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.