soft start regualtor

Hi all,

I did a quick test circuit to soft start a supply rail using a P-chan
mosfet, bascially using a capacitor on the gate, so assuming capacitor is a
short when empty pulls the gate to the supply rail, and a charge resistor
charges the capacitor lowering the voltage on the gate by 12 volts.. limited
by a zener... a discharge resistor is also placed across the capacitor.....
while this works its hard to control on and off times., lowering the
discharge resistor helps for the off times, but you then need more current
to charge it up since the resistor is bypassing some of the current, so you
have to make the charge up resistor lower in value also.. so then your back
to square one but with different values....

Anyway, there are some soft start regualtors out there, I could use on of
those to power the mosfet gate, can't seem to find a simple one though... So
food for thought has anyone got any ideas on how to build a similar thing ?

I've been toying with using a thermistor, somehow, when its cold it will be
one resistance, and as it warms up change value and alter voltage on the
mosfet gate... would hopefully only take a couple of second to cool down to
"reset".... I dont like using those things in that way though...

Can't think of anything else I could try...

The voltages on the gate must rise from 0volts to 6V over a few seconds,
then the mosfet will bascially be full on about 6volts, though a rise to 12V
could be better, 6-12volts does nothing so that could rise a lot faster than
the 0-6V rise... The off time needs to be a few seconds also.... I'd rather
the supply rail shutdown before the smoothing caps discharge (won't go into
the whys of that now though).

Any clues or ideas anyone ?

Cheers,
Chris


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