Choosing an AC/DC power supply can be a daunting task. Should it be internal or external? Will it be compliant if my product ships to other regions of the world? Does my medical product need a power supply that conforms to medical safety standards? These are just some of the questions that engineers might ask as they focus their efforts on the power supply aspects of their new product design.
Determining The Type Of Power Supply Required
The decision to select an AC/DC external power supply may be down to practical space constraints within the product. Depending on the power requirements, internal power supplies can occupy valuable space, making the end-product unnecessarily larger than it needs to compared to other designs on the market. The footprint a product occupies is a crucial consideration for many consumers, especially when it might be on a counter-top or alongside a media system. Selecting an external supply, whether a desktop power supply or a wall plug unit (Figure 1), can be a prudent choice for some practical reasons as it keeps the end-product as small as possible.
Figure 1: CUI’s SWI product family offers a compact, efficient and economical solution for ac-dc wall adapters (Source CUI)
Wall plug units tend to suit products that need a DC supply from just a few watts up to approximately 50 Watts. Anything more than this will require a desktop unit (Figure 2).
For convenience, many designers opt to power their product from a standard 5 VDC supply, so selecting a wall plug unit that has a standard USB-A socket makes a simple choice for products drawing up to 2 Amps. Likewise, designing your product to work off the popular nominal voltages of 12 VDC, 15 VDC, or 24 VDC makes for ease of selection since most manufacturers provide standard units for these voltages. Also, what type of power input socket do you intend to use in your product? Is it available as standard or at an additional cost from the power supply manufacturer? Don’t forget to check the peak power requirements of your design, since that will determine the adapter you need to choose. Opting to use a unit that doesn’t offer enough peak output power, particularly those that have an inductive load such as a motor might result in overheating of the power supply, causing unnecessary component stress and lowering product reliability.
Compare Your Requirements Against the Datasheet Specifications
Make sure to study the datasheets of potential power supplies carefully, since some factors might not be so apparent as the output voltage and current rating.
Many wall plug power supplies provide a DC output that is floating with respect to ground. Termed Class II, these supplies do not provide access to a protective ground connection, you’ll need a Class I supply for that, and these tend to be desktop units.
Other specifications to check on the datasheet include line regulation, ripple and noise, protection features, and approvals.
Regulation gives a measure of how close a power supply can maintain its output voltage against the stated nominal output. Adequate regulation may be +/- 10 %, but if your application is sensitive to voltage input changes, this feature needs further consideration. For a nominal 12 VDC output, a +/- 10 % regulation means the output voltage could range from 10.8 VDC to 13.2 VDC. Ripple and noise will usually be present on the DC output, and again, sensitive embedded circuitry may be prone to excessive noise on the power lines. The use of additional filtering components may assist but bear in mind that those will add to your bill of material cost and will occupy valuable PCB space.
While looking at the electrical specifications, check to see what types of protection features the power supply has as standard. Output over voltage, output over current, and over temperature are popular protections, but if your product may function erratically should the input voltage fall below a given voltage, an under voltage lock-out requirement may also be important.
Safety and Energy Efficiency Standards
Power supply approvals incorporate aspects of safety, energy efficiency, and electromagnetic compatibility. The following standards all apply to not only the power supply unit but also the product connected to it as a whole. Internal standard IEC-62368 stipulates safety factors such as the maximum permitted leakage currents and input to output isolation voltages for commercial, IT and AV equipment. Figure 3 illustrates the potential safety hazards and the isolation testing requirements. Isolation prevents the lethal AC line voltage being passed through to the DC output.
Figure 3: Isolation test requirements for an AC/DC power adapter (source CUI)
The IEC-62368 will become law in December 2020, having been determined previously by two standards IEC-60950 (IT systems) and IEC-60065 (AV equipment). This safety standard also stipulates the minimum distances relating to voltage stresses within the power supply specified in terms of creepage (the surface path between two points) and clearance (the direct air gap distance). Any power supply used for powering medical appliances that have potential to contact the patient, such as blood pressure monitors, diagnostic equipment, or oximeters needs to conform to IEC 60601-1 4th edition.
Energy efficiency is an essential aspect of any power supply. The European Ecodesign 2019/1782 and the US Department of Energy Level VI standards stipulate the minimum average active efficiency requirements; the average of the efficiency measured at 25%, 50%, 75%, and 100% load. Ecodesign further requires documentation of the efficiency at 10% of full load. See Figure 4 for the timeline evolution of energy efficiency standards. These also have specific limits on energy consumption when there is no load, or the end-product is in standby.
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Figure 4: Energy efficiency standards timeline (Source CUI)
Energy efficiency has recently benefited from advances in semiconductor process technologies, with silicon carbide (SiC), and Gallium Nitride (GaN) proving to be well suited to power conversion applications.
The final aspect of approvals covers how susceptible a power supply may be to the effects of external electromagnetic inference and also if the power supply itself generates EMI above acceptable levels. The FCC Part 15 Class B, CISPR 32, and IEC 61204-3 standards apply.
The selection of an external power supply adapter need not be an erroneous task. A thorough review of the power supply requirements and a diligent comparison of datasheet features can simplify the project significantly.
|Ron Stull is a Power Systems Engineer at CUI Inc. Ron has gathered a range of knowledge and experience in the areas of analog and digital power as well as ac-dc and dc-dc power conversion since joining CUI in 2009. He has played a key role on CUI’s Engineering team with responsibilities including application support, test and validation, and design. Outside of power engineering Ron can be found playing guitar, running, and touring the outdoors with his wife, where their goal is to visit all of the U.S. National Parks.|
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