The STMicroelectronics STD2805T4 is a 5 A, 60 V PNP bipolar power transistor in the DPAK (TO-252-3) surface-mount package. It is designed for medium-power switching and linear applications such as load drivers, relay and solenoid actuation, and output stages in power supplies up to 15 W. The 150 MHz transition frequency keeps switching losses low in circuits running a few hundred kilohertz. The DPAK footprint with the exposed tab carries the heat to the PCB copper — the 15 W max power assumes a properly sized pad and via stitch.
The 60 V collector-emitter breakdown voltage (Vceo) sets the maximum DC rail this transistor can block — fine for 24 V and 48 V industrial buses, but leave margin for transients. The 5 A continuous collector current (Ic) covers most solenoid and relay loads; the 600 mV Vce saturation at 5 A means the on-state voltage drop is under a volt, so conduction loss at 5 A is about 3 W — within the 15 W package limit if the board pulls the heat away. The DC current gain (hFE) of 200 minimum at 100 mA gives good base-drive efficiency; a microcontroller pin can drive it through a 1 kΩ resistor without a Darlington pair.
The DPAK (TO-252-3) has two leads plus the exposed tab, which is the collector.