Soft start circuit with Mosfet?

[guscrown]

I want to use a soft-start circuit for my design, as I am currently experiencing some inrush/sparking issue whenever I hotplug the external power-supply brick into my board. I reduced it by using some NTC Thermistors but the 2 minute reset time of the Thermistor is not going to be something we can work with.

I know how the concept of the soft-start works, but I don't know how to put it down on the schematic. Can you help me out with some sample schematics?

Here are my specs:

Vin: 24V Imax: 8A P-Mosfet P/N: NTD20P06LT4G
Delay time: 500ms

Any ideas?

[AndyC_772]

We'll need to start with your schematic to see how it might be adapted. Soft-start is rarely something that's bolted on as a separate feature.

[NewBeginner]

A possible inrush current limiting circuit could be the one in the attached image.
But please note that I am a beginner in electronics so this circuit may not be fully correct or it may not be 100% safe. So use it at your own risk.
Maybe other, more experienced and more knowledgeable, members of the forum can shed a light on this.


R2 along with C2 sets the turn-on time. The D1 zener ensures that Vgs will not be lower than -12V (PMOS) so the MOSFET will not get damaged.
I believe that the zener should be selected based on your MOSFET specs. If a Vgs of -12V is too big for your selected MOSFET then a smaller value zener should be used to ensure that Vgs is kept within the specified limits of the MOSFET, but in the same time ensuring that Vgs will have a proper value to fully turn on the MOSFET.
I think that the turn-on time can also be affected by the MOSFET used, increasing or decreasing based on the amount of gate charge of the selected MOSFET.
Also the selected MOSFET should be able to handle the power dissipation, especially during the transition period (from off to on).
Note that in this setup if the supply voltage (Vs) is decreased the turn-on time will increase.

But again, I am a beginner so this my not be 100% correct. Please correct me if I am wrong.

Thank you

[guscrown]

I've prepared the following circuit. Feedback is very much welcomed.

[guscrown]

My only problem with the above circuit is no matter how slow I ramp that Vgs voltage on M1, I am not getting a ramp-type voltage on Vds. It just goes from 0V to 24V instantly.

Any ideas why?

[hlavac]

I would simply have a big resistor (10 ohm ish) in series with the power rail that is shorted by a MOSFET once the voltage rises enough.