While looking through some of my accumulated AC/DC wall adapters, I came across an AC-line input, 5-volt, 2-amp output linear power supply from “back in the day.” The Analog Devices model 956 encapsulated-brick module is serious business, as it weighs almost a kilogram (just under two pounds). I have used it occasionally as a bench supply when testing or fixing various small products.
According to its date code, it was made in 1997 (it’s nearly 50 years old!), yet it still works fine. Its output is rock solid despite load transients and has no visible output noise. I couldn’t find a data sheet online, even at legacy-documentation sites, but the solid epoxy block (10.2 × 6.8 × 5.1 cm/4.0 × 2.7 × 2.0 inch) seen in Figure 1 reinforces the definite impression that it is electrically and physically “bulletproof.”
The no-nonsense screw terminals for AC input and DC output connections are clearly marked, as shown in Figure 2. It attaches to a chassis or reinforced PC board via #4-40 screws, which go into threaded metal inserts in each of its four corners.
Going to a switching approach
Thinking again about this supply’s size and weight relative to modern ones with comparable ratings, I started to wonder: we all know that IC feature and transistor sizes have shrunk dramatically over the decades as adjunct of Moore’s “law,” but what about power supplies? While the improvement has certainly not been as dramatic, the development of switching power-supply technology (formally called a switched-mode power supply, or SMPS) and its many variants over the past decades has dramatically shrunk the size, weight, and losses of AC/DC supplies.
I did a quick search for some state-of-the-art switching supplies with the same ratings and found that the fully encapsulated, PC-board mount PSK-10B-S5 (5.08 × 2.54 × 2.1 cm/2 × 1 × 0.83 inch) from CUI/Bel Fuse was a representative 5V/2A AC/DC unit, shown in Figure 3. There are even smaller ones available that operate in the hundreds of kilohertz and into the low-megahertz range, but this 85-kilohertz unit is a good example due to its overall performance specifications.
Note that I deliberately chose not to consider mass-market consumer units such as standard USB adapters/chargers, even though they might be even smaller and lighter than the CUI unit. My reasoning was that their quality is usually inferior to a “proper” AC/DC supply in terms of DC output stability and transient response, operating-temperature range, and overall electrical and mechanical ruggedness.
Table 1 shows the numbers for the CUI PSK-10B-S5 compared to those for the AD956, illustrating the reality of a switching supply versus a linear one with the same basic ratings. In a word: the differences in volume, weight, and efficiency are truly impressive.
Further, like nearly all of today’s AC/DC supplies, the CUI unit must meet a very long list of complicated and detailed regulatory mandates covering EMI/EMC performance, galvanic-isolation voltage, various safety demands, and ROHS, all in addition to protection against adverse thermal and short-circuit conditions. Many of these requirements were not in place in 1977 or were much “looser” then.
Given the many real and critical advantages of switching AC/DC power supplies compared to linear designs, you might think that linear ones are obsolete, but that’s not the case at all. For example, Acopian Technical Company offers several standard families of linear supplies with outputs ranging from 40 watts to 1200 watts, with voltage outputs from 1.5 VDC to 150 VDC; vendors offer even higher power, and custom units are available.
Acopian doesn’t try to convince prospective users that linear supplies are the better choice in all applications. They note that “a 250-W linear power supply would require 600 in3 of mounting space and weighs 26 pounds, while a comparable AC/DC switching power supply would require 60 in3 of mounting space and weighs 2 pounds.” Further, they present a lengthy table with clear numbers comparing the attributes of these two supply topologies. In addition, they acknowledge that switching supplies can be used to both step up and down the input voltage, while linear ones can only provide a step-down output.
Given the many tangible benefits of a switching supply, why would an engineer even consider selecting a linear one? Sometimes, the positive attributes of the linear supply —lack of switching clock, nearly zero noise, fast transient-recovery time, longer hold-up time, and tight line regulation — outweigh the negatives and are critical factors in an application such as high-resolution X-ray machines. As in nearly all engineering endeavors, making a choice requires a balance among priorities.
Once again, designers first look at conventional wisdom, but don’t necessarily exclude unpopular alternatives. Sometimes they must go against the flow when the technical facts require consideration of less-conventional and perhaps even presumed-obsolete alternative solutions. It’s the classic engineering-design conundrum revolving around the trade-offs, technical costs, and realities of the situation.
References
Power Supply Comparisons: Difference Between Linear and Switching Power Supply, Acopian Technical Company
PSK-10B Series Data Sheet, CUI/Bel Fuse
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