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Re-visiting a prior question I asked a while back. I have a circuit that looks like the following:

schematic

simulate this circuit – Schematic created using CircuitLab

... and from what I surmised from my previous question, a possible way for me to reduce power-on surge current from my downstream sensor module would be to add an RC filter on the P-FET gate, and maybe a (e.g. 5.6V) zener diode in parallel with the load (though I doubt I need a Zener diode in this application).

schematic

simulate this circuit

  1. Have I correctly understood the guidance in the previously reference question?
  2. Is a 10k x 0.1uF = 1ms RC time constant a useful / good choice of values to limit in-rush current?
vicatcu
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  • Related : https://electronics.stackexchange.com/questions/294061/p-channel-mosfet-inrush-current-limiting – Long Pham Sep 09 '18 at 16:33
  • Q2: I think it depends on how much capacitance your load has. – Long Pham Sep 09 '18 at 16:38
  • All your problems are due to insufficient de-coupling of power rails. In all schematics the bulk bypass capacitor is missing, not shown, which means that you are not considering this critically important part of power rail network. Your power rail suffers current starvation when your sensors are turned on. The controller board power rail must be de-coupled from sensor rail. Put a 1000 uF capacitor for beginning ( better 5x220uF or something), and/or use Schottky diode on the MCU side to de-couple the MCU power from sensors. – Ale..chenski Sep 09 '18 at 17:59
  • I think the suggestion of adding 1000uF of capacitance on the 5V rail is not warranted in this case, but I will consider adding more capacitance to the 5V rail (from which the switching circuit draws its power). – vicatcu Sep 09 '18 at 18:59

1 Answers1

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It depends on what is the load.

My guess is that you have caps on your subcircuit and when you switch on the circuit, the caps are basically in short and it resets your MCU as the rail voltage is getting too low.

The capacitor on the mosfet gate might not be the best solution for two reason. 1. the mosfet will be in transiant state and you will have to oversize the mosfet and 2. it will be very temperature dependant, your circuit might stop working at different temperature as the gate threshold may vary a lot regarding temperature.

A common and simple way to reduce inrush current is to add an inductor in serie with the load. You may also want to add a flyback diode.

The inductor will have a high "resistance" to the current going through when you switch on your circuit, thus letting go very little current, but once in DC state it will have very small "resistance". Choose a inductor with high inductance and low resistance value (thick wire, high saturation current).

Calculate the value of the inductor accordingly.

schematic

simulate this circuit – Schematic created using CircuitLab

Damien
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