BY LEN CRANE,
Director of Technical Marketing
Coilcraft, Inc.
www.coilcraft.com

While DC/DC conversion circuitry has matured to the
point that there are “cookbook” design aids as well as software to help,
selecting the right power inductor is a critical aspect of converter design.
This requires a good understanding of inductor performance and of how desired
in-circuit performance relates to the information available in supplier data
sheets.

Inductors that can be used in DC/DC converters come in
a wide variety of shapes and sizes (Fig. 1). In order to
compare types and choose the optimal part for the application, a designer must
rely on correctly understanding published specifications.

1117_Special_Passives_Coilcraft-Fig-1

Fig. 1: Inductors
come in many shapes and sizes: Thin
versions enable low-profile converter designs.

Inductor performance can be described by relatively few
numbers. A typical data sheet excerpt for a surface-mount power inductor
intended for DC/DC converters is shown (Table 1).

Table 1: Typical data sheet excerpt for
a surface-mount power inductor.

Part number L ±20% (µH) DCR max (mΩ) SRF typ (MHz) Isat (A) Irms (A)
XAL4020-102 1.0 14.6 79 8.7 9.6

 

Inductance: Per Table 1, the inductance (L) is the main
parameter that provides the desired circuit function and is the first parameter
to be calculated in most design procedures. It is calculated to provide a
certain minimum amount of energy storage (or volt-microsecond capacity) and to
reduce output current ripple. Using less than the calculated inductance causes
increased AC ripple on the DC output. Using much greater or much less
inductance may force the converter to change between continuous and
discontinuous modes of operation.

Because it is not practical for a data sheet to show
performance for all possible sets of operating conditions, it is important to
have some understanding of how the ratings would change with different
operating conditions.

Tolerance: Fortunately, most DC/DC converter applications do not require
extremely tight tolerance inductors to achieve these goals. It is, as with most
components, cost-effective to choose standard tolerance parts, and most
converter requirements allow this. The inductor in Table 1
is shown specified at ±20%, which is suitable for most converter applications.

Test conditions: These are critical, and designers need to pay special
attention to voltage, wave shape, and test frequency. For example, most catalog
inductance ratings are based on “small” sinusoidal voltages, and the use of
sinusoidal voltage is a standard instrumentation test condition. With regard to
frequency, most power inductors do not vary dramatically between 20 kHz and 500
kHz, so a rating based on 100 kHz is quite often used and suitable. However, inductance
eventually decreases as frequency increases. As switching frequencies increase
to 500 kHz, 1 MHz, and above, it becomes more important to consider ratings
based on the actual application frequency.

DC resistance (DCR): This is based strictly on the wire diameter and length
and is specified as a “max” in the catalog but can also be specified as nominal
with a tolerance. DCR varies with temperature, so it is important that the DCR
rating also notes the ambient test temperature. The temperature coefficient of
resistance for copper is approximately +0.4% per degree C. So the part shown rated at
0.009 Ω max would have to have a corresponding rating of 0.011 Ω max at 85°C —
only a 2-mΩ difference in this case, but a total change of about 25%. The
expected DCR versus temperature is shown (Fig. 2).

1117_Special_Passives_Coilcraft-Fig-2

Fig. 2: Expected DCR versus temperature
curve based on 0.009 Ω max at 25°C.

AC resistance: This is not commonly shown on inductor data sheets and
is not typically a concern unless either the operating frequency or the AC
component of the current is large with respect to the DC component.

When trying to minimize the size of the component, the
designer should try to select the part with the largest possible resistance. Typically,
to reduce the DCR means having to use larger wire and probably a larger overall
size. So optimizing the DCR selection means a tradeoff of power efficiency,
allowable voltage drop across the component, and component size.

Self-resonant frequency (SRF): Every inductor winding has some associated distributed
capacitance, which, along with the inductance, forms a parallel resonant tank
circuit with a natural self-resonant frequency. For most converters, it is best
to operate the inductors at frequencies well below the SRF. This is usually
shown in the inductor data as a “typical” value.

Current rating: This is perhaps the rating that causes the most
difficulty when specifying a power inductor. Current through a DC/DC converter
inductor is always changing throughout the switching cycle and may change from
cycle to cycle depending on converter operation, including temporary transients
or spikes due to abrupt load or line changes. This gives a constantly changing
current value with a sometimes very high peak-to-average ratio. It is the
peak-to-average ratio that makes specification difficult. Look for an inductor
that has two current ratings: one to deal with possible core saturation from
the peak current and one to address the heating that can occur due to the
average current.

Saturation current (Isat): One effect of current through an inductor is core
saturation. Frequently, DC/DC converters have current wave shapes with a DC
component. The DC current through an inductor biases the core and can cause it
to become saturated with magnetic flux. The designer needs to understand that
when this occurs, the inductance drops and the component no longer functions as
an inductor. A typical L versus current curve for a gapped ferrite core is
shown (Fig. 3).

1117_Special_Passives_Coilcraft-Fig-3

Fig. 3: A typical L versus DC bias
current curve for a gapped ferrite core showing the point of current
saturation.

It can be seen that this curve has a “knee” as the
inductor moves into the saturation region. Definition of where saturation
begins is, therefore, somewhat arbitrary and must be defined. In the example of
Table 1, saturation is defined at the point at
which the inductance drops by 10%. Definitions in the range of 10% to 20% are
common, but it should be noted that some inductor catalogs might use figures of
50% inductance drop. This increases the current rating but may be misleading as
far as the usable range of current is concerned.

While there is more to be said on this topic, suffice it
to say that it is typically desirable to operate with current peaks near the
saturation rating because this allows the smallest possible inductor to be
chosen.

RMS current (Irms): The second major effect of current is component self-heating.
The Irms is used to give a measure of how much average current can continuously
flow through the part while producing less than some specified temperature
rise. In this case, the data sheets usually provide a rating based on
application of DC or low-frequency AC current, so this does not include heating
that may occur due to skin effect or other high-frequency effects. The current
rating may be shown for a single temperature rise point as in the example, or
some suppliers provide helpful graphs of temperature rise versus current or
factors that can be used to calculate temperature rise for any current.

Temperature rise due to self-heating may cause the
inductor to be at a temperature higher than the rated range. This is normally
acceptable provided that the insulation ratings are not exceeded.

As with other parameters, it is important to know the
inductor temperature rise so that this can be traded off with other parameters
when making design choices. If lower temperature rise is desired, a larger size
component is most likely the answer.

Conclusion
It can be seen that inductors for DC/DC
converters can be described by a small number of parameters. However, each
rating may be thought of as a “snapshot” based on one set of operating conditions
that may need to be augmented to completely describe expected performance in
application conditions.