It must be one of the difficulties of switching power supply to make a good PCB board (the PCB design is not good, it may lead to the situation that no matter how the parameters are debugged, it is not alarmist). The reason is that there are still many factors to consider in PCB layout, such as: electrical performance, process route, safety requirements, EMC impact, etc.; among the factors considered, electrical is the most basic, but EMC is the most difficult to understand , The bottleneck of the progress of many projects is the EMC problem; the following will share with you the PCB layout and EMC from twenty-two directions.

1. The EMI circuit of PCB design can be easily carried out only if you are familiar with the circuit

The impact of the above circuit on EMC can be imagined. The filters at the input end are all here; the voltage sensitivity to prevent lightning strikes; the resistance R102 to prevent inrush current (with the relay to reduce losses); the key differential mode X capacitors and and Inductor and Y capacitor for filtering; there is also a fuse that affects the safety board; each device here is very important, and the function and function of each device should be carefully tasted. When designing the circuit, it is necessary to consider the EMC severity level of the design, such as setting several stages of filtering, the number and position of the number of Y capacitors. The choice of the size and quantity of the pressure sensitive is closely related to our demand for EMC. We welcome you to discuss the seemingly simple EMI circuit that actually contains profound truths for each component.

2. Circuit and EMC: (the most familiar flyback main topology, see which key places in the circuit contain the EMC mechanism)

Circle some parts in the circuit in the figure above: the influence on EMC is very important (note that the green part is not), such as radiation, everyone knows that electromagnetic field radiation is spatial, but the basic principle is the change of magnetic flux, which involves the effective cross-sectional area of the magnetic field , which is the corresponding loop in the circuit. Electric current can generate a magnetic field, which produces a stable magnetic field, which cannot be converted into an electric field; but a changing current produces a changing magnetic field, and a changing magnetic field can generate an electric field (in fact, this is the famous Maxwell equation, which I use in plain language), changing The electric field can produce a magnetic field in the same way. So be sure to pay attention to those places where there is a switch state, that is one of the EMC sources, here is one of the EMC sources (one of which will be discussed here, of course, other aspects will be discussed later); for example, the dotted loop in the circuit is the opening of the switch. When designing the circuit, not only the switching speed can be adjusted to affect the EMC, but also the area of the layout circuit loop has an important impact! The other two loops are the absorption loop and the rectification loop. Let's understand it in advance and talk about it later!

3. The relationship between PCB design and EMC

1. The impact of the PCB loop on EMC is very important, such as the main power loop of the flyback, if it is too large, the radiation will be poor.

2. The effect of filter wiring. The filter is used to filter out interference, but if the PCB wiring is not good, the filter may lose its intended effect.

3. In the structural part, the poor grounding design of the radiator will affect the grounding of the shielded version, etc.;

4. The sensitive part is too close to the source of interference. For example, the EMI circuit is very close to the switch tube, which will inevitably lead to poor EMC. A clear isolation area is required.

5. The trace of the RC absorption circuit.

6. The grounding and wiring of the Y capacitor, and the position of the Y capacitor are also critical, etc.!

and many more. Think of this and say these things first, and we will discuss them in detail later. Let’s start with an introduction.

Here is a small example:

As shown in the dotted box in the picture above, the X capacitor pin traces have been shrunk inwards. You can learn how to make the capacitor pin traces external (using squeezing current traces). In this way, the filtering effect of the X capacitor can reach the best state.

4. Preparations for PCB design: (only when you are fully prepared, you can design steadily step by step to avoid overturning the design)

There are roughly the following aspects, which are all considered in the design process. All the content has nothing to do with other tutorials, but is only a summary of their own experience.

1. Appearance and structure dimensions, including positioning holes, air duct flow direction, input and output sockets, need to be matched with the customer's system, and also need to communicate with the customer about assembly problems, height limits, etc.

2. Safety certification, what kind of certification is made for the product, where the basic insulation creepage distance should be left enough, and where the reinforced insulation should be left enough distance or slotted.

3. Package design: There is no special period, such as custom parts package preparation.

4. Process route selection: single-sided double-sided selection, or multi-layer board, according to the schematic diagram, board size, cost and other comprehensive evaluation.

5. Other special requirements of customers.

The structural process is relatively more flexible, and the safety regulations are still relatively fixed. What certifications are required, and what safety standards have passed. Of course, there are some safety regulations that are common to many standards, but there are also some special products, such as medical treatment, which are more stringent.

In order not to be dazzled by new entry engineer friends;

Next, some common products are listed. The following are the specific layout requirements summarized by IEC60065. It is necessary to keep in mind the safety regulations, and specific products should be dealt with in a targeted manner:

1. The safety requirements for the distance between the input fuse pads are greater than 3.0MM, and the actual layout is 3.5MM (simply speaking, the creepage distance is 3.5MM before the fuse, and then the creepage distance is 3.0MM)

2. The safety requirements for the front and rear of the rectifier bridge are 2.0MM, and the board layout is 2.5MM.

3. There is generally no requirement for safety regulations after rectification, but the distance between high and low voltage should be kept according to the actual voltage, and it is customary to leave more than 2.0MM for 400V high voltage.

4. The primary inter-stage safety regulations require 6.4MM (electrical clearance), and the creepage distance is 7.6MM as the best. (Note that this is related to the actual input voltage, you need to look up the table for specific calculations, the data provided is for reference only, subject to the actual situation)

5. The cold ground and the hot ground for the primary and secondary use are clearly marked; the L, N mark, the input AC INPUT mark, the fuse warning mark, etc. need to be clearly marked.

If you have any questions about the above, you can also discuss and learn from each other!

It is reiterated that the actual safety distance is related to the actual input voltage and the working environment. It is necessary to check the table for specific calculation. The data provided is for reference only, and the actual situation shall prevail;

5. Consider other factors in the safety regulations of PCB design

1. Understand what certification your product is, and what product category it belongs to. For example, medical, communication, electricity, TV, etc. are different, but there are many similarities.

2. In the area close to the PCB layout in the safety regulations, understand the characteristics of insulation, which areas are basic insulation and which areas are reinforced insulation, and the insulation distances of different standards are different. It is best to check the standard and calculate the electrical distance and creepage distance.

3. Pay attention to the safety components of the product, such as the relationship between the transformer magnetism and the primary and secondary sides;

4. The distance between the radiator and the surrounding area, the grounding of the radiator is not the same, the insulation condition is also different, whether it is connected to the ground or the cold ground, and the insulation of the hot ground is also the same.

5. Pay special attention to the distance of insurance, where the requirements are the strictest. The distance between the front and rear of the fuse is the same.

6. The relationship between Y capacitance and leakage current and contact current.

The follow-up will explain in detail how to keep the distance and how to meet the safety requirements.

6. Power layout of PCB design

1. First measure the size of the PCB and the number of components, so that the density is consistent, otherwise one piece is dense, and one piece is sparse and ugly.

2. The circuit is modularized, the core device is the center, and the key devices are placed first, and the devices are placed at one time.

3. The device is placed vertically or horizontally. One is beautiful, and the other is convenient for plug-in operation. In special cases, tilting can be considered.

4. The layout needs to consider the wiring, and place it in the most reasonable position to facilitate subsequent wiring.

5. Minimize the loop area as much as possible during layout. The four major loops will be explained in detail later.

To achieve the above points, of course, it must be used flexibly, and a more reasonable layout will be born soon.

The following is the first virgin PCB board I drew. It was done many years ago. It was very hard to complete at that time. There may be small problems in the middle, but the general layout is still worth learning:

The power density of this picture is still relatively high. The control part of the LLC, the auxiliary source part and the BUCK circuit driver (high-power multiple output) part are on the small board, so I didn't take it out. Let's take a look at the layout characteristics of the main power:

1. The input and output terminals are fixed and cannot be moved. The board is rectangular. How to choose the main power flow?

Here, the layout is from bottom to top, from left to right, and the heat dissipation depends on the shell.

2. The EMI circuit still has a clear flow direction, which is very important. Otherwise, it will be messy and not good for EMC.

3. The position of the large capacitor should consider the PFC loop and the LLC main power loop as much as possible;

4. The current on the secondary side is relatively large. In order to carry the current and dissipate heat from the rectifier tube, such a layout is adopted. The rectifier tube is on the top and the MOS tube of the buck circuit is on the bottom, so the heat dissipation effect is good; just.

Each board has its own characteristics and of course its own difficulties. How to solve it reasonably is the key. Can you understand the meaning of rational selection of layout?

7. Appreciation of PCB examples

You can check this board according to the PCB layout points discussed earlier, whether it is done well, I think it is a better place, of course, there will always be flaws, you can also bring it up, the single panel is so compact, it can be done like this. It is not easy, you can use this board to learn and discuss! I will explain and learn about this board later, so please enjoy it first.

8. Understanding of the four major loops in PCB design: (The basic requirement of PCB layout is that the area of the four major loops is small)

In addition, the absorption loop (RCD absorption and RC absorption of MOS tube, RC absorption of rectifier tube) is also very important, and it is also a loop that generates high-frequency radiation. If you have any questions about the above picture, you are welcome to discuss, not afraid of any doubts, As long as it is a question for the problem, discussing and learning together can make greater progress!

Nine, PCB design hotspots (floating potential points) and ground lines:

Precautions:

1. Special attention must be paid to hot spots (high-frequency switching points), which are high-frequency radiation points, and the layout and traces have a great impact on EMC.

2. The loop formed by the hot spot is small, the trace is short, and the trace is not as thick as possible, but enough for enough current.

3. The ground wire should be grounded at a single point. The main power ground and the signal ground are separated, and the sampling ground is taken separately.

4. The ground of the radiator needs to be connected to the main power ground.

10. EMC rectification experience

All are personal understandings, and may be different from traditional materials and textbooks. Please consider for yourself. Anyway, I feel that many general textbooks are not used by myself, and I boast. I want to say a lot, it may be a little messy, but it's all practiced!

How to understand the EMC generation and the results measured during the test: in short, how to prescribe the right medicine, how to get the first round of test results in many cases, and how to compare the results with the power supply; the theme ideas are as follows:

1. For conduction, the standard test range is 15K- 30M , and the common EN55022 is 150K. How did the source of conduction come about? For low frequencies, mainly the switching frequency and its multiplier (there will be diagrams later), this cannot be solved from the source, and the switching frequency cannot be eliminated. Of course, you can change the switching frequency, which only moves the test results, and Not really eliminated. It can only be solved by a filter. Generally speaking, the high magnetic flux material such as R10K has a good effect for low frequency. The size of the magnetic ring is related to your power. Generally, the inductance reaches 10MH, or even 20MH. Capacitors are generally easy to solve, low frequency is not a problem; the real difficulty is high frequency. Personally, I think the cause of high frequency is much more complicated. There are switches, transformers, and inductances. That is, everything has an on-off state. There may be places where there is a problem (how to judge the specific location, follow-up explanation), it needs some exploration here; finding the source may not be able to solve the problem, there may be improvements, or it is necessary to cooperate with the filter. For high frequency, use low magnetic flux material, such as nickel-zinc ring, the inductance is generally UH level, with suitable Y capacitors (complex power supply, it is recommended to leave a few more Y capacitor positions when laying out the board, which is convenient for rectification);

2. For some coordination methods, many textbooks mention increasing the X capacitor to judge differential mode or common mode. It has a certain meaning and may not be helpful in reality. Generally, we will put the X capacitor at an appropriate value during design. And increasing the X capacitor can solve the differential mode problem, which is also nonsense, so many textbooks provide a certain meaning guide, and I personally think it is useless. I think there are several better methods: 1. Comparing the difference between grounding and incomprehensible grounding, no grounding may be worse, because there are fewer conduction paths in the system structure; it may also be improved, indicating that it is conducted to the port through the ground loop . The specific solution is to adjust the point Y capacitance of the circuit grounding and add magnetic beads. 2. Set a magnetic ring on the input port. If the low U-ring is improved, adjust the first-stage filter inductance. 3 For complex systems, pay attention to shielding measures for EMI circuits. If the measures have no effect, reflect on the PCB design, which will be discussed in the PCB design.

3. For radiation: The source must be found to solve it, and the first test result must be observed. If it exceeds 30M, it is related to grounding. Find the grounding on the system, and judge whether the grounding is good during the test. Sometimes the input line has an impact. 2. Within 40M-100M, it is generally caused by the on-off of the MOS tube. Sometimes, it is not easy to directly judge whether it is on or off, and the observation results can be rectified to verify (of course, this will cost money, and I will explain how to use it later). Oscilloscope to judge, this is a secret trick). 3 100M and above are mostly caused by diodes, rectify the diode absorption capacitance, and some high-power ones may be synchronous rectification, change the MOS tube absorption loop, remember that sometimes when adjusting C, you have to cooperate with R rectification.

There is too much to say. I will add more specific examples later. I will fight so much first. Anyway, I have fought hard enough, and it is difficult to resonate. After all, everyone’s rectification experience is much different, so let’s give some inspiration to newcomers and friends. Examples will follow!

10. Filter capacitor traces for board traces

The wiring of the filter capacitor plays a vital role in the filtering effect. If it is not well connected, it may lose its proper filtering effect.

Figure 1 shows the secondary-side rectification and filtering method, so that the effect of the two capacitors is shared to avoid the failure of the second capacitor in the rectifier loop.

Figure 2: The wiring for the output filter capacitor must not be plugged in (that is, bypassed).

12. Layout and EMC of LLC circuit

The LLC circuit is most familiar to everyone. The dotted circle is the driving circuit, which is placed close to the MOS tube during circuit design. That is to say, the driver provided by the IC only needs to lead two wires to the driving circuit, and the driving circuit is close to the MOS tube to avoid Disturbed (at the same time, pay attention to the drive to interfere with sensitive signals when routing, which is both a sensitive signal and a source of interference); once the drive is interfered with the power supply can be imagined.

In the same way, the MOSFET driver of synchronous rectification should also be close to the synchronous rectifier tube. When designing the schematic diagram, it can be well understood by placing it like this picture. If you give this circuit to the PCB engineer, he will be very intuitive on how to layout the wiring. , if you draw in a mess, many PCB engineers may easily layout the wrong board if they understand the circuit thoroughly.

In addition: there is an important loop on the primary side, the loop area formed by the PFC capacitor, the MOS tube, the transformer, the resonant inductor and the resonant capacitor is small;

The secondary side rectification filter loop is equally important, and the wiring of the capacitor is also very important as mentioned before;

When routing, pay attention to the distance between high and low voltage. In some places, the voltage is floating and must be treated as a high voltage, such as the upper tube driver and the corresponding reference voltage.

As for EMC, the turn-on of LLC is a soft switch. Turn-on has almost no effect on EMC. The focus is on the effect of the turn-off speed on EMC; and the junction capacitance of MOS transistors has a great influence on EMC, so the choice of capacitor is not appropriate, or Whether it is not added (the MOS tube itself also has junction capacitance) may have an impact on EMC, this is the key point to pay attention to; this picture has no Y capacitor, and the positive or negative anti-Y capacitor of the MOS tube can also filter out the switching interference very well;

13. PFC of circuit design and layout

The figure above is a typical BOOST PFC circuit:

The green box on the left is the driver circuit. Like the previous LLC topology driver, it is placed close to the MOS, and it is reflected in the schematic diagram.

The green dashed box on the right is the MOS tube turn-off peak absorption circuit, which is the same as the MOS tube to form the smallest loop;

The other two important loops, one is the MOS tube turn-on loop (dotted line in red), and the other is the MOS tube turn-off loop (solid line in red); the loop area is as small as possible;

14. The Magical Use of Magnetic Rings in EMC

For some products, EMC is difficult to deal with at the source. Magnetic ring filtering can be used. Of course, the magnetic ring I am talking about here has two meanings. On the one hand, it is the filter inductance of the input and output terminals, which uses magnetic rings of different materials and different turns. There will be corresponding effects. On the other hand, it means that the magnetic ring is directly placed on the input and output lines. Sometimes it can be used for magic, but it can not be used in all occasions, at least it can be used as a basis for judgment;

The blue and black lines in the picture above are the positive and negative ends of the output, and a magnetic ring is placed on it to solve the high-frequency end excess caused by the output rectifier; sometimes the interference of the port may not be effective by adding a filter on the PCB board. Putting the magnetic ring online has unexpected effects.

15. Key signals of PCB traces

Notice:

1. CS signal (sampling signal): Pull out from the sampling resistors R25 and R26. Note that the ground wire of the IC is based on the sampling resistor, and the positive and negative differential traces of the sampling resistor pull down the IC CS pin and the IC GND pin.

2. The drive signal pulls down the IC drive pin from the drive circuit, be careful not to interfere with the CS pin; as shown in the figure, the three lines are routed side by side, and the ground line is routed between the drive first and the CS line to play a certain shielding role;

3. It is best to shield the IC with a ground layer on the double-sided panel. The grounded network must be drawn from the IC GND. The non-critical signal GND can be directly punched through holes. The key signal ground needs to be grounded at a single point and directly connected to the IC;

4. FB feedback network signal, pay attention to check the wiring and connect to the IC at a single point;

5. The RCD absorption network should not be placed in the main circuit;

6. The rectifier and filter ground of VCC needs to be connected to the main power ground, and the secondary filter can be connected to the IC ground;

7. The Y capacitor wiring should be connected separately, and should not be confused with the main power to avoid interference;

16. Wiring diagram of main power and control part

Many people may see this picture, which is cloudy and foggy, so I will briefly introduce it:

1. The driver of the PFC and the IC are connected to the PFC tube, more specifically, the ground of the sampling resistor;

2. The drive ground and control ground of the DC-DC part are connected to the sampling ground of the DC switch tube part;

3. The auxiliary source part of the control ground is connected to the auxiliary source MOS tube for sampling, and the MOS tube ground is connected to the main power ground;

4. The power supply ground of each IC is connected to the IC ground through the auxiliary source EC filter, pay attention to the RC filter close to the IC;

Summary: Pay attention to the single-point grounding of each, and the grounding line is not messy, which is one of the most important places for wiring! ! !

17. Analysis of Electromagnetic Field Shielding Mechanism

Figure 1: Principle of Magnetic Field Shielding

As shown in the figure: the input and output electric field interference can be coupled through capacitive transmission. If the shielding plate is added, the size of C4 will be increased, and C1 will also be reduced, which will attenuate the electric field interference;

Figure 2: Principle of Magnetic Field Shielding

As shown in the figure: The characteristics of magnetic field shielding are different from that of magnetic field. It needs shell shielding, and electric field only needs a plane shielding plate. Therefore, the radiator shielding brings electric field shielding, and some use shell-enclosed power supply to play a certain magnetic field shielding;

The principle of magnetic field shielding, the magnetic field will change the magnetic circuit through the shielding cover, causing the magnetic field lines to spread to the surrounding, and the interference of the intermediate magnetic field achieves the purpose of shielding;

18. Switching devices and EMC

The understanding of the device is also of great significance to EMC, such as MOS tube, the main switch MOS is one of the important EMC sources, and the turn-on and turn-off of the rectifier tube will also generate high-frequency radiation (the principle is that the current generates a magnetic field, The changing current generates an electric field); of course, the semiconductor switching devices are mainly introduced here, and other inductive transformers will not be explained;

Which parameters of the switching device have an important impact on EMC, we often say that the fast tube, what is the reference for the slow tube? We all know that the fast tube has low turn-on loss, and we like to use it for high efficiency, but in order to pass the EMC smoothly, we have to give up the efficiency and reduce the switching speed to reduce the switching radiation;

For the MOS tube, the turn-on speed is determined by the drive resistance and the input junction capacitance; the turn-off speed is determined by the output junction capacitance and the internal resistance of the tube;

Referring to the above two pictures, it is a different type of MOS tube, compare the input junction capacitance and output junction capacitance, 2400PF and 800PF; 780PF and 2200PF; at a glance, we know that the first specification is a fast tube, and the second is a slow tube. At this time, the switching speed should be matched with the driving resistance. In general, there are many driving resistances in the range of 10R-150R. The driving resistance is related to the junction capacitance. The driving resistance of the allegro can be appropriately increased, and the driving resistance of the slow tube can be appropriately reduced;

For diodes, there are Schottky diodes, fast recovery diodes, ordinary diodes, and a less-used SIC diode. The switching speed of the SIC diode is almost zero, which means that there is no reverse recovery, the switching radiation is minimal, and the loss is also minimal. , the only disadvantage is that it is expensive, so it is rarely used; the second is the Schottky diode, the forward voltage is reduced, the reverse recovery time is short, followed by fast recovery and ordinary diodes; it is necessary to compromise between loss and EMC; generally Measures such as improving absorption and covering magnetic beads can be taken to rectify EMC;

Nineteen, EMC filter

The influence of the filter architecture selection on the filter is very important. In different occasions, the filter achieves the filtering effect according to the impedance matching. You can choose how to filter according to the principle of this figure; for example, the most commonly used output rectifier bridge uses π type filtering and LC filter at the output end;

The material of the filter is also crucial to the design of the filter inductance. Materials with different initial permeability will work in different frequency bands. If the wrong material is selected, the desired effect will be completely lost;

20. EMC flyback high frequency equivalent model analysis

First understand EMC from the simplest model:

The path of EMC, of course, the space radiation is related to the loop, and the loop is also constructed by the path; the high-frequency equivalent model of the flyback is analyzed to help understand the mechanism of EMC formation; our test receiving equipment will receive from the L and N terminals. Conduction, in order to reduce the interference received, it is necessary to let the interference flow through the ground loop instead of flowing from the L and N ports to the receiving device; at this time, our EMI inductance and Y capacitor can be achieved through impedance matching; in addition, the interference of the primary side can be Through the Y capacitance of the primary and secondary sides, the transformer stray capacitance and the ground coupling to the secondary side, more loops are formed; of course, some junction capacitance parameters, such as MOS tube junction capacitance and heat sink junction capacitance, can also form a flow path;

21. The form and frequency distribution of radiation

This diagram may be a bit abstract, but it is difficult for EMC to be specific, and we need to give us some inspiration. We know that the differential mode radiation exists in the form of a loop, while the common mode radiation is emitted in the form of an antenna; therefore, it just proves that As mentioned earlier, when we lay out the board, the layout of the switch loop and the wiring should not take an acute angle. The conventional route is 45 degrees, and it is best to use a circular arc. Of course, the routing efficiency will be relatively low;

The basic knowledge of these principles is well understood, and it is very useful for actually dealing with EMC work and layout. If there is no such awareness, it may be useless, because it cannot provide a direct method and needs to be combined with other knowledge;

Moreover, many of the principles mentioned here cannot be seen in many EMC materials, and they are not so concentrated, so you need to experience them repeatedly!

As shown in the figure: the relationship between some frequency terminals and the parts of the switching power supply, this is just a general rule, don't believe it completely; it is a rule but you can't fully believe it, why? The rule is not true in all cases, and the difference between different power sources is also very large, so the principle is to help you analyze, not to apply the method;

22. EMC Examples

According to the conduction example, the key point of the frequency distribution is the relationship between the specific data and the fundamental frequency. After this test, it is necessary to speculate on the law of these values, and what clues may be found; of course, how to solve these frequencies through the filter. The means have been mentioned before.