PCB Blog - Switching power supply printed board design and PCB board layout

1. PCB boardlayout: The pulse voltage connection should be as short as possible, in which the input switch tube connects to the transformer, and the output transformer connects to the rectifier tube. The pulse current loop is as small as possible, as the input filter capacitor is positive to the transformer to the switch tube and the return capacitor is negative. The output part of the transformer output terminal to the rectifier tube to the output inductor to the output capacitor return to the transformer circuit X capacitor should be as close as possible to the input terminal of the switching power supply, the input line should be avoided parallel to other circuits, should be avoided. The Y capacitor should be placed on the chassis ground terminal or FG connection end. Keep a certain distance between the common touch induction and the transformer to avoid magnetic coupling. If it is not easy to handle, a shield can be added between the common-mode inductor and the transformer. The above items have a greater impact on the EMC performance of the switching power supply. Generally, two output capacitors can be used, one close to the rectifier tube and the other close to the output terminal, which can affect the output ripple index of the power supply. The parallel effect of two small-capacity capacitors should be better than using one large-capacity capacitor. The heating device must be kept a certain distance from the electrolytic capacitor to extend the life of the whole machine. The electrolytic capacitor is the key to the life of the switching power supply, such as transformers, power tubes, and high-power resistors. Keep a distance from the electrolysis, and leave space for heat dissipation between the electrolysis, It can be placed in the air inlet if conditions permit. Attention should be paid to the control part: the connection of the high impedance weak signal circuit should be as short as possible, such as the sampling feedback loop. During processing, try to avoid interference. The current sampling signal circuit, especially the current control circuit, is not easy to handle. Accident.

2. Switching power supply printed board design and PCB board layout

2.1 Switching power supply printed board wiring principle
Line spacing: With the continuous improvement and improvement of the printed circuit board manufacturing process, there is no problem with the line spacing equal to or even less than 0.1mm in general processing plants, which can fully satisfy most applications. Taking into account the components and production processes used in the switching power supply, generally the double-sided board line spacing is set to 0.3mm, the single-sided board line spacing is set to 0.5mm, pad and pad, pad and via or via and via, The spacing is set to 0.5mm to avoid "bridging" during the welding operation. In this way, most board factories can easily meet the production requirements, and can control the yield rate very high, and can also achieve reasonable wiring density and have a more economical cost. The line spacing is only suitable for signal control circuits and low-voltage circuits with a voltage lower than 63V. When the line-to-line voltage is greater than this value, the line spacing can generally be selected according to the empirical value of 500V/1mm. In view of the fact that some related standards have clear regulations on the line spacing, it must be strictly implemented in accordance with the standards, such as the connection between the AC inlet end and the fuse end. Some power supplies have high volume requirements, such as modular power supplies. Generally, the distance between the input side wires of the transformer is 1mm and it is proved to be feasible in practice. For power products with AC input and (isolated) DC output, the stricter regulations are that the safety distance must be greater than or equal to 6mm. Of course, this is determined by the relevant standards and implementation methods. Generally, the safety distance can be referenced by the distance on both sides of the feedback optocoupler, and the principle is greater than or equal to this distance. Slots can also be made on the printed board under the optocoupler to increase the creepage distance to meet the insulation requirements. Generally, the distance between the wiring on the input side of the AC input side or the components on the board and the non-insulated shell and radiator should be greater than 5mm, and the distance between the wiring or components on the output side and the housing or radiator should be greater than 2mm, or strictly follow the safety regulations.

Common methods:
Method 1. The circuit board slotting method mentioned above is suitable for some occasions where the spacing is not enough. By the way, this method is also commonly used to protect the discharge gap, which is common in the tail plate of the TV kinescope and the AC input of the power supply. . This method has been widely used in modular power supplies, and good results can be obtained under potting conditions.
Method two, pad insulation paper, can use insulation materials such as blue shell paper, polyester film, polytetrafluoroethylene oriented film, etc. Generally, green shell paper or polyester film is used for general power supply to pad between the circuit board and the metal casing. This material has high mechanical strength and has a certain ability to resist moisture. Polytetrafluoroethylene oriented film is widely used in module power supply due to its high temperature resistance. An insulating film can also be placed between the component and the surrounding conductor to improve the insulation resistance. Note: The insulation covering of some devices cannot be used as an insulating medium to reduce the safety distance, such as the outer skin of an electrolytic capacitor. Under high temperature conditions, the outer skin may be heat-shrinked. Leave a space at the front end of the large electrolytic explosion-proof tank to ensure that the electrolytic capacitor can unimpeded the pressure in an extraordinary situation.

2.2 Precautions for PCB copper wire routing
Trace current density: Now most electronic circuits are made of insulating board bound copper. The thickness of the copper skin of the commonly used circuit board is 35μm, and the current density value can be taken according to the 1A/mm empirical value for the wiring. For the specific calculation, please refer to the textbook. In order to ensure the mechanical strength of the wiring, the line width should be greater than or equal to 0.3mm (other non-power circuit boards may have a smaller line width). The thickness of the copper skin is 70μm, and the circuit board is also commonly used in switching power supplies, so the current density can be higher. To add, the commonly used circuit board design tool software generally has design specification items, such as line width, line spacing, dry plate via size and other parameters can be set. When designing the circuit board, the design software can be executed automatically in accordance with the specifications, which can save a lot of time, reduce part of the workload, and reduce the error rate. Generally, double-sided boards can be used for circuits with high reliability requirements or high wiring density. It is characterized by moderate cost and high reliability, which can meet most applications. Some products in the module power supply ranks also use multilayer boards, which are mainly convenient for integrating power devices such as transformer inductors, optimizing wiring and power tube heat dissipation. It has the advantages of good craftsmanship and good consistency and good heat dissipation of the transformer, but its disadvantages are high cost and poor flexibility, which is only suitable for industrialized large-scale production. Single-panel, general-purpose switching power supplies circulating in the market almost all use single-sided circuit boards, which have the advantage of low cost, and some measures in design and production technology can also ensure its performance. Today, I will talk about some experiences of single-sided printed circuit board design. Because single-sided board has the characteristics of low cost and easy manufacturing, it is widely used in switching power supply circuits. Because it has only one side of copper bound, the electrical connection of the device and the mechanical fixation are required. Relying on that layer of copper, you must be careful when handling it. In order to ensure good welding mechanical structure performance, the single-sided board pad should be slightly larger to ensure a good binding force between the copper skin and the substrate, and the copper skin will not peel off or break off when subjected to vibration. Generally, the width of the solder ring should be greater than 0.3mm. The diameter of the pad hole should be slightly larger than the diameter of the device pin, but it should not be too large. Ensure that the solder connection distance between the pin and the pad is short, and the size of the disk hole does not hinder normal inspection., The diameter of the pad hole is generally larger than the pin diameter by 0.1-0.2mm. Multi-pin devices can also be larger to ensure smooth inspection. The electrical connection should be as wide as possible, and in principle, the width should be greater than the diameter of the pad. In special cases, the wire must be widened when the connection meets the pad (commonly known as teardrop generation) to avoid breaking the wire and the pad under certain conditions. In principle, the line width should be greater than 0.5mm. The components on the single panel should be close to the circuit board. For devices that require overhead heat dissipation, sleeves should be added to the pins between the device and the circuit board to support the device and increase insulation. It is necessary to minimize or avoid external impact on the connection between the pad and the pin. The influence of, enhance the firmness of welding. The heavy components on the circuit board can increase the supporting connection points and strengthen the connection strength with the circuit board, such as transformers and power device radiators. The single-sided soldering surface pins can be kept longer without affecting the distance from the shell. The advantage is that the strength of the soldering part can be increased, and the soldering area and the phenomenon of virtual soldering can be found immediately. When the pin is long and cuts the leg, the welding part receives less force. In Taiwan and Japan, the process of bending the device pins at a 45-degree angle with the circuit board on the soldering surface and then soldering is often used, and the principle is the same as above. Today, I will talk about some matters in the double-sided board design. In some applications with higher requirements or higher trace density, double-sided printed boards are used, and their performance and various indicators are much better than single-sided boards. The double-sided board pad has a higher strength because the holes have been metallized, the solder ring can be smaller than the single-sided board, and the hole diameter of the pad hole can be slightly larger than the pin diameter, because the solder solution can penetrate to the top layer through the solder hole during the soldering process. Pads to increase soldering reliability. But there is a drawback. If the hole is too large, part of the device may float up under the impact of the jet tin during wave soldering, resulting in some defects.

2.3 For the treatment of high-current traces, the line width can be processed according to the previous post. If the width is not enough, it can generally be solved by tinning the traces to increase the thickness. There are many methods.
1. Set the trace to the pad attribute, so that the trace will not be covered by solder resist during the circuit board manufacturing, and will be tinned during hot air leveling.
2. Place a pad on the wiring, set the pad to the shape that needs to be routed, and pay attention to setting the pad hole to zero.
3. Place the wire on the solder mask. This method is flexible, but not all circuit board manufacturers will understand your intentions, and you need to use text to explain. No solder resist will be applied to the solder mask where the wire is placed.
Several methods of tinning the circuit are as above. It should be noted that if the wide traces are all tinned, a large amount of solder will be bonded after soldering, and the distribution will be very uneven, which will affect the appearance. Generally, a slender strip of tin-plated width is 1~1.5mm, and the length can be determined according to the circuit. The tin-plated part is separated by 0.5~1mm. The double-sided circuit board provides great selectivity for layout and wiring, which can make the wiring more Tends to be reasonable. Regarding grounding, the power ground and signal ground must be separated. The two grounds can be merged at the filter capacitor to avoid accidental factors of instability caused by large pulse currents passing through the signal ground connection. The signal control loop should be grounded as far as possible. There is a trick, try to place non-grounded traces on the same wiring layer, and lay ground wires on another layer. The output line generally passes through the filter capacitor first, and then to the load. The input line must also pass through the capacitor first, and then to the transformer. The theoretical basis is to let the ripple current pass through the filter capacitor. Voltage feedback sampling, in order to avoid the influence of large current passing through the wiring, the sampling point of the feedback voltage must be placed at the end of the power supply output to improve the load effect index of the whole machine. The wiring change from one wiring layer to another wiring layer is generally connected by vias, and it is not suitable to be realized through the device pin pads, because this connection relationship may be destroyed when the device is inserted, and when every 1A current passes through, There should be at least 2 vias, and the diameter of vias should be greater than 0.5mm in principle. Generally 0.8mm can ensure processing reliability. Device heat dissipation. In some low-power power supplies, the circuit board traces can also serve as heat dissipation. Its characteristic is that the traces are as wide as possible to increase the heat dissipation area. No solder resist is applied. If possible, the vias can be evenly placed to enhance the PCB board.