This part is a kind of semiconductor called STRS5707.

Function of this product has OFF-LINE SWITCHING REGULATORS - WITH BIPOLAR SWITCHING TRANSISTOR.

Manufacturers : Allegro MicroSystems

Image and pinout :

Some of the text within the PDF file :

STR-S5707 AND STR-S5708 OFF-LINE SWITCHING REGULATORS – WITH BIPOLAR SWITCHING TRANSISTOR COLLECTOR 1 COMMON 2 FAULT The STR-S5707 and STR-S5708 are specifically designed to meet the requirement for increased integration and reliability in off-line quasiresonant flyback converters with indirect feedback. Each device incorporates the primary control and proportional drive circuit with a third-generation high-voltage bipolar switching transistor. Crucial system parameters such as maximum ON time and OFF time are fixed during manufacture. Local control circuit decoupling and layout are optimized within each device. Cycle-by-cycle current limiting, under-voltage lock-out with hysteresis, over-voltage protection, and thermal shutdown protect these devices during all normal and overload co [ ... ]

STRS5707 PDF Datasheet Download