BASIC VARIANTS
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Figure 26. Peak-to-peak ripple current as function of duty cycle. Scaled values can be converted to real
values by multiplying Udc/(fsw*L)
Interleaved switching helps to reduce the output ripple, but does not affect the single choke ripple.
The choke losses are thus relatively big, when compared to e.g. active front end (AFE). To keep the
losses reasonable, it is recommended to have peak-to-peak ripple one fourth of the nominal choke
current. Thus the proposed formula to calculate the required inductance is:
Using the above filter dimensioning, the relative output ripple is:
Example with a ~ 3% peak-to-peak: The ripple current is a triangle wave and the RMS value is peak-
to-peak divided by 2
, i.e. 0,8% RMS.
A more general formula for inductance calculation is:
This can be used for example with powder core chokes which tolerate more ripple. With liquid
cooling peak-to-peak ripple can be approximately 35-45% of the nominal current. A bigger ripple in
the choke also directly increases the output side ripple which in this case would be approximately
4-5% peak-to-peak.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
0.05
0.1
0.15
0.2
0.25
Ripple current: Choke I
L,pp
(blue) / Output I
Out,pp
(black)
Duty cycle
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