What this NPN transistor is and where it was used
The STMicroelectronics STD127DT4 is a high-voltage NPN power transistor in a DPAK (TO-252-3) surface-mount package. It was specified for offline switch-mode power supplies, flyback converters, and ballast circuits where a 400 V collector-emitter breakdown and 4 A continuous collector current were needed. With a maximum power dissipation of 35 W and a saturation voltage of 1.3 V at 1 A base drive and 4 A collector current, it fit medium-power switching stages in compact board layouts.
The STD127DT4 is officially obsolete per the manufacturer's product status.
The 400 V V(BR)CEO and 4 A Ic are the primary selection anchors. A replacement NPN must match or exceed these with a similar or lower Vce(sat) to avoid thermal runaway in the same switching application. The 35 W power limit at a 150°C junction temperature sets the package's thermal budget — any substitute in DPAK or a larger footprint (DPAK+ / IPAK) needs comparable or better RθJA. The low DC current gain (minimum 10 at 1 A, 5 V) means the base drive circuit was designed for a relatively high base current; a replacement with higher hFE may let you reduce the base resistor value, but verify drive current limits on the controller side.