Solid state soft start for psu

[Analog_Fan]

2K2 into 47uF has a pole at 1.5Hz. However two stages gives 40dB/decade, so at the gate of the MOSFET, your 120Hz (or 100Hz) ripple is over 80dB down, leaving a very clean output on the source of the MOSFET...in addition to the slow start behavior.

I use the exact same circuit in various tube amp projects for screen and preamp B+ supplies. With a high voltage MOSFET, it's very stable with a 1K gate resistor, and can even be made to withstand shorts or overcurrents (shorted output tubes, etc) with a simple current clamp. It's also easy to make a negative version for bias supplies by flipping everything upside down, and switching to a PMOS (since bias supplies generally don't need to provide much if any DC current, the PMOS can even be small like a TO-92).

Thnx, I'll do the same accept with a totem pole config to get the voltage from there rather then the low pass filter and use 1K as well, as everyone does.

The current config i have using this (2K2 into 47uF) makes the voltage rise steper than i had before but much more current rushing, like 1.5 Amp, the inrush maxes now at 270mA.
reviewing the datasheet of Rohm SH8MB5, witch i have, you could be right, 8.5 Amp.

To bad i have no rail 2 rail opamp.

 

[Matador]

Here is a version with current limiting:

http://www.smithsofweston.com/Assets/Images/AMPS/PPP70W/CapMultiplier.jpg

 

[Analog_Fan]

Here is a version with current limiting:

http://www.smithsofweston.com/Assets/Images/AMPS/PPP70W/CapMultiplier.jpg

Google (YouTube) served me a video:


You (me) thought to know something, but apparently not.
: )

https://en.wikipedia.org/wiki/Euler's_formula

 

[ccaudle]

Here is a version with current limiting:

http://www.smithsofweston.com/Assets/Images/AMPS/PPP70W/CapMultiplier.jpg

I don't understand how that circuit works. How do you ever get the positive gate-source voltage needed to turn on M1?

 

[Matador]

Not sure I understand: the source starts off at 0V, and the R2/R6/U1 charge up C1 until M1 turns on (around 4V threshold), so the source follows the gate upwards (minus the 4V) until steady state is reached.

It's a standard MOSFET capacitance multiplier topology.

 

[ccaudle]

the source follows the gate upwards (minus the 4V)

I misunderstood the circuit as a proposed soft start. If the output is intended to be lower than input then it makes sense (assuming you can tolerate the voltage drop).

 

[Matador]

The soft start is a bonus feature.