Voltage regulator modules: What determines whether one uses 5V or 12V in?

[Kwad Guy]

I see that there are two types of Voltage Regulator
Modules readily available. Some support 5V in, some
support 12V in, and some support both voltages.

My question: What determines this voltage? Is this
an arbitrary (or explicit) decision on the part of the board
designer? Or does it depend solely on the types of
processors supported by the board?

Will a 12V VRM fry a MB that expects a 5V, or vice-versa?

Thanks,

Kwad

 

[jamarno]

Switching-mode regulators are more limited in their regulation range than linear regulators are, at least for efficient operation (linears get worse the bigger the in-out voltage difference), because they change the frequency or duty cycle of their oscillator to maintain regulation. So if you feed one a higher voltage, its duty cycle goes down or its frequency rises, maybe out of range for efficient operation of its coil. Most likely you can operate a 5V-input regulator from 12V, but its maximum output power will be less.