Datasheet

MAX OUT
ΔI
I = 1.15 × I +
2
æ ö
ç ÷
è ø
IN OUT
V V
D
I =
L s
-
D
¦
AC
e
L I
B =
A N 2
D
´
2
2
LRMS O UT
I
I = I +
12
D
UCD7242
www.ti.com SLUS962B –JANUARY 2010–REVISED AUGUST 2012
Powdered iron has the advantage of lower cost and a soft saturation characteristic; however, its losses can be
very large as switching frequencies increase. This can make it undesirable for a UCD7242 based application
where higher switching frequency may be desired. It’s also worth noting that many powdered iron cores exhibit
an aging characteristic where the core losses increase over time. This is a wear-out mechanism that needs to be
considered when using these materials.
The powdered alloy cores bring the soft saturation characteristics of powdered iron with considerable
improvements in loss without the wear-out mechanism observed in powdered iron. These benefits come at a cost
premium.
In general the following relative figure of merits can be made:
Ferrite Powdered Alloy Powdered Iron
Cost High Medium Low
Loss Low Medium High
Saturation Rapid Soft Soft
When selecting an inductor with an appropriate core it’s important to have in mind the following:
1. I
LRMS
, maximum RMS current
2. ΔI, maximum peak to peak current
3. I
MAX
, maximum peak current
The RMS current can be determined by Equation 6:
(6)
When the 40% ripple constraint is used at maximum load current, this equation simplifies to: I
LRMS
≈I
OUT
.
It is widely recognized that the Steinmetz equation (P
fe
) is a good representation of core losses for sinusoidal
stimulation. It is important to recognize that this approximation applies to sinusoidal excitation only. This is a
reasonable assumption when working with converters whose duty cycles are near 50%, however, when the duty
cycle becomes narrow this estimate may no longer be valid and considerably more loss may result.
P
ƒe
= k × ƒ
α
× B
AC
β
(7) (7)
The principle drivers in this equation are the material and its respective geometry (k, α, β), the peak AC flux
density (B
AC
) and the excitation frequency (ƒ). The frequency is simply the switching frequency of the converter
while the constant k, can be computed based on the effective core volume (V
e
) and a specific material constant
(k
ƒe
).
k = k
ƒe
× V
e
(8) (8)
The AC flux density (B
AC
) is related to the conventional inductance specifications by the following relationship:
(9)
Where L is the inductance, A
e
, is the effective cross sectional area that the flux takes through the core and N is
the number of turns.
Some inductor manufactures use the inductor ΔI as a figure of merit for this loss, since all of the other terms are
a constant for a given component. They may provide a plot of core loss versus ΔI for various frequencies where
ΔI can be calculated as:
(10)
I
MAX
has a direct impact on the saturation level. A good rule of thumb is to add 15% of head room to the
maximum steady state peak value to provide some room for transients.
(11)
For example for a 10A design has the following:
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