5
vacon • 27
BASIC VARIANTS
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In most cases, a duty cycle d = 0,5 should be used in calculations. This will guarantee that worst case
ripple is taken into consideration. For example, if the application operates with duty cycles 0.7-0.9,
it could be possible to decrease the inductance, and in that way increase the choke ripple (see
Figure 27 below). However, this would also increase the output ripple, and it might not be
acceptable. On the other hand, if output ripple is not important, duty cycle optimization can lead to
significant savings.
Figure 27. Always consider possible duty cycle window
A big DC-link voltage requires more inductance. If the DC/DC converter is sometimes used to boost
DC-link higher than nominal, it must be noted that ripple increases.
In order to minimize size (inductance) of the filter choke switching frequency and current, ripple
should be as high as possible. An optimal design is a compromise between these and power losses.
When the design is ready, switching frequency decrease is not allowed, otherwise ripple will
increase and this can cause temperature problems. In an LCL-filter structure, a smaller switching
frequency can also cause resonances.
The inductance of a traditional laminated iron core choke usually remains constant (L
NOM
) up to
saturation point (I
NOM
,
PEAK
in Figure 28 below). After saturation point, the inductance starts to
decrease. If overload ability is needed, ripple increases in saturation region and must be considered
in the design. The inductance of a powder core choke behaves differently. Usually inductance as a
function of current decreases continuously which means that with small currents the inductance is
bigger. This is an advantage because ripple with partial loads will be smaller. On the other hand, it
must be verified that the core is not going to saturation too fast, if overload ability is needed. How
big the initial inductance is depends on choke design but typical values range approximately from
20% to 50% over nominal.
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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