Forced ventilation, metal PCB and component placement for switching power supply thermal design

1 Natural air cooling and forced air cooling

In the actual design of switching power supplies, two forms of cooling are usually used: natural air cooling and fan forced air cooling. When installing a heat sink for natural air cooling, the blades of the heat sink should be placed vertically upward. If possible, several ventilation holes can be drilled around the heat sink installation position on the PCB to facilitate air convection.

Forced ventilation cooling uses fans to force air convection, so the air duct design should also make the heat sink blade axis consistent with the fan's exhaust direction. In order to achieve good ventilation, the larger the heat dissipation of the device, the closer it should be to the exhaust fan. In the case of an exhaust fan, the thermal resistance of the heat sink is shown in the table.

Table Thermal resistance of heat sink with exhaust fan

2 Metal PCB

With the miniaturization of switching power supplies, surface mount components are widely used in actual production. At this time, it is difficult to install heat sinks on power devices. The main method to overcome this problem is to use metal PCB as the carrier of power devices, mainly aluminum-based copper-clad boards, iron-based copper-clad boards, and metal PCBs. The heat dissipation performance is much better than that of traditional PCBs, and SIVD components can be mounted. There is also a copper core PCB, the middle layer of the substrate is a copper plate insulation layer using a high thermal conductivity epoxy glass fiber cloth adhesive sheet or a high thermal conductivity epoxy resin. It can mount SMD components on both sides. High-power SND components can directly weld the SMD's own heat sink on the metal PCB, using the metal plate in the metal PCB to dissipate heat.

3. Arrangement of heating elements

The main heating elements in the switching power supply include high-power semiconductor devices and their heat sinks, power conversion transformers, high-power resistors, etc. The basic requirement for the layout of heating elements is to arrange them from small to large according to the degree of heat generation. The smaller the heat generation, the more it should be arranged in the upwind direction of the switching power supply air duct, and the larger the heat generation, the closer it should be to the exhaust fan.

In order to improve production efficiency, multiple power devices are often fixed on the same large heat sink. At this time, the heat sink should be placed as close to the edge of the PCB as possible. However, there should be at least 1 cm distance from the housing or other components of the switching power supply. If there are several large heat sinks on a circuit board, they should be parallel to each other and parallel to the wind direction of the air duct. In the vertical direction, the devices with less heat generation are arranged at the lowest level and the devices with greater heat generation are arranged at a higher level.

Heat-generating devices should be placed as far away as possible from temperature-sensitive components, such as electrolytic capacitors, on the PCB layout.