Selecting the Best
Inductor for Your
DC-DC Converter
Leonard Crane
Understanding the Data Sheet
Proper inductor selection requires a good understanding
of inductor performance and of how desired in-circuit
performance relates to the information available in supplier data sheets. This article walks both the experienced
power conversion specialist and nonspecialist through
the inductor catalog and the important specifications.
The use of dc-dc converters is increasing. As electronic
systems become more miniaturized, mobile, complicated,
and popular, the power requirements become more
varied. Available battery voltages, required operating
voltages, size, and shape requirements are ever changing, leaving equipment designers constantly in need of
new power conversion solutions. As product requirements constantly drive performance improvement and
size reduction, optimization is crucial. A “one size fits all”
approach to power conversion does not fit all applications. For example, low profile components as shown in
Figure 1 are much in demand.
continues to evolve rather slowly. Because of this it has
become quite practical and useful for authors to create
“cookbook” design aids by which equipment designers
can create their own converter design. Software is also
readily available to facilitate these designs1.
After deciding on a circuit topology, one of the key design
tasks is component selection. Many circuit design programs produce a list of the required component values.
The task for the designer then is to get from knowing
the desired inductance value to selecting an available
component to do the job. Inductors that can be used in
dc-dc converters come in a wide variety of shapes and
sizes. Figures 2 and 3 show just two of the possible
inductor shapes. In order to compare types and choose
the optimal part for the application, a designer must rely
on correctly understanding published specifications.
Not only is the market for purchased converters growing,
but also many circuit designers now design their own
dc-dc conversion circuits instead of relying on power supply specialist companies. This increases the number of
circuit designers involved in selecting components. Basic
dc-dc conversion circuitry is fairly mature technology and
Figure 2. E-Core Inductor with Flat Wire
Figure 1. Thin Inductor Shapes Allow Low Profile
Converter Design
Figure 3. Molded inductors are mechanically rugged and
magnetically shielded for use in high density circuits
Document 469-1 Revised 05/23/16
DC-DC Converter Requirements
Simply stated, the function of a dc-dc converter is to
provide a stable dc output voltage from a given input voltage. The converter is typically required to regulate the dc
output voltage given a range of load currents drawn and/
or range of input voltage applied. Ideally the dc output is
to be “clean”, that is with ripple current or voltage held
below a specified level. Furthermore, the load power is to
be delivered from the source with some specified level of
efficiency. Power inductor selection is an important step
to achieving these goals.
Power Inductor Parameters
Inductor performance can be described by a relatively
few numbers. Table 1 shows a typical data sheet excerpt
for a surface mount power inductor intended for dc-dc
Table 1. Typical Inductor Catalog Excerpt2
L ±20%a
Part number
DCR max SRF typ
100 9.0 6.8
a. Inductance tested at 100 kHz, 0.1 Vrms
b. Inductance drop = 10% typ. at Isat
c. For 40° C temperature rise typ. at Irms
d. All parameters tested at 25° C.
L – Inductance The primary functional parameter
of an inductor. This is the value that is calculated by
converter design equations to determine the inductors ability to handle the desired output power and
control ripple current.·
DCR – DC Resistance The resistance in a component due to the length and diameter of the winding
wire used.
SRF – Self Resonant Frequency The frequency
at which the inductance of an inductor winding
resonates naturally with the distributed capacitance
characteristic of that winding.
Isat – Saturation Current The amount of current
flowing through an inductor that causes the inductance to drop due to core saturation.·
Irms – RMS Current The amount of continuous
current flowing through an inductor that causes the
maximum allowable temperature rise.
Test Conditions
To use the ratings properly, one must understand how
they were derived. Since 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
■ Voltage. The inductance value rating should note
the applied frequency and test voltage. Most catalog
inductance ratings are based on “small” sinusoidal
voltages. This is the easiest and most repeatable
method for the inductor supplier, and suitably indicates the inductance for most applications.
Inductance (L)
■ Wave shape. The use of sinusoidal voltage is a standard instrumentation test condition, which usually
serves quite well to ensure that the inductance value
calculated from the design equations is delivered.
Inductance is the main parameter that provides the
desired circuit function and is the first parameter to
be calculated in most design procedures. Inductance
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.
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
■ Test 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. It must be remembered that inductance
eventually decreases as frequency increases. This
can be due to the frequency roll off characteristic of
the core material used or it may be due to the selfresonance of the winding inductance resonating with
its self capacitance. As most converters operate in
the 50 kHz to 500 kHz range, 100 kHz has been a
suitable standard test frequency. As switching frequencies increase to 500 kHz, 1 MHz, and above, it
will be more important to consider ratings based on
the actual application frequency.
Document 469-2 Revised 05/23/16
DC Resistance (DCR)
DCR varies with temperature in the same manner as
the resistivity of the winding material, typically copper.
It is important that the DCR rating makes note of the
ambient test temperature. The temperature coefficient of
resistance for copper is approximately +0.4% per degree
C3. So the part shown rated at 0.009 Ohms max would
have to have a corresponding rating of 0.011 Ohms max
at 85°C, only a 2 milliOhm difference in this case, but a
total change of about 25%. The expected DCR versus
temperature is shown in Figure 4.
DCR (Ohms max)
Frequency (kHz)
Figure 5. ACR/DCR for #22 AWG Round Copper Wire
This has not been necessary for most applications
working below 500 kHz. As can be seen from Figure 5
the ac resistance does not become comparable to the
dc resistance at frequencies below about 200 kHz. And
even above that frequency the ac resistance will not
be an issue if the ac current is not large compared to
the dc component. Nevertheless at frequencies above
200-300 kHz it is recommended to ask the supplier for
loss versus frequency information to supplement the
published information.
The designer should try to choose the component that
has the largest possible resistance if the size of the component is to be minimized. 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.
DCR is simply a measure of the wire used in the inductor. It is based strictly on the wire diameter and length.
Normally this is specified as a “max” in the catalog but
can also be specified as a nominal with a tolerance. This
second method can be a little more instructive by giving
a better indication of the nominal or expected resistance,
but also may unnecessarily tighten the specification as
almost always no harm is done by a part having too little
90 100
Temperature (°C)
Figure 4. Expected DCR Based on 0.009q Max at 25°C.
AC Resistance
This is a parameter that 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.
The resistance of most inductor windings increases with
operating frequency due to skin effect. If the ac or ripple
current is relatively small compared to the average or
dc current then the DCR gives a good measure of the
resistive loss to be expected. The skin effect varies with
wire diameter and frequency3, so to include this data
would require a full frequency curve for each inductor
listed in a catalog.
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
Current Rating 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 sometime a very high peak-to-average ratio.
It is the peak-to-average ratio that makes specification
difficult. If one takes the highest possible instantaneous
peak current and looks for an inductor with this “current
Document 469-3 Revised 05/23/16
Saturation Current
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.
Figure 6 shows a typical L vs current curve for a gapped
ferrite core. 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 10-20% are
common, but it should be noted some inductor catalogs
may 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.
L (µH)
w / core saturation
Inductor Current (A)
rating” the inductor is likely to be overkill for the application, yet if one looks for a current rating for the average
current, the inductor may not perform well when passing
the peak current. The way to address this problem is to
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.
w / o core saturation
Switching Cycles
Figure 7. Inductor Current Waveform With and Without
Core Saturation.
with current peaks near the saturation rating because
this allows the smallest possible inductor to be chosen.
Increasing the saturation current rating typically means
using a larger size component or selecting a smaller
inductance value in the same size.
RMS Current
The second major effect of current is component selfheating. The RMS current 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 almost
always 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 as mentioned earlier
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.
Inductor core saturation can often be observed directly
in the converter current waveform where di/dt is inversely
proportional to inductance. As inductance drops due to
core saturation, the current slope increase rapidly. This
can cause noise and damage to other components.
The Irms rating should include the ambient temperature
at which it was measured. Normally an inductor specification includes an operating temperature range. This
is the range of ambient temperature environment within
which the inductor is expected to be used. 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 the insulation ratings are not
exceeded. Most inductors presently use at least 130°C
or 150°C insulation types.
If the inductor is operated at currents only slightly exceeding the saturation current rating, however, the problem
may not be so dramatic. In many cases a slight rise in
the slope of the current waveform is acceptable. Despite
the potential pitfalls, it is typically desirable to operate
As with other parameters it is important to know the
inductor temperature rise so 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.
Current (Adc)
Figure 6. L vs DC Bias Current for Coilcraft DO3316P-103
Document 469-4 Revised 05/23/16
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 which may need to
be augmented to completely describe expected performance in application conditions. Table 2 summarizes the
ratings that should appear in a power inductor data sheet.
1. Switchers Made Simple, an Expert System for the Automated
Design of DC to DC Converters using Simple Switcher Power
Converters Version 4.1, National Semiconductor.
2. Magnetics for RF, power, filter and data applications, p32,
Coilcraft Inc, Cary, IL, USA, June 2013.
3. Reference Data for Radio Engineers 6th Edition, Howard W.
Sams & Co., Inc, Indianapolis, Indiana, USA, 1975.
4. McLyman, Colonel William T., Designing Magnetic Components for High Frequency dc-dc Converters, Kg Magnetics, Inc.
San Marino, CA, USA, 1993.
Table 2. Summary of Important Inductor Ratings
Rating Should Include
•Nominal value
•Test frequency
•Test voltage
•Ambient test temperature
DCR: The wire resistance.
•Nominal with tolerance or max value
•Ambient test temperature
SRF: The frequency at which the winding self capacitance resonates with the inductance.
•Typical or nominal value
Isat: The current at which inductance drops due to
core saturation.
•Minimum or typical value.
•Definition of saturation.
Irms: The current which causes a specified amount of
temperature rise.
•Minimum value.
•Ambient test temperature.
Document 469-5 Revised 05/23/16