Key Considerations When Choosing an External Power Adapter

A system is only as good as its weakest link, and the adapter or external power supply has to match the quality and performance of the end product to avoid an unhappy customer. More than just producing volts and amps, an adapter must also meet international statutory standards for safety and electromagnetic compatibility, which can vary with the end application and geographical point of use. Other parameters such as efficiency, regulation, peak output power, hold-up on AC failure, temperature rating, moisture ingress rating, no-load/standby losses, aesthetics, and more may also be critical to usability and successful operation of the end-equipment.

This article explains the headline and not-so-obvious specifications of a power adapter and why there can be a significant variation in size and cost for parts that might otherwise seem to have similar ratings. The sometimes-confusing array of markings is explained, and some of the pitfalls to avoid are outlined when choosing an adapter. Examples are given of parts available that demonstrate state-of-the-art design and user benefits.

Starting with the overall system power requirements

As a new product design starts to take shape, the overall system power requirements tend to emerge. Part of the decision-making process can focus on whether to build in a discrete or modular power supply into the product or use an external power supply. The type of application, physical size, and the overall power budget required will form the selection criteria. While both approaches are viable, the use of an external power supply tends to offer more flexibility, particularly when the product may be used around the world. However, while it might appear a prudent choice, selecting the wrong external power supply might have implications for the reliability of the product it is powering. Not only might this cause inconvenience, frustrating customers, but in the longer-term can result in damage to the product manufacturer's brand image and reputation.

The initial criteria

The engineering requirements stipulate the voltage and current the product requires, these being the starting criteria from which to select the power supply. In the majority of cases, the product will require a single DC input voltage, say, 12 VDC, and be connected to the AC line mains voltage. The maximum current the product requires should include any connected peripherals or motors that might be used with and powered by the product during use. For example, if the maximum current required is 2 A, then the total output from the power supply will be 24 Watts. Deemed the full-load power output, the power supply needs to operate reliably, supplying the product continuously without overheating. When selecting an external power supply for the above parameters, the engineer will encounter several other considerations that will add a degree of complexity to the selection process.

Broadly speaking, the additional considerations will fall into three categories; mechanical, electrical, and safety standards and compliance.

Mechanical

Depending on the power requirements, you may be able to choose between an external desktop-style power supply or a wall plug format that plugs directly into the power outlet. Typically providing power outputs in the range of 3 Watts to 45 Watts, wall plug supplies are convenient and compact. The output may either be directly wired or for standard voltages such as the USB 5 VDC output, provide an industry-standard socket. For wired outputs, will the lead length be sufficient to reach your product for the majority of use cases? Also, what type of DC power socket do you envision on your product, and is it available as standard option on the selected wall plug? The manufacturer's datasheet should show the DC plug options available, which typically include standard barrel jacks, either straight or right-angled, a USB power delivery socket, and EIAJ connector. Is a locking option available, or do you intend to have a custom power socket in which case can the manufacturer fit the corresponding plug or supply the output lead stripped and tinned? Don't forget to check the polarity of the plug arrangement, and where within your product will the socket be located, and how will it be secured? The AC input side of a wall plug offers several options too. Firstly, if the product is to be used globally, the availability of interchangeable plug configurations, either two- or three-pin, make for simple product packaging. The earth/ground pin of a wall plug is usually not used, and the DC output is floating with respect to ground; this is referred to as a Class II supply. A useful reference guide to the various plug styles can be found on the IEC website. The AC input voltage range should also be confirmed, although most wall and desktop power supplies available from reputable manufacturers accommodate the universal AC voltage range from 90 to 264 VAC and 50/60 Hz.

Desktop power supplies have a similar set of mechanical input and output connector options. Desktop supplies are not usually as physically constrained as their wall plug counterparts, so they are generally available with power ratings up to 250 Watts, an example being the CUI SDI250-U series. The input plug will usually be a standard IEC320/C14 plug/socket arrangement, but check that line cords are available with all the country and regional-specific plug types where your product may be used. With the higher output power rating, and the need to ship optional country-specific power cords, desktop power supplies may impose additional size and weight considerations on your overall product packaging criteria.

The mechanical construction of the power supply will also have an impact on where your product can be used, of which the environmental conditions of the application, amongst others, are crucial. These factors may include moisture ingress and operating temperature and are covered in the standards section below.

A final note on potential ways to customize your power supply. We are all familiar with the black plastic construction used for external power supplies, but product design has become sleeker and more thoughtful even at the adapter level. Think about how the power supply will match the overall product esthetics. Is it possible to have your company logo or brand name printed on the case, potentially have the power adapter in your company colors, and even have bespoke packaging for the adapter? From the consumer perspective, small details like these make a huge positive impact on the visual aspects of their overall experience when opening the product packaging for the first time.

Electrical specifications

Some critical electrical parameters need checking in addition to the input voltage, current, and frequency mentioned above. From the line input perspective, the supply inrush current should be quoted on the datasheet, this being the instantaneous peak current of a 'cold-start' and may cause unnecessary tripping of a customer's line circuit breaker. Also, is the leakage current within specification - see the standards section. The DC output attributes need a careful inspection; the load regulation, ripple, and noise are critical. Can the power supply maintain the output voltage within an acceptable range across all load conditions and AC input voltage ranges? Regulation of +/- 5% is usually adequate, meaning a nominal 5 V output is maintained within 4.75 - 5.5 VDC. Also, does the power supply require a minimum load to operate? Ripple and noise are particularly important criteria for sensitive semiconductor devices such as FPGAs and microcontrollers, and a high noise level may require additional filtering components within your product to maintain reliable operation. For example, the CUI SDM36-U 5 VDC supply has a maximum quoted ripple and noise of 80 mV p-p.

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Figure 2 - Graph depicting regulation across different load and input voltage conditions (Source: CUI)

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Figure 3 - Global energy efficiency standards timeline (Source: CUI)

Figure 4 illustrates the minimum average efficiency and maximum no-load power consumption for a single low output voltage power supply. By comparison, the European Ecodesign (2019/1782) further includes a low load metric, which requires that the efficiency at 10% of the stated full load rating be documented by the manufacturer.

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Figure 4 - DoE Level VI requirements for external AC-DC power supplies (Source: CUI)

Recent advances in the semiconductor process technologies used for the power converter's switching transistors are delivering improved efficiency, reduced weight, and decreased size for external power supplies. For example, the CUI SDI220G-Uexternal desktop power supply uses wide-bandgap gallium nitride (GaN) semiconductors to increase power efficiency to 96 %, reduce weight by 32 %, and reduce the overall size by more than half compared to previous models.

In addition to the above safety and energy efficiency requirements, power supplies also need to conform to electromagnetic compatibility standards for both conducted and radiated noise of which FCC 20870, CISPR22, and IEC 61204-3 are the most commonly quoted. It should also be noted that for medical applications, the IEC -1 3^rd edition includes electromagnetic immunity criteria, which is specified in the collateral standard IEC 60601-1-2, which is in its 4^th edition.

Conclusion

External desktop and wall plug AC-DC power adapters provide a convenient method of powering a wide variety of consumer, healthcare, and industrial equipment. Aside from the essential voltage and current requirements and their commodity appearance, engineers need to pay attention to the not so apparent specifications of the adapter with a careful reading of the adapter's datasheet and making sure it completely suits the product's requirements. External power adapters can vary significantly in size, weight, and cost, despite appearing to have similar ratings.

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