high temperature of power supply heatsink

Hello,

iMac: 21,5 - inch; Late 2012; 2,9 GHz Intel Core i5; NVIDIA GeForce GT 650M 512 MB; OS X El Capitan (10.11.3)

Problem:

Sometimes, when I'm working (Safari, text documents, music) wiht my Mac I notice high temperature of power supply heatsink (128),

maximum speed of fans, Mac turned off and turned on.