400V, 4A dual NPN in an 8-DIP – what you need to know for a replacement
The STD845DN40 is a through-hole dual NPN transistor from STMicroelectronics, rated for 400V collector-emitter breakdown and 4A continuous collector current, with a maximum power dissipation of 3W in the 8-DIP package. It is designed for medium-voltage switching applications where two NPN devices are needed in one package – common in offline power supplies, motor drive pre-drive stages, and industrial relay or solenoid drivers. The 150°C junction temperature rating suits it for environments with elevated ambient heat, such as enclosed power boards or near hot-side heatsinks.
Obsolete – plan for a replacement or surplus sourcing
STMicroelectronics lists the STD845DN40 as obsolete. For new builds or repair stock, you will need to either source from the surplus/broker channel or qualify a substitute. The dual-NPN configuration in 8-DIP is the key constraint – most modern high-voltage transistors are single-die or SMD, so a direct drop-in is not common. A parametric match for 400V Vce, 4A Ic, and 3W in a dual through-hole package is the starting point for a replacement search.
The Vce saturation is specified at 500mV maximum at 1A base current and 4A collector current – a relatively high saturation voltage for a 4A device, which means conduction losses will be noticeable at full load. That low gain means the base drive circuit must supply substantial current (hundreds of mA) to keep the transistor in saturation at rated current. If you are replacing this part, pay attention to the base drive capability of your existing circuit: a replacement with higher hFE could reduce base-drive power, but one with lower hFE may not saturate properly with the same drive.