This part is a kind of semiconductor called K14A55D.
Function of this product has TK14A55D.
Manufacturers : Toshiba
Image and pinout :

Some of the text within the PDF file :
TK14A55D TOSHIBA Field Effect Transistor Silicon N Channel MOS Type (π-MOSⅦ) TK14A55D Switching Regulator Applications • Low drain-source ON-resistance: RDS (ON) = 0.31 Ω (typ.) • High forward transfer admittance: ⎪Yfs⎪ = 6.5 S (typ.) • Low leakage current: IDSS = 10 μA (max) (VDS = 550 V) • Enhancement mode: Vth = 2.0 to 4.0 V (VDS = 10 V, ID = 1 mA) Unit: mm Absolute Maximum Ratings (Ta = 25°C) Characteristics Drain-source voltage Gate-source voltage Drain current DC (Note 1) Pulse (t = 1 ms) (Note 1) Drain power dissipation (Tc = 25°C) Single pulse avalanche energy (Note 2) Avalanche current Repetitive avalanche energy (Note 3) Channel temperature Storage temperature range Symbol VDSS VGSS ID IDP PD EAS IAR EAR Tch Tstg Rating 550 ±30 14 56 50 521 14 [ ... ]