# DC link capacitor

Discussion in 'General Electronics Chat' started by Cerkit, Nov 3, 2015.

1. ### Cerkit Thread Starter Active Member

Jan 4, 2009
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Hi.

Can someone explain how a dc link capacitor helps smooth ripple current?

Thanks

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Looks like the "experts" in the solar industry came up with a new name for what we call a "bypass" or "filter" capacitor...

3. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
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Hmm. The explanation in the link hasn't helped me much.

Sorry I don't really understand.

Nov 30, 2010
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5. ### MikeML AAC Fanatic!

Oct 2, 2009
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Solar panel(s) cannot supply a large momentary current;they are very current-limited, and normally operate such that their peak current equals their average current, as in charging a battery.

Placing a large capacitor between the output of the panel(s) and the input of a switch-mode converter provides a means of storing energy (0.5*C*V^2) so that the current peaks required by the smps come out of the capacitor instead of the panel(s). The panel(s) can deliver their average current to the capacitor; the capacitor delivers the current peaks at the duty-cycle dictated by the operation of the smps. It is a means of letting the panel(s) operate such that their average current is equal to their peak current...

This is the inverse of what a filter capacitor does in a linear power supply, where the rectified AC line delivers high peak current pulses to charge the capacitor, and the the capacitor provides DC to the input of the linear regulator. The amount of ripple across the capacitor would be similar in either case.

A capacitor is a capacitor is a capacitor...

Last edited: Nov 3, 2015
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6. ### Papabravo Expert

Feb 24, 2006
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Try reading the following wiki article:

https://en.wikipedia.org/wiki/Capacitor

The really important point is that the voltage across a capacitor cannot change instantaneously. The time rate of change of voltage across a capacitor is proportional to the current. Since we can't have infinite current we cannot have instantaneous voltage change. Is that enough understanding or is more required?

7. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
Ok. So if I understand correctly, for the example of an smps. When the gate switches at the output side open and close really fast the current spikes and in order for there to not be a voltage dip the capacitor helps to maintain the voltage? Such as the capacitors below?

8. ### PeterCoxSmith Member

Feb 23, 2015
148
38
Yes a DC Link capacitor refers to the capacitance on the DC bus in a motor drive in which you convert the fixed frequency AC input into a DC voltage that is fed to the inverter in your example circuit, the output of which will be a variable frequency AC.

9. ### nsaspook AAC Fanatic!

Aug 27, 2009
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We use DC link capacitors in high energy 600vdc+ plasma generation protection circuits. Specifically the ones with a polypropylene dielectric as a substitute for electrolytic capacitors of similar capacity. For us the ability to take abuse from intermittent cathode arcing without exploding from internal heating was the major factor.

"A capacitor is a capacitor is a capacitor..." but some are more equal than others.

10. ### wayneh Expert

Sep 9, 2010
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Who knows, maybe it will help to consider the water analogy. A capacitor is like a big water tank with a pipe in and a pipe out. If the supply pressure (voltage) jumps up or down, the output pressure only rises or falls as the level in the tank (charge on the capacitor) changes - ie. slowly. Big pipes (big currents) require a bigger tank (capacity) to do the smoothing job.

11. ### #12 Expert

Nov 30, 2010
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Apparently I've been using Link Capacitors in vacuum tube guitar amplifiers for over 40 years. In the course of doing that, I also found that polypropylene film capacitors are rather good at their job.

12. ### MikeML AAC Fanatic!

Oct 2, 2009
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"A capacitor is a capacitor is a capacitor..."

13. ### #12 Expert

Nov 30, 2010
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Yep. And they all have the same dielectric absorbtion, DC leakage characteristics, temperature coefficient, price per uf, uf per cubic inch, and self healing properties as each other.

14. ### nsaspook AAC Fanatic!

Aug 27, 2009
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We know what you mean but 'Capacitance' is capacitance is capacitance. A capacitor is a physical device with many properties other than pure capacitance.