PCB Technical - Switching power supply circuit board PCB design points

1. Wiring principle
1. The small signal trace should be kept away from the high current trace as far as possible, and the two should not be close to the parallel trace. If it is unavoidable to be parallel, a sufficient distance should be kept to avoid interference of the small signal trace.
Small signal traces should be as far away from large current traces as possible, and the two should not be close to parallel 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.
Critical small signal traces, such as current sampling signal lines and optocoupler feedback signal lines, etc., minimize the area enclosed by the loop.

3. There should be no excessively long parallel lines between adjacent ones (of course, parallel routing of the same current loop is possible), and the upper and lower layers should be crossed vertically as far as possible, and the routing should not be suddenly cornered (ie: ≤90°), Right angles and acute angles will affect electrical performance in high frequency circuits.
There should be no excessively long parallel lines between adjacent ones (of course, parallel routing 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 cornered suddenly (ie: ≤90°), right-angled, Acute angles will affect electrical performance in high-frequency circuits

4. Pay attention to the separation of power loop and control loop, and use single-point grounding method, as shown in Figure 9 and Figure 10.
The components around the primary PWM control IC are grounded to the ground pin of the IC, and then lead 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.
The power circuit and the control circuit should be separated, and the single-point grounding method should be adopted.

The power circuit and the control circuit should be separated, and the single-point grounding method should be adopted.

5. Do not place wires on the first layer of high-frequency components (such as transformers and inductors). It is also best not to place components on the bottom surface of high-frequency components directly opposite. If it is unavoidable, shielding can be used. Layer, the control circuit is facing the Bottom layer, pay attention to the first layer where the high-frequency components are coated with copper for shielding, as shown in Figure 11, so as to prevent high-frequency noise radiation from interfering with the control circuit on the bottom.
High-frequency components (such as transformers, inductors) should not be routed on the first layer underneath. It is also best not to place components on the bottom surface of high-frequency components directly opposite. If it is unavoidable, shielding can be used.

6. Pay special attention to the routing of the filter capacitor, as shown in Figure 12. On the left, a part of the ripple & noise will be routed out, and the filtering effect on the right will be much better. The ripple & noise are completely filtered out by the filter capacitor.
Pay special attention to the routing of the filter capacitor, as shown in Figure 12. Some of the ripple & noise in the left picture will be routed out, and the filtering effect on the right will be much better. The ripple & noise are 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, as shown in Figure 13.
The wiring of the power cord and ground wire 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. Electric clearance: the shortest distance measured along the air between two adjacent conductors or a conductor and the surface of an adjacent conductive casing. Creepage distance: The shortest 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 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 minimum slot 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 manufacturer with high processing accuracy. Of course, the cost will increase.
If the PCB space of the module is limited and the creepage distance is not enough, slotting can be used

The relationship between general power module voltage and minimum creepage distance can refer to the following table:
The relationship between general power module voltage and minimum creepage distance

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, due to the small size and height of the components, and the low input and output voltages, 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.
If the width of the trace into the round pad or via 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 is required, otherwise, due to the fast heat dissipation during reflow soldering, it is easy to cause false soldering or desoldering.
When the pins of the SMD device are connected to a large area of copper foil, thermal isolation is required. Otherwise, due to the fast heat dissipation during reflow soldering, it is easy to cause false soldering or desoldering

5. When the PCB 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. As shown in Figure 17, it can be slotted appropriately to reduce the stress when breaking the PCB. 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 away from V- than component A CUT slot, the stress during breaking is also smaller than that of component A.
It can be slotted appropriately to reduce the stress when breaking the PCB. 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 the stress during breaking Also smaller than component A

Of course, the above are just some personal experience in switching power supply PCB design, and there are many details or other aspects of knowledge that need to be paid attention to. Finally, I want to talk about PCB design. In addition to principle requirements and experience knowledge, the most important The point is to be careful and then careful, check and check again.