AC-DC power supplies are usually provided in an open frame format known in the industry. The product usually described by an open frame is a component power supply with a pure PCB structure, which is designed to be installed in the terminal device application that provides a housing for the entire product.

AC-DC power supplies are usually provided in an open frame format known in the industry. The product usually described by the open frame is a component power supply with a pure PCB structure, which is designed to be installed in the terminal device application that provides a housing for the entire product.

Figure 1. An example of an open power supply.

Another common format for power supplies integrated into terminal equipment is U-shaped channels, in which the power supply PCB is installed in a U-shaped (usually aluminum) chassis, which is usually used as part of the thermal management of power semiconductors, such as equipment manufacturing The supplier offers a variety of fixing options to install the power supply into the final assembly. There are several considerations when installing open frame and U-channel power supplies; these are mainly related to safety, electromagnetic compatibility (EMC) and thermal management, and this article will discuss these areas of concern.

Another important consideration is the detailed specifications of the power supply, especially the temperature and input voltage derating when compared to the data sheet title rated power. The best products maintain rated power at ambient temperatures up to 50°C and AC input as low as 90 V, while some products advertise the title power rating at low voltage with a 20% derating and derating available power at ambient temperature 40°C lower, which may make these products unsuitable for the end application.

When installing the open frame power supply into the equipment enclosure, the creepage distance and electrical clearance required from the equipment enclosure to all sides of the power supply must be observed. In a category 1 system, this means ensuring that there is a distance of 3 or 4 mm between any grounded metal part and any major part of the power supply, depending on whether the final application is industrial or medical, which may require the use of insulators around the power supply assembly.

When using a Class 1 power supply, the safety ground connection of the power supply is an integral part of the electrical safety system and must be firmly connected to the equipment safety ground. This connection is usually achieved through one of the mounting holes, the AC input connector, or a snap-on label on the power supply PCB.

It is likely that more than one ground connection is required to the component, which will affect the electrical emission and sensitivity performance, which will be discussed later.

In the case of using a category 2 power supply, the creepage distance and electrical clearance in the metal enclosure may need to be larger, although in the case of using these units, the equipment enclosure is usually non-conductive.

The U-shaped channel structure alleviates the problems surrounding safety because the U-shaped channel chassis is connected to the safety ground of the power supply and can be directly connected to the equipment enclosure along with the power supply safety ground connection. The design meets the safety clearance requirements between the PCB and the surrounding U channels. However, the end of the U-shaped channel and the top surface of the component are usually still open, and attention must be paid to these areas to ensure that sufficient creepage distance and clearance distance are maintained.

Figure 2. U-channel power supply with and without cover.

The U-shaped channel structure has the additional advantages of easy operation and easy installation. The U-shaped chassis provides a more sturdy structure and includes threaded mounting holes for installers, simplifying the mounting hardware to simple screw fixing. Care must be taken to observe any maximum screw insertion depth to maintain a safe creepage distance and clearance distance.

Another advantage of the U-shaped channel structure is that by conducting cooling of the device casing, the power components can be additionally cooled, thereby reducing the temperature of the bonded components, thereby reducing the overall temperature in the U-shaped channel structure.

Both open frame and U-channel power supplies contain one or sometimes two (for products designed for medical equipment applications) input fuses, which are also an integral part of the overall product safety system design and provide protection in the event of a fire disaster Sexual failure. This fuse is usually permanently installed in the power supply and is not designed for replacement, because the only reason to clear the fuse is a failure of the power supply component.

Since these two structures require input cables, the terminal equipment also needs additional fuses to prevent potential fire hazards caused by the combination of connectors, indicators, switches, and the cable itself. The size of the output cable must be suitable for the maximum power capacity of the power supply, including the maximum tolerance of its overload protection specification, to ensure safe operation when the device itself fails.

Because certain safety-critical components have the highest rated temperature, thermal factors also need to be considered-this will be discussed in more detail in the following thermal management.

Open power supplies usually require two or even three installation points to be grounded. As mentioned above, in a Class 1 system, one of these connections is usually required for safety grounding and is located on the input side of the component. This connection will also connect the line-to-ground and neutral-to-ground common mode filter capacitors, also known as Y capacitors.

These Y capacitors work with the common mode inductance in the power supply assembly to attenuate the noise associated with rapid changes in the power stage voltage of the power supply. The other or others are usually on the secondary side and connect the output common mode filter capacitor to ground. The differential components of the filter are designed to attenuate the noise associated with rapid changes in current, and are included in the line and neutral connections of the power supply.

Figure 3. Mechanical drawing of the open frame power supply showing the ground connection at the mounting point.

This output common mode capacitor is an indispensable part of the EMC performance of the power supply and must be connected to obtain the best EMC performance. In the case of devices using metal casings, this rarely becomes a problem. In a plastic enclosure, whether it is a Class 1 or Class 2 configuration, other measures need to be taken to connect these points together to ensure EMC compliance. The points that need to be grounded or connected to each other are usually identified in the power supply data sheet, as shown in Figure 3.

The best way to connect these points is to mount the open frame power supply on a metal plate that does not need to be connected to anything else, but provides a low-impedance path with low parasitic components for the filter capacitor to be connected. If this installation method is impractical, other methods must be used to connect these installation points, such as multi-strand cables.

In the U-shaped channel structure, all ground connections are made in the U-shaped channel housing, which simplifies the installation of the power supply from the EMC perspective. A good electrical connection from the U-shaped channel chassis to the equipment housing through multiple fixed points is also beneficial, which can minimize parasitic elements-see Figure 4.

In both cases, the input and output cables should be kept at a good distance and avoid close to open components. This eliminates the potential problem of radiation from switch components and magnetic components in the power supply being induced into the system, thereby creating potential conduction and radiation emission problems for the terminal device.

Figure 4. A typical mechanical diagram of the U-channel power supply, detailing the fixing and connection.

Open power supplies may sometimes have rated power in convection cooling, forced air cooling, or both. In the case of U-channel power supplies, there may also be conduction cooling ratings, using the equipment housing or external radiator to further cool the components.

The installation location, orientation, available surrounding space, applied loads and surrounding components, as well as any system air cooling, are unique for each application. It is very important to check the operating temperature of key components in the power supply assembly after installation to ensure that the safety-critical components do not exceed the maximum ratings specified in the safety approval report, and to ensure that the reliability and service life of the power supply are not compromised.

Figure 5. Tables and graphs showing safety limits and estimated service life for 24/7 operation.

Data sheets for open frame and U-channel power supplies designed for integration into terminal equipment will generally identify critical safety components and their maximum temperature ratings, which vary from power source to source based on the type of insulation system used. They also usually provide an estimated service life curve based on the temperature of the key electrolytic capacitor (Figure 5), which is the only component in the power supply that has a wear mechanism.

The service life prediction is based on the design life of the electrolytic capacitor at its highest rated temperature and the average temperature experienced by the final application in its task curve. Obviously, the maximum temperature rating cannot be exceeded under any circumstances or any extreme operation.

All electrolytic capacitor life calculations are based on the Arrhenius equation, where the reaction rate is halved, so every 10 degrees Celsius decrease in temperature doubles the life, which makes it a key factor in the life or service interval of the entire final application.

The lifetime calculations performed by the power supply manufacturer will include factors based on the applied ripple current, but since this is impractical in the finished power supply assembly, it can be achieved by measuring the component case temperature and applying the Arrhenius equation to the specified temperature and design life.

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