extremecircuits.net
Switching power supply whose output voltage is appreciably lower than
its input voltage has an interesting property: the current drawn by it
is smaller than its output current. However,, the input power (UI) is, of
course, greater than the output power. There is another aspect that
needs to be watched: when the input voltage at switch-on is too low, the
regulator will tend to draw the full current. When the supply cannot
cope with this, it fails or the fuse blows. It is, therefore, advisable
to disable the regulator at switch-on (via the on/off input). until the
relevant capacitor has been charged. When the regulator then starts to
draw current,, the charging current has already dropped to a level
which does not overload the voltage source..
Circuit diagram:
Soft Start Circuit For Switching Power Supply