AC/DC buck boost converter design

Design a cellphone charger that takes an input voltage of 90-240 V
the output voltage 5V (standard USB connection)
Test cases 90V, 150V, 240V

i have to design it using a buckboost converter without a controller..

Can someone explain to me how i should start.

Well, yes.

If I where to do that, I would:
1: Set up a current limiter and a regular opamp voltage follower to ensure a stable output voltage.
2: Design a Buck-boost converter, include an oscillator circuit with an input that regulates the oscillator duty cycle output.
3: Melt the two designs together.
4: Ensure that the design can withstand at least 240*1.5*sqrt(2) volts (1.5 for least safety, but that you can increase)