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Installing open frame and U-channel PSUs for safety and performance

Many (if not most) AC/DC power supply units (PSUs) are delivered in an open frame format, meaning that they are not fully enclosed. As they are typically installed inside equipment, this approach allows any forced air cooling to be used most efficiently and save cost. As these devices require professional installation, they are considered to be components and, therefore, rely on the product housing for safety and to protect them from the elements.

The U-channel format is a variation on this theme and consists of an open frame PSU mounted into a U-shaped aluminum chassis. The aluminum will generally contain multiple pre-drilled and threaded holes that allow installers a range of options for installation. From a thermal management perspective, the PSU manufacturer can thermally bond the major semiconductors to the aluminum, thereby creating a better heat conduction path, improving both the performance and reliability of the PSU.

When planning the installation of any type of PSU, including open frame and U-channel, designers must consider a few factors. Inevitably, safety, thermal management and electro-magnetic compatibility (EMC) come to the top of the list of considerations.

PSU specifications can be complex, especially deciphering the difference between a best-case rating and the reality when derating for variations in the line voltage and temperature effects are considered. Good quality PSUs will not require derating of the power with line (input) voltages as low as 90VAC and can maintain full power in ambient temperatures of at least 50⁰C. PSUs with less design margin may require a 20% derating if the line voltage drops to 90VAC, and may struggle to deliver full power above 40⁰C, all of which can catch a designer unawares and limit the performance of the final application.

Safety is paramount

Safety considerations can appear complex as they relate to the individual PSU’s power and voltage ratings, as well as the end application – with medical applications being among the most stringent. Standards will define creepage and clearance distances that must be maintained to all faces of the supply; these relate to the distances between any primary part of the PSU and any earthed metal part. As an example, Class I systems require this distance to be 2mm (2.5mm for medical applications). In order to ensure compliance, insulators are often added to the PSU mounting.

Class I PSUs provide a safety ground connection and, as this is an important part of the overall system’s safety system, it must be properly connected to the equipment safety ground. There are several methods for achieving this which commonly include using the AC mains input connector, using a ‘spade’-type FASTON connector or one of the mounting bushings. To achieve the susceptibility and emissions requirements for the system, it is not unusual for several earth points to be provided – these are not options, and must all be connected to ensure proper performance and compliance with legislation.

In Class II systems, the spacing distances are generally bigger than Class I, although these are often negated by the use of non-metallic (therefore non-conductive) housings.

Figure 1

Open frame power supply

Open frame power supply

One good reason for using a U-channel PSU is that it can significantly simplify many of the issues surrounding safety. For example, the PSU manufacturer will bond the safety ground to the chassis, with the installer only needing to bond the chassis to the equipment enclosure with fixings that are often supplied. As the manufacturer also mounts the open frame PSU in the channel, all clearances are dealt with; however, installers should pay close attention to the top of the PSU assembly and the ends of the U-channel as these are exposed and will need to ensure that no safety distances are violated.

Figures 2a and 2b

U channel power supply (b)with and (a)without cover

U channel power supply (b)with and (a)without cover

Not only does the U-channel make safety easier, it also facilitates far simpler installation methods. When mounting a bare PCB, standoffs and other hardware must be added to the bill-of-materials (BoM) and sourced. By comparison, with threaded mounting locations, U-channel PSUs just require standard machine screws. However, manufacturers provide guidance for maximum screw lengths and these must be complied with to avoid degrading any safety-critical creepage and clearance distances.

As the U-channel can be fixed directly to the equipment chassis, and PSU manufacturers bond the heat-generating power semiconductors to the chassis, a low-resistance thermal path is created to ambient. This will reduce the temperature of the components and the ambient inside the enclosure, thereby prolonging component lifetimes.

All types of PSU include an input fuse and PSUs intended for medical applications often include two fuses that are required for general safety and as a means of fire protection should a catastrophic failure occur. In most cases the fuse(s) are an integral part of the PSU and are not user-replaceable.

On the subject of fusing, most open frame and U-channel PSUs need wiring to carry the line input to the input of the PSU itself. Along with the associated indicators, connectors and switches, the cable is another area with the potential for catastrophic failure and, therefore, must be fused.

When selecting the output wiring, installers must consider not only the normal rated output of the PSU, but also what levels of current and voltage could be present in a fault condition, and select wiring accordingly to ensure that safety is maintained under all conditions.

The final safety consideration is to ensure that the PSU is mounted in such a way as to maintain its thermal ratings. Many safety-critical components have a maximum temperature rating that must be adhered to.

EMC noise performance

While ground connections are important for safety, they also have a valuable role to play in regulatory compliance for EMC which is why many open frame PSUs will have two or three earthing points. The point located near the input (in a Class I system) earths the line and neutral via the Y-capacitors (common mode filtering capacitors), as well as providing a safety ground connection. The main role of the Y-capacitors is (along with common mode inductors) to reduce noise spikes due to voltage spikes within the power supply.

There are also common mode capacitor filters on the output stage to reduce current-related noise and the second ground connection is provided for the purpose of grounding these directly. As these filters are critical to EMC performance, it is essential for regulatory compliance that all grounding points are connected with a low impedance path to earth.

In any application with a metal housing this is simple to achieve, but the requirement for EMC-related grounding remains even if the housing is made from a non-conductive material, such as plastic. In these cases, installers must find an alternative means of grounding these points.

While this may seem complex, PSU manufacturers will always identify points requiring grounding (or communing together) on their datasheets, so this is a good source of information, as shown in Figure 3.

Figure 3

Mechanical drawing of open frame supply showing mounting point ground connections

Mechanical drawing of open frame supply showing mounting point ground connections

Ideally, these connections are made without wires by fixing the PSU to a grounded metal plate. In unusual cases this may not be possible or practical and so wires are used. If using wires, then they must be specified (or over-specified) carefully to ensure low impedance as well as ensuring the parasitic elements do not affect the performance of the filter. Multi-strand cable is generally a good option, but direct mounting to a plate is always preferred.

With U-channel PSUs, the PSU manufacturer will generally provide the unit with all relevant ground connections made to the chassis, thereby simplifying the installer’s task. In order to ensure the chassis has a good electrical connection to the enclosure, several mounting points must be used, using conductive screws as this minimizes parasitic elements as well.

Figure 4

Typical mechanical details of a U-channel power supply detailing fixings and connections

Typical mechanical details of a U-channel power supply detailing fixings and connections

The cables used for the mains feed and the output can radiate and be susceptible to noise. For that reason, they must be neatly wired to avoid each other and any open aspects of the PSU where they could pick up switching noise which could increase system noise above levels permitted by legislation.

Temperature-related considerations

The amount of power that can be delivered by a PSU can vary greatly, depending on the environment in which it is placed, how it is mounted and the availability of any moving air. As a result, PSU manufacturers qualify their products in several situations and, for open frame and U-channel PSUs, often provide ratings for both convection and forced air cooling.

Each application and installation are different, and the same PSU may perform very differently in different applications, so each one must be fully assessed. Installers need to consider where the PSU is mounted and its orientation, as well as the space envelope around it and whether the PSU is to be placed in a forced air stream. The maximum load is also a factor as are any heat-generating components nearby.

During system prototyping, key safety components should be checked for temperature, preferably using a non-invasive method such as an infrared thermometer – thermocouples can act as a heatsink and affect measurements. Most manufacturers will identify several key components, their location and the maximum temperature they should be allowed to reach (under any operating condition) and this will ensure that no safety ratings are exceeded, and the anticipated component lifetimes are not adversely affected.

As open frame and U-channel PSUs do not normally contain fans, the principal wear-out mechanism is the large electrolytic capacitors. Manufacturers often provide a service life estimate for these components in the datasheet.

Figure 5

Table and graphs showing safety limits and estimated service life for 24/7 operation

Table and graphs showing safety limits and estimated service life for 24/7 operation

These estimates take into consideration the design lifetime of the capacitor from the manufacturer at both its maximum temperature rating and the anticipated temperature it will experience during normal operation of the PSU.

Factors such as ripple current will have an impact on capacitor lifetime but, as this is difficult to quantify, its effects are commonly ignored. Instead, the Arrhenius relationship is used to relate lifetime to operating temperature. Simply put, Arrhenius states that for every 10C rise in temperature, the expected lifetime halves. This puts into clear context the importance of managing the temperature of these key components during the working life of the PSU.

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