Ripple current rating of Film Capacitor?

[cupoftea]

Hi

The attached (LTspice sim and PDF schem) is a 3.6kW Half Bridge SMPS. 3 Phase 240VAC input. 3 phase , 6 pulse rectifier, 12V, 300A output.
The Half Bridge primary side rail splitter caps each have 26Arms of ripple current in them.

They are as follows…
Film capacitor: 12uF MAB MKP 12/400 Siemens
https://www.rgbautomatyka.pl/media/docs/en/96/12UF MAB MKP 12-400 SIEMENS - product information.pdf

Here is a picture of it on ebay...

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A proper datasheet cannot be found. (also they dont weigh anything like 1Kg) Do you know how much ripple current they are rated for?

 

[Warpspeed]

These types of capacitors are usually used for (small) motor start/run capacitors and power factor correction at power line frequencies. The reactance at 60Hz is 221 ohms, so even if connected directly across 220v, the current can never be much more than one amp. They would consist of a simple wound low cost light foil construction and are simply unsuitable for very high ripple current, or high frequency operation, which is why current is not specified.

What you really need are heavy duty premium quality low ESR electrolytics or some of the newer ceramic capacitors. These come fully specified for ripple current at both power line and inverter switching frequencies.
Series inductance, and ESR will also be fully specified.
They are specifically designed for your type of application.

 

[FvM]

I agree with Warpspeed. These are 50/60 Hz motor capacitors. There's no technical reason why they should be designed for high current rating. www.capacitor.de/app/download/6739008/MKP_400_0612_final.pdf

 

[Easy peasy]

I find your assertion of 26A rms ripple in each cap a little surprising

ignoring the slight ripple current from the mains for the moment, the converter is 3.6kW ( you say ) allowing for 88% efficiency we have 4.1kW

divided by the HVDC bus, 565VDC gives 7.5A, for half bridge we get double this in the mosfets and caps, i.e. 15A as there are 2 caps operating half of the time each ( like the fets ), both supplying current at switching freq - say 20kHz, then we can say 7.5A in each due to switching.

Now adding in the ripple from the mains, 15% ripple on a 3 ph rectified line, and a PF of 0.95 say, ripple is 300Hz, ( 360 in the USA )

The caps have to supply 4100/565 = 7.2 amps ave ( from both ) over half of 300Hz, 167uS, from 607V down to 522V , this is 7.2 amps from the series combo of the two caps. Thus the rms current due to mains will be about 7.2A in each cap.

Adding vectorially, for a single cap we get SQRT ( 7.2^2 + 7.5^2 ) = 10.4 amps rms each ( approx ) at full power.

As they are rated for 1.5A ( 50Hz ) on a 400V ac line .... they are probably a bit under-rated for this app

N.B. at 26A rms each ( 6x heating effect as for 10.4A ) - they would get very noticeably warm/hot at full power ....

 

[cupoftea]

Adding vectorially, for a single cap we get SQRT ( 7.2^2 + 7.5^2 ) = 10.4 amps rms each ( approx ) at full power.

Thankyou...Easy Peasy is of course, correct about the ripple current in each Half Bridge rail splitter capacitor....ie approx 10-11 Amps. However, that is what it would be for a "standard" Half Bridge converter.

What we have here is not "standard". We have a Half Bridge SMPS which is like in the attached LTspice simulation....it has no output inductor (just seven torroids round the busbar on the way to the output caps). Since it has such ridiculously low output inductance, it relies heavily on use of the leakage inductance in the transformer. -And as you can see from the attached simulation, the ripple current in the rail splitting capacitors is approx 30A.
How long do you think these capacitors would last with 30Arms of ripple current in them?
The thing i dont yet know of course, is how many hours or minutes per day this converter runs for.

 

[cupoftea]

So the ripple current rating is given by VAC(rated)/Xc?

Where Xc = Xc at 50Hz and with 12uF

Presumably the ripple current ratign is some three times the 50Hz level at 20Khz.

I've heard some people in other places saying that Film caps dont have a ripple current rating , but just a dv/dt rating?

 

[Easy peasy]

I / C = dv /dt hence max rms current rating

if you are correct about design then equip is not designed for continuous running - this may have been taken into account for cap temp rise ...

 

[Warpspeed]

So the ripple current rating is given by VAC(rated)/Xc?

Where Xc = Xc at 50Hz and with 12uF

The intended application of these capacitors is to create some phase lead for particular motor windings, and for power factor correction to the 50/60 Hz mains supply voltage. The ripple current will always be within some safe assumed range limited by Xc. They are quite adequate for that.

They have a wound foil type of construction, with a single connection to one end of the foil. As a result there will be significant series inductance and series resistance. The performance at higher frequencies will not be good.
If you start running very high pulsed current, it will very likely fail open circuit due to the very high current density right at the start of the foil winding.

 

[Easy peasy]

often these types of caps are end sprayed with a hot Zinc sputtering to make contact to the Al on the foil - if not done well it is a failure point for higher rms & peak currents ...

 

[cupoftea]

Thanks, it really does sound to me from what you say that the converter of the LTspice sim of post #5 shouldnt be anywhere near these caps

 

[Warpspeed]

I doubt if you can make simple assumptions about rms ripple current. There can be very high crest factors, and perhaps even resonant circulating current which can significantly change things.

Anyhow, I would steer clear of any component that does not have a critical operating parameter specified.
It usually means that the component is basically unsuitable for the application.

 

[cupoftea]

Thanks, our case is a pair of these caps being used as rail splitting caps in a 3.6kw half bridge smps.....with vin = 565v and vout = 12v....fsw=20khz.......ripple current is 30Arms as its a half bridge that uses leakage inductance as opposed to big output inductor.

 

[Warpspeed]

After a rather quick internet search, it seems dc energy storage capacitors of only 12uF in that voltage range have ripple current ratings way lower than you require. Hardly surprising really.

There is only one application for this type of thing that I am aware of.
Induction heating tank resonating capacitors.

These have to tolerate continuous massive resonant circulating current, and are rated appropriately.
Only one company I know of that will supply these where you only want one or two, ex stock, at short notice.
They are massively overrated for your application, and they are very large and expensive.
But if you become truly desperate...

Celem are an Israeli company, and their smallest capacitors are the C500T range.
https://www.celem.com/Uploads/Conduction cooled capacitors/2_spec.pdf
The closest available value is 10uF, voltage rating is 700v rms (which is usual in ac resonant circuits).
And it will cruise along quite happily at 1,000 amps of ripple at 20Khz.
I have used these in an induction heating project, and they work very well.

Its laughably larger than you require.
Sorry, but its the best I can do.

 

[FvM]

Dedicated DC link capacitors 10 uF/400V have typically around 10 Arms rating, e.g. Kemet C4AQ series. Capacitors with higher current capability like Kemet R76 achieve the 30 A range with two in parallel.

If you need even larger currents, you can go for GTO capacitors, e.g. WIMA GTO MKP series.

Generally, it becomes obvious that designing a converter with excessive ripple current involves a trade-off. The seeming advantage can easily turn into an issue.

 

[cupoftea]

They have a wound foil type of construction, with a single connection to one end of the foil. As a result there will be significant series inductance and series resistance. The performance at higher frequencies will not be good.
If you start running very high pulsed current, it will very likely fail open circuit due to the very high current density right at the start of the foil winding.

Thanks, this is the crucial bit, that blasts these motor capacitors out of the water as rail splitter caps for a 565vin 3.6kw half bridge smps....but then there's that other side of me that wonders.....maybe motor capacitors are just tough capacitors, that can survive ....and the 28Arms of ripple current will end up "water off a ducks back"?.....and the transformer has loads of leakage inductance anyway, so maybe a bit extra ESL in the cap isnt gonna hurt too much? Just wondering how much of a biggy it really is about these motor caps being "not right for the job"?

I mean, by rights , they have 1.5A of current rating at 50Hz, and yet, they dont blow up straight away on max load...with 28A+ of rms ripple current in them...maybe motor capacitors are just tough old birds?