Power Supply Voltage regulation question

[robbee]

Hi everyone,

I'm reading about AC-DC and DC-DC converters and there's something I cannot quite grasp. A lot of AC-DC bricks output to 19V, and a lot of DC-DC units and motherboards take 19v as input. But then they regulate that down to 12v, 5v and 3.3v.

Is there a reasoning behind this 19v step between the 2 units? There are also AC-DC units that output 12v, which seems more efficient to me, as you don't have to regulate it again in the DC-DC unit. But I have no technical knowledge whats however about this. Anyone who straighten this out for me? Thanks!

 

[jeshikat]

It's easier to step down, so if you use a 12V AC-DC and it's outputting low it's harder for the DC-DC to compensate.

Also, Voltage x Amperage = Wattage so a 19V output means less current so laptop brick makers can get away with thinner gauge wire. And 19V is easier to use for charging laptop batteries, hence why it's so common on laptops, which means there are way more 19V adapters to choose from compared to 12V.

 

[confusis]

That^

 

[robbee]

So if i get it right, even without batteries, 19v input can be usefull for more stability. That's useful information, thanks!