The STM VIPer 53 is an off-line primary switch. It combines a PWM controller with a HV MOSFET that can switch the mains voltage (85-265V). I looked at a Power Integrations (LNK306) competitor to this device I found in a stove control board back in April.
After depot there is a very obvious difference between the parts. The ST VIPer has separate MOSFET and PWM controller logic chips. The Power Integrations is a single 5mm2 die with the MOSFET integrated.The HV MOSFET die is bigger (3.8mm x 2.5mm) than the controller die (3.2mm x 2.2mm) and the combined silicon area at 16.5mm2 is over 3x the area of the Power Integrations die. The STM VIPer 53 datasheet indicates it is a functional equivalent to the PI LNK306. There are some differences the power levels are slightly different between dies and the PI die is limited to 66kHz whereas the STM VIPer can switch up to 300kHz. The STM FET has an impressive 0.9Ω ON-resistance vs. 12Ω for the PI version I looked at, but the PI has better off-state leakage 70μA vs 150μA specified for the ST VIPer.
And this is the STM VIPer 53 block diagram from the datasheet, showing there is a lot going on