PCB circuit board design switching power supply copper wire routing needs to pay attention to
PCB circuit board design switching power supply copper wire routing needs to pay attention to, switching power supply is a voltage conversion circuit, the main work is boost and buck, widely used in modern products. Because the switching transistor always works in the "on" and "off" states, it is called the switching power supply.
Isolated power supplies can be divided into two broad categories according to their structural form: forward and flyback.
The flyback means that the secondary side is cut off when the primary side of the transformer is turned on, and the transformer stores energy. When the primary side is turned off, the secondary side is turned on, and the energy is released to the working state of the load. Generally, the conventional flyback power supply has a single tube and the double tube is not common. Forward mode means that the primary side of the transformer is turned on and the secondary side induces a corresponding voltage output to the load, and the energy is directly transmitted through the transformer. According to the specifications, it can be divided into regular forward, including single tube forward and double tube forward. Half-bridge and bridge circuits are all forward circuits.
Both forward and flyback circuits have their own characteristics, and they can be used flexibly in order to achieve optimal cost performance in the process of designing circuits. Flyback is usually available in low power applications. A slightly larger single-tube forward circuit can be used, a medium-power dual-tube forward circuit or a half-bridge circuit can be used, and a push-pull circuit is used at a low voltage, which is the same as the half-bridge operation state. High-power output, generally using bridge circuit, low-voltage can also use push-pull circuit. Because of its simple structure, the flyback power supply eliminates an inductor similar in size to the transformer, and is widely used in small and medium power supplies. In some introductions, the flyback power supply can only achieve tens of watts, and the output power exceeds 100 watts, which has no advantage and is difficult to implement.
The pulse voltage connection is as short as possible, in which the input switch is connected to the transformer and the output transformer is connected to the rectifier connection line. The pulse current loop is as small as possible as the input filter capacitor is positive to the return capacitance of the transformer to the switch. Output part of the transformer output to the rectifier to the output inductor to the output capacitor return transformer circuit X capacitor should be as close as possible to the input end of the switching power supply, the input line should avoid parallel with other circuits should avoid.
The Y capacitor should be placed at the chassis ground terminal or FG connector. The total inductance is kept at a certain distance from the transformer to avoid magnetic coupling. The output capacitor can generally be used with two ones close to the rectifier and the other should be 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 a large-capacity capacitor. The heating device should be kept at a certain distance from the electrolytic capacitor to extend the life of the whole machine. The electrolytic capacitor is the bottle strength of the switching power supply life. For example, the transformer, the power tube, the high-power resistor must be kept away from the electrolysis, and the heat dissipation space must be left between the electrolysis. , conditions allow it to be placed at the air inlet.
Some matters of PCB board copper wire routing
Trace current density: Most circuits are now made of insulating plated copper. The copper thickness of the common circuit board is 35μm, and the current density value can be taken according to the empirical value of 1A/mm. For the specific calculation, please refer to the textbook. In order to ensure the mechanical strength of the routing, the line width should be greater than or equal to 0.3mm. The copper thickness is 70μm. The circuit board is also commonly used in switching power supplies, so the current density can be higher. Some products in the module power supply line also use multi-layer boards, which are mainly convenient for integrating power devices such as transformer inductors, optimizing wiring and power tube heat dissipation. The utility model has the advantages of good process consistency and good heat dissipation of the transformer, but the disadvantage is that the cost is high and the flexibility is poor, and it is only suitable for industrial large-scale production.
Single-panel, market-distributed general-purpose switching power supplies almost all use single-sided circuit boards, which have the advantage of low cost, and some measures in design and production process can also ensure their performance. In order to ensure good soldering mechanical structure performance, the single-panel pad should be slightly larger to ensure good adhesion between the copper and the substrate, without being peeled off or broken when subjected to vibration. Generally, the width of the welding 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, so that the soldering distance between the pin and the pad is the shortest. The size of the hole does not hinder the normal inspection. The diameter of the pad hole is generally larger than the pin. The diameter is 0.1-0.2mm. Multi-pin devices are also larger to ensure a smooth check.
The components on the single panel should be close to the circuit board. For devices that require overhead heat dissipation, it is necessary to add a sleeve to the pin between the device and the circuit board to support the device and increase the insulation. To minimize or avoid external force impact on the pad and pin connection. The effect is enhanced to enhance the firmness of the weld. The heavier components on the board can increase the support connection point and strengthen the connection strength with the circuit board, such as transformers, power device heat sinks. The double-pad pad has a higher metallization strength than the hole, and the solder ring can be smaller than the single-layer. The hole diameter of the pad hole is slightly larger than the diameter of the pin because the solder solution penetrates into the top layer through the solder hole during the soldering process. Pads to increase soldering reliability.