Soft Start Circuit for Negative Power Supply

[ymliao]

Hi,

Does anyone have any idea on designing a soft start circuit for -5V power supply input? Any suggestion is greatly appreciated!

Thanks!

 

[cbarn24050]

Hi,what do you mean by soft start, if you mean a fast ramp then you can use a resistor before the main capacitor, if you need something slower you could try an emmiter follower with a RC network on the base but you would loose 0.6v

 

[Cappels]

Alternatively, you could use an N-chennel FET connected as follows:


Source to -5V
Collector connected to load (and decoupling to ground).
Gate connected to ground through a large resistor.
Capacitor connected from Gate to Source.
A diode can be added across the resistor (cathode to ground, anode to the gate) to dump the capacitor -here take the dv/dt of the -5V supply and the capacitor value into account so you don't smoke the diode when power is switched off.


Make sure the VGS threshold is sufficently low and that the FET is appropriately sized to handle the peak dissipation, and on resistance should be low enough so voltage drop won't be a problem.

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