Inrush current: Additional capacitor or NTC to handle voltage spikes?

Question

I will power an ESP32-S3 using a LDO (MCP1825S) which will require a 1 µF capacitor on the input and a 10 µF capacitor on the output. Both of them are ceramic ones, so typically have a low ESR:

In a related question soon a user (thanks to Justme) mentioned in a comment that I need to protect against voltage spikes if I hotplug the power connector, since my power/USB cable might be about 2 meters long. This was a very important hint for me, since I thought 1–10 µF from the so small ceramic capacitors would not be a problem, but it seems to turn out that this — with the long cables — can be already a critical value.

I hope it is okay if I quote the comment in the answer to the related question from bobflux here:

bobflux: It's the LC resonant circuit formed by wire inductance and low ESR capacitor like MLCC at the input of the LDO (not output). To avoid a spike it needs to be damped by resistance, a good method is to use a capacitor with high-ish ESR like "general purpose aluminium" at the input. For example you can use 100µV 16V and 1µF or 100nF MLCC in parallel.

I have a few questions to that, so it makes more sense to open a dedicated question:

• Is what bobflux describes nothing more than the inrush current? Is inrush current always caused by these LC resonant circuits (L = inductor, C = capacitor)? I assume that the long wires are the inductors here. But what if the wire is e.g. 1 mm long and the capacitor is large, is the result the same, a high inrush current? Can both of these elements alone (long cable and small capacitance vs. short cable with high capacitance) cause a high inrush current?

• In order to prevent spikes bobflux said, a general purpose aluminium electrolytic capacitor with a high ESR could be used, e.g. 100 µF (16 V) in my case. Why is that? Is it that this higher ESR capacitor is to charge the smaller capacitor with lower ESR? Also, isn't that really huge (about 100 times of the smaller capacitor)? Also, how would the schematics look like for such an additional 100 µF capacitor? Is it just a second capacitor before the 1 µF input capacitor (C4) such as below?

• Can I also use a NTC resistor? I found out that such resistors exist as SMD 0805 versions, the size I usually use. However, only a EPC/TDK B57421V2103 with 210 mW at 10 kΩ which does not sound suitable. A PTC instead can't be used I think (?).

Answer 1

Inrush current is caused by other things as well (consider the startup current of a motor, for instance), but capacitive inrush is probably the most common. This is due not to the LC circuits you reference (which cause problems in another way, by causing the input voltage to briefly spike to as much as double the supply value), but simply to charging a capacitor from zero volts to the supply voltage. Adding more capacitance alone will simply worsen this, because it's more capacitance to charge up.

Note that adding a capacitor with high ESR can actually mitigate the LC overshoot. It won't do much for inrush current itself, though.

NTCs actually are a common way to limit inrush current. Ones sold for this purpose are called inrush current limiters or ICLs. They're not too expensive, but they can be bulky and they do constantly waste power.

Another common way is to have a resistor in series with the supply, then short that resistor out with a relay or MOSFET after enough of a delay to allow the capacitors to partially or fully charge. This is common in high power applications where currents too high to easily limit by other means are involved, such as motor drivers (which may even use a centrifugal switch to determine when to switch out the resistor).

You can also use a semiconductor device in its linear mode to limit current to a fixed maximum value. This method is commonly included in smart power switch ICs, like the AP22653 or the MIC20xx.

Answer 2

So a few things - plugging in a capacitor to a supply causes an inrush current. The series resistance defines how large the current can possibly be, and the series inductance defines how quickly the current is available.

Inrush current is thus limited both by the ESR and ESL, but as well as the ESL limits the rise of the current, it limits the fall of the current. So if you plug a scratcy/flaky/bouncy connector (or push a push button), you get large current transients as the caps charge, but also the voltage peaks are huge when current jumps between 0 and the nominal value, the di/dt of the current is large and thus the induced voltage is large.

Why a large capacitor with high ESR works is that it damps the LC oscillations by dissipating the energy as heat.

And no you cannot put a 100uF cap on USB VBUS input, that violates the USB specs as the maximum is defined as 10uF capacitive load (along with some resistive load) so if you have 10uF on input and some other capacitances on regulator output, the total energy of a hotplug will be violated.