I'd like to solve the above-mentioned problem by using a buck-boost or SEPIC DCDC converter. I haven't successfully found any suitable ICs though. SEPIC converter with input voltages from 4.5V to twenty-some volts doesn't seem the best choice as it would be necessary to have the aimed 5V 300mA output as soon as possible. (Minimum input of 2...3V would be better.) I was thinking on designing a voltage doubler / multiplier between the generator and the converter so that the input voltage is more convenient (4.5V input is reached faster). Unfortunately I don't have any previous experience on voltage multipliers, so I'm not quite sure it would solve my problem.
Efficiency is vital, so I'd prefer keeping it as high as possible. The voltage source will gain the power from the rotating shaft of a small car, thus the voltage will increase monotonously in time until reaching the maximum value of approx. 6V.
Thanks in advance for your help!