The power envelope is a characteristic value of power supplies. It is a characteristic curve that shows the relationship between the current that can be drawn at a certain output voltage. Such power envelopes can be rectangular, but also show a linear or hyperbolic relationship of the two output quantities voltage and current.
Rectangular power envelopes have a nonlinear relationship between output current and voltage: as the output current increases, the output voltage remains constant up to the maximum current. In a power envelope consisting of several rectangles, the current remains constant up to a first maximum current, after which it drops to a second voltage value, which it maintains until the second, lower maximum current.
It is different with a linear relationship between voltage and current, where the power curve shows a continuous relationship between current and voltage. At high voltage, only a certain current can be drawn, and this current increases continuously as the voltage drops. The relationship between voltage and current is therefore inverse. The power curve has a maximum voltage value and a maximum current value.