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I realized that I don't have much clue when it comes to bulk capacitance

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Is there a rule of thumb to use to determine how much bulk capacitance is needed for a board? From what I can gather, bulk capacitance is used both to smooth out poor quality power supplies and also to provide a low impedance source for sudden changes in load.

A few examples:

* On one board I have, I'm using the LM2738XMY which switches at 1.6 MHz which allows the use of ceramics. So I have 10 µF at the input. Should I have added more bulk capacitance? Or is the capacitance enough? When you see electrolytics used, they usually have a rather high capacitance but from what I can tell, this is mostly because high capacitance often means low ESR. There aren't many sudden load changes on this board though I suppose the power supply powering it could be crappy?
* I'm building a driver board for RGB LED strips which is going to switch up to about 5A using MOSFETs. Should I add some bulk capacitance to the anode terminal so that there charge available at low impedance?
This post may sound a bit confused but it's just that I started thinking about this now and realized I don't really know what I'm doing.

armandas:

--- Quote from: shadewind on October 15, 2011, 07:30:06 pm ---Is there a rule of thumb to use to determine how much bulk capacitance is needed for a board?
--- End quote ---
Is there a rule of thumb for how long a piece of string should be? :)

The capacitance required depends on how much current "backup" you need and how often. Bigger capacitors can supply more current, but take longer to recharge. If you want to do the calculations, you can make use of the following formula:
I = C(dv/dt).

Once you begin to deal with high frequencies, you have to start considering other factors, such as inductance.

At any point, you should find some application notes and read through them.

--- Quote from: shadewind on October 15, 2011, 07:30:06 pm ---from what I can tell, this is mostly because high capacitance often means low ESR
--- End quote ---

Absolutely not.

IanB:

--- Quote from: shadewind on October 15, 2011, 07:30:06 pm ---When you see electrolytics used, they usually have a rather high capacitance but from what I can tell, this is mostly because high capacitance often means low ESR.
--- End quote ---
Most application notes say that electrolytic capacitors have to be derated at high frequencies, so you need a value at least ten (or one hundred) times larger than a tantalum part for the same duty.

SiBurning:
It is a bit complicated at high frequency and high load current.

A simpler form of the equation armandas gave is
C = I / V * t
where I = dc load current,  V = acceptable ripple voltage, t =~ (dis)charge rate, probably 2 * the switching frequency, but could be 4*, with some additional factor for non-sinusoidal waves. Keep in mind that the ac current (peak or instantaneous) is much higher than the dc load current.

With high load current, ripple current is just as important. The capabilities of the physical part are frequency & temperature dependent, so you need to be careful, and can't rely on a single Ir and ESR figure.

Just to add to what armandas said (again)--or, really, put it in terms dummies like me can understand easier--you use larger values of capacitance to get more charge stored, but the slower speed of charge/discharge also smooths the voltage by slowing the changes in ripple voltage. Note in the equation how ripple voltage is inversely proportional to capacitance.

--- Quote from: armandas on October 15, 2011, 08:06:05 pm ---
--- Quote from: shadewind on October 15, 2011, 07:30:06 pm ---from what I can tell, this is mostly because high capacitance often means low ESR
--- End quote ---

Absolutely not.

--- End quote ---

By looking at at the data sheet for a typical low ESR cap (http://www.panasonic.com/industrial/components/pdf/pic_fk_series.pdf), it is clear that there is a relationship between the ESR and the capacitance. So it seems to me that the reason to choose a high value capacitor for a switcher isn't so much the capacitance itself but that higher capacitance of results in lower ESR which is the dominant factor in filter stages for switchers, for example. But maybe that's not what you mean by "Absolutely not."?

--- Quote from: SiBurning on October 15, 2011, 08:18:38 pm ---It is a bit complicated at high frequency and high load current.

A simpler form of the equation armandas gave is
C = I / V * t
where I = dc load current,  V = acceptable ripple voltage, t =~ (dis)charge rate, probably 2 * the switching frequency, but could be 4*, with some additional factor for non-sinusoidal waves. Keep in mind that the ac current (peak or instantaneous) is much higher than the dc load current.

With high load current, ripple current is just as important. The capabilities of the physical part are frequency & temperature dependent, so you need to be careful, and can't rely on a single Ir and ESR figure.

Just to add to what armandas said (again)--or, really, put it in terms dummies like me can understand easier--you use larger values of capacitance to get more charge stored, but the slower speed of charge/discharge also smooths the voltage by slowing the changes in ripple voltage. Note in the equation how ripple voltage is inversely proportional to capacitance.

--- End quote ---
Yes, I know the theory, that's not the problem. I don't know how to apply this in real life. I don't know how bad/good a typical wall wart voltage rail is and so on.

Regarding ripple voltage, in switchers, the ESR is usually the dominant term unless you're switching at low frequencies.