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All professional DC, BLDC or PMSM motor controllers that I have seen (Sevcon, etc.) have large numbers of DC bus capacitors connected in parallel. Their capacitances range around 100 µF - 220 µF. Wouldn't a single capacitor of a large value, like 4700 µF or 10000 µF, be more convenient?

Is it because of the large surge current whenever these controllers are connected to batteries or other high current power sources?

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Sure having enough capacitance is one parameter. But capacitors have series resistance which limits how much peak current can be drawn from a capacitor. Capacitors also have series inductance which limits how fast you can get the peak current out. Having multiple smaller capacitors in parallel reduces both series resistance and inductance.

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    \$\begingroup\$ Maybe you should add that big capacitors have more inductance than small ones, so a bunch of small caps whose total capacitance equals that of a single larger cap will have lower overall inductance than the single, larger cap. \$\endgroup\$ – DKNguyen Aug 29 at 23:35
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Higher ripple current capability, lower ESR and sometimes better form factor (eg. shorter) to fit in a convenient spot in the enclosure are likely reasons.

More surface area of the capacitor means more power dissipation capability, all other things being equal.

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Other answers have already mentioned the main factors which determine that choice: lower total ESR, lower total inductance, better heat handling capability, etc.

I'll add one more aspect that has been neglected: reliability.

If you have just one big capacitor, once it fails, you are left with a nonworking system. Moreover, a bigger cap can do more damage to nearby components if it fails spectacularly.

Having multiple caps in parallel helps mitigate the effects you have when a cap fails open, because the others will still be there. You could even design the system with redundancy in mind, i.e. adding more caps than the minimum you would need given the other constraints.

There are also issues with endurance against vibrations (this is particularly relevant when dealing with big motors). A single, big capacitor can be stressed mechanically more heavily when subjected to vibrations. The big mass of the cap can resonate mechanically and exert a bigger stress on its terminals or its mounting points, leading to mechanical failure of the cap itself or the PCB it is attached to.

Smaller capacitors, since they have less mass, have less inertia, so they experience and cause less mechanical stress due to vibrations or shocks. Therefore it's also easier (and cheaper) to design appropriate strain reliefs to avoid mechanical stress and shocks causing problems.

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  • \$\begingroup\$ I guess with enough capacitors in parallel, a short fail will self clear... in a way. \$\endgroup\$ – rackandboneman Aug 30 at 21:29
  • \$\begingroup\$ Vibration isn't the only source of physical stress. Capacitor plates are mechanically deformed by electrostatic force. \$\endgroup\$ – Peter Wone Sep 2 at 0:02
  • \$\begingroup\$ @PeterWone You are right, but those forces are accounted for in the design of the capacitor, since they depend on the voltage the capacitor is charged to. So if a capacitor is rated for 250Vac(rms) I assume the designers took those forces into account when rating the capacitor. BTW, I never heard capacitors could get damaged by those forces, but my knowledge may be biased because my experience is essentially with "lowish" voltage parts ("240Vac mains or low voltage DC). On the other hand, some datasheets will have data about vibration and mechanical shock withstanding capability of the part. \$\endgroup\$ – Lorenzo Donati supports Monica Sep 2 at 14:27
  • \$\begingroup\$ @LorenzoDonati things that affect the lifespan of a component are taken into account to the extent that they affect warranty obligations. There's a cost/benefit trade-off. You are right about it not mattering at low values. \$\endgroup\$ – Peter Wone Sep 3 at 23:58
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The capacitors help in filtering and decoupling noise. But each single value of capacitor is only good at one particular frequency. It has least ESR (higher ability to mitigate noise) Using a range of values provides that good filtering ability over wide frequency range.

Reduced Heating due to ESR. As the ripple currents flow through the capacitors to and fro, the ESR opposes the current flow (similar to resistor). The higher ESR means higher power dissipation (as heat). This effectively raises the temperature of the capacitors. Higher the temperature lower the capacitance they can provide. Hence, low ESR over multiple frequency band is one desired parameter which can be effectively received by combining multiple capacitors than one single bug capacitor.

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  • \$\begingroup\$ Where did you get that impedance graph? It doesn't actually work out that way where you always get the capacitor with the best performance being dominant. There are anti-resonance spikes formed by the interactions of the LC circuit introduced by each capacitor. \$\endgroup\$ – DKNguyen Aug 29 at 18:21
  • \$\begingroup\$ electronics.stackexchange.com/questions/320363/… \$\endgroup\$ – DKNguyen Aug 29 at 18:22
  • \$\begingroup\$ @DKNguyen I wanted to only present benefit of sharing multiple of capacitors so that the effective impedance is less. I agree it comes with side effect of antiresonance too.let me not confuse. Will remove it. \$\endgroup\$ – Umar Aug 29 at 18:32
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This could also be a production optimization thing. If a product already uses 220uF capacitors, using them instead of an additional 4700uF may make sense (though replacing one cap with 20 seems a bit extreme). A 4700uF cap is likely to be through-hole, and if it's the only through-hole component in a product, you save a whole manufacturing step if you can avoid it. Even if it's not, your stock becomes easier to manage because there are less part types to order, and you reduce the risk of having to redesign a product because that capacitor model goes out of production.

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A single, custom capacitor optimized for the needs of that drive would probably have some advantages, if that was the only product you were building. But if you build dozens of different drives, as all drive manufacturers do, you want to optimize the supply chain across the entire product line. That means standardizing on as few building blocks as possible, and using them in various combinations to get the voltage and capacitance ratings you need.

This model needs two caps in parallel, another needs two in series, another needs four, another needs twenty, but you still only have to stock one part. You get economies of scale in purchasing, lower likelihood of running out of a part you need, and lower stocking costs overall. Bonus points if it's the same part a dozen other drive manufacturers are using, since they're probably building exactly the same drive frame sizes you are.

Now, if we could just get the power magnetics industry to work this way...

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  • \$\begingroup\$ I hope you'll expand on that throwaway remark about power magnetics. Do you see an ignored opportunity for standardisation? \$\endgroup\$ – Peter Wone Sep 23 at 21:42
  • \$\begingroup\$ My experience with high frequency power magnetics is that they're all bespoke designs for specific applications. It would have saved me a lot of time in a previous life if anyone just had some set parts to choose from. \$\endgroup\$ – Stephen Collings Sep 23 at 22:32
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I think it is the best option for the manufacturer. After all, whatever cost less I think will be the preferred option for it.

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  • \$\begingroup\$ It is not always a case of what is cheapest, generally there is a number of trade-offs that have to be met, as that one special component that does everything you want is generally expensive, the size of the components effect the size and weight of the device which gets out of hand for big capacitors, equally throwing hundreds of the cheapest capacitors down makes manufacturing more difficult. and may come back to bite you with warranty. \$\endgroup\$ – Reroute Aug 31 at 7:40

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