In quadcopter/drone ESCs post-BLHeli, it has bcome common to use arrays of multilayer ceramic capacitors (MLCC) as the DC link capacitance. A MLCC farm like this has become a familiar sight.
There is some considerable merit behind this. MLCCs have extremely low ESR and ESL, comparable to plastic film caps and any other type of non-electrolytic capacitor, and could do the job with much less capacitance than an electrolytic, which actually really suck at being DC link caps, despite always being used historically for DC link caps. More reading on the subject. Beyond that, an array of MLCCs on a board packages flat and thin and avoids dangly bits, and in terms of overall package length, the length of an existing lytic can added to the PCB is enough area to fit plenty of MLCCs.
On the cost front, it isn't too bad. A 10uF 50V X7R 1206 part, likely similar to the 18 parts on the above nerfish-scale board, from the vendor I usually use for MLCCs (Samsung Electro-Mechanics) can be had for 10 to 15 cents at this point in moderate quantity. If the Salcone-Bond whitepaper example cases are taken to frame a rule of thumb, certainly 18 to 20 such parts totalling 180 to 200 uF should suffice in most cases to replace the lytic(s) outright, at least in the current handling and bus ripple sense.
There are a couple of reasons I have been very reluctant about adopting either this strategy or a hybrid one with a smaller lytic and a handful of MLCC though, instead favoring the tried and true, dirty great big electrolytic, solution.
One is that MLCCs are brittle and sensitive to stress, whether mechanically induced (external forces or flexing applied to boards) or due to thermal expansion. And when these things crack, they often become shorted. High-current, deeply parallel MLCC farms directly across a lipo and immediately next to a heat source (FETs) give me pause for this reason - there are a lot of parts there to multiply the failure probability, they are attached to beefy, heavy (inflexible) copper planes, they get heated a whole lot at installation to solder them down to said planes, and they and the differentially expanding PCB are being constantly thermal cycled by FET heat in operation. If one dies, it likely causes a rather high-energy smokey unconfined failure, destroying the PCB and possibly resulting in combustion. I worry enough about the single big MLCCs that are across the battery on my ESCs and S-Core boards already. Is it really worth such fear? It is paranoia? I don't know, drones don't seem to be nuking ESCs all that frequently, I guess. Meh.
Another is that a pure MLCC solution has no "damping" the way lytics do. Small motor controllers normally don't have precharge setups for the caps at startup, and just get switched up with a huge inrush into the caps (and the familiar spark). Between the source inductance and the several hundred uF of basically-zero ESR caps, there could be spiking when powering up with just ceramic caps. It's like setting a car suspension spring vertically on the ground and hitting it with a sledgehammer. Pololu warns about this with their DRV8825 carrier boards, and their answer is to always have a lytic nearby on the same bus (as the 4.7uF 1206 input cap on the board). Advice to do similar things is common. So perhaps it is apt to use a hybrid solution at most, not purely MLCCs. Or perhaps a TVS on the bus could guard the FETs and logic power supply against overvoltage spikes sufficiently.
The third is that a MLCC farm to replace lytics turns 1 or 2 parts into 15-20 or more for a larger board, and a hybrid solution adds 4-6 parts and still doesn't completely remove the lytic cans that have to stick out somewhere, only make them smaller.
The fourth is that often, a huge surplus of capacitance is beneficial. The Salcone-Bond view of DC link caps assumes that they are solely there to source ripple current and stabilize the bus voltage at the input. In the real world, big lytics are often doing something else. In VFDs and appliance motor drives, they are much more a bulk energy store, since the input is rectified 50/60Hz mains and there are big valleys in the voltage that need filling in. In blasters, that's irrelevant, but they sometimes are doing the same thing to cache some local charge and prevent marginal battery and wiring conditions from causing the floor to fall out of the bus voltage as easily during a motor current transient (possibly to the extent of causing MCU resets and other woes). It's all the more important when batteries are NOT lipos (lipos are much more non-inductive than usual due to the stacked construction) and/or there is a long cable between the battery and the inverter, which appears multiple places in blaster design. Blasters can have much less optimal battery situations than RC stuff - just the matter that people play HvZ where/when there is snow on the ground and they are NOT going to be keeping a pack warm, for instance.
On the flip side, I do suspect my layouts with just the lytic at the end of the board are suboptimally decoupled and could benefit from a few MLCCs very, very near the actual devices in addition. And I kinda want to do a lytic-less board sometimes just for variety and to have an option that can be as thin as possible.
Edit/footnotes: https://www.rcgroups.com/forums/showpost.php?p=38006136&postcount=1220 https://www.reddit.com/r/Multicopter/comments/7nb2zs https://www.rcgroups.com/forums/showpost.php?p=41652761&postcount=16 https://www.rcgroups.com/forums/showpost.php?p=41660297&postcount=17 More interesting accounts of electrolytics and MLCCs and I wasn't surprised to see the exploded rail of MLCCs with a 1000uF lytic bodged in its place!
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