35DC-DC Converter Design and Magnetics
example (keeping everything else the same), doubles the DC level of the inductor current
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
is 10 A. And if I
is increased to 10 A, I
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
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
), then almost coincidentally , V
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
We should also note from Figure 3.2 that I
. Therefore, sometimes in our
discussions that follow, we may refer to the DC level of the inductor current as I
, and
sometimes as the average inductor current I
, but they are actually synonymous.
In particular, we should not get confused by the subscript L in I
. The L stands for
inductor , not load . The load current is always designated as I
. Of course, we do realize
that I
for a buck , but that is just happenstance.

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