Labeled D53 on PCB board for diode, SOT-89 package with marking "43."
It's part of a pulse-generator section on a DC power supply, it's right by an opto-coupler. This picture looks exactly like it, but I can't find any standard distributors in the U.S. I'm scared to throw something from Amazon.com on the board.
This Amazon link has a picture that looks exactly like the chip.
Best Answer
Page 27 has your answer on bottom right.
http://www.bitsavers.org/components/centralSemiconductor/_dataBooks/1997_Central_Semiconductor_SMD_Data_Book.pdf
I searched using D53 diode codes SOT-89. 43 is just for mfg traceability or date code last digits YM or YYWW