How to Ensure Optimum Results When Using Open-Frame AC/DC Power Supplies

By Bill Schweber

Contributed By Digi-Key's North American Editors

AC/DC power supplies—sometimes referred to as “off-line” supplies—are widely used in lighting, display, information technology, and industrial applications. They are a standard building block of nearly all electronic systems, with the exception of those which are powered by batteries alone.

Versions of these supplies are delivered as open-frame units to be embedded in OEM systems as basic, unenclosed pc boards, and rely on the final product packaging for the requisite overall enclosure. These supplies operate over a wide range of AC line voltages and are offered in many combinations of output voltage, current, and power.

Although they are functionally complete and relatively easy to use, there are nonetheless some design-in considerations engineers must be aware of when using them. These include:

  • Electrical safety/regulatory
  • Thermal management and derating
  • Electromagnetic compatibility

This article examines these considerations in the context of open-frame supplies from XP Power and its LCE80 family of convection cooled, 80 watt (W) supplies.

Power supplies: Make or buy?

Historically, one of the first questions associated with the need to choose one of these supplies was “should we make or buy?” The rationale is that designing and building one or a few units of a basic, functioning <100 W supply is not difficult, at least in principle.

But doing so in practice is a far more complex and multifaceted situation, requiring a design and construction which:

  • Works to specifications under all operating conditions including AC line high/low, transient performance, and temperature range
  • Has the needed protection features such as overvoltage protection, under voltage lockout, and thermal cutoffs
  • Be aware of and meet the many complex worldwide regulatory mandates for safety, efficiency, and quiescent power
  • Addresses the various shock and vibration requirements
  • Includes a plan to test, verify, and certify performance

The reality is that it is extremely challenging for even a team of skilled engineers with experience in this area to do a successful design in a reasonable time, and at acceptable up-front non-recurring engineering (NRE), bill of materials (BOM), production set-up, and test and qualification costs.

Even when requirements cannot be satisfied with a standard unit, most AC/DC power supply vendors offer customization services where they modify a standard supply to meet unique requirements, while still incorporating the many technical and regulatory demands.

Begin with open-frame implementation

An open-frame supply is the industry’s designation for a board-only construction which functions as a single, complete component such as those in the LCE80 family (Figure 1). It is installed into the end equipment application, and that end product provides both the physical and electrical protection enclosure for the supply. Open-frame supplies offer installation flexibility, excellent performance, meet regulatory standards and mandates, and are cost-effective solutions that allow the design team to focus more on the rest of the system design and its differentiation.

Image of XP Power LCE80 series of 80-watt open-frame power suppliesFigure 1: The LCE80 series of 80-watt open-frame power supplies has all needed components mounted on a single pc board. (Image source: XP Power)

An open-frame supply is not the same as another widely used AC/DC supply called the U-channel, where the power supply circuit board is installed in a U-shaped chassis that is usually made of aluminum (Figure 2). A good example is the VCS100US12 100-watt supply from XP Power. The chassis also provides multiple options for the equipment manufacturer to install the supply into the final assembly, and often includes a removable cover that provides electrical and physical protection, and is perforated for airflow.

Image of XP Power 100-watt VCS100US12 U-channel power supplyFigure 2: The 100-watt VCS100US12 U-channel power supply includes a removable protective cover. (Image source: XP Power)

Although the open-frame supply is complete and ready to use, there are still considerations related to electrical safety/regulatory issues, thermal performance and limits, and installation and electromagnetic compatibility (EMC).

Electrical safety/regulatory compliance: Users of open-frame supplies must be aware of clearance and creepage requirements. Clearance is the shortest distance in air between two conductive parts, while creepage distance means the shortest distance along the surface of a solid insulating material between two conductive parts (Figure 3). The required minimums for these two factors is a function of the supply voltage, as well as operating conditions such as anticipated pollution inducing dust, moisture, and other particu­late matter in the air surrounding, or on the surface between, high-voltage nodes.

Diagram of circuit board designs must meet minimum dimensions for clearanceFigure 3: Circuit board designs must meet minimum dimensions for clearance, the shortest distance in air between two conductive parts, and creepage, the shortest distance along the surface of a solid insulating material between two conductive parts. (Image source: Altium Limited)

Supplies are also divided into several IEC classes, based on the end application:

  • Class I: User protection from electric shock is achieved through a combination of insulation and a protective ground
  • Class II: User protection from electric shock is achieved through two levels of insulation (either double or reinforced)
  • A Class I system requires three or four millimeters (mm) between any earthed metal part and any primary part of the power supply, depending on whether the end application is industrial or medical. This may require additional insulators around the open-frame power supply assembly; Class II supplies may need larger creepage and clearance distances.

    When a Class I supply is used, the safety ground connection to the supply is an integral part of the electrical system and it must be securely connected to the equipment safety ground. Also, there is likely to be more than one required earth connection to the assembly, which affects the electrical emissions and susceptibility performance (discussed further below).

    Both open-frame and U-channel power supplies include an integral fuse; for medical equipment applications, two fuses are needed.

    The fuses are usually permanently installed in the power supply and are not designed for field replacement, as the only reason for a fuse to activate (open) is a failure of the supply, which must be repaired or replaced before using the system again. There may also be additional requirements for system-wide fusing for protection against problems with interconnect cables and connections, as well as other circuitry which is unrelated to the supply.

    Thermal management and derating: Heat is a well-known concern in all electronic systems, as it is the primary cause of component fatigue and stress-induced failures, including fractures due to thermal cycling. Regardless of the specific voltage and current ratings of the supply, designers are primarily concerned with the total power in watts which the supply delivers.

    Vendors often design a family of supplies for a specific maximum delivered power rating, and then set the voltage and current pairings to match. For example, all units in the XP Power LCE80 series are rated for 80 W, with the lowest voltage unit, the LCE80PS05, delivering 5 volts at up to 12 A, while the highest voltage LCE80PS54 provides 54 volts at up to 1.48 A. In between are eight more DC output options of 12 volts, 15 volts, 20 volts, 24 volts, 30 volts, 36 volts, 42 volts, and 48 volts.

    The supplies operate over an input voltage range of 90 to 305 volts AC, with full load power available even at a low line of 90 volts. Efficiency is very close to 90%, meaning that only 8 W is dissipated by the supply; the remaining 72 W is available for the system needs. All family members measure 101.6 mm × 50.8 mm × 27.9 mm. The operating temperature range is -40°C to +70°C, with full power available from -30°C (-40°C at high AC line) to +50°C. The calculated mean time between failures (MTBF) is 300 kilohours, per MIL-HDBK-217F.

    All units in the series meet the many relevant regulatory standards including (but not limited to) EN55032 Class B for conducted and radiated emissions; EN55035, EN61547, and EN61000-4-2/3/4/5/6/8/11 for EMC immunity; EN61000-3-2 harmonic current Class C for 50 W load and above. Safety approvals include CB IEC62368-1 (ITE), IEC60950-1 (ITE), UL62368-1 (ITE), TUV EN62368-1 (ITE), EN61347 (lighting), and UL8750 (lighting).

    The efficiency of any supply is critical as it determines how generated heat is managed. Open-frame power supplies can be cooled using passive convection, active forced air (fan), or a combination of both. Many designers prefer to choose supplies which are specified to function to their ratings using passive air cooling alone and not use a fan, for a long list of reasons, including:

    • It saves on direct BOM cost and reduces product assembly time.
    • It eliminates a potential source of failure—the fan—which would have a ripple effect of inducing overheating and greatly shortening the lifetime of the supply.
    • It avoids issues associated with fan speed and operation management, usually based on the sensing of ambient temperature.
    • It is obviously quieter, an important factor in many situations.
    • It avoids the prospect of the end-user unintentionally causing overheating issues caused by blocking the fan inlet or outlet.

    In short, eliminating the need for a fan significantly enhances the reliability of the overall system, simplifies mechanical design, and reduces cost. In order to go fanless, it’s necessary for designers to look at the power supply datasheet to see if forced air is needed to meet the stated specifications, or if passive convection alone will be sufficient.

    This examination includes checking the maximum temperature for which the vendor guarantees performance to all specifications, as well as the derating curve which defines how much the output power decreases beyond a threshold temperature. A well-designed supply will maintain the rated power to 50⁰C ambient temperature, as well as down to 90 volts AC input. In contrast, some products promote a “headline” power rating but quickly derate up to 20% at low AC line, and derate the available power beginning at ambient temperatures as low as 40⁰C. For the LCE80 series, full performance is guaranteed up to 50⁰C, derating linearly to 50% up to a maximum temperature of 70⁰C (Figure 4).

    Graph of derating curve for the XP Power LCE80 seriesFigure 4: This derating curve for the LCE80 series shows that these supplies maintain their 80 W performance rating up to 50⁰C, and then decrease by 50% to 40 W at a 70⁰C maximum operating temperature. (Image source: XP Power)

    The mounting position, orientation, available surrounding space, applied load and surrounding parts, along with any air cooling, are unique to each application. It’s important to model and measure the temperature at the open-frame supply, and not somewhere else in the system enclosure since there can be wide, highly localized variations

    A critical factor in determining the estimated service life of a supply is a lifetime curve based on the temperature of key electrolytic capacitors, which are the only parts with a wear-out mechanism. All electrolytic capacitor lifetime calculations are based on the Arrhenius equation, where the rate of reaction doubles—and hence the lifetime is cut in half—for every ten degree Celsius increase in temperature (Figure 5). A good indication of service life can be determined by measurement of the component case temperature and application of the Arrhenius equation to the specified temperature and design lifetime.

    Graph of thermal derating curves for two typical electrolytic capacitors (click to enlarge)Figure 5: The thermal derating curves for two typical electrolytic capacitors shows their halving of life for every 10⁰C increase in temperature, as per the Arrhenius equation (right). (Image source: XP Power)

    Electromagnetic compatibility issues: Open-frame power supplies typically require two and sometimes three mounting points to be connected to Earth ground to meet standards. In a Class I system, one of these connections is required for safety ground and is located on the input side of the assembly. This connection will also connect the line-to-ground and neutral-to-ground common-mode filter capacitors, also known as Y capacitors (Figure 6).

    Diagram of Y capacitors function as a common-mode filterFigure 6: The Y capacitors function as a common-mode filter and are used on the input side of the power supply, connecting the line and neutral to ground. (Image source: www.blogranya.blogspot.com)

    These capacitors work with the common-mode inductors in the power supply to attenuate noise associated with rapid changes in voltage in the supply’s power stage. These output common-mode capacitors are critical to the EMC performance of the power supply and must be connected for optimum EMC performance.

    It is necessary to connect these points together to ensure EMC compliance with open-frame supplies. The points which require connection to ground or together are usually identified in the power supply datasheet, and the best way to connect these points is by mounting the supply on a grounded metal plate (Figure 7).

    Diagram of mounting holes marked on the drawing with the earth ground symbol (click to enlarge)Figure 7: The mounting holes marked on the drawing with the earth ground symbol must be connected to safety earth in Class I applications, or connected together in Class II applications. (Image source: XP Power)

    This plate does not need to be connected to anything else, as its function is to provide a low-impedance path with low parasitic elements for the filter capacitor connections to ground. Mounting holes marked with the earth ground symbol must be connected to safety earth in Class I applications, or connected together in Class II applications.

    As a general guideline, all input and output cables of the supply should be kept apart and avoid close proximity to the open assembly. This minimizes potential issues where the electromagnetic radiation generated within the power supply induces conducted and radiated emissions into the end equipment.

    Conclusion

    Designers can shorten and enhance their design-in process by concentrating on a single family of open-frame supplies with different voltage/current ratings while keeping all other factors unchanged. Doing so simplifies mounting, grounding, EMC, and thermal analysis, derating considerations, performance-envelope calculations, physical connections, and cabling.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

About this publisher

Digi-Key's North American Editors