Going from linear to switcher designs has made a huge difference in size, but what about robustness?
I was looking for a 5-V AC/DC adapter module (aka "wall wart") to power a small project the other day, and found 4 units in my collection, two at 1A, two at 2A. One was definitely a linear unit, two were switchers, and I am pretty sure the last one is a linear unit. The dramatic span of their sizes and weights really caught my attention.
Since I had too much idle time on my hands, I decided to do the engineering thing and analyze the four units and their key performance attributes. The four units are pictured here:
(Click on image to enlarge)
Going clockwise from upper left, (do most people even know what "clockwise" means, anymore?), they are:
- a 1-A unit that came with an Iomega Zip drive, probably 15 years old. I assume it's a linear supply, since it has several louver slots on 3 slides for cooling.
- a 1-A switcher unit I received from Power Integrations, also about 15 years old, showing how much smaller a AC/DC unit could be using their control IC. Amazingly, it had the same DC plug as the Iomega power unit, so I could swap wall warts.
- a 2-A switcher that I got with some USB product in the last year or so
- and a 2-A linear supply I acquired in a previous life, an Analog Devices 956, with a 1977 date code
I took some size and weight measurements, and summarized it all in a spreadsheet:
What's impressive is the linear vs. switcher power/weight ratio, in both the 1- and 2-amp versions: it's roughly 10:1, to give a quick sense of the situation. The power-density difference is also impressive, and the switchers can handle 120/240 VAC inputs, while the linear units are 120 VAC only.
Of course, these are only first-order specifications, and indicate nothing about the dynamic performance—factors such as line/load regulation—or tolerance for environmental or electrical abuse; and then there's also physical robustness of the units.
No doubt, the AC/DC module power world has made real progress with genuine benefits to the user who has to carry these adapters, while also saving energy (but it is nice to warm your hands on cold days on that slightly warm linear unit!).
I'll say this: when size and weight are not issues in my around-the-house test projects, that ADI linear unit inspires the most confidence. Maybe it's all a matter of appearance, but the solid epoxy brick with its substantial heft (nearly two pounds) looks as if it could withstand a direct lightning hit and maybe even a bullet.
But today's reality is that smaller, lighter, more efficient is the way to go, and less costly in initial materials as well as operating expense. Once again, engineers should step forward to get some recognition—but I'm afraid the chances of that are pretty close to zero! ♦