Speaking of the difficult problem of switch power supply, PCB cloth plate problems is not very difficult, but if want to cloth up a refined PCB must be one of the difficulties in switching power supply (PCB design is not good, may cause no matter how to debug parameter debugging cloth out of the situation, so not scaremongering) when reason was the PCB board consideration or a lot of, such as: Electrical performance, process route, safety requirements, EMC impact, etc.; Among the factors to be considered, electrical is the most basic, but EMC is the most difficult to understand, and the bottleneck of many projects lies in EMC. The following from 22 directions to share the PCB board and EMC.

1, mature circuit can be leisurely PCB design EMI circuit

The impact of the above circuit on EMC can be imagined, the input filters are here; Lightning-proof pressure sensitivity; Resistance R102 to prevent shock current (with relay to reduce loss); Key error mode X capacitance and Y capacitance with inductor filtering; There are fuses affecting the safety board; Each of these devices is of vital importance, and the function and action of each device should be carefully appreciated. The EMC severity level should be considered when designing the circuit, such as the number of filters to be set, the number and location of the y-capacitor quantity. The choice of pressure-sensitive size and quantity is closely related to our requirements for EMC. Welcome 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 parts of the circuit contain EMC mechanism)

The circled parts in the circuit in the figure above are very important for EMC (note that the green part is not), such as radiation. It is known that electromagnetic field radiation is spatial, but the basic principle is the change of magnetic flux, which involves the effective cross-sectional area of magnetic field, namely the corresponding loop in the circuit. The electric current can produce a magnetic field, which is stable and cannot be converted into an electric field. But a changing electric current produces a changing magnetic field, and a changing magnetic field can produce an electric field (in fact, this is the famous Maxwell equation and I’m using plain language), and a changing electric field can also produce a magnetic field. So make sure you pay attention to the places where there are on/off states, that’s one of the sources of the EMC, and this is one of the sources of the EMC. For example, the dotted line loop in the circuit is the opening and closing loop of the switch tube. Not only the switching speed can be adjusted during the design of the circuit, but also the area of the wiring loop of the layout board has an important influence on EMC! The other two loops are absorption loops and rectifying loops, first understand in advance, and then talk!

3. Association between PCB design and EMC

1.PCB loop has a very important influence on EMC, such as flyback main power loop. If it is too large, the radiation will be poor.

2. Filter wiring effect, filter is used to filter out interference, but if PCB wiring is not good, filter may lose the effect it should have.

3. The structure part, the radiator design grounding is not good will affect the grounding of the shielded version;

4. If the sensitive part is too close to the interference source, such as EMI circuit and switch tube, it will inevitably lead to poor EMC, and a clear isolation area is required.

5. Wiring of RC absorption loop.

6.Y capacitor grounding and wiring, and the position of Y capacitor is also critical!

I’m going to talk about this, and I’m going to talk about it more, but I’m going to give you a lead.

Here’s a quick example:

As shown in the dotted box in the figure above, the X capacitor pin wiring has been indented. You can learn how to make the capacitor pin wiring external (using the squeeze current wiring). In this way, the filtering effect of X capacitor can reach the best state.

4. Preparation for PCB design :(if you are fully prepared, the design can be steady step by step to avoid design overturning and starting again)

There are roughly the following aspects, are their own design process to consider, all the content has nothing to do with other tutorials, are just their own experience summary.

1. Appearance structure size, including positioning hole, air channel flow direction, input and output socket, need to match with the customer system, also need to communicate with the customer assembly problems, height limit and so on.

2. Safety certification, products do what kind of certification, where do the basic insulation creepage distance to leave enough, where do strengthen the insulation to leave enough distance or slot.

3. Packaging design: there is no special period, such as preparation for customized packaging.

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

5. Other special requirements of customers.

The structure and process will be relatively more flexible, safety regulations or relatively fixed part, what certification to do, what safety standards, of course, there are some safety regulations are common in many standards, but there are also some special products such as medical treatment will be more stringent.

For the new entry engineer friends are not dazzled;

Next list some general products general, the following is summarized for IEC60065 specific cloth requirements, for safety needs to keep in mind, encounter specific products will be targeted processing:

1. The distance of the input fuse pad is greater than 3.0mm as required by safety regulations, and the actual plate is 3.5mm (simply speaking, the creepage distance of the fuse is 3.5mm before and 3.0mm after).

2. Before and after the rectifier bridge, the safety requirements are 2.0MM, and the plate layout is 2.5MM.

3. After rectification, safety regulations generally do not require, but the distance between high and low voltage is left according to the actual voltage, and 400V high voltage is left above 2.0mm.

4. The safety regulations for the first stage require 6.4mm (electrical gap), and the creepage distance should be 7.6mm. (Note that this is related to the actual input voltage, need to refer to the table for specific calculation, the data provided for reference only, subject to the actual situation)

5. Cold ground and hot ground are clearly marked for the first stage; L, N mark, INPUT AC INPUT mark, fuse warning mark and so on should be clearly marked;

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

5. Consider other factors for PCB design safety

1. Understand what certification their products do and what product categories they belong to. For example, medical treatment, communication, electricity, TV and so on are different, but there are also many similarities.

2. In safety regulations, understand the insulation characteristics of the close place with PCB board, which place is basic insulation, which place is reinforced insulation, different standard insulation distance is not the same. It is best to check standards, and can calculate electrical distance, creepage distance.

3. Focus on the safety devices of the product, such as the relationship between the magnetism of the transformer and the original side;

4. Radiator and surrounding distance problem, radiator insulation is not the same as the ground is not the same, the ground is cold, hot insulation is the same cloth.

5. Special attention should be paid to the distance of insurance, requiring the strictest place. The distance between front and rear of the fuse is consistent.

6. Relationship between Y capacitance and leakage current and contact current.

And so on, it will explain in detail how to leave the distance, how to do safety requirements.

6, PCB design of power supply layout

1. First measure the size of PCB and the number of components, so as to achieve a good density, or a dense, sparse will be ugly.

2. Modularize the circuit, take the core devices as the center, and place the key devices first.

3. The device is vertical or horizontal anti-positioning, one is beautiful, the other is convenient plug-in operation, special circumstances can consider tilt.

4. Take cabling into consideration and arrange the layout in the most reasonable position for subsequent cabling.

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

Do the above points, of course, flexible use, more reasonable layout will be born soon.

The following is the first virgin PCB I drew, many years ago, it was very hard to finish, there may be a small problem in the middle, but the general layout is worth learning:

In this figure, the power density is still relatively high. The control part of LLC, the auxiliary source part and the BUCK circuit driver (high-power multi-channel output) part are on the small board, which is not taken out. Let’s take a look at the layout characteristics of the main power:

1. Input and output terminals are fixed and cannot be moved. The board is rectangular.

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

2.EMI circuit is still clear flow direction, which is very important, otherwise it is not beautiful and bad for EMC.

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

4. The current of the side side is relatively large. In order to run the current and dissipate the heat of the rectifier tube, this layout is adopted. The top layer of high power generally goes negative, and the bottom layer goes positive.

Each board has its own characteristics, of course, also has its own difficulties, how to reasonably solve the key, we can understand the layout of reasonable selection of meaning?

7. Appreciation of PCB examples

I think it is a good place to do it. Of course, there will always be defects, which can also be pointed out. It is not easy for a single panel to be so compact, so you can use this board to learn and discuss! Behind will also be for this board to explain learning, we first enjoy.

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

In addition, the absorption loop (RCD absorption, RC absorption of MOS tube and RC absorption of rectifier tube) is also very important, which is also the loop that generates high frequency radiation. If you have any questions about the above figure, you are welcome to discuss them. We are not afraid of any questions.

9. PCB design hot spot (floating potential point) and ground wire:

Matters needing attention:

1. Pay special attention to hot spots (high-frequency switching points), which are high-frequency radiation points. Cable layout has a great impact on EMC.

2. The loop formed by hot spots is small and the wiring is short, and the wiring is not as thick as possible, but as long as the current is enough.

3. The ground cable must be grounded at a single point. Main power ground and signal ground separate, sampling ground go separately.

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