In the industrial field, AC-DC power modules are usually supplied in two forms: open frame and U-slot. These modularly designed power supplies are widely used in terminal devices that need to be embedded and installed, and are protected by the device housing. Below we will discuss the key considerations when installing open frame and U-slot power supplies from the aspects of safety, electromagnetic compatibility (EMC) and thermal management, and discuss their practical applications in combination with product specifications.

Before discussing, we need to understand what power supplies are? What types are there?

A power supply is a device or system that converts one form of electrical energy into another. It is a core component of the operation of electronic equipment and provides stable and reliable energy support for the circuits of the equipment. The power supply can obtain energy from a variety of input sources (such as AC, DC, batteries or solar energy) and output the required voltage, current and frequency. Commonly available on the market are 12v switch mode power supplies (SMPS), open frame power supplies, U-slot power supplies, etc., and the two types of power supplies we are discussing today are the latter two.

Open frame power supply

Open frame power supplies usually consist of only printed circuit boards (PCBs) and components, without additional housings. This type of power supply needs to be installed inside the device and is protected by the device housing. Its advantages are simple structure and light weight, but special attention should be paid to the safety and thermal management of the installation environment.

U-slot power supply

The U-slot power supply adds a U-shaped shell, usually made of aluminum, to the PCB. This U-shaped shell not only helps to dissipate heat, but also provides a variety of fixed installation options. Compared with open frame power supplies, U-slot power supplies are easier to install and the overall structure is more robust.

Safety

Creep distance and electrical clearance

When installing an open frame power supply, it is necessary to ensure that all sides of the power supply meet the necessary creepage distance and electrical clearance requirements from the equipment housing. For example, in Class I systems, a distance of 3-4 mm is usually required between the primary circuit and the grounded metal parts, depending on the application scenario (such as industrial or medical equipment). If these distance requirements cannot be met, isolation may be required using insulating materials.

Ground connection

For Class I power supplies, the ground connection is an important part of the electrical safety system. The ground connection is usually achieved through mounting holes, AC input connectors, or quick-connect terminals on the PCB. In U-slot power supplies, the U-shaped shell is usually connected to the safety ground of the power supply and can make good electrical contact directly with the equipment housing. This design simplifies grounding connections while improving overall safety.

Fuse and cable protection

Open frame and U-slot power supplies usually have one or two input fuses built in as part of the overall safety system. These fuses cannot be replaced because a blown fuse usually means irreversible damage to the power supply. To prevent the risk of fire caused by faults in connectors, cables, etc., terminal equipment needs to be equipped with additional protective fuses.

Electro Magnetic Compatibility (EMC)

Grounding and filtering design

Open frame power supplies usually require multiple grounding points. One of the grounding points is located at the input end, which is used to connect to the safety ground and serve as the grounding for the common mode filter capacitor (Y capacitor). The Y capacitor and the common mode inductor jointly suppress the rapid voltage change noise generated by the switching circuit inside the power supply. Other grounding points are usually located at the output end, which are used to connect the common mode filter capacitor at the output end. The grounding points of the U-slot power supply are concentrated on the U-shaped housing, further simplifying the EMC installation requirements.

Cable routing

Whether it is an open frame or U-slot power supply, the input and output cables should be laid as far apart as possible to avoid being close to the switching components and magnetic components inside the power supply. This helps to reduce conducted and radiated interference.

Thermal Management

Heat Dissipation and Temperature Monitoring

Open frame power supplies are usually rated for natural convection cooling and forced air cooling. U-slot power supplies may also support conduction cooling through the device housing or external heat sinks. During installation, it is necessary to check the operating temperature of key components to ensure that they do not exceed safe temperature limits based on factors such as the installation location of the power supply, the surrounding environment, and the load conditions.

Life of key components

The life of the electrolytic capacitors in the power supply is usually the determining factor of its life. According to the Arrhenius formula, the life of electrolytic capacitors approximately doubles every 10°C as the temperature decreases. Therefore, the operating temperature of the capacitors should be minimized during installation to extend the service life of the power supply.

Selecting the right power supply

The specifications and operating environment of the power supply need to be carefully checked during installation. For example, some products can still provide full power at low input voltages (such as 90VAC) or high ambient temperatures (such as 50°C), while other products may need to be derated at low input voltages or high ambient temperatures. These parameters directly affect whether the power supply is suitable for the final application.

Conclusion

Open frame and U-slot AC-DC power supplies have their own advantages and disadvantages in structure, installation and performance. When selecting and installing, safety, electromagnetic compatibility, thermal management and product specifications should be considered comprehensively to ensure the safe and reliable operation of the power supply in the equipment.