What the 80 V / 4 A rating means on the bench
The BD680 is a PNP Darlington transistor rated for 80 V collector-emitter breakdown and 4 A continuous collector current, with a maximum power dissipation of 40 W. The Darlington pair inside gives it a DC current gain (hFE) of at least 750 at 1.5 A and 3 V — so a modest base drive of 30 mA can switch a 1.5 A load without the driver stage breaking a sweat. The Vce saturation is 2.5 V max at 30 mA base and 1.5 A collector current — that 2.5 V drop times the load current is the conduction loss you budget for in the heatsink calculation. At 1.5 A, that's 3.75 W of dissipation before you even add the base drive power.
Package and board-fit — SOT-32-3 / TO-225AA
Housed in a SOT-32-3 (also known as TO-225AA or TO-126-3) through-hole package, delivered in a tube. The three-lead layout is standard for medium-power Darlington transistors — the tab is the collector, and the metal back provides a direct thermal path to the heatsink or PCB copper pour. Operating junction temperature is rated to 150 °C, which is the ceiling for the silicon die itself — the case temperature at full 40 W dissipation will be lower, governed by the thermal resistance of the mounting interface.