[SOLVED] Sinsuoidal Current: Difference Between terms 'ripple' and 'Irms'

[chiques]

I see two terms often used for capacitors: 'ripple current' and 'Irms'. What is the difference between these two?

 

[KlausST]

Hi,

please give a link to a document, so we can discuss about the same thing.

Technically "I-RMS" just says the it is "current" and it is calculated using "RMS" method.
Theroetically you can calculate any current with RMS method. But it sometimes simply makes no sense.

"ripple" just means that the current is not constant. It is the AC part of a current.
usually this ripple refers to a frequency. Sometimes mains frequency, or the switching frequency of an SMPS.

Klaus

Besides "RMS" current there is "average" current (and maybe others).
RMS mainly is used needed in AC systems whereas average current is used in DC systems.
If you are interested to go deeper in this topic I recommend to read an applicatio not about the calculation of power dissipation of an SCR.
There you need both: RMS and average value to get the almost realistic result.
Neither "RMS" alone, nor "average" alone leads to a precise result.

 

[chiques]

Hello

KlausST​

Per your request, attached are two MLCC capacitors. One rates the current referring to Irms and the other refers to the current as ripple current.

 

[KlausST]

Hi,

the first document is a snippet of something. Not really useful at all.
The second defines I_rimple as current in RMS.
The one does not exclude the other.

I try to find an example where we use similar terminology. like RMS:
You may define distance in kilometers or miles. Or the usual mile and a nautic mile.
or dB, dBm. dbA... dBm is decibel referred to 1 mW, dBA is A weighted (filtered) decibel calculation.

both are distances.
But there are differnt distances. (similar to different currents)
you can measure the distance from Rome to Paris differntly. The distance via roads, or the air distance.
Both in km or miles.

Klaus

 

[FvM]

Per your request, attached are two MLCC capacitors. One rates the current referring to Irms and the other refers to the current as ripple current.

There's no difference and the documents make it pretty clear. Both specify the capacitor AC current to be measured in Arms.

 

[chiques]

There's no difference and the documents make it pretty clear. Both specify the capacitor AC current to be measured in Arms.

That's what I take away from all of this. Two different terms for the same parameter.

 

[dick_freebird]

In a purely AC system the two are the same thing.

In applications like DC-DC converters, there is an AC
component (the ripple) riding on top of the DC current
of interest. The "DC" in the inductor in fact toggles
between sources so outside the inductor, is not DC.

Anyhow, a filter capacitor on the output of a converter
would see the ripple current only. The input filter
capacitors would see RMS current of roughly IOUT
divided by the "on" duty cycle.

In either case Irms is the "thermal" value of the current
through the capacitor. Other expressions like "average"
insist on a specific waveshape and its figuring. Irms is
the measurement. It could be applied to ripple or to
some other through-current source.

 

[KlausST]

. Two different terms for the same parameter.

The datasheet informations are the same.
But the terminology I_RMS has not the same meaning as I_ripple.
You could also give the ripple in Peak-to-peak instead of RMS.

* I_Ripple says "what" is measured
* I_RMS says "how" it is measured

Klaus

 

[chiques]

The datasheet informations are the same.
But the terminology I_RMS has not the same meaning as I_ripple.
You could also give the ripple in Peak-to-peak instead of RMS.

* I_Ripple says "what" is measured
* I_RMS says "how" it is measured

Klaus

But it would then be called 'Ipeak', not Irms or ripple

 

[KlausST]

Hi,

You can say I-Ripple is 5App
or you can say I-Ripple is 223mA RMS

Klaus

 

[std_match]

I_RMS includes the DC component, if any. If you feed it thru a resistor, the dissipated power is I_RMS^2 * R.
I_ripple is the RMS value for the AC component.
There is no DC component for the current thru a capacitor, so I_RMS and I_ripple are the same.

 

[KlausST]

Hi,

I_RMS includes the DC component

I have a Fluke 114 handheld true RMS meter:
On 5V 50Hz input it shows
* 5V in AC mode
* 0V in Dc mode

On 5V DC input it shows:
* 0V in AC mode
* 5V in DC mode

On 5V DC + 5V 50Hz added it shows:
* 5V in AC mode
* 5V in DC mode

In my eyes:
* in DC mode that show "average"
* in AC mode they show RMS excluding DC

Agilent 34401A does it the same way.

Klaus

 

[std_match]

You need a multimeter with an "AC+DC" range:

 

[D.A.(Tony)Stewart]

I see two terms often used for capacitors: 'ripple current' and 'Irms'. What is the difference between these two?

Irms is the rated max current due to fusing or thermal limitations.

Ripple voltage is the common result from load current x source impedance which is dynamic or feedback current ripple in a regulator.