900 V drain-source — the spec that picks the application
The STP21N90K5: The 900 V Vdss rating suits high-voltage offline converters such as PFC boost stages and flyback supplies.
On-resistance and gate charge — the switching loss pair
That's the conduction loss anchor: at 9 A the dissipation is I²R = 24 W, which the 250 W package rating can handle with a proper heatsink. Gate charge totals 43 nC at 10 V — a moderate figure for a 900 V part, meaning the driver doesn't need to push hard. Input capacitance is 1645 pF at 100 V drain-source, which sets the switching speed ceiling; the driver's peak current and the gate resistor together determine the rise and fall times.
Temperature range and what it buys the board
The silicon handles avionics cold starts and engine-bay heat soak.