DC/DC buck regulator regularly emits an audible clicking noise

Question

Relevant TI forum post with no resolution: here.

I am using an AOZ2264NQI-11 chip to convert 24 V down to 12 V. I originally designed this circuit for use with an AOZ2261NQI-12 chip, which has an identical footprint/pinout and a mostly identical application circuit (for more info read my previous question). I switched to the new chip to get the higher current output capability, 15 A vs 8 A.

The problem I am seeing - or rather, hearing - is that once the load reaches ~2.5-3 A, the circuit begins to make an audible clicking noise. The noise is relatively loud and occurs roughly every 1.2 seconds, although it becomes more frequent as a higher load is applied. By the time the load hits ~8 A, the noise is frequent enough to be more of a hum. I haven't captured that behavior yet so I couldn't give a frequency but I am working on it.

When I first tested these PCBs with the lower-current chip, I also experienced the clicking but much less often. It would sometimes occur at a load of between 5.5-6 A, with roughly the same frequency.

I am including the schematic, layout, and captures of relevant voltages when clicking: V_IN, V_LX, V_OUT, and V_PROJ, which is just the load voltage and differs from V_OUT because there is a 0.03 Ω current sensing resistor in line with the load. Note: I am using a 100 kΩ resistor for R_OCS, not the 20 kΩ value in the EDA snips.

My next step is to examine the EN, PGOOD, and BST lines, and try to capture the clicking frequency at higher loads. I suppose I have two main questions:

1. What buck regulator shutdown behavior causes an audible clicking? I don't believe it is thermal shutdown, using an IR gun the chip package sits at around 33°C even at higher loads.
2. Why would the higher-current chip have this behavior at a lower load than the other chip? Apart from the R_OCS value, the passive calculations are identical.

Answer 1

Ah, ceramic input capacitor strikes again.

...Most likely. I'll explain that a bit more, later. First, the capacitor itself:

The AVX KGM15CR51C226MT does not have a datasheet (AVX provide sadly little data on their ceramic capacitors), but we can look up a comparable 22uF 16V 0603 X5R part, such as the Samsung CL10A226MO7JZN, which does:
https://weblib.samsungsem.com/mlcc/mlcc-ec.do?partNumber=CL10A226MO7JZN

You appear to be applying 24V which is unsafe for the 16V part anyway, but the dielectric and construction will not differ significantly from this part and we can assume there's a flea-fart's capacitance left at this voltage.

For a 10A class converter, I would expect more like 4-8 x 1206 or even 1210 chips, or a couple 0805s plus a polymer, and in either case an electrolytic would be welcome to absorb further ripple current plus dampen any input-circuit resonances.

Because type 2 ceramic capacitance depends on voltage, it's neither economical nor practical to select the largest possible value in a given voltage rating and chip size. Indeed, Digikey presently shows only three items of comparable rating, out of nearly a million items in the category; you selected quite the rare part.

More typical would be 4.7 to 22uF, 25 to 50V, 1206 or 1210, and such values will be available in X7R, which tends to have a flatter curve, also in terms of temperature (105°C rated).

As for function, SMPS controls typically assume a low and stable input impedance, or a stable input voltage. Clearly, neither of those are true here..!

There are two significant consequences for an under-damped input filter:

1. Hot-plugging; inrush surge. This is a very common cause of failure among SMPS questions here (hence my comment).

   Even if the capacitor were linear, hot-plugging a voltage source to a lossless series LC circuit generates a peak voltage twice the input. Since C depends on V, as V rises, C decreases, dV/dt accelerates, V rises even further, C decreases even further... The resulting peak voltage can be five times the input, easily.

2. Oscillation due to the converter's negative input impedance.

   That is, current draw decreases as voltage rises -- it conserves power more or less, so this is simply a natural consequence: negative (incremental) resistance. Depending on control type, this can manifest at frequencies as high as Fsw, or disappears above some fraction thereof (at frequencies where the control loop can't act fast enough to maintain constant input power), but in any case applies from DC up to some at least modest AC frequency which can include the input filter resonant frequency. Those frequencies overlap, the negative resistance exceeds loss resistance at that frequency, and off she sings.

So, simply dampening that with a bulk capacitor of nominal ESR, such as an electrolytic capacitor, keeps the regulator input happy. 100uF in polymer or electrolytic would be welcome, with more like 1000uF electrolytic being typical.

While it's true this may not be the explanation for the totality of observed behavior, including the clicking or bouncing or restarting, solving this is a prerequisite for further analysis.

By the way, did you catch this juicy little bit? A closer look...

That peak looks suspiciously flat-topped, and if I'm not mistaken, that's a peak around 32V. On a device rated merely 28V... methinks you've observed avalanche breakdown, and are damn lucky the thing didn't fail shorted already!

24V is very tight to be operating a 28V converter at, anyway; I would advise keeping it under 20V, and using a TVS and electrolytic at the input, to dampen overshoot and ringing, and clamp overshoot to within the regulator's ratings. Else, get a higher-voltage rated regulator. Headroom around 40% (rated vs. nominal-max operating voltage) is recommended, give or take application.

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Further reading: recently I simulated the transient, by itself as a startup (inrush) surge (not including the converter here), and collected a few related questions on the phenomena, in another answer:
https://electronics.stackexchange.com/a/713473/311631

Answer 2

Just to illustrate the Tim Williams comment. Below is an example from TI part with embedded inductor. Please note the number and spectrum of capacitors on input and output: You should practice something similar.