What is percentage ripple current?

[treez]

If I have a boost converter SMPS operating in Boundary conduction mode (aka critical conduction mode)...then is that 200% ripple on the input current?, or do you call that 100% ripple?

 

[mrinalmani]

Ripple Percentage is give by
[(I_RMS² - I_DC² )^1/2]/I_DC*100
Where,
I_RMS = RMS value of current being measured
I_DC = Average value of that current

In the case of "Boundary Mode Conduction", you can approximate the current waveform to be triangular. Now you must perform appropriate integrations to find the RMS and the average value.
By the way, I think you rather wanted to ask " What is the DENOMINATOR of the equation of ripple percentage". Well it is I_DC and NOT I_RMS

 

[treez]

surely a BCM boost has 200% ripple?

(ripple current pk-to-pk/ average inductor current) * 100 = 200%

 

[mrinalmani]

Peak to peak ripple current does not have to do anything with the ripple factor. The RMS value of ripple must be considered.
I_ripple² = I² - I_dc²
where
I_ripple = RMS value of ripple current
I = RMS value of current whose ripple must be found
I_dc = Average value of current whose ripple must be found

 

[treez]

I think we are talking at crossed purposes.
You are right
I am right
We are talking about different things.