PCB Blog - The design points of switching power supply PCB board

In the design of switching power supply, PCB boarddesign is a very critical step, which has a great impact on the performance of the power supply, EMC requirements, reliability, and manufacturability. With the development of electronic technology, the volume of the switching power supply is getting smaller and smaller, the operating frequency is getting higher and higher, and the density of internal components is getting higher and higher, which makes the anti-interference requirements of PCB board layout and wiring more and more stringent., Reasonable and scientific PCB board design will make your work twice the result with half the effort.

1. Layout requirements
The layout of the PCB board is more exquisite, not just put it on and just squeeze it down. General PCB board layout should follow several points:
(1) The first principle of the layout is to ensure the wiring rate, pay attention to the connection of the flying wires when moving the device, and put the devices with the connection relationship together.
(2) Take the components of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB circuit board, so that it is not only beautiful, but also easy to install and solder, and easy to mass produce. Minimize and shorten the leads and connections between components; the oscillator circuit, filter decoupling capacitors should be close to the IC, and the ground wire should be short.
(3) When placing components, consider future soldering and maintenance. Try to avoid placing short components between two high-height components. This is not conducive to production and maintenance. The components should not be too dense, but with the development of electronic technology With the development, the current switching power supply is becoming more and more miniaturized and compact, so it is necessary to balance the degree between the two, not only to facilitate welding and maintenance, but also to take into account the compactness. There is also the need to consider the actual chip processing capability. According to the IPC-A-610E standard, consider the accuracy of the component side deviation, otherwise it is easy to cause the tin connection between the components, and even the component distance is not enough due to the component deviation.
(4) Photoelectric coupling devices and current sampling circuits are easy to be interfered. They should be far away from devices with strong electric and magnetic fields, such as high-current wiring, transformers, and high-potential pulsating devices.
(5) When placing components, give priority to the loop area of high-frequency pulse current and large current, and reduce it as much as possible to suppress the radiation interference of the switching power supply.
(6) The area where the high-frequency pulse current flows should be far away from the input and output terminals, and the noise source should be far away from the input and output ports, which is beneficial to improve EMC performance. The transformer is too close to the entrance, and the electromagnetic radiation energy directly acts on the input and output ends. Therefore, the EMI test fails. After changing to the method on the right, the transformer is far away from the entrance, and the distance between the electromagnetic radiation energy and the input and output ends is increased, the effect is improved significantly, and the EMI test is passed.
(7) The layout of heating elements (such as transformers, switch tubes, rectifier diodes, etc.) should consider the effect of heat dissipation, so that the heat dissipation of the entire power supply is even, and key components (such as IC) that are sensitive to temperature should be far away from the heating elements and generate more heat. The device should have a certain distance from the electrolytic capacitor and other devices that affect the life of the whole machine.
(8) Pay attention to the height of the bottom element when laying out the board. For example, for potted DC-DC power modules, because the DC-DC module itself is relatively small, if the height of the bottom component is unbalanced on all four sides, the height of the pins on both sides will be high while the other is low during potting.
(9) Pay attention to the antistatic ability of the control pins during layout, and the distance between the corresponding circuit components should be sufficient, for example, the Ctrl pin (low-level shutdown), the circuit does not have the same capacitance as the input and output terminals. Filtering, so the anti-static ability of the whole module is weak, so you must ensure that there is enough safety distance.

2. Wiring principle
(1) Small signal traces should be kept away from high current traces as much as possible, and the two should not be close to parallel traces. If it is unavoidable to be parallel, a sufficient distance should be kept to avoid interference of small signal traces.
(2) The key small signal wiring, such as current sampling signal line and optocoupler feedback signal line, etc., minimize the area enclosed by the loop.
(3) There should not be too long parallel lines between adjacent ones (of course, parallel wiring of the same current loop is possible), and the upper and lower layer wiring should be crossed vertically as far as possible. The wiring should not be suddenly cornered (ie: ≤90° ), right angles and acute angles will affect electrical performance in high frequency circuits.
(4) The power circuit and the control circuit should be separated, and the single-point grounding method should be adopted. The components around the primary PWM control IC are grounded to the ground pin of the IC, and then led from the ground pin to the large capacitance ground wire, and then connected to the power ground. The components around the secondary TL431 are grounded to pin 3 of the TL431, and then connected to the ground of the output capacitor. In the case of multiple ICs, a parallel single-point grounding method is adopted.
(5) Do not route high-frequency components (such as transformers and inductors) on the bottom layer, and do not place components on the bottom surface of high-frequency components directly opposite. If it is unavoidable, shielding can be used, such as high-frequency components on the top layer, control The circuit is facing the Bottom layer. Pay attention to copper shielding on the layer where the high-frequency components are located, so as to prevent high-frequency noise radiation from interfering with the control circuit on the bottom.
(6) Pay special attention to the routing of the filter capacitor. A part of the ripple & noise will go out through the routing. The filtering effect of the picture on the right will be much better. The ripple & noise will be completely filtered out by the filter capacitor.
(7) The power line and the ground line are as close as possible to reduce the enclosed area, thereby reducing the electromagnetic interference caused by the cutting of the external magnetic field loop, and at the same time reducing the external electromagnetic radiation of the loop. The wiring of the power and ground wires should be as thick and shortened as possible to reduce the loop resistance, the corners should be smooth, and the line width should not change suddenly.
(8) A large area of bare copper can be used for heat dissipation under components with large heat (such as TO-252 packaged MOS tubes), which can improve the reliability of the components. The narrow part of the power trace copper foil can be used for tinning with bare copper to ensure the flow of large current.

3. Safety distance and process requirements
(1) Electrical clearance: the short distance measured along the air between two adjacent conductors or a conductor and the surface of an adjacent conductive housing. Creepage distance: The short distance measured along the insulating surface between two adjacent conductors or a conductor and the surface of an adjacent conductive housing. If the module PCB board space is limited and the creepage distance is not enough, slotting can be used. As shown in Figure 14, an isolation slot is opened at the optocoupler to achieve good primary and secondary isolation. Generally, the slotting width is 1mm. If you want to open a smaller slot (such as 0.6mm, 0.8mm), you generally need special instructions. Find a PCB board manufacturer with high processing accuracy. Of course, the cost will increase.
(2) The distance requirement from the component to the edge of the board. The components located at the edge of the circuit board are generally no less than 2mm away from the edge of the circuit board. For miniaturized DC-DC modules like 10W or less, because the component volume and height are relatively small, and the input and output voltages are not high, in order to meet the miniaturization It is required to leave at least a distance of 0.5mm or more. The distance between the large-area copper foil and the outer frame should be at least 0.20mm or more. Because it is easy to mill the copper foil when milling the shape, the copper foil will lift up and the flux will fall off.
(3) If the width of the trace into the round pad or the via hole is smaller than the diameter of the round pad, then teardrops should be added to strengthen the adsorption force to prevent the pad or via from falling off.
(4) When the pins of the SMD device are connected to a large area of copper foil, thermal isolation should be performed, otherwise, due to the fast heat dissipation during reflow soldering, it is easy to cause false soldering or desoldering.
(5) When the PCB board is assembled, it is necessary to consider the feasibility of sub-boarding, to ensure that the distance between the components and the edge of the board is sufficient, and at the same time, consider whether the stress of the sub-board will cause the components to warp. It can be slotted appropriately to reduce the stress when breaking the PCB board. Component A is placed parallel to the direction of the V-CUT slot, and the stress during breaking is smaller than that of component B; component C is farther from the V-CUT slot than component A, and when breaking The stress is also smaller than that of component A. Of course, the above are just some personal experience in switching power supply PCB boarddesign.