TT Electronics Power Supply Solutions
Installation Considerations for Internal Power Supplies
AC/DC power supplies can be classified into one of two primary families: internal or external. Internal power supplies are those which will be installed within some end device as a component; external power supplies accompany an end device as a stand-alone sub-assembly. Internal and external power supplies vary greatly in the degree of engineering effort required to successfully implement the power source as an element of the final system.
When designing an internal AC/DC power supply into a system, several factors must be considered surrounding the safety, thermal, and electromagnetic compatibility (EMC) implications of the installation. This article outlines the caveats associated with utilizing an internal power conversion solution in opposition to an external one and provides guidance on achieving a proper installation.
Internal vs external
External power supplies offer several benefits and conveniences to original equipment manufacturers (OEMs) that are not afforded by internal converters. For this reason, there continues to be a trend toward the use of external adaptors, particularly in the industrial and commercial markets.
Utilizing an external adaptor places the onus of expensive safety compliance design, testing, and certification on the power-supply manufacturer. Electrical hazards are mitigated for downstream safety extra low voltage (SELV) circuits, meaning the OEM does not (necessarily) need to address the safety concerns associated with high-voltage circuitry within their design.
OEMs are also relieved of the responsibility to manage power conversion-related heat dissipation within their enclosures. Today’s most efficient power conversion topologies can still be expected to generate heat at roughly one BTU per hour for every three watts of power drawn by the end device; a noteworthy amount of heat in many medium- and higher-power applications.
External converters augment end-device serviceability. OEMs do not need to send a technician out to a remote site to replace an external power adaptor. Rather, component replacement can be facilitated by untrained consumers, reducing system downtimes and OEM service costs.
Power supplies rely on the use of magnetic components and heat sinks, neither of which are acclaimed for being small or light-weight elements. Positioning these large and heavy materials outside the device itself can help differentiate it from a marketing perspective.
Despite the abundance of benefits associated with external power conversion, there are still plenty of applications that fundamentally demand the implementation of an internal power supply. These applications are often fixed or semi-permanent installations and/or utilize more power than is available from standard off-the-shelf external supplies, which become exceptionally scarce above 350W. In such cases, the aforementioned benefits of external power conversion become implementation challenges.
Internal power supplies are components, not standalone devices. This means many aspects of the product’s safety are dependent on how it is used rather than simply how it is built.
An internal power supply cannot be properly evaluated against many safety standard clauses until that supply is installed within another device. Consider IEC 60950-1 for example, a standard commonly used to evaluate the safety of power supplies intended for industrial applications. The following clauses would be difficult or even impossible to evaluate with the internal power supply alone, but may indeed warrant evaluation in the end application:
- Portions of clauses 1.5, 1.7, 3.3, and 3.4 concerning the use of interconnecting cables and disconnect devices:
Internal power supply I/O typically consists of either headers or terminal blocks that are fixed to the PCB. When installing the PSU, wiring harnesses are needed to bring the input and output ports out to some standard AC inlet and down to the PCB respectively. Many internal PSUs have additional ports for alarms, control signals, or auxiliary power rails. All of these peripheral connections need to be evaluated for safe practice in the final assembly.
- Portions of clause 2.1 concerning accessibility of energized parts:
Internal power supplies, even those with a case or U-channel assembly, do not provide adequate protection from electrical shock. The end-product assembly should deliver such protections, disallowing operators from inadvertently making physical contact with hazardous voltages. When using an external supply, the plastic enclosure surrounding the PCB affords these protections.
- Portions of clause 2.6 concerning earthing and bonding provisions:
Class I internal power supplies provide a means for connecting the supplies’ protective earth node to the system’s protective earth. One or more of an internal PSU’s mounting holes is usually electrically tied to the protective Earth node of the power conversion circuit, such that if the end device chassis is earthed, mounting the PSU down with conductive hardware completes the required earth ground connection. Other common configurations include an earth ground pin on the input header, or a separate ground tab emerging from the PCB. The installation will need to undergo a safety evaluation to ensure that these connections are properly made (proper conductor sizes and impedances, proper insulating materials and insulation colors, etc.).
- Portions of clause 2.10 concerning creepage and clearance distances, and insulation:
Clause 2.10 is fully considered in the safety evaluation of an internal power supply. However, care must be taken not to compromise these compliance items during the installation. Proximity of the PSU to nearby conductors could potentially decrease effective creepage and clearance distances. Consider the following hypothetical situation whereby an optocoupler is used to transfer output voltage data to a control card on the primary side of a power converter.
In this example configuration, the optocoupler bridges the safety gap between primary and secondary (Figure 1). Where the through hole leads protrude on the bottom of the PCB, the high voltage and SELV leads are separated by 7 mm of air below the FR4. This configuration is compliant with the 60950-1 6 mm clearance mandate.
Figure 1 This example configuration shows clearance distance in an internal PSU installation.
If an external item is affixed to the chassis (or the chassis affixed to something else) with a (poorly-placed) machine screw, the compliance of this configuration with clause 2.10 can be compromised, as shown in Figure 2. Note that for the purposes of creepage and clearance measurements, unconnected conductive parts that divide creepage and clearance distances are counted as zero distances.
Figure 2 A machine screw could compromise clearance distance across the optocoupler.
The machine screw protruding into the chassis, despite not making contact with any connected electrical part, has effectively decreased the clearance distance across the optocoupler from 7 mm to 5.5 mm. The assembly is therefore no longer compliant with clause 2.10. This is one reason why PSUs in U-channel assemblies specify maximum screw lengths for mounting the U-channel. While protruding machine screws are a common culprit for this type of installation issue, they are not the only unconnected conductive parts that must be considered when installing an internal power supply.
- Portions of clauses 4.1 and 4.2 pertaining to mechanical stability and strength:
The onus of ensuring that mechanical stresses do not cause safety hazards falls on the end assembly. Internal power supplies are not intended to be used as standalone devices, and so their standalone mechanical integrity is not often evaluated at the safety agency. What matters is how well the final installation holds up to mechanical stresses and supports the internal PSU. Internal power supplies must of course be designed to be mechanically sound so that they do not become the weakest link in the final assembly, but the associated safety assessments cannot be made at the component level.
- Portions of clauses 4.6 and 4.7 concerning enclosure openings and resistance to fire:
Electrical equipment must be enclosed in a manner that helps mitigate the spread of fire in the event of a catastrophic failure. While steps are taken to ensure the internal power supply design is done in such a way that fire is not a probable result of a failure, a fire enclosure must still be implemented in the final assembly.
Heat is a power converter’s foremost nemesis. At high operational temperatures, thermal runaway can cause semiconductors to overheat and burn out, component temperatures may exceed those permitted by applicable safety standards, and a devices’ operational lifetime can rapidly degrade as chemical processes are accelerated (particularly in electrolytic capacitors). Further complicating this matter is the fact that power supplies generate heat as a biproduct of normal operation. The heat generated by a power supply is related to its operational efficiency, according to Equation 1.
Where Qd is the heat dissipated in watts, POUT is the output power in watts, and η is the efficiency. POUT and η have been represented in Equation 1 with like subscripts to make clear the fact that the operational efficiency varies with output (O) power and is not just some fixed value.
If 100% of the heat generated by a given component is transferred to its environment (indicative of a hypothetical junction to ambient thermal impedance of 0°C/W), that component’s temperature will not rise. On the other hand, if there is a disparity between the heat generated and the heat transferred, the device’s temperature will rise according to its thermal impedance. An internal power supply must be installed in such a way that it is allowed to transfer its heat to its environment.
Most standard internal PSUs are designed to facilitate this heat transfer via convection, either natural or forced. Some internal PSUs may also offer conduction paths for heat to be pulled away from the device through contact with some external heat sink. It is important that the PSU manufacturer’s specified cooling requirements be considered when installing the power supply. For anything more than very-low power applications (circa 50W or less), care should be taken to ensure there is at least a path for air to flow across the power converter.
If a PSU is specified to supply a given amount of power under natural convection conditions, there is an implication that there is a means for allowing that natural convection to occur in the final assembly under the influence of thermal gradients. Vents and some amount of free space should be available to allow air to naturally circulate around the warm PSU components, removing heat from them. Natural convection is not the same thing as perfectly still air and one should be careful not to “suffocate” the internal power converter.
As power levels increase, natural convection often becomes insufficient for the removal of heat from sensitive PSU components. In this case, forced-air cooling is often required. Forced-air cooling involves using a fan to push or pull greater volumes of air from outside the enclosure across the hot power-supply components per unit time. The more air that passes over these components, the more heat can be removed. If forced-air cooling is required, the power-supply manufacturer should specify the volume per time and flow direction required to achieve optimum cooling performance.
There are some applications in which forced-air cooling cannot be deployed due to audible noise concerns or the absence of sufficient ventilation. In these cases, a conduction path must be provided to transfer heat from the sensitive PSU components to the outside world. It should be noted that not all internal power supplies are designed to facilitate conduction cooling.
Interestingly, the physical orientation of the power supply relative to the pull of Earth’s gravity can sometimes be a significant thermal consideration. Hot air rises away from Earth as it is replaced by denser, cooler air in a convection cycle. Sometimes an ill-oriented power supply can promote hot air moving toward the more heat-sensitive components in the design. Consider an instance whereby the power supply’s main switching transistor (heat source) is near an electrolytic capacitor (heat sensitive). Positioning the power supply such that the electrolytic capacitor is physically above the transistor should be avoided if possible (Figure 3).
Figure 3 This illustration shows the influence of physical orientation on natural convection heat transfer.
In a similar vein, an assessment should be made regarding the proximity and relative orientation of heat-generating and heat-sensitive subassemblies within the enclosure. The PSU itself is both heat-generating and heat-sensitive.
EMC certifications can occasionally present a challenge for any system integration, regardless of whether the PSU is internal or external. While PSU manufacturers work hard to get their emissions margins as high as possible, to allow room for end-device circuitry to emit energy without causing a system-level failure, it is still important for OEMs to understand that having a compliant power supply does not always guarantee system-level compliance. This is especially true for internal power supplies, where installation decisions can greatly impact the radiated and conducted emissions profiles of the final system. Frequent errors include improper functional grounding and lax wiring.
Earth ground is useful for more than shunting fault currents away from unsuspecting users. Ground is also a great place to drop off unwanted high-frequency (HF) energy as desired/intended currents enter and exit the PSU. If high-frequency artifacts from switching elements couple onto input and/or output wires, they can wreak havoc on both radiated and conducted emissions profiles. In response, most PSU designs incorporate low-impedance AC paths to earth ground for high-frequency currents from both the input and output conductors. Making sure these paths are properly connected is a critical factor in the PSU installation.
It is easier to make a functional grounding mistake when the mounting hardware is not already used for protective grounding functions. In the event that the PSU is not conductively mounted, it will still often be necessary to create an electrical connection between all of the mounting holes as the PSU design regularly assumes they will be continuous. One or both mounting holes on the secondary side of the converter are usually capacitively coupled to DC+ and Return to shunt HF noise to ground, as illustrated in Figure 4.
Figure 4 Mounting holes on the secondary side of the converter are capacitively coupled to DC+ and Return to shunt HF noise to ground.
In Figure 4, the user is responsible for securing the functional connections indicated by the dashed lines. If this is not accomplished through the use of conductive mounting hardware, drain wire will need to be manually run to each applicable mounting hole. Otherwise, the shunt paths will become open circuits and the HF energy will have nowhere to go but out through the DC wiring harness.
Even if the shunt paths on both sides of the converter are indeed properly connected, some amount of HF energy will still make its way out onto the I/O wiring. If need be, filtering can be improved by adding common-mode inductance to the wiring harnesses. In any event, care should be taken to keep I/O runs as short as possible, and to avoid running I/O wires near components that are expected to radiate electromagnetic energy.
A slew of (albeit simple to mitigate) complications are introduced to a power supply design-in effort when an internal power supply is to be used. Between safety, thermal, and electromagnetic installation considerations, a lot can go wrong. When embarking upon a new design that will use an internal power supply, or if you are having integration issues with your existing solution, keep these guidelines in mind.