Ripple current rating of DC bus caps

ripple current calculation

i apologise for my inquisition, but i ask of your assistance on the ripple current on the output of a transistion mode power factor corrector chip.

in this case...the PFC chip is L6561...
http://www.st.com/stonline/books/pdf/docs/5109.pdf

Here is an application note on it

http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXyyvyr.pdf

......but it does not make clear how to get the ripple current into the cap on the 400V dc bus thats supplied by the PFC stage.

My peak PFC current is 0.7 Amps.........but this peak obviously varies sinusoidallyt over the mains cycle.

I wish to know the ripple current as i need to specify the ripple current rating of the bus caps.

For me, is it .....I rms^2 = I ripple ^2 + I dc ^2 ? (1)

where
Irms = mains input rms current
I ripple = (what i wish to find -ie the ripple current into the 400v bus cap)
Idc = the dc current supplied out of the 400v DC bus cap to the next stage.

the thing is, how do i calculate Irms in equation (1)?

...since my boost inductor current is lots of triangles, the peaks of witch vary with the input rectified mains value.

dc bus cap

As a rough estimation, the 100 or 120 Hz RMS ripple current value at the bus capacitor is half of the DC bus current. The additional PWM frequency componenent mainly depends on the inductor dimensioning, but should be expected considerably lower than the low frequency part. Cause the total ripple current is formed by adding the squared components, the high frequency part can be neglected in many cases.

For an exact calculation, you have to consider the fourier series of the full-wave rectified current waveform.

i ripple

how to get the ripple current into the cap

Click to expand...

Well, ripple current is there because of voltage waveform on the cap, you don't have to "get the ripple current into the cap". You start selection of capacitor based on how much ripple voltage on the DC bus you can have in your application. You select capacitor size and ESR rating based on this. Then you calculate how much ripple current your capacitor(s) on the bus will have with those values using formulas provided in application note you posted.