Re: Chosing a High Ripple Current Capacitor

On 22 abr, 20:29, D from BC <myrealaddr...@xxxxxxxxx> wrote:
In spice I have these waveforms across the capacitor:

Voltage: 5Vpp 100 Khz triangle wave on 270VDC
Current: 2App (swings -1A to +1A.) square wave at 100Khz.
Capacitance: 1uF
Max. Ambient temp: 30C

Would a polypropylene film or electrolytic be better? There's lots of
spare PCB space..I prefer picking the longer life component..

I've read that heat ages electrolytics and one way to heat up an
electrolytic is with lots of ripple current to make I^2R heat.

When I looked at the polypropylene data sheetshttp://www.vishay.com/docs/28124/mmkp383.pdf
I found AC vs frequency derating graphs.
What is this derating based on?? Heat? Material breakdown?
My app only has 5Vpp of AC and so a 400VDC pp film cap falls into the
safety zone at 100Khz..
But will it outlast an electrolytic?
How do I figure out if this cap gets toasty from the pulse current?
Do I look at the tangent of loss angle spec?
D from BC
Hello D from BC

About derating of foil capacitors,

For relative low frequencies, the maximum AC voltage is limited
because of internal corona discharge in air pockets, that may erode
the dielectric. At high frequencies, heat generation inside the
dielectric is the limiting factor. At even higher frequencies, the
limiting factor is current carrying capability of the contacts.
Mostly the graphs are based on 10 degrees internal temperature rise.

You also have to derate when the temperature will be higher (for
example because of heat generated by nearby components.

Electrolytic capacitors.
You need a big cap (several 100 uF to mF) to handle a ripple current
of about 600mA rms. The larger the size, the higher the ripple current
capability. Ripple current is mostly based on 5..10 degrees
temperature rise with respect to ambient. Same current at lower
temperature gives somewhat higher temperature rise because of higher
ESR at low temperature. The life time is specified at given
temperature, rated voltage and rated ripple current (I thought).

When you stay below the maximum ripple current, you gain a factor 2
for every 10 degrease lower temperature then specified in the data
***. Like with the foil capacitor, check the actual temperature of
the electrolytic capacitor. When you use them according to the
specification, they have a predictable lifetime. Life time can be
shortened significantly by wrong treatment (for example certain
chemicals for cleaning). Most manufacturers have good information on
how to calculate the life time based on ripple current, frequency,
ambient temperature and voltage and how to treat their capacitors (for
example ELNA, Nichicon, Panasonic).

Do you also check your design for inrush current and overload
situations? Standard or SMPS capacitors are not made for fast
discharging.

When the value of the capacitor is of importance, a foil capacitor is
a good option.

Best Regards,

Wim
PA3DJS



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