Indeed, it appears that the actual meaning of the question is lost in the translation.
Having said this, I do have seen low cost, low power supplies using a 555. But that was back in the early 1990s. The driven Mosfet was quite small, the frequency only about 25 Khz. Under those circumstances it was an acceptable, but not optimal solution.
Nowadays I don’t think anyone would use a 555. Although it can produce a PWM waveform, the duty cycle range is limited, it doesn’t have an integrated reference, and lacks integrated protection features like UVLO and current sense. Even the ancient SG3524 is better suited than a 555.