Boost converter - ripple current

[sprunger]

Whilst looking at the possible parts for a boost converter I came across references to making sure that the capacitor could handle the "ripple current".

I looked a bit further and can see 2 possibilities.
1) The ripple in the capacitor current itself ie the difference between the minimum and maximum capacitor current.
2) The other possibility is that there is a ripple current called delta i,L which is mainly affected by the size of the inductor for given input voltage and duty cycle.

I found a capacitor which lists its "ripple current" rating as 0.8A/800 mA. Does this mean it can handle a ripple of 0.8A in the inductor current or can its own current only vary by +/- 0.8A?

The source of my confusion may be misunderstanding a page here - http://www.analog.com/library/analogdialogue/archives/45-09/boost.html

"The current ripple, shown as ΔIL ...The average inductor current flows into the load, while the ripple current flows into the output capacitor"

 

[Baluncore]

The ripple current is the average current flowing in the capacitor leads.

If the load current is continuous and a converter produced bursts of higher current, then the ripple current is the average of the absolute tidal current.

Say output current is 10A and current pulses from converter are 40A, (for 25% of the cycle), then output capacitor ripple current is ((40A – 10A) * 0.25 ) + (0.75 * 10A) = 7.5A + 7.5A = 15A (Note that the two terms have the same value).

Capacitor ESR is also important in analysis because it produces step changes in capacitor voltage, rather than the ramp due to charge and discharge. The voltage ripple should include the ESR step component.

 

[sprunger]

Thank you very much. It was a little more complicated than I thought but your explanation was very helpful.

 

[meBigGuy]

The ripple current in the capacitor is also important because it causes power dissipation. When they say a capacitor is rated for 800ma ripple current it means it can handle the power dissipation associated with that current and it's effective series resistance.
For example, in this data sheet there is a 100uF low esr capacitor with 1 ohm ESR and is rated at 100ma ripple current at 125C. The capacitors with lower ESR have higher ripple current ratings.
http://www.mouser.com/ds/2/315/ABA0000CE101-183196.pdf

If you exceed the ripple current rating the capacitor will degrade and fail. Choosing the proper input and output capacitors is an important part of switching converter design.

 

[alphy]

I would first like to clarify that a boost converter is a type of DC-DC converter that steps up the input voltage to a higher output voltage. In this process, the input current is converted into an output current with a higher voltage and lower current.

Now, to address the issue of ripple current in a boost converter, it is important to understand that this is a common phenomenon in all switching converters. Ripple current refers to the AC component of the output current, which is superimposed on the DC component. This is caused by the switching action of the converter, where the input current is chopped into small pulses to create the desired output voltage.

In a boost converter, the ripple current is mainly affected by the size of the inductor, as mentioned in your research. The inductor stores energy during the on-time of the switching cycle and releases it during the off-time. This results in a fluctuation in the inductor current, which is known as ripple current.

Now, in terms of the capacitor's ability to handle this ripple current, it depends on the capacitor's rated "ripple current" as you have mentioned. This rating indicates the maximum allowable ripple current that the capacitor can handle without overheating or failing. Therefore, in the case of the capacitor you have found with a rating of 0.8A/800mA, it means that it can handle a maximum ripple current of 0.8A without any issues.

To further clarify, the ripple current rating of a capacitor does not refer to the variation in its own current, but rather the maximum allowed ripple current that can pass through it. So, in a boost converter, the capacitor will experience a ripple current of 0.8A, while the inductor's ripple current can vary based on its size and the input voltage and duty cycle.

In summary, when designing a boost converter, it is important to choose a capacitor with a suitable ripple current rating to ensure its proper functioning and longevity. I hope this helps to clarify any confusion you may have had regarding ripple current in a boost converter.