Flyback converter VIPER12A

Hi,
I've been trying to come up with a really small design that will take mains line voltage in the range of 90-250 volt a/c - 50/60 Hz and output
5 / 3.3volt at < 500mA.

I don't want to use a big transformer+linear regulator because of the size !

I've seen implementations like Microchip AN954, (capacitive and resistive PSU):



Then I located the ViPer family of devices from ST, specifically the VIPER12A which can handle a bulk and flyback configurations.
It provides some sort of protection and I like them, they come in DIP package and most important I can source them in my town !

From the app note "AN1484":
The circuit is a standard Flyback converter with secondary current and voltage regulation driving the VIPer12A feedback pin through an optocoupler.

This is the design I was looking for !, it's small, safe and it's been used all over the world but I know nothing about them  :o,
I feel I'm going through the rabbit hole and things get complicated every step of the way.
First and most important it's very hard to find the correct flyback transformer, the two vendors I found don't have a sales webpage they work only
through distributors, and even then I'm not sure about the stock.

How To Make Induction Cooker

The induction cooking design consists of a small number of simple blocks.
The isolated power supply is obtained directly from the mains, 220 V AC 50 Hz. 15 volts are
used to supply the IGBT driver, fan, relay and feedback circuitry, while 5 volts are needed to
supply the rest of the ICs, including the MCU.
The ST7FLITE09Y0 microcontroller controls the whole process and communicates with the
user interface (buttons and display), drives the fan and the relay, receives feedback from the
cooking element (referred to in this document as “plate” for simplicity) and generates the
PWM signal to drive the IGBTs.

Schematic
Although the schematic is not very complex, this section presents the different parts as
separate topics:

    Mains, DC link and zero voltage switching
    Isolated power supply
    Power stage
    Feedbacks
    MCU pin configuration

Isolated power supply
An isolated power supply is connected immediately after the mains filtering, without passing
through the safety relay. A VIPer22A and a simple voltage regulator provide 15 and 5 volts
respectively. The power supply ground is isolated from the system ground.

NE555 Basic Monostable

Here the popular NE555 timing IC, is wired as a monostable. The timing period is precise and equivalent to:

1.1 x R1 x C1

With component values shown this works out at approximately 1.1msec.The output duration is independant of the input trigger pulse, and the output from the 555 is buffered and can directly interface to CMOS or TTL IC's, providing that the supply voltages match that of the logic family.

The timing diagram above shows the output pulse duration, the trigger input and the output at the discharge terminal of the IC.