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Choosing the Inductor for a Buck Converter

In this article, Dr. Ridley examines how the value of a buck inductor should be
turn out to be much more complicated than expected, and the range of allowable induc

Design rules for choosing the inductor

Five or six times a year, I teach a class in power supply design to 24 working engineers
involves a buck converter, and the design starts with the choice of the inductor value. I
inductor should be used, or how much ripple current should be in the inductor. (Ripple c
the peak-to-peak value of the inductor current at high line, divided by the maximum loa
from a value of 10% up to perhaps 30%.
A traditional value of inductor current ripple is 10%, and you will find this in several boo
a starting point for design, but does suggest at the end of the article that the value can
desired output ripple. Further examination of literature shows a huge range of
suggest ranges from 2.5% to 50%.

References [14, 15] select the inductor according to the minimum load, with
continuous-conduction mode (CCM). This can lead to a very large inductor if the light lo

Which one of these is the correct value?

As is often the case in designing power supplies, there is no one correct answer and it
specific converter that you are working on at the moment.

Where do the rules come from?

We usually find in power electronics that design “rules-of-thumb” arise from some pract
several different factors that drive the choice of inductor value.
1. Output Ripple Voltage – older output capacitor types tend to drive inductor values t
innovations in capacitor design have driven the ESR down to very low values, and rare
factor in choosing the inductor.

2. Inductor Loss – Higher ripple current leads to higher RMS current in the
proximity losses. Core losses will also increase with larger ripple current. A higher value
higher dc conduction loss.

3. Switch Conduction Loss – the RMS current in the switch climbs with inductor curre

4. Rectifier Conduction Loss – the RMS current in the rectifier climbs with inductor cu
factor when using a synchronous rectifier, but less important when using a

5. CCM operation – References [14, 15] choose the inductor to give a ripple which is
avoid DCM operation at light load. In the early days of power supply design, this was
transient performance under all conditions. With a modern power supply, using current
all to keep the converter in CCM operation.

Buck Converter Design Example

Searching deeper into the literature than the short list included with this article will not le
regarding the right value of inductance. For every converter design that you do, the
very specific set of circumstances. And for each specific case, a detailed design must b
coming to any conclusions about the right value.

For example, Figure 1 shows a specific design case. The switching frequency

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output specification is 5A at 12 V from a 18-36 V input. The output capacitor is preselec

The choice of the output capacitor can be as wide ranging as the choice of the inductor
more detail in this article due to space constraints.

Buck Converter with Parameter Values

The ripple current in the converter is maximum at high line, and the value of
inductance of 160 µH which gives a current ripple ratio of 10%.

Inductor Current Waveforms 10% Ripple.

The peak-to-peak value of the current is 0.5 A, and this results in a 5 mV ripple on the o
practical design cases, the output ripple at the switching frequency is not the main drive
much lower than is needed. It is almost always far lower than the high-frequency switch
There nothing special about choosing the 10% ripple value for Figure 1. It is just one va
used. For each value, we must optimize the design of the inductor before we can really
This can be a time consuming process, but for this study, it was automated using the d

The inductor designed for this case had the following practical design constraints applie
inductor:
1. Magnetics Inc. RM8 core with R material was used.
2. Turns were set to the nearest integer value.
3. Maximum flux level was designed for 0.3 Tesla, with 10% overcurrent
4. Maximum wire size was 20 awg (0.9mm diameter) to limit mechanical stress on bo
5. Multiple strands of wire and multiple layers were used as appropriate.
6. Winding loss includes proximity loss as predicted by Dowell’s equations
7. Core losses include advanced modeling techniques [17].

There are hundreds of cores that could have been chosen, but the RM8 was convenien
necessarily be the optimal choice since the definition of “optimal” varies for every user.

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Table I shows the results for designs, with a range of inductance from 640
ratio ranging from 2.5 % (as in the example of [11]) up to 200%, the boundary of discon
load.

L Ripple Turns DCR ACR Winding

(µH) Ratio Ω Ω Loss (W)

640 2.5 221 1.7 53.8 43

160 10 58 0.112 5.5 2.94
80 20 31 0.30 1.5 0.88
40 40 17 0.10 0.4 0.41
20 80 10 0.006 0.14 0.23
10 160 7 0.003 0.03 0.24
8 200 6 0.002 0.02 0.25

Inductor and Switch Loss with Different Ripple Values Using RM8 Core

It is clear from the table that for this design case, with the chosen core size, the largest
with a loss of 43 W in the inductor windings. The 10% ripple case also has a higher dis
designs from 40% ripple to 200% ripple were all excellent.

It is interesting to see that the big change in ripple from 10% to 200% (20
conduction loss of the power FET. The increase in the RMS current value does not cha
expect.

Core losses were low for all designs, as would be expected when using a
operate at high ripple current without a big penalty. (This would not be true for many of
in the standard component designs.) Overall, the overall lowest loss value of inductor g
current waveform for this case is shown in Figure 3.

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Inductor Current Waveforms 80% Ripple.

Summary

Choosing the proper value of an inductance can have a tremendous impact on

converter. There is, however, no single specific value that is correct for all converters. T
freely selected over a practical range from 10% to 200%. In every power supply, the sp
design choice will be unique, and the optimal value for the inductor must be found by tr
testing the designs in the circuit.

The design example given in this article had an optimal ripple of 80%. If I had to choose
started, I would agree with Reference [2] and begin with a ripple factor of 40%, but with
not a problem to move significantly away from this number, and the number should not

Other Recommendations
1. Jerry Foutz, “Switching-Mode Power Supply Design Tutorial
Simple Switching Topologies”, http://www.smpstech.com/tutorial/t03top.htm
2. Sanjay Maniktala, “Switching Power Supplies A to Z”, 40% ripple.
Semiconductor Vendors
3. Micrel, “MIC4574 application note”, http://www.micrel.com/_PDF/mic4574.pdf
(Figure 1).
4. GMOS Technology Corporation, “GT1512 Datasheet”, 15% ripple.
5. Microchip, “MCP1612 Datasheet”, 16.5% ripple.
6. Texas Instruments, “TPS4000 controller reference design”, 25% ripple.
7. Analogic, 30-40% ripple.
8. Sanjaya Maniktala, National Semiconductor, “Current Ripple Ratio Simplifies Selec
Buck Converters” http://powerelectronics.com/mag/power_current_ripple_ratio/
9. Maxim application note – 30% ripple.
10. National Semiconductor, “Selecting Inductors for Buck converters”,
40% ripple.

Magnetic Component Vendors

11. Douglas R. Kokesh, Tyco Electronics Power Components, CoEv Magnetics Grou
to employment of switch mode power supplies”, http://www.designfax.net/archives/0304
ripple.
12. Coiltronics, 10-30% ripple.
13. GB International Custom cores, “Using GBI’s 4400 Series to Design Buck Regula
http://www.gbint.com/Files/Apps/General Apps/GB-4400-001.htm, 2 x minimum load.
14. Wurth Electronics, “Power Inductors 8 Design Tips”, 30% ripple.
15. Jim Holdahl and Terry VanConant, CoEv, “Demystifying Buck Inductors
http://powerelectronics.com/mag/power_demystifying_buck_inductors, 2 times
Magentics Design Example References
16. Ray Ridley, “Proximity Loss in Magnetics Windings”, www.switchingpowermagaz
Loss.pdf
17. Ray Ridley and Art Nace “Modeling Ferrite Core Losses”, www.switchingpowerma
Ferrite Core Losses.pdf
18. Ridley Engineering, “Power 4-5-6 Simulation and Design Software”,

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