Inrush Current Limiting Circuit - Would this work?

I came up with an idea that I think might manage some inrush current I'm trying to limit, but I'm concerned because I didn't come across this when I was searching for ways to manage inrush current on the internet. It consists of just an N-channel MOSFET, a resistor, and a capacitor. The principle is that the resistor and capacitor cause the gate voltage to rise slowly, and eventually the MOSFET opens and allows all of the current through.

As I said, I'm concerned because this seems so simple, but I didn't come across this in my searching. Is there some reason that this wouldn't work? I don't design a lot of MOSFET circuits, so maybe I'm overlooking something obvious here. Any input is appreciated. The schematic below is for illustrative purposes only, don't mind the component values.

SmartSelect_20230316-120247_EveryCircuit.jpg
 
I like it. I think there will be a voltage drop across the mosfet, dependent on how much gate drive voltage is needed to sustain the current in the load (all dependent on the mosfet specs, the supply volts and the load.. For this circuit, with no resistive load, the voltage on the output capacitor should eventually come up to the supply voltage as the gate voltage gradually decreases and the mosfet shuts off.
 
Thanks for the input! I built this circuit last week, and got some interesting results. It does indeed limit the inrush of current, but it also leads to weird behavior. I think that when Vgs gets high enough to allow current through the MOSFET, the capacitor discharges slightly, and then there's a little back and forth as Vgs tries to reach an equilibrium. That leads to the motor starting and stopping a few times before slowly building up speed like it's supposed to. This design works, but needs improvement.
 
With a 24Vdc motor across the output capacitor, I'm imagining at very low rpm during startup, the back emf from the motor would throw the output voltage up and down as current shifts back and forth between charging the output capacitor and raising the current in the motor. This would also affect the voltage across the gate as you describe, and the effects would diminish as the motor comes up to speed. Maybe a capacitor from source to gate would help smooth out the startup ?
 
But there could be another problem.
The MOSFET should have 600V rating, but also the Vgs should handle a big voltage, and this could be a problem.
Thanks for your input, AND your simulation. I did mention to ignore the component values in my schematic, so please disregard those values. I'm more curious about the general idea of slowly charging a capacitor to bring the gate of the transistor up to its threshold value and controlling the inrush current by this means. I should have also mentioned that the capacitor load should really be a 24 VDC fan for what I'm doing, or any capacitive load.
 
Maybe a capacitor from source to gate would help smooth out the startup ?
I tried this out, and it did seem to fix the issue of the motor teetering between on and off. I'd say there's probably a good solution buried somewhere in that circuit, but sorting out the proper R and C values is a bit tricky. A more experienced engineer could probably make this work well.
 
What your circuit shows is diode connected mosfet. The voltage drop across the FET will be equal to the Vgs threshold voltage. You also need to clamp the Gate to source voltage with a zener.

If you want a low voltage drop, say tens of milli-volts. You need to use one of these circuits. The N-channel FET will have a lower voltage drop than the P-channel FET for the same device size. The downside is that the load is not referenced to ground and may pose a safety hazard.

The P-channel has a ground referenced load and is probably better from a safety standpoint since the load is not floating at the input voltage when the FET is off.

I did not show this, but you will have to discharge the cap with a switch or bleed resistor to turn off the FET(s).

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An incandescent light bulb will give you the opposite of an inrush limiter because the resistance of the filament starts out low and increases as it heats up.
 
Your circuit depends on the FET operating in its linear zone--if the FET turns on hard there's no VGS. That's probably what you're seeing with the motor oscillating. You'll necessarily be wasting power in the FET, so size it properly and make sure it's got a good heat sink, otherwise the FET could exceed its max junction temperature. This topology will be fussy operating over a wide temperature range. You'll see variation in turn on times. You'll probably see unacceptable variation between the manufacturing lots of FETs, if not in-lot variation. FET turn-on is very non-linear, so a little delta V due to temperature variations etc. can make a big delta I. Not something I'd leave to chance.
gmorita's low side switch is better, but if you ask me, the schmancy hot-swap/motor controllers with the charge pumps and current feedback and OV/UV lockouts are short money for what they do, which is making your phone not ring. After all, do you want it to work or what? ;)
 
Your circuit depends on the FET operating in its linear zone--if the FET turns on hard there's no VGS. That's probably what you're seeing with the motor oscillating. You'll necessarily be wasting power in the FET, so size it properly and make sure it's got a good heat sink, otherwise the FET could exceed its max junction temperature. This topology will be fussy operating over a wide temperature range. You'll see variation in turn on times. You'll probably see unacceptable variation between the manufacturing lots of FETs, if not in-lot variation. FET turn-on is very non-linear, so a little delta V due to temperature variations etc. can make a big delta I. Not something I'd leave to chance.
gmorita's low side switch is better, but if you ask me, the schmancy hot-swap/motor controllers with the charge pumps and current feedback and OV/UV lockouts are short money for what they do, which is making your phone not ring. After all, do you want it to work or what? ;)
Definitely agree with Peter1.evans about the hot-swap controllers with all the protective circuitry. Is this for a one off project or production?
 
Definitely agree with Peter1.evans about the hot-swap controllers with all the protective circuitry. Is this for a one off project or production?
This was going to be for small production runs, about 1000 per year, but I'm no longer spending time investigating this. It was originally a small part of a larger circuit, but I've abandoned this and am using an ICL thermistor instead. I came up with this gate-delay MOSFET current limiter when I thought that our operational temperatures wouldn't allow for the thermistor, but it turns out I was wrong and we can use the thermistor after all. Thank you for your input, though!

One last note, my final circuit was a success, but it also wound up being unnecessary so it will not be used.
 
Yes, the inrush current limiting circuit you described would work. The circuit would use a thermistor to limit the inrush current to the DC fast charger. The thermistor would have a high resistance when it is cold, which would limit the current flow. As the thermistor warms up, its resistance would decrease, allowing more current to flow. This would prevent the DC fast charger from drawing too much current when it is first turned on.

The circuit would work as follows:

  1. When the DC fast charger is turned on, the thermistor would be cold and have a high resistance.
  2. This would limit the current flow to the DC fast charger.
  3. As the thermistor warms up, its resistance would decrease.
  4. This would allow more current to flow to the DC fast charger.
  5. Once the thermistor has reached its operating temperature, the current flow would be limited to the charger's maximum current rating.
The circuit would be effective in limiting the inrush current to the DC fast charger. However, it is important to select a thermistor with the correct characteristics for the application. The thermistor should have a high resistance at cold temperatures and a low resistance at operating temperatures. The thermistor should also have a fast response time to ensure that the inrush current is limited quickly.

Here are some additional considerations for designing an inrush current limiting circuit for a DC fast charger:

  • The thermistor should be placed as close to the DC fast charger as possible to ensure that it is accurately measuring the temperature of the charger.
  • The thermistor should be rated for the maximum current that the DC fast charger can draw.
  • The thermistor should be connected to the DC fast charger in a way that does not create a fire hazard.
By following these considerations, you can design an inrush current limiting circuit that is effective and safe.
 
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