I've been working on a soft-start circuit to solve an inrush current problem that is causing some sparking when I hotplug my power supply into my board. I played around with different solutions and this is the one I like the best so far. It adds a delay of 250ms between Vin and Vout on the power MOSFET on my load.
My schematic:
My simulation results:
I want to get some opinions from the PROS out there.
Now, since I'm only using M1 to drive M2, there is no reason to use a power MOSFET, am I correct? I could use a small current MOSFET to drive the other one.
Edit:
Updated shcematic thanks to Kaz:
Edit #2:
I'm working on a soft-start circuit to avoid some sparking when I plug it my external power-supply to my board. I'm using 2 P-channel MOSFETS and some RC circuit to control the gate on one of them whicl will help me control the gate on the power mosfet that willl feed my board.
This is my little circuit:
R3 = My Load. I'm using max 8A hence I am representing it with a 3ohm resistor.
R2 and R4 are there to bias my MOSFET's gate with a Vgs goign from 0V to -12V.
The problem I am having is on biasing M1. Currently Vgs is going from -24V to 0V, and I need it to go from -12V to 0V, otherwise I might end up damaging the device. a Vgs of -24V is too high.
The only thing I could think of was shifting the reference point of C1 up by 12V using a voltage divider but I don't know if this is recommendable.
Like this:
My only concern now is the behaviour of the MOSFET, no matter how slow I ramp the Vgs voltage, it is still acting like an ON-OFF switch. Any ideas?
Best Answer
There are a large number of off-the-shelf solutions to this sort of problem. The advantage that these schemes have is not only do they manage the hot-plug, they limit the inrush current so that the power supply isn't overloaded.
Your simulation result shows a wicked slew rate after 250ms - that doesn't look at all like soft-start to me. The voltage rise should be a ramp, not a straight edge.