PCB Technical - Switching power supply PCB design of the complete process and key considerations

Switching power supply PCB design of the complete process and key considerations

As a professional PCB engineer, you need to pay special attention to the following important matters when carrying out PCB layout and wiring of switching power supply:

Processing after power supply access: After power supply access, it is first purified by filter capacitors, and then delivered to the subsequent equipment to be used. This is because the PCB alignment is not an ideal conductive channel, they have a certain resistance and distribution inductance. If power is taken from before the filter capacitors, this will result in a larger ripple component in the power supply, which will reduce the filtering effect.

Line design details: In the design, should be as wide as possible rather than thin lines, avoid the use of sharp chamfers and right-angle turns. For the ground line, should be designed as wide as possible, and give priority to the use of large-area copper, which can significantly improve the grounding effect.

Layout principles for capacitors: Capacitors are mainly used to provide the necessary capacitive support for switching devices (e.g. gate circuits) or other components that require filtering/decoupling. Therefore, during layout, it should be ensured that these capacitors are located as close as possible to the components they serve, in order to avoid excessive distances that could lead to capacitor failure.

When designing the PCB layout of a power supply board, in conjunction with safety regulations, special attention needs to be paid to the following key points:

For the AC power inlet section, the minimum safe spacing between the two power lines should be ensured to be no less than 6 mm before the fuse is installed in position, while the minimum safe distance between these two power lines and the chassis shell or the grounded portion of the chassis should also be maintained at more than 8 mm.

The alignment design after fuse installation needs to be carefully designed to ensure that the minimum creepage distance between the zero wire and the fire wire is 3 mm or more to prevent electrical short circuits and safety hazards.

A minimum creepage distance of at least 8 mm should be maintained between high voltage areas and low voltage areas. If this distance is equal to or less than 8 mm, a safety slot of 2 mm width shall be provided between the two areas to further ensure electrical isolation and safety.

The high-voltage area shall be equipped with a visible high-voltage warning mark in the form of a triangular symbol containing an exclamation mark and printed on the PCB by means of silkscreen. In addition, the high-voltage area should be marked with a screen-printed frame no less than 3 mm wide to highlight its special characteristics and to remind operators to pay attention to safety.

In the high-voltage rectifier filter circuit, the minimum safe distance between positive and negative poles should be no less than 2 mm to prevent electrical breakdown or short circuit problems caused by too close a distance.

The design process of switching power supply PCB:

Schematic design: first, according to the design requirements, use professional electronic design software to draw the schematic diagram of the switching power supply. This is the starting point of the entire design process, providing the basis for subsequent steps.

Network Table Generation: After the schematic design is completed, the software will automatically generate the corresponding network table by compiling and checking if the schematic is correct. The network table records in detail the connection relationship between each component in the circuit, which is an important basis for subsequent PCB layout and wiring.

Physical border setting: Next, you need to set the physical border of the PCB in the design software (Keepout Layer), in order to clarify the size of the PCB form factor and boundary limitations, to ensure that the subsequent layout and wiring will not exceed the scope.

Component and network import: the schematic components and network relationships into the PCB design environment for the subsequent layout and wiring ready.

Component Layout: Component layout is a very critical step in PCB design. Reasonable layout can not only improve product life and stability, but also enhance electromagnetic compatibility. Layout should follow the following principles:

Placement order: first place the components that are closely related to the structure of the fixed position, such as power sockets, indicators, switches, etc., and use the locking function of the software to fix them. Then place special components and large components on the line, such as heat generating components, transformers, integrated circuits, etc. Finally place the small components.

Heat dissipation considerations: For high-power circuits, special attention should be paid to heat dissipation. Heat-generating components should be scattered layout, avoid centralised placement, and not close to high capacitance, in order to prevent premature aging of the electrolyte.

Wiring: After the component layout is completed, the wiring work begins. Wiring needs to consider the integrity of the signal, the size and direction of the current, and electromagnetic interference and other factors.

Adjustment and refinement: After the completion of the wiring, you need to adjust the text, individual components, alignment, etc., and copper treatment. Copper laying is usually used to fill in the blank areas left after wiring, either by laying copper foil for the ground (GND) or for the power supply (VCC) (but be aware of the risk of short-circuiting). In addition, for signal lines with special requirements, ground wrapping can be used to surround them with two earth wires to prevent interference.

Checking and verification: Finally, the entire PCB design needs to be carefully checked and verified to ensure that the network relationship is consistent with the schematic diagram and that there are no omissions or errors. This is an important step to ensure the quality of the design and smooth production.

After the above steps, the PCB design of the switching power supply is completed. The accuracy and completeness of the design should be reconfirmed before submitting it to the plate maker.

During the synergy between PCB design and mechanism design, harmony between the two needs to be ensured:

In response to the height limit requirements, the layout of components must be carefully planned to avoid any interference in the assembly process. At the same time, the shape design of the PCB, the location and size of the positioning holes and mounting holes need to be closely matched with the structural design to ensure that the PCB manufacturing and installation smoothly.

The selection of devices needs to fully take into account the structure and processing errors to ensure that the selected devices can adapt to the processing accuracy of the structural components. In PCB layout, the assembly process should be optimised to enhance production efficiency. Designers need to assess whether the board shape design is too complex, can be simplified through the design (such as multilayer or double-sided board replaced by a single-panel) to reduce the problems in the assembly process.

Each side of the PCB should be completed in a single assembly process as far as possible to reduce the use of hand soldering and increase automated production. Where possible, cartridge components should be replaced by SMD components to reduce production costs and increase production efficiency.

The package of the components must be consistent with the real thing, and the pitch and size of the pads need to meet the design requirements. Components should be evenly distributed, especially high-power devices should be dispersed to avoid PCB local overheating stress, which affects the reliability of the solder joints. For high-power devices, special consideration should also be given to thermal design.

Where conditions permit, similar components should be arranged in the same direction, and modules with the same function should be centrally arranged. Components of the same package should be placed at equal distances to facilitate component placement, welding and testing. Finally, the silkscreen should be clearly legible, with clear indications of polarity and direction, and to ensure that it will not be obscured by the devices after assembly is complete.

PCB (printed circuit board) version of the material variety, for the switching power supply field commonly used materials:

Flame-retardant grade materials: In PCB materials, 94V-0 and 94V-2 are flame-retardant grade materials, of which 94V-0 is the top flame-retardant material in this category.

Organic materials: These materials are mainly composed of organic matter, commonly including phenolic resins, glass fibre reinforced epoxy resin (referred to as glass fibre board), Polyimide (polyimide) and BT / Epoxy (bismaleimide triazine / epoxy resin) and so on. These materials are widely used in PCB manufacturing due to their good electrical properties and processability.

Inorganic materials: Unlike organic materials, inorganic materials are mainly composed of inorganic substances such as aluminium, copper-invar-copper (a special sandwich-structured copper alloy material for high-precision, low coefficient of expansion applications) and ceramic. These materials offer unique advantages in specific areas such as high frequency, high temperature or special environmental applications.

Aluminium PCBs: Aluminium substrates are a special type of PCB material that uses aluminium as a base material, and the circuit layers are tightly bonded to the aluminium base by a special process. Aluminium substrate not only has excellent heat dissipation performance, but also can effectively reduce the size and weight of the PCB, so it is widely used in switching power supplies and other electronic devices that require efficient heat dissipation.

Material Recognition Process:

Firstly, we perform individual tests on the samples and issue ‘Sample Test Report’. For some items that require specialised instruments, we can refer to the test results provided by the manufacturer. For crystal semiconductors, plastic parts and packaging materials of well-known foreign brands, individual testing can be exempted, but samples of various materials need to be tested for actual installation and use, and the results will be used as an important basis for final judgement.

Next, using the test results as a key basis, the R&D department will compare the test results of the samples with the specifications in the acknowledgement to ensure the consistency between the two and to check the completeness of the contents of the acknowledgement.

If it is found that a single test fails or the acknowledgement does not meet the requirements, the purchasing department will be asked to re-supply the sample and acknowledgement.

For certain critical materials, after the R&D department passes the monomer test, it is also necessary to apply for a small batch trial casting. At this time, the R & D department to apply, the production department to lead the test casting work, while the quality control department is responsible for verifying the quality of the test casting materials.

Switching power supply PCB design is a complex and delicate process, the choice of suitable PCB material is also a switching power supply design can not be ignored. Only a comprehensive grasp of the design process, key points and material selection and other aspects of knowledge, in order to design a high-quality, high-performance switching power supply PCB, for the stable operation of electronic equipment to provide a strong guarantee.