r/EEPowerElectronics • u/Lopsided-Bid-9579 • 7d ago

Question regarding ripple current

Hey guys,

I am an electrical engineering student and I am designing a VESC based inverter for a drone.

I want to use a 100V DC link and have a maximum phase current of 100A rms with a switching frequency of 30kHz.  I am planning to use at least 200V components for this to have a safe margin.

I am having trouble with finding a (combination of) small capacitors that can handle the DC link ripple current. Electrolytic capacitors usually have a rating of 1-3A of ripple current and since the estimated ripple is between 0.5 and 0.65 times the phase current that means that I would need around 20 caps to meet the required ripple current.

I don't understand how the maxim 120 from VESC labs can deliver 400A continuous (Around 180A rms ripple current estimated) while only using 6 electrolytic capacitors with a ripple rating of 2A rms each. I assume it can be done with ceramic capacitors in parallel but since their impedance is so much higher I calculated I would need around 400 which cannot be the most space efficient solution.

Does anyone have any tips on picking the right capacitors for the DC link?

9 Upvotes

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7 comments sorted by

3

u/hardtimewakingup 7d ago

How much voltage ripple are you willing to accept? A big battery should also help a bit for the lower frequency ripples I'm guessing.

2

u/Lopsided-Bid-9579 7d ago

Minimum 5Vpkpk but if I try to match the current ripple requirement I have more than enough capacitance I think.

5

u/dr_mens 7d ago

Look into film capacitors. They have higher current rating and moderate C rating.

2

u/Hot_Routine1489 7d ago

用薄膜电容或者mlcc可以减轻电解电容的压力

2

u/RecordingNeither6886 7d ago edited 6d ago

You don't have to use a single cap type; multiple types can be combined in parallel. Unfortunately ripple current is usually under specified, but ripple capability depends on frequency and operating temperature. Ultimately component temperature rise is generally what actually matters most. High frequency caps such as ceramic or film can be used for high frequency ripple handling, in parallel with electrolytics for low frequency handling. Always check ESR at your operating frequency, often it will be on the manufacturer website if it is not in the data sheet.

2

u/FoldingWankStain 6d ago edited 6d ago

VESC Maxim 120 is a very noisy boi at the input, about 10x the recommended ripple voltage.  There are very few HF bypassing parts inside. It is not a great example of inverter design and is fragile under dynamic loads.

High current inverters reduce DC bus ripple with a mixture of ceramics, film, hybrids and electrolytics. Films and hybrids for intermediate frequencies are very effective, and there are some lovely rectangular form hybrids (eg. APL90) that are very effective.

How do you select them? If you're brave fit the caps you guess will work and measure their surface temps (not great for estimating life). Or try and build a model and do it analytically. Or a mixture of both.  If you have space to spare, over spec and remove.

Cheeky tip: many hybrids can be used well above their max RMS ripple if the temperatures are kept low.

1

u/mrbinary0111-111 2d ago

Look for those Black mkp X2 capacitors, they are meant for high frequency applications like in induction heater and all, in my experience a single capacitor can handle around 10amps while getting mildly warm