layout:
The input switching transistor is connected to the transformer, and the output transformer is connected to the rectifier transistor. Pulse voltage connections should be as short as possible. Pulse current loops should be minimized. For example, the input filter capacitor should be positive for the transformer and negative for the return capacitor. The X capacitor (from the transformer output to the rectifier output, then to the output inductor, and back to the transformer circuit) should be as close as possible to the input of the switching power supply, and input lines should avoid parallel connection with other circuits. The Y capacitor should be placed at the chassis ground terminal or FG connection terminal. Maintain a certain distance between the common-mode inductor and the transformer to avoid magnetic coupling. If this is difficult to achieve, a shielding layer can be added between the common-mode inductor and the transformer. All of the above factors have a significant impact on the EMC performance of the switching power supply.
Typically, two output capacitors can be used, one near the rectifier and the other near the output terminals. This will affect the power supply's output ripple index. The effect of two small-capacity capacitors in parallel is better than that of a large-capacity capacitor. Heating devices must be kept a certain distance from electrolytic capacitors to extend the overall lifespan of the machine. Electrolytic capacitors are critical to the lifespan of switching power supplies. For example, transformers, power transistors, and high-power resistors must be kept away from electrolytic capacitors, and there must be sufficient space for heat dissipation between them. If possible, they can be placed in the air inlet.
Note the control section: Connections to high-impedance, weak-signal circuits should be as short as possible, such as sampling feedback loops. During processing, please minimize interference. Current sampling signal circuits, especially current control circuits, are not easy to handle.
Line spacing: With continuous improvements in printed circuit board manufacturing processes, line spacing of 0.1mm or less is generally sufficient for most applications. Considering the components and manufacturing processes used in switching power supplies, the minimum line spacing for double-sided boards is typically set to 0.3mm, and for single-sided boards, it's 0.5mm. The minimum spacing between pads, and between pads and vias, is also set to 0.5mm to avoid bridging during soldering. This allows most PCB manufacturers to easily meet production requirements, maintain high yield rates, achieve reasonable wiring density, and maintain cost-effectiveness. Minimum line spacing is only applicable to signal control circuits and low-voltage circuits with voltages below 63V. When the voltage between lines exceeds this value, the line spacing can typically be selected based on the empirical value of 500V/1mm.
Given that some relevant standards have specific requirements for line spacing, it is essential to strictly adhere to these standards, such as the connection between AC input terminals and fuse terminals. Some power supplies have stringent size requirements, such as modular power supplies. Typically, a line spacing of 1mm on the transformer input side has proven feasible. For power supply products with AC input and (isolated) DC output, a stricter rule is that the safety distance must be greater than or equal to 6mm. Of course, this depends on the relevant standards and implementation methods. Generally, the safety distance can be expressed as the distance between the two sides of the feedback optocoupler, based on the principle that it must be greater than or equal to this distance. Slots can also be cut on the printed circuit board below the optocoupler to increase the creepage distance and meet insulation requirements. Generally, the distance between the AC input side wiring or board assembly and the non-insulated housing and heat sink of a switching power supply should be greater than 5mm, and the distance between the output side wiring or assembly and the housing or heat sink should be greater than 5mm. A distance greater than 2mm, or strictly adhering to safety regulations, is also acceptable.
Common Method: The above-described slotting method is suitable for situations where the spacing is insufficient. Incidentally, this method is also commonly used to protect discharge gaps, which is frequently seen in the AC input and power supply of television picture tube tail plates. This method has been widely used in modular power supplies and yields good results under potting conditions.
Method 2: Use insulating paper. Insulating materials such as blue-shell paper, polyester film, and PTFE oriented film can be used. Typically, green-shell paper or polyester film is used as a general power supply to fill the gap between the circuit board and the metal casing. This material has high mechanical strength and a certain degree of moisture resistance. PTFE oriented film is widely used in modular power supplies due to its high-temperature resistance. Insulating films can also be placed between components and surrounding conductors to improve insulation resistance.
