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35DC-DC Converter Design and Magnetics
example (keeping everything else the same), doubles the DC level of the inductor current
(whate
ver it was to start with). So in a boost with a duty cycle of 0.5, for example, if we
have a 5 A load, then the I
DC
is 10 A. And if I
O
is increased to 10 A, I
DC
will become 20 A.
Summarizing, changing the input/output voltages (duty cycle) does affect the DC level of
the inductor current for the boost and the buck-boost. But changing D affects the swing
I in all three topologies, because it changes the duration of the applied voltage and
thereby changes the voltseconds.
Changing the duty cycle affects I
DC
for the boost and the buck-boost.
Changing the duty cycle affects I for all topologies.
Note: The off-line forward converter transformer is probably the only known
exception to the logic presented above. We will learn that if we, for example,
double the duty cycle (i.e., double t
ON
), then almost coincidentally , V
ON
halves, and
therefore the voltseconds does not change (and nor does I ). In effect, I is then
independent of duty cycle.
Based on the discussions above, and also the detailed design equations, we have
summarized these variations in Table 3.2 . This table should hopefully help the reader
eventually develop a more intuitive and analytical feel for converter and magnetics
design, one which can come in handy at a later stage. We will continue to discuss certain
aspects of this table, in more detail, a little later.
3.3 Defi ning the AC, DC, and Peak Currents
In Figure 3.2 , we see how the AC, DC, peak-to-peak, and peak values of the inductor
current waveform are defi ned. In particular we note that the AC value of the current
waveform is defi ned as
I
I
AC
2
(3-3)
We should also note from Figure 3.2 that I
L
I
DC
. Therefore, sometimes in our
discussions that follow, we may refer to the DC level of the inductor current as I
DC
, and
sometimes as the average inductor current I
L
, but they are actually synonymous.
In particular, we should not get confused by the subscript L in I
L
. The L stands for
inductor , not load . The load current is always designated as I
O
. Of course, we do realize
that I
L
I
O
for a buck , but that is just happenstance.

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