Nothing comes to mind that really needs exactly 19V, with USB-C on board which can provide 20V it seems even a bit of a drawback to step up the voltage rather than increasing the PSU voltage.

  • \$\begingroup\$ The 19 V has been in use for years as the supply voltage for laptops and small form factor PCs. The 20 V of USB-C is much newer. 19 V or 20 V isn't that much of a difference, a 19 V product that cannot cope with 20 V is a poor design. \$\endgroup\$ Feb 26 '20 at 16:59

Your assumption that if the supply was 20 V, it could be used directly for USB-C doesn't take into account the fact that the there is tolerance on the supply voltage.

That tolerance depends on what supply you're using. If you're using a 20 V supply that for some reason outputs 19.5 V then how could the USB-C output deliver 20 V? It would be unable to do so and would then be "out of spec."

To guarantee 20 V on the USB-C output, a guaranteed 20 V (or more) is needed, that means that it would typically need to be around 21 to 22 V to account for tolerances. That means 1 to 2 volt has to be "burned off" (using a linear regulator) which is inefficient.

A switched converter could be used as well but that generally makes little sense for small voltage differences. You'd really need a buck/boost converter to support a wide range of input voltages.

Since a buck/boost is needed anyway it can be more efficient to make 12 V from 19 V (easy using a buck converter and that would work over wide tolerances, if your 19 V supply was only 17 V that could still be enough) and then use a buck/boost converter to make 5 V, 9 V, 20 V for USB-C.

  • \$\begingroup\$ So, 19V was chosen in order to have 7V of headroom to provide power to the 12V rails or does something actually draw more than 12V? I can also imagine going for a higher voltage to reduce amperage and heat generation on the connectors. 19V still seems a bit arbitrary though. Wouldn't 24V be more common? Haven't read enough datasheets to tell if 24V rather than 19V excludes maximum ratings for common buck converters. \$\endgroup\$
    – Horst
    Feb 27 '20 at 8:05
  • \$\begingroup\$ The 19 V is probably (I wasn't there when 19V was chosen) the result of amount of power, safety, price and many other factors. Wouldn't 24V be more common? Why? Because that's 2 x 12 V which is car battery voltage? Also an arbitrary choice. And supporting 24 V means the electronics need to be able to handle up to 28 V, that makes them more expensive. Maybe today 24 V buck converters are cheap, they might not have been long ago when 19 V was chosen. \$\endgroup\$ Feb 27 '20 at 8:18

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