Electrical – Simplest way to get regulated 12V from 10-18V input

There are several low quiescent current adjustable output low drop-out linear regulators that meet the criteria given in the question. The parametric search on the web sites of several of the usual LDO manufacturers would yield some options.

For instance, Texas Instruments TPS76201-Q1:

• Adjustable Output Voltage: 0.7 V to 5.5 V
• 27 uA Quiescent Current at 100 mA load
• 1 uA Quiescent Current if the enable pin is driven high

The typical application circuit is simple as well:

At the stated operating conditions, power dissipation in such linear regulators is:

P = (4.1V - 2.05V) * 0.127 mA = 0.26035 mW in a worst-case calculation.

P = (4.1V - 2.05V) * 0.1 mA + (4.1V * 0.027 mA) = 0.3157 mW in a worst-case calculation. (Thanks, markrages, for the correction)

Still well under the 1 mW power budget specified in the question.

Firstly, your use of a 7812 voltage regulator for you initial circuit may have seemed fine when you tested it but the 7812 needs at least 14V (or above) for it to provide a regulated output of 12V - if you check what the output is when the input drops significantly below 14V you will see that I'm correct. At 11.58V, it's likely that your "regulated" output is less than 11V.

I'm saying this because you have a misconception as to how linear voltage regulators work and you are in danger of believing you can apply this to higher voltage supplies like the 48V in your question.

You have a desired input voltage range of 43.2V to 67.2V and your required output is 48V. Without compromising your intended requirements, the only way this can be achieved is with a buck-boost switching regulator.

In addition to this, powering fans in series will likely destroy one fan - it will go open circuit (or burning) and then nothing will work. To simplify this, consider using a switching buck regulator that converts your 43.2-67.2V to +12V, then power your fans in parallel.

Maybe try one of these: -

I'm assuming you don't need more than 1A at 12V output as per your original design.

EDIT A higher output version is this and now, you will have to use external transistors on the device: -

I would also seriously consider the LTC3810. Here's a picture and also see figure 19 in the data sheet: -