Package and mounting
The IRF432 is an N-Channel HEXFET power MOSFET rated for a 500 V drain-source breakdown voltage and 4 A continuous drain current, with a 75 W power dissipation ceiling. The HEXFET structure is International Rectifier's original cellular-layout process, known for rugged avalanche energy handling — useful when a flyback or motor-drive snubber lets the drain ring above the rail.
Sizing the heatsink and the load budget
The 75 W dissipation limit is the absolute maximum at 25 °C case temperature. In a real 500 V switching application — say a 300 W off-line flyback primary — the switching losses at 50 kHz and conduction losses at the 4 A peak current will push junction temperature well above ambient. The through-hole package (likely a TO-3 or TO-247 tab-mount) expects a bolted heatsink; the thermal pad area under the mounting tab sets the RthJA, and without a heatsink the part derates sharply. The 4 A continuous rating is a DC limit. In a pulsed application like a motor-drive output stage, the pulsed drain current can be higher — but the average power must stay inside the 75 W boundary. The N-channel polarity means the load connects between the drain and the positive rail, with the source tied to ground through a low-side sense resistor or directly.
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