기 시작했습니다.

SMPS 회로를 이해하고 직접 만들고 싶다는 생각으로 무작정 부딪혔습니다.. 하지만 아쉽게도 국내에는 정보 공유가 활발하지 않아 많은 어려움을 겪었습니다. 시간이 흘러 조금씩 트랜스포머를 이해하고 직접 설계를 할 수 있는 능력이 생겼으며 이러한 과정을 공유하고 싶다는 생각이 들었습니다. 

전원회로, SMPS 에 대한 내용을 주로 다룰 것이며 그와 함께 PCB 수리나 전자제품의 내부 회로에 대한 이야기를 다루고 싶습니다. 제 채널에서 많은 도움을 받으셨으면 좋겠습니다. 질 좋은 정보를 공유드리기 위해 노력하겠습니다. 감사합니다.
