This part is a kind of semiconductor called AP3988.

Function of this product has PRIMARY SIDE POWER SWITCHER.

Manufacturers : Diodes

Image and pinout :

Some of the text within the PDF file :

A Product Line of Diodes Incorporated AP3988/89/90 PRIMARY SIDE POWER SWITCHER FOR OFF-LINE SMPS Description The AP3988/89/90 is a performance enhanced power switcher for power supplies with better conversion efficiency, better voltage & current accuracy, and improved protection functions. Typical applications include charger, adapter for ADSL, home appliance power supply, LED lighting power supply and PC auxiliary power supplies. The controller regulates the output voltage and current in the primary side by piece-wise Pulse Frequency Modulation (pPFM) in discontinuous conduction mode (DCM). The system operating frequency reduces linearly from heavy load to light load in each interval of the p-PFM, and enters constant current mode when the load current equals to the maximum system output [ ... ]

AP3988 PDF Datasheet Download