[Foreword] Speaking of the difficult problem of switching power supply, PCB layout problem is not very difficult, but if you want to lay out a good PCB board must be one of the difficulties of switching power supply (PCB design is not good, may lead to no matter how to debug The parameters are debugged out, so it is not alarmist. The reason is that there are still many factors to consider when planning PCB layout, such as: electrical performance, process route, safety requirements, EMC impact, etc. The most basic, but EMC is the most difficult to understand, the bottleneck of the progress of many projects lies in the EMC problem; below, from the twenty-two directions to share the PCB layout and EMC.
1, familiar with the circuit can be easily carried out PCB design EMI circuit
The influence of the above circuit on EMC can be imagined, the filter at the input is here; the pressure sensitivity of lightning protection; the resistance R102 to prevent the inrush current (to reduce the loss with the relay); the key difference mode X capacitance and The inductor is matched with the filtered Y capacitor; there is also a fuse that affects the layout of the layout; each of the devices here is critical, and the function and function of each device should be carefully evaluated. The EMC severity level to be considered when designing the circuit is designed with ease, such as setting several levels of filtering, the number of Y capacitors, and the position. The choice of pressure sensitive size is closely related to our demand for EMC. Welcome to discuss the seemingly simple EMI circuit that each component contains profoundly.
2, circuit and EMC: (the most familiar flyback main topology, see which key parts of the circuit contain the mechanism of EMC)
The circuit in the above figure is looped in several parts: it is very important for EMC (note that the green part is not). For example, radiation everyone knows that electromagnetic field radiation is spatial, but the basic principle is the change of magnetic flux, and the magnetic flux relates to the effective cross-sectional area of ​​the magnetic field. That is, the corresponding loop in the circuit. The current can generate a magnetic field, producing a stable magnetic field that cannot be converted to an electric field; but the changing current produces a changing magnetic field, and the changing magnetic field can generate an electric field (in fact, this is the famous Maxwell equation I use in plain language), change The electric field is similar to generate a magnetic field. Therefore, we must 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 the following will of course talk about other aspects); for example, the dotted loop in the circuit, the switch is opened And the loop that is turned off, not only the switching speed can be adjusted to the EMC when designing the circuit, but also the layout area of ​​the layout routing has an important impact! The other two loops are the absorption loop and the rectification loop. Let's know ahead and talk about it later!
3. PCB design and EMC
The impact of the PCB loop on EMC is very important, such as the flyback main power loop, and if it is too large, the radiation will be poor.
Filter trace effect, the filter is used to filter out interference, but if the PCB trace is not good, the filter may lose its effect.
In the structural part, the grounding of the heat sink design will affect the grounding of the shielded version;
The sensitive part is too close to the source of the interference. For example, the EMI circuit is close to the switch tube, which will inevitably lead to poor EMC and a clear isolation area.
The trace of the RC absorption loop.
The Y capacitor is grounded and routed, and the position of the Y capacitor is also critical!
First think of this and say this, the follow-up will be discussed in detail, starting with a primer.
Here's a small example:
As shown in the dotted line in the above figure, the X capacitor pin routing has been retracted. You can learn how to make the capacitor pin wire plug-in (using a squeeze current trace). In this way, the filtering effect of the X capacitor can be optimally achieved.
4, PCB design preparations: (prepared enough, in order to design step by step, to avoid design overturning)
There are some aspects in the following, all of which are considered in the design process. All the content has nothing to do with other tutorials. It is just a summary of their own experience.
Appearance structure dimensions, including positioning holes, air flow direction, input and output sockets, need to match the customer system, and also need to communicate with the customer on the assembly problems, height limit and so on.
Safety certification, which kind of certification for products, where to achieve basic insulation creepage distance should be enough, and where to strengthen insulation to stay enough distance or slot.
Package design: There are no special periods, such as custom package preparation.
Process route selection: single-panel double-panel selection, or multi-layer board, based on the schematic and board size, cost and other comprehensive evaluation.
Other special requirements of the customer.
The structural process is relatively more flexible, and the safety regulations are still relatively fixed. What certifications are used, what safety standards have been passed, and of course some safety regulations are common in many standards, but there are also some special products such as medical treatments that are more demanding.
For the new entry engineers, friends are not dazzled;
Next, list some common products. The following are the specific layout requirements for IEC60065. For safety regulations, you need to keep in mind that specific products should be targeted:
The distance of the input fuse pad is more than 3.0MM, and the actual layout is 3.5MM (simply say that the fuse is 3.5mm creepage distance before the fuse, then according to the 3.0MM creepage distance)
The safety regulations of the rectifier bridge are 2.0MM, and the layout is 2.5MM.
After rectification, the safety regulations are generally not required, but the high and low voltages are kept at a distance according to the actual voltage. It is customary to use 400V high voltage to stay above 2.0MM.
The safety requirements between the primary and secondary are 6.4MM (interval), and the creepage distance is best according to 7.6MM. (Note that this is related to the actual input voltage, you need to check the specific calculation, the data is for reference only, subject to the actual occasion)
The primary and secondary use cold ground, hot and clear identification; L, N logo, input AC INPUT logo, fuse warning logo, etc. need to be clearly marked;
Again, the actual safety distance is related to the actual input voltage and the working environment. It is necessary to check the specific calculations. The data is provided for reference only, whichever is the actual situation.
5, PCB design safety considerations consider other factors
Understand what kind of product certification, what kind of product, such as medical, communication, electricity, TV, etc., but there are many similar places.
In the place where the PCB is tight in the safety regulations, understand the characteristics of the insulation, what is the basic insulation, and which places are the reinforced insulation. The insulation distance of different standards is different. It is best to check the standard and calculate the electrical distance and creepage distance.
The safety device of the product pays attention to, such as the magnetic relationship between the transformer and the primary and secondary sides;
The problem of the distance between the radiator and the surrounding area is different from that of the ground where the radiator is connected. The ground is still cold, and the insulation is also the same.
The distance of insurance is especially important, and the most demanding places are required. The distance between the front and rear of the fuse is the same.
Y capacitance and leakage current, contact current relationship.
Wait, the follow-up will explain in detail how to stay and how to do the safety requirements.
6, the power layout of the PCB design
First measure the PCB size and the number of devices, so that the density is dense, or else a piece of dense, a piece of sparse is difficult to see.
The circuit is modularized, centered on the core device, and the key device is placed on the principle of placing the device at a time.
The device is vertically or horizontally placed, one is beautiful, and the other is convenient for plug-in operation. In special cases, tilt can be considered.
The layout needs to take into account the traces and place them in the most reasonable position for subsequent routing.
Minimize the loop area during layout, as explained in the four major loops.
To achieve the above points, of course, to be flexible, a more reasonable layout will soon be born.
The following is the first virgin PCB board I drew. Many years ago, it was very difficult to accomplish at the time. There may be minor problems in the middle, but the general layout is worth learning:
The power density of this figure is still relatively high. The control part of the LLC, the auxiliary source part and the BUCK circuit drive (high power multi-output) are not on the small board. Look at the layout characteristics of the main power:
The input and output terminals are fixed dead, can't move, the board is rectangular, how to choose the main power flow? Here, it is laid down from the bottom to the top, and the heat is relied on the outer casing.
The EMI circuit is still in a clear flow direction. This is very important. If it is not confusing, it is not good for EMC.
The position of the large capacitor takes into account the PFC loop and the LLC main power loop as much as possible;
The secondary side has a relatively large current. In order to take current and heat dissipation of the rectifier, this layout is adopted. The rectifier is on the upper side, and the BUCK circuit MOS tube is under the heat dissipation effect. The top layer of the high power is generally negative and the bottom layer is positive.
Each board has its own characteristics, and of course it has its own difficulties. How to solve it properly is the key. Can you understand the meaning of reasonable layout selection?
7, PCB example appreciation
According to the PCB layout points discussed before, check this board, whether it is done in place, I think it is a better place, of course, there will always be, you can also propose that the single panel is so compact It is not easy, you can learn from this board! Later, I will explain the learning for this board. Let's enjoy it first.
8. Recognizing the four major loops of PCB design: (The basic requirement of PCB layout is that the four major loop areas are small)
In addition, the absorption loop (RCD absorption and RC absorption of the MOS tube, RC absorption of the rectifier) ​​is also very important, and is also a loop for generating high-frequency radiation. Any doubt about the above figure is welcome, and there is no fear of any doubt. As long as it is a questioning of the problem, it is a better progress to discuss and study together!
9, PCB design hotspot (floating potential point) and ground line:
Precautions:
For hotspots, special attention must be paid to (high-frequency switching points), which are high-frequency radiation points, and layout traces have a great impact on EMC.
The loop formed by the hot spot is small, the trace is short, and the trace is not as thick as possible, but enough to take enough current.
Ground wire should be grounded at a single point. The main power ground is separated from the signal ground, and the sampling ground is separated.
The ground of the radiator needs to be connected to the main power ground.
10, EMC rectification experience
All are personal understanding, perhaps different from the traditional materials teaching materials, please consider it yourself, anyway, I feel that many common textbook results are not used by my own summary, boasting. I want to say a lot, maybe some chaos, all are practiced!
1. How to understand the results of EMC production and testing: In simple terms, how to prescribe the right medicine, many cases get the first round of test results, how to compare the results with the power supply; the main ideas are as follows:
1. For conduction, the test range is standard 15K-30M, and the common EN55022 is from 150K. How did the source of conduction come about? For the low frequency, mainly the switching frequency and its multiplier (followed by the diagram), this source can not be solved, the switching frequency can not be eliminated, of course you can change the switching frequency, it just moves the test results, and There is no real sense of elimination. Can only be solved by the filter, generally for the low frequency R10K high magnetic flux material has a good effect, the size of the magnetic ring is related to your power, generally up to 10MH sense, even larger to 20MH, with Y Capacitance can generally be solved very well, low frequency is not difficult; the real difficulty is high frequency, personally think that the cause of high frequency is much more complicated, there are switches, there are transformers, there is also the possibility of inductance, that is, everything exists in the switch state The place may exist (how to judge the specific location, follow-up explanation), here need to find some way; find the source may not be able to solve the source, there may be improvement, or the matching filter. For high frequency, low magnetic flux material, such as nickel-zinc ring, the sensitivity is generally UH level, with the appropriate Y capacitor (more complicated power supply, it is recommended to leave a few Y capacitor position when laying the board, easy to rectify);
2. Some cooperation methods, many textbooks mention increasing the X capacitance to judge the differential mode or the common mode. There is a certain meaning that the actual help is not great. Generally, our X capacitors will be put into appropriate values ​​during design. And increasing the X capacitor can solve the differential mode problem, but it is also awkward, so many textbooks provide a certain meaning guidance, personally feel that it is useless. I think there are several better means: 1. Summarize the differences against grounding and incomprehensibility. Ungrounding may be worse, because the system has less conduction paths; it may also be improved, indicating that it is conducted to the port through the ground loop. . Specific solutions, adjust the point Y capacitance of the circuit ground and add magnetic beads. 2. Set the magnetic ring on the input port. If the low U ring is improved, adjust the first stage filter inductor. 3 complex systems pay attention to the shielding measures of EMI circuits. If the measures have no effect, reflect on the PCB design, this aspect will be mentioned in the PCB design.
3. For radiation: the source must be found to solve the problem, and the first test result is observed. If it is beyond 30M, it is related to grounding, the system is grounded, and it is necessary to judge whether the grounding is good during testing, and sometimes the input line has an influence. Within 2.40M-100M, it is usually caused by the turn-off of the MOS tube. Sometimes it is judged whether it is turned on or off for the sake of the site. It can be verified by correcting the observation results (of course, this will cost money, and the follow-up will explain how to use The oscilloscope judges, this is a secret trick). 3 100M or more is mostly caused by diodes, rectify the diode absorption capacitance, and some of the high power may be synchronous rectification, change the MOS tube absorption loop, remember that sometimes it is necessary to adjust R with R.
Too much to say, follow-up to specific examples to add it, first hand to play so much, anyway, I played hard enough, can cause resonance is difficult, after all, everyone's rectification experience is much worse, just give some new friends a revelation, Follow-up examples!
11. Filter capacitor trace for layout trace
The trace of the filter capacitor plays a crucial role in the filtering effect. If it is not well taken, it may lose its proper filtering effect.
Figure 1 shows the secondary side rectification and filtering method to spread the effect of the two capacitors to avoid the failure of the second capacitor in the rectifier circuit.
Figure 2: For the output filter capacitor trace, it must not be plugged (that is, bypassed), and the output ripple is not good.
12. Layout and EMC of LLC circuit
The LLC circuit is the most familiar to everyone. The dotted circle is the driving circuit. When the circuit is designed, it is placed close to the MOS tube. That is to say, the driver provided by the IC only needs to draw two wires to the driving circuit. The driving circuit is close to the MOS tube to avoid Be disturbed (at the same time, when driving the line, pay attention to the drive to the sensitive signal, which is both the sensitive signal and the interference source); once the drive is disturbed, the power supply can be imagined.
Similarly, the MOS tube drive of the synchronous rectification should also be close to the synchronous rectifier. The design schematic diagram can be well understood as shown in this figure. If you give this circuit to the PCB engineer, he is very intuitive how to layout the trace. If you paint very messy, many PCB engineers may have a good understanding of the circuit and it may be easy to make a mistake.
In addition: the primary side has an important loop, the PFC capacitor and the MOS tube and the transformer, the resonant inductor, and the resonant capacitor form a small loop area;
The secondary side rectification and filtering loop is also important. It is also important to talk about the capacitors before they are routed.
Pay attention to the high and low voltage distance when routing. In some places, the voltage is floating and must be treated as high voltage, such as the upper tube drive and the corresponding reference voltage.
As for the opening of the LLC aspect of the LLC, the soft switch has almost no effect on the EMC. The focus is on the speed of the shutdown speed on the EMC. The capacitance of the MOS junction capacitor has a great influence on the EMC. The capacitor is not suitable, or Is not added (MOS tube itself also has junction capacitance) may have an impact on EMC, this is the key point of attention; this figure does not have Y capacitor, the positive or negative anti-set Y capacitor in MOS tube can also filter the switch interference well;
Any questions about this circuit can be discussed and grow up in the discussion!
13, circuit design and layout of the PFC
The picture above is a typical BOOST PFC circuit:
The green box on the left is the drive circuit, which is placed close to the MOS as in the previous LLC topology drive, and is shown on the schematic.
The green dotted box on the right is the MOS tube turn-off spike absorbing circuit, which is the smallest to form a loop with the MOS tube;
The other two important loops are the MOS tube open loop (dashed red graph) and the other MOS tube turn-off loop (solid red graph); the loop area is as small as possible;
14, the magnetic ring in the EMC magic
Some products EMC is difficult to deal with at the source, you can use magnetic ring filter, of course, the magnetic ring I said here has two levels of meaning, on the one hand is the filter inductance of the input and output, using different material magnetic ring, different number of turns There will be a corresponding effect, and on the one hand, it means that the magnetic ring is directly placed on the input and output lines, and 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 above are the positive and negative ends of the output, and a magnetic ring is placed on the top to solve the high-frequency end of the output rectifier. In some cases, the interference of the port on the PCB does not necessarily have an effect. The magnetic release ring on the line has an unexpected effect.
15, the key signal of PCB trace
note:
CS signal (sampling signal): Pull out from the sampling resistors R25 and R26. Note that the ground of the IC is based on the sampling resistor. The positive and negative differential traces of the sampling resistor pull down the IC CS pin and the GND pin of the IC.
The driving signal pulls down the IC driving pin from the driving circuit, taking care not to interfere with the CS pin; as shown in the figure, the three wires are arranged side by side, and the ground wire is placed in the middle of the driving first and the CS line to play a certain shielding effect;
The double panel is best to shield the ground layer of the IC. The network of the grounding must be drawn from the IC GND. The non-critical signal GND can be directly punched through the hole. The key signal ground needs to be grounded at a single point and directly connected to the IC.
FB feedback network signal pay attention to check the trace and connect the IC at a single point;
RCD absorption network should not be placed in the main circuit;
The rectification and filtering ground of VCC needs to be connected to the main power ground, and the secondary filtering can be connected to the IC ground;
The Y capacitor traces are connected separately and cannot be confused with the main power to avoid interference.
16. Main power and control part wiring diagram
Many people may see this picture, in the fog, the general introduction:
The PFC driver and IC are connected to the PFC tube in common, and the specific point is the ground where the sampling resistor is connected;
The driving ground and the control ground of the DC-DC portion are connected to the sampling ground of the DC switching tube portion;
The auxiliary source part is controlled to be connected to the auxiliary source MOS tube sampling, and the MOS tube ground is connected to the main power ground;
The power supply ground of the respective ICs is connected to the IC ground through the auxiliary source EC filter, and it is noted that the RC filter is close to the IC.
Summary: Pay attention to the single point grounding, the ground line is not chaotic, is one of the most important places to go! ! !
17. Electromagnetic field shielding mechanism analysis
Figure 1: Principle of magnetic field shielding
As shown in the figure: the electric field interference of the input and output can be coupled by capacitive transmission. If the shielding plate is added, the size of C4 is increased, and C1 is also reduced, which attenuates the electric field interference;
Figure 2: Principle of magnetic field shielding
As shown in the figure: the characteristics of the magnetic field shielding are different from the magnetic field. The outer casing is shielded. The electric field only needs the plane shielding plate. Therefore, the shielding of the radiator brings the electric field shielding, and some closed-type power supply with the casing plays a certain magnetic field shielding.
The principle of magnetic field shielding, the magnetic field passing through the shield will change the magnetic circuit, causing the magnetic lines to spread to the surrounding, and the magnetic interference in the middle reaches the shielding purpose;
18, switching devices and EMC
The understanding of the device also has important significance for EMC. For example, MOS tube, main switch MOS is one of the most important EMC sources, and the opening and closing of the rectifier also generates 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 device is mainly introduced here, and other inductive transformers are not described;
Which parameters of the switching device have an important impact on EMC, we often say that the fast tube, the slow tube is what is the reference? We all know that the fast-opening loss is small, and we like to use it for high efficiency. However, in order to pass the EMC smoothly, we have to abandon the efficiency and reduce the switching speed to reduce the switching radiation.
For MOS transistors, 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 figures, different types of MOS tubes, compared to the input junction capacitance and output junction capacitance, 2400PF and 800PF; 780PF and 2200PF; at first glance, the first specification is the fast tube, the second is the slow tube, this When the switch speed is determined to match the drive resistance; in general, the drive resistance is more in the 10R-150R. The drive resistance is related to the junction capacitance. The drive resistance of the Allegro can be increased appropriately, and the drive resistance of the slow tube can be appropriately reduced.
For diodes, there are Schottky diodes, fast recovery diodes, common diodes, and a relatively small number of SIC diodes. The switching speed SIC diode is almost zero, which is equivalent to no reverse recovery, minimum switching radiation, and minimal loss. The only drawback is that it is expensive, so it is rarely used; the second is Schottky diode, the forward voltage is reduced, the reverse recovery time is short, followed by fast recovery and ordinary diode; need to compromise between loss and EMC; The EMC can be rectified by measures such as changing absorption and magnetic beads;
19. EMC filter
The choice of filter architecture is very important to the filter. In different occasions, the filter is filtered 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 adopts π Type filtering and LC filter at the output;
The material of the filter is also very important for designing the filter inductor. Materials with different initial permeability will work in different frequency segments, and the wrong material will completely lose its proper effect.
20. Analysis of EMC's flyback high frequency equivalent model
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 flyback high frequency equivalent model 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 received interference, it is necessary to let the interference flow through the ground loop without flowing from the L, N port to the receiving device; at this time, our EMI inductance and Y capacitor can be achieved by impedance matching; Through the primary secondary side Y capacitor, 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 constitute a circulation path;
21. Form of radiation and frequency distribution
This picture may be somewhat abstract, but just as EMC is difficult to be specific, we need to give us some inspiration. It can be seen that the differential mode radiation exists in the form of a loop, and the common mode radiation is emitted in the form of an antenna; therefore, it just confirms In the front, when we lay out the layout of the switch loop and do not take the acute angle when routing, the conventional walk is 45 degrees, preferably the arc trace, of course, the trace efficiency will be relatively low;
The basics of these principles are well understood. They are useful for actually handling EMC work and layout. If you don't have this kind of awareness, it may be useless. Because you can't provide direct methods, you need to combine with other knowledge.
And many of the principles mentioned here are not visible in many EMC materials, and they are not so concentrated, and need to be repeated!
Figure: The relationship between some frequency ends and the location of the switching power supply. This is just a general rule. Don't believe it completely. Is it a law and can't believe it? Why? The law is not established in all situations, the difference between different power sources is also very large, so the principle is to help you analyze, rather than follow the method to hard set;
22, EMC instance
According to the conduction example, the key point of the frequency distribution point is the relationship between the specific data and the fundamental frequency. After this test, it is necessary to speculate on the laws of these values, and it may be possible to find any clues; of course, how these frequencies are solved by filters The means have also been mentioned before;
Here is to add some seemingly mysterious EMC how it came, feel no longer mysterious, not just a confused filter to solve the problem!
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