Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Electrolytic capacitors are unexpectedly rare at 100uF/450V spec?

Status
Not open for further replies.

Flyback

Well-Known Member
Hello,
We are designing a 60W flyback, (90-265VAC in; 24Vout; 100W peak power).
Unfortunately there is no ambient temperature spec yet, and the SMPS will be mounted in a totally sealed container.
The load is a diaphragm motor (whale gulper 220 pump). The current draw of this is 2.5A average but every 400ms, the current peaks up to 5A for a some milliseconds.
Anyway, the problem is in finding a post diode bridge high voltage smoothing capacitor with enough ripple current rating. The ripple current in this capacitor is 1.7A.
We have identified the Epcos B43505C5107M000 electrolytic capacitor by Epcos….
B43505C5107M000 (£3 UK for 1000 pces)
https://media.digikey.com/pdf/Data%20Sheets/Epcos%20PDFs/B41505__B43505.pdf
….however, page 15 of the datasheet does not give information on ESR for the low 2.5Hz frequency of our load. To make matters worse, the ESR graph on page 15 shows the ESR increasing exponentially for ripple currents below 100Hz. We can only predict that ESR will be very significantly high for ripple currents in the 2.5Hz range. –However, we don’t know how high as the datasheet doesn’t say, -As such , it is very difficult for us to give a lifetime expectancy for this capacitor.
It is possible that we may be able to operate the pump on a 5 minutes ON , 5 minutes OFF basis, (or such like basis) but would really like to know the lifetime of this electrolytic when ON all the time continuously.
Cost:
Since there are so many unknowns here (not in the datasheet) and there is the possibility of being able to turn the pump ON/OFF/ON…, we believe that we could just use a much cheaper capacitor here and just do the ON/OFF/ON thing.
We could for example use the EKMQ451VSN101MQ25S capacitor at £1.13 for 100 pces, though of course, we would have to turn the pump ON/OFF/ON….at a suitable duty in order to allow this capacitor to have a decent service life. There s no data in the datasheet which could help us to find the optimum ON/OFF/ON.. regimen.

EKMQ451VSN101MQ25S (100uF, 450V)
https://www.chemi-con.co.jp/e/catalog/pdf/al-e/al-sepa-e/005-snapin/al-kmqlug-e-140701.pdf
Mains harmonics:
We find that there are capacitors with much larger capacitance values obviously having higher ripple current ratings, however, as you know, one cannot use too large a primary DC bus capacitance due to the problem of overly large mains input current harmonics (300Hz, 900Hz…etc)
Why so few?
It is rather strange that there is such a shortage of 450V rated , 100uF electrolytic capacitors with high ripple ratings, because you would have thought that it was common for people to get SMPS’s designed at power levels of 60-70W …because it is just underneath the 75W level above which a PFC stage is needed. Therefore, why are there so few high ripple current rated electrolytic capacitors available in the 100uF, 450V rating?
 
I expect that most commercial supplies in your power range are designed with 400Volt caps. 450Volt caps are typically only used when you expect a significantly higher than normal line voltage or transient voltages. That may be part of the design spec for your supply, but it's the exception, not the norm.

As for the availability of multiple sources of caps, manufacturers make what their customers ask for. If there are enough customers looking for caps of a combination of factors, then they will make them. But generally the demand, or expectation of demand, drives the offering.


But, I think you are over-analyzing your needs. Your cap really won't see any any ripple current below 100 hz.

The discharge current pulses of the capacitor will be at your PWM switching frequency. The charge current the cap will be at twice your line frequency. The variation of your load current will modulate the magnitude and duration of discharge current pulses, but will still be at the PWM frequency.
 
I see a lot of 450 volt caps here:

http://www.nichicon-us.com/english/products/alm_mini/index.html

and here I see more, including some "high ripple" types:

http://www.nichicon-us.com/english/products/alm_larg/index.html

You asked in another forum about the problem of large rise in ESR at very low frequency, and I suggested you do a calorimetric experiment.

Get yourself two of the candidate capacitors; connect them in series opposing, like you were making a non-polarized capacitor; you could connect a diode (1N4007) in parallel with each cap to guarantee the cap won't experience any reverse voltage. Tape a thermocouple to the outside of one (or both) of them.

With an ammeter in series with the two caps, using a variac apply enough line frequency AC voltage to the two caps to cause a current through them equal to the rated ripple current. You might wrap some thermal insulation around the cap with the thermocouple to get a higher temperature rise above ambient. Shield the setup from strong air currents. Wait several hours and see how hot (temperature rise above ambient) the cap gets.

Now connect the cap to your power supply (with the same thermal conditions experienced by the cap, insulation, lack of air currents, etc.) and operate the pump for several hours as it will be operated by the customer. If the cap temperature rise above ambient isn't higher than in the first test with rated ripple, you're good to go.

Doing this test will take into account any rise in ESR at your lower frequency repetition rate.
 
Simulation shows the low frequency component of the cap ripple current is negligible. Ripple is dominated by components at mains frequency and harmonics thereof.
The sim below uses an assumed pump current profile (I1), a worst-case scenario of 90V AC supply, and an assumed ideal inverter efficiency of 100%. Thus a 2.5A load current at 24V DC equates to a 0.666A RMS current draw at 90V RMS AC.
Ripple.gif
 
The SIM pump model is flawed as it is a planetary gear with cyclic torque and current load at 2x the rotation speed.

Peak load is at least 50% more than average assuming excitation losses in the motor and gear are high (20% @no load)

Therefore this 60W motor will not be capable of driving at constant RPM with a half cycle of 400ms or a full cycle of 800ms or 1.25 Hz thus the rotational speed will cycle up and down and the average speed of sludge will be significantly reduced even if it doesn't stall and the motor running longer will cause more thermal rise and fatigue, with longer interruptions of duty cycle usage. It will be under strain going chug chug at a slower speed like 0.6Hz at full load and take 2~4x longer to complete the job at dips to 6V.

The sealed plan of enclosure is also flawed with lack of thermal design.

Peak Power in any Cap must be supplied beyond what the SMPS cant provide before OCP triggers and drops voltage then the motor continually changes speed on each half cycle. with less than full torque and thus much slower than expected. Which mind you on a marine system with lead acid batteries will be 14.2 x2 or 28.4V at source.

Thus this SMPS concept will be flawed in comparison to battery power. It will behave like a weak battery in -40'C weather turning over a car motor.. arrr.. arr..arrhgh

Do I need to prove this with an accurate simulation or can you manage this yourself?
 
Last edited:
The SIM pump model is flawed
No doubt, but even major changes to the model make little difference to the low frequency component of the cap ripple.
 
The model of the power source is also innacurate as there is zero resistance and no current limiting including the ESR of the Motor, which is not infinite current sink averaging 0.6A but a switched ESR of probably 2 Ohm that averages to 12 Ohm average impedance at 24V/2A if that is accurate. giving start currents 6x average and running currents smoothed to 2Hz bandwidth going from 1 to 3A which would required a Cap of 100k uF.

And this is being conservative with a 48W average load.

Furthermore the Big Cap will have an ESR more than the cheap small cap suggested of 1 Ohm resulting in huge voltage swings.

In fact if the SMPS current limit is 3A @24V or equivalent to an 8 ohm load and the motor ESR is 2 Ohms or less and the Cap ESR is 1 ohm or less , the voltage source and current available for torque and power will drop more than 50% ~70%below spec. based on quick calculations, which is unacceptable in my books.

If you cant understand no quick explanation possible, if you do, no further explanation needed.
 
Last edited:
The model of the power source is also innacurate as there is zero resistance and no current limiting including the ESR of the Motor, which is not infinite current sink averaging 0.6A but a switched ESR of probably 2 Ohm that averages to 12 Ohm average impedance at 24V/2A if that is accurate. giving start currents 6x average and running currents smoothed to 2Hz bandwidth going from 1 to 3A which would required a Cap of 100k uF.

And this is being conservative with a 48W average load.

Furthermore the Big Cap will have an ESR more than the cheap small cap suggested of 1 Ohm resulting in huge voltage swings.

By "Big Cap", do you mean the "Cap of 100k uF" you referred to at the end of the first paragraph? Why would a Cap of 100k uF have an ESR more than 1 Ohm?
 
Because the CV product = 0.045 FV in an electrolytic
100uFx450V
 
That is not the same body. I used original your datasheet which supports 0.66A @105'C of ripple and cannot dissipate much more than 1W @ 90'C hence >=1 Ohm ESR. probably 4Ohms

This new part is 20x the size and not the same CV product /volume is proprtional to ESR. and varies with dielectric.


35V vs 450V

**broken link removed**

I can back up everything I say., Get the film cap and run at 30~50kHz or get a 100F battery and trickle charger. or buy a 5A unit off the shelf for $15. period.
 
Last edited:
That is not the same body. I used original your datasheet which supports 0.66A @105'C of ripple and cannot dissipate much more than 1W @ 90'C hence >=1 Ohm ESR. probably 4Ohms

This new part is 20x the size and not the same CV product /volume is proprtional to ESR. and varies with dielectric.


35V vs 450V

**broken link removed**

I can back up everything I say., Get the film cap and run at 30~50kHz or get a 100F battery and trickle charger. or buy a 5A unit off the shelf for $15. period.

The datasheet you refer to wasn't my datasheet; it was Flyback's.

The source of confusion here is that in post #7 you never said that you were assuming that the 100k uF capacitor you spoke of was to be in the same size can as the one from Flyback's datasheet.
 
Sorry I mixed you with flyback who is actually treez on EDA. Apology.

Just remember CV/vol is proportional to ESR. (but not precisely)


I guess if he needed 450V he would 13 and they would not fit in his little box.


We used to buy 100k uF "Computer grade Caps" for mainframes in surplus stores in late 60's that were half the size of a paper roll. for $10 Nice low ESR arcs. but at least 20% memory so after shorting it would charge up again to 20%V partly due to the double-layer effect. which in most cases is lossy parallel capacitance. but in modern caps with smaller gaps is a much higher k but low V.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top