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Old 02-23-2006, 06:27 AM   #1
yanz
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Question What does ripple current mean?

What does "ripple current" mean?


It always bugs me whenever I think about it. I know that the definition (from panasonic website):

  • Ripple Current
    Ripple current is the rms value of alternating current flowing through a capacitor. This current causes an internal temperature rise due to power losses within the capacitor.

  • ESR (Equivalent Series Resistance)
    Equivalent series resistance causes heat generation within the capacitor when AC ripple current flows through the capacitor. Maximum ESR is normally specified at 120Hz, 20C.


So, there is a current flowing through a capacitor, and its value is alternated. The higher the rms value of this ripple current the worse the effect to the capacitor. The internal heat of capacitor will rise as the result of resistance. So this is the use of low esr caps, it's allowed the ripple current flowing through with minimal generated heat in the caps thanks to its low resistance..

The higher the ripple current rate for the capacitor, the more "strong" the caps can sustain the higher excessive ripple current, and the lower the heat within the caps.

Do I understand it right?


What happens if the ripple current is higher beyond the allowed value? Will it change into the heat instead of flowing through the caps?

And why the caps should allow the ripple current? Isn't its job to smooth the ripple? If the smoothing jobs doesnt run good what is the effect to voltage? I mean, if the ampere decreased, the voltage will increase to keep the same wattage, right?

I know the relation between the capacitance and transient performance, but is there any relation between low esr and charging/discharging time for a capacitor? What does impedance involve here?

What is more important, the ripple current rate or the low esr rate? Or does the low esr rate automatically translate to higher ripple current rate?

What the relation between capacitance and impedance. Now Im completely lost about impedance, what does impedance mean?

Did I ask the same question again and again? lol.. Sorry... Your explanation will be appreciated to shed a lot of confusion I have...


Maybe this is all just a simple theory. So, pls help me to understand it simply better...

Thanks...
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Old 02-23-2006, 10:08 PM   #2
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Default Re: What does ripple current mean?

Good Qs, yanz! And you have a lot of it right.

In a switching power supply, the voltage coming out of the O/P rectifier and into the O/P inductor is a positive-polarity square wave. The O/P inductor and O/P capacitor(s) filter that into a DC voltage. The current through the inductor is a DC level (which goes to the load) with triangle-shaped AC current riding on the DC level. The AC current is due to the charging of the inductor (during the switch "On" time) and discharging of the inductor (during the switch "Off" time). If things are designed properly and operating normally, the inductor never "fully" charges (saturation), nor discharges fully. Almost all of the AC current, several amps rms, goes through the O/P capacitors rather than to the load (if it did, the ripple voltage would be fairly high). While a capacitor's DC resistance is very high, its impedance to AC depends on the frequency of the AC. In the 100KHz range, that impedance is milliohms (thousandths of an ohm). By way of contrast, the effective resistance on the load in the computer is much higher (e.g., if the O/P voltage is 2V and the load current is 20A, the DC "resistance" is 100 milliohms). So it is entirely normal for the capacitors to be conducting several amps of ripple current, as this is due to the capacitors' smoothing action.

What is impedance? In a capacitor, it has 3 basic components in series with each other: an ideal capacitor; the ESL, equivalent series inductance, of the leads; the equivalent series resistance of the leads, the foils, and the electrolyte. Impedance is the vector sum of the capacitive reactance [1/(6.28xFxC)] X(C), the inductive reactance (6.28xFxL) X(L), and the ESR. At relatively low frequencies, X(C) is basically the impedance. As frequency increases, the impedance falls until the ESR is greater than the X(C), and the ESR is basically the impedance. As frequency continues to rise, the X(L) becomes greater than the ESR, and the impedance is basically the X(L). P/Ss and VRMs operate in the frequency range where the impedance and ESR are approximately the same.

If the capacitor is conducting ripple current, it is dissipating power (I^2)(ESR), which is heat. The higher the ripple current, the more the heat. If you exceed the ripple current rating, the cap will overheat, unless the ripple current is so high that hydrogen gas is being generated. So the failure mechanisms with excessive ripple current are electrolyte evaporation and evaporation- or gas-related venting.
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Old 02-23-2006, 10:22 PM   #3
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Default Re: What does ripple current mean?

Quote:
I know the relation between the capacitance and transient performance, but is there any relation between low esr and charging/discharging time for a capacitor?
The rate of charging or discharging a capacitor is the product of the capacitance and the resistance through which it is charged or discharged. The ESR of the cap only becomes relevant when it becomes a significant part of the overall resistance in the circuit. In a P/S O/P circuit. however, the charging and discharging rate are determined by the I/P voltage to the inductor, the inductor itself, and the load current.
Quote:
What is more important, the ripple current rate or the low esr rate? Or does the low esr rate automatically translate to higher ripple current rate?
The ripple current rating is related to the ESR, the stability of the electrolyte, and the ability of the cap to dissipate heat to the ambient environment. If you are comparing caps of identical case size and temperature rating, it basically comes down to ESR.
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Old 02-24-2006, 12:07 AM   #4
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Default Re: What does ripple current mean?

I had an instructor way back who used the electricity to water analogy.
It works surprisingly well.

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Old 02-24-2006, 01:23 AM   #5
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Default Re: What does ripple current mean?

A good descriptions Pete. I have been wondering if it would be profitable for this forum to create a VRM block diagram/equivalent circuit with input caps, converter, output inductor, output caps(electro and ceramic) and load. To include ESR and ESL, conversion ratio (12V in/1.6V out) and how the load on the input caps is changed by the ratio. How the ratio changes the effective ESR need of the input caps compared with the output caps etc.

Pictures can help.
What do you think? Can you supply such a diagram?
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Old 03-02-2006, 04:38 AM   #6
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Default Re: What does ripple current mean?

Pete, that explains a lot, thanks! While i feel i only understand 70% of it, it answer a lot of my initial question.

Quote:
Pictures can help.
That would be excelent..

Quote:
I had an instructor way back who used the electricity to water analogy.
It works surprisingly well.
Yup i use it to explain to some friends of mine, but when it is about pawor factor, water analogy doesnt help.
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