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Chapter 3
during a very small interval. The current can therefore overshoot the set current
limit threshold by a v
ery large amount, thereby endangering the switch. That is
why, especially when we come to off-line applications, it is actually customary to
select a core large enough to avoid saturation at the current limit threshold. And
that usually gives enough time for the current limit circuitry to act— before the
slope of the current has gone completely out of control.
Note, however, that the copper windings still only need to be proportioned to handle
the continuous current (i.e., based on the maximum operating load).
In effect, what we are therefore always implicitly doing in off-line applications is
setting the I
SAT
of the transformer higher than its I
DC
rating. That is clearly not what
we usually do in low-voltage DC-DC converter design.
g) Generally speaking, in most low-voltage applications (i.e., V
IN
typically less than
about 40 V), the inductors are selected based only on the maximum operating
load current. The current limit is therefore, in effect, virtually ignored! This is
the usual industry practice for DC-DC converter design, though it is probably
not clearly spelled out in this way most of the time. But luckily, it seems to have
worked!
3.14 The Spread and Tolerance of the Current Limit
Any specifi cation, including the current limit, either set by the user or fi xed internally in
the IC, will have a certain inherent tolerance band, which includes spreads over process
variations and over temperature. All these variations are combined together inside the
electrical tables of the datasheet of the device, under its MIN and MAX limits. In a
practical converter design, a good designer learns to pay heed to such spreads.
But let us fi rst summarize the general procedure for selecting the inductance for
a switching power converter. Then we will look at the practical issues concerning
spreads/tolerance.
The normal procedure is to determine the inductance by requiring that the current ripple
ratio is about 0.4, because we know that that represents an optimum of sorts for the entire
converter. But there may be another possible limitation when dealing with switcher ICs,
especially those with internally set (fi xed) current limits: if our normal operating peak
currents are close to the set current limit of the device (i.e., we are operating close to the
maximum current capability of the switcher IC), we need to ensure that the inductance is
large enough not to cause the calculated operating peak current (within any given cycle)
to exceed the current limit. Otherwise foldback will obviously occur at the current limit
threshold, and so the desired maximum output power cannot be guaranteed.

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