Actually, this is quite common in transistor switching applications.
Transistors (whether FETs, Bipolars, IGBT's whatever) have have "Safe Operating
Areas" which is a technical term for the range of simultaneous voltage and
current
levels it can safely sustain.
For example, the venerable 2N2222A is rated at 75 Volts VCBO, 40 Volts VCEO
and 0.6A max Collector Current...but the max power dissipation is 0.5 watts.
Obviously you cannot sustain 75V at 0.6A SIMULTANEOUSLY. Manufacturers
specify a safe operating area--usually in chart form--showing how much current
the
transistor can pass for given voltage conditions--both in forward and reverse
bias conditions. It's important to note that these charts do not necessarily
correspond to a simple curve of the maximum power dissipation.
Combined with safe operating areas is the consideration of junction
temperature. Almost all of these specs are given for device junctions at 25
degrees C
(about room temperature). Of course, you can imagine, the ratings degrade at
higher temperatures. Overtemperature and overvoltage are the two most common
transistor-killers.
So, yeah, the Alpha guy is probably right...you can't get both max voltage
and max current simultaneously....and it is not that simple.
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