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Coilcraft CST2020 Series Current Sense Transformers

CST2020 Series Current Sense Transformers

Courtesy of Coilcraft : CST2020 Series Current Sense Transformers

  • Sensed current up to 40 Amps
  • For use between 400 Hz to 1 MHz
  • Very low primary resistance
  • AEC-Q200 Grade 1 (−40°C to +125°C)
  • Meets Reinforced Insulation per UL 60950-1.
  • 4000 Vrms, one minute isolation (hipot) winding to winding
  • RoHS-compliant. 260°C compatible. Tin-silver over tin over nickel over phos bronze terminations

Core material Ferrite
Terminations Tin-silver over tin over phos bronze (pins 1 – 3);
Tin-silver over tin over nickel over phos bronze (pins 4 – 5)
Weight 7 – 8.5 g
Ambient temperature –40°C to +125°C
Maximum part temperature +165°C (ambient + temp rise)
Storage temperature Component: –40°C to +165°C.
Tray packaging: –40°C to +80°C
Moisture Sensitivity Level (MSL) 1 (unlimited floor life at <30°C /
85% relative humidity)
Failures in Time (FIT) / Mean Time Between Failures (MTBF)
38 per billion hours / 26,315,789 hours, calculated per Telcordia SR-332
Packaging 100 per tray
PCB washing Tested to MIL-STD-202 Method 215 plus an additional
aqueous wash. See Doc787_PCB_Washing.pdf.

Coilcraft LPD8035V

Miniature 1:1 coupled inductors provide 1500 Vrms isolation between windings

Courtesy of Coilcraft : LPD8035V High-Isolation Miniature Coupled Inductors

LPD8035V Series Miniature 1:1 coupled inductors provide 1500 Vrms, one-minute isolation (hipot) between windings.

  • Ultra-small package size 8.0 × 6.4 × 3.5 mm
  • 1500 Vrms, one minute isolation (hipot) between windings
  • Provides significant size and cost reductions over conventional bobbin-wound alternatives
  • 13 inductance values ranging from 4.7 to 150 µH
  • Peak current ratings up to 2.7 Amps – a 40% increase over previous generation products
  • Tight coupling coefficient: K > 0.97
  • Ideal for Flyback, SEPIC and isolated-Buck converter designs
  • AEC-Q200 Grade 3 qualified (-40°C to +85°C)

Click here for full specs and free samples!

Coilcraft 0201AF

Coilcraft 0201AF (0603) Wirewound Ferrite Beads

Courtesy of Coilcraft : 0201AF (0603) Wirewound Ferrite Beads

  • Higher performance than other surface mount ferrite beads in the market
  • High impedance across a wide bandwidth; up to GHz band
  • Ferrite construction and heavy gauge wire for high current handling
  • Extremely low DCR for high current applications – as low as 0.15 Ohms
  • Eliminates high frequency noise in power supplies or RF signal isolation applications
  • RoHS Compliant, halogen free. 260°C compatible. Matte tin over nickel over
    silver-platinum-glass frit
  •  PDF data sheet

SPECIFICATIONS

What are Ferrite Beads?

Ferrite beads are used as low pass filters to eliminate high frequency noise while allowing low frequency signals or DC current to pass through a circuit. The noise may come from any number of sources including high-frequency switching noise from a power-supply circuit or RF noise in an RF signal-isolation circuit that must be minimized to ensure both signal integrity and antenna efficiency.

Ferrite beads, whether chip or wirewound, are used to filter electromagnetic interference (EMI). You may be surprised to discover that wirewound ferrite beads provide a high magnitude of attenuation over a wide frequency range, whereas traditional thick-film chip ferrite beads have limited options for both attenuation and frequency range.

Wirewound Versus Traditional Chip

In addition to offering better attenuation and frequency performance than their chip counterparts, wirewound ferrite beads also provide lower DC resistance (DCR) and higher current ratings without core saturation, resulting in the highest possible performance in the smallest size.

Wirewound versus Traditional Chip

Coilcraft wirewound ferrite beads come in standard package sizes from 0201 (0603) to 1812 (4532), all providing extremely low DCR while maintaining high filtering impedance over the broadband frequency range. These features enhance the performance of the choke circuit while potentially reducing board space by replacing a larger chip ferrite bead with an equivalent, or higher-performing wirewound ferrite bead.

Figures 2 and 3 demonstrate how Coilcraft wirewound ferrite beads provide superior broadband performance compared to both low- and high-DCR chip ferrite beads. Figure 2 compares the Coilcraft 0402DF-12 wirewound ferrite bead to the lowest-DCR 0402-sized chip ferrite bead. The 0402DF offers higher impedance across the frequency range, providing a greater measure of filtering from 100 MHz and above. Compared to typical high-DCR chip ferrite beads, the Coilcraft wirewound ferrite bead maintains the same high frequency attenuation while providing higher current ratings and 40 percent better DCR.

Wirewound versus Traditional Chip Wirewound versus Traditional Chip
Figure 2 Figure 3

What Types of Ferrite Beads does Coilcraft Manufacture?

  • Ferrite construction for higher current handling
  • Higher inductance values than other inductors
  • Low Losses for low frequency filter applications
  • RoHS Compliant, halogen free, 260°C compatible
  • Samples are available in Coilcraft Designer’s Kits
  • Click on a table row below for detailed information
    about each ferrite bead series
Series Size Max Height Impedance Range (Ohms) AEC-Q200
0201AF 0201 (0603) 0.18 mm 19.89 – 118.6 @ 100 MHz
159.9 – 1,089 @ 900 MHz
026011F 0201 (0603) 0.25 mm 91.52 – 320.5 @ 100 MHz
747.2 – 3,435 @ 900 MHz
0402DF 0402 (1005) 0.66 mm 11.98 – 5,270 @ 100 MHz
93.76 – 7,628 @ 900 MHz
0402AF 0402 (1005) 0.66 mm 11.23 – 331.3 @ 100 MHz
82.23 – 3,593 @ 900 MHz
0603AF 0603 (1608) 0.91 mm 8.83 – 3,807.7 @ 100 MHz
48.14 – 6,332 @ 900 MHz
0603LS 0603 (1608) 1.12 mm 28.21 – 5,760 @ 100 MHz
27.89 – 3,815 @ 900 MHz
Grade 3
0805AF 0805 (2012) 1.52 mm 65.61 – 7,069 @ 100 MHz
26.14 – 2,451 @ 900 MHz
0805LS 0805 (2012) 1.60 mm 48.54 – 7,765 @ 100 MHz
28.82 – 2,265 @ 900 MHz
Grade 3
1008AF 1008 (2520) 1.91 mm 6.09 – 66.51 @ 1 MHz
59.29 – 721.8 @ 10 MHz
1008LS 1008 (2520) 2.03 mm 6.56 – 682.9 @ 1 MHz
63.95 – 7,511 @ 10 MHz
Grade 3
1812LS 1812 (4532) 3.43 mm 77.85 – 7,167 @ 1 MHz
770.7 – 191,219 @ 10 MHz

Tools

Ferrite Bead Finder

Ferrite Bead Comparison

Datasheets

S-Parameter and Spice Models

Designer’s Kits

Related Application Notes

Inductors as RF Chokes

LC Filter Reference Design

Ultra Low-loss Power Inductors

These Ultra Low-loss Power Inductors are Ideal for High Switching Frequencies

Courtesy of Coilcraft : These Ultra Low-loss Power Inductors are Ideal for High Switching Frequencies

Coilcraft’s new XEL50xx Family of high-performance, molded power inductors offer exceptionally low DC resistance and ultra-low AC losses, greatly improving power converter efficiency at high switching frequencies (2 to 5+ MHz) and high ripple current. 

The XEL5020 Series is available in six inductance values from 0.10 to 1.0 µH, with current ratings up to 39 Amps. Learn more …

The XEL5030 Series has 10 values from 0.13 to 4.7 µH, with current ratings up to 44 Amps. Learn more …

Both series feature soft saturation characteristics to withstand high current spikes and have no thermal aging issues.

They are also qualified to AEC-Q200 Grade 1 standards (-40° to +125°C ambient) with a maximum part temperature of +165°C,
suitable for automotive and other harsh-environment applications.

Download datasheets and order Free Samples here!

Coilcraft XEL50xx

Coilcraft Introduces XEL50xx Ultra Low-loss Power Inductors for High-frequency Applications

Courtesy of Coilcraft
Cary, IL, USA – Coilcraft introduces its new XEL50xx Family of high-performance, molded power inductors that offer exceptionally low DC resistance and ultra-low AC losses, greatly improving power converter efficiency at high frequencies (2 to 5+ MHz) and high ripple current.XEL50xx Family inductorsThe use of high switching frequencies or high ripple current allows a corresponding lower inductance value, which results in a physically smaller part for the same electrical specs. XEL50xx Family inductors measure just 5.28 x 5.48 mm with a maximum height of 3.2 mm.
The XEL50xx Family is currently available in two models, the XEL5020 and XEL5030. A third model, the XEL5050 is expected late Q2 2018. The XEL5020 is available in six inductance values from 0.10 to 1.0 µH, with current ratings up to 39 Amps. The XEL5030 has 10 values from 0.13 to 4.7 µH and current ratings up to 44 Amps. All models offer soft saturation characteristics to withstand high current spikes and have no thermal aging issues.

XEL50xx Family inductors are qualified to AEC-Q200 Grade 1 standards (-40° to +125°C ambient) with a maximum part temperature of +165°C, making them suitable for automotive and other harsh-environment applications. They feature RoHS-compliant, tin-silver-over-copper terminations and are halogen free. Their composite construction also minimizes audible buzzing.

As with all Coilcraft parts, free evaluation samples of the XEL50xx Series are available online at www.coilcraft.com. For more information, contact Len Crane, +1-847-639-6400, lcrane@coilcraft.com.

About Coilcraft

Headquartered outside of Chicago in Cary, Illinois, Coilcraft is a leading global supplier of magnetic components including high performance RF chip inductors, power magnetics and filters. In addition to a large selection of standard components, Coilcraft also designs and builds custom magnetics to fit a customer’s exact electrical requirements.

Engineers and buyers consider Coilcraft a preferred supplier because of its reputation for quality, reliable delivery, engineering support and the superior performance of our products. In independent surveys, engineers consistently rank Coilcraft the number one magnetics company they would recommend to a friend.

Coilcraft
1102 Silver Lake Road
Cary, IL 60013 USA
+1-847-639-6400
Fax +1-847-639-1469

Common Mode Chokes

A Guide to Understanding Common Mode Chokes

Read full article on Coilcraft website here: A Guide to Understanding Common Mode Chokes – Written by Chris Hare

What is a Common Mode Choke?

A common mode choke is an electrical filter that blocks high frequency noise common to two or more data or power lines while allowing the desired DC or low-frequency signal to pass. Common mode (CM) noise current is typically radiated from sources such as unwanted radio signals, unshielded electronics, inverters and motors. Left unfiltered, this noise presents interference problems in electronics and electrical circuits.

How do Common Mode Chokes Work?

In normal or differential mode (single choke), current travels on one line in one direction from the source to the load, and in the opposite direction on the return line that completes the circuit. In common mode, the noise current travels on both lines in the same direction.

Common mode chokes have the two or more windings arranged such that the common mode current creates a magnetic field that opposes any increase in common mode current. This is similar to how single line (differential) inductors function. Inductors create magnetic fields that oppose changes in current.

In common mode, the current in a group of lines travels in the same direction so the combined magnetic flux adds to create an opposing field to block the noise, as illustrated by the red and green arrows in the toroid core shown in Figure 1. In differential mode, the current travels in opposite directions and the flux subtracts or cancels out so that the field does not oppose the normal mode signal.

How do I Choose a Common Mode Choke?

The main criteria for selecting a common mode choke are:

  • Required impedance: How much attenuation of noise is needed?
  • Required frequency range: Over what frequency bandwidth is the noise?
  • Required current-handling: How much differential mode current must it handle?

High-Speed and Super-Speed Data Line EMI Chokes

Coilcraft USBRA6870, and CM1394 high-speed and Super-Speed data line common mode chokes effectively reduce common mode noise in high-speed interfaces like USB 2.0, USB 3.1 Gen 1, HDMI, IEEE 1394, LVDS, HDBaseTTM, MOST® bus, etc. They maintain excellent signal integrity for high-speed communications with -3 dB differential mode cutoff frequencies up to 6.5 GHz. Most provide greater than 30 dB common mode attenuation at 500 MHz and 25 dB in the GHz band.

Data Line Common Mode EMI Chokes

Coilcraft CJ5100CQ7584, and CR7856 surface mount data line common mode chokes are designed to attenuate common mode noise up to 100 MHz. The PDLF Series can reduce noise by a factor of 32 from 15 MHz to 300 MHz and are available in 2, 3 and 4 line versions. The PTRF Series is optimized for FCC and ITU-T (formerly CCITT) requirements. These parts provide 15 to 25 dB attenuation, greater than 1000 Ohms impedance and 1500 V isolation between windings. M2022 can suppress common mode noise up to 500 MHz in a compact 1812 package.

Data/Power Line Common Mode EMI Chokes

Coilcraft LPDMSD and PFD Families are low profile, miniature footprint common mode chokes that can be used to attenuate common mode noise or differential mode noise in both data and power line applications.

Surface Mount Power Line Common Mode EMI Chokes

Coilcraft low-cost, high performance surface mount power line common mode chokes come in a variety of sizes and packages. They are designed to eliminate AC line-conducted common mode noise across a broad range of frequencies, with up to 1500 Vrms isolation. These common mode chokes can operate over a wide range of current from 0.06 Amps to 15 Amps, providing attenuation where line filtering is needed, such as in switch-mode power supplies.

Through-Hole Power Line Common Mode EMI Chokes

Coilcraft low-cost through-hole BU Series high efficiency choke coils are designed to eliminate line conducted common mode noise across a broad range of frequencies. The BU9S and BU9HS are ideal for signal line applications; the other BUs can be used in switching power supplies and power supply circuits. For low profile applications, the BU9 and BU9S filters are available in a horizontal configuration that reduces their height to under half an inch (12.5 mm).

CMT Common Mode EMI Chokes

Coilcraft CMT toroid style common mode chokes are designed to provide the highest common mode impedance over the widest frequency range. These parts are ideal for any application requiring a high DC current bias and are well suited for use in switch-mode power supplies. These common mode chokes are most effective in filtering supply and return conductors with in-phase signals of equal amplitude. Differential mode inductors are available for filtering out-ofphase or uneven amplitude signals.

What Types of Common Mode Chokes does Coilcraft Manufacture?

Coilcraft designs and manufactures a variety of common mode chokes for many applications. Use one of our tools to find the right common mode choke for your application.

Common Mode Choke Finder: https://www.coilcraft.com/apps/finder/cmffinder.cfm

Data Line Common Mode Chokes: https://www.coilcraft.com/prod_emi.cfm

Power Line Common Mode Chokes: https://www.coilcraft.com/prod_cmc_power.cfm

Telecom Tip and Ring Filters: https://www.coilcraft.com/ptrf.cfm

Related Application Notes

Selecting Common Mode Filter Chokes for High Speed Data Interfaces: Doc1009_cm_chokes_hi_speed.pdf

Data Line Filtering: Doc155_DataLineFilt.pdf

Common Mode Filter Design Guide: Doc191_CMFiltDesign.pdf

Common Mode Filter Inductor Analysis: Doc200_CMFiltAnalysis.pdf

LC Filter Reference Design: Doc124A_LC_Filter_appnote.pdf

Coilcraft to Announce New High-voltage Coupled Inductors at APEC 2018

Courtesy of Coilcraft

For Editorial Use:
Word Doc  |   Image
Coilcraft Press Room

Cary, IL, USA – Coilcraft will debut its new LPD8035V Series of miniature, high-voltage 1:1 coupled inductors at the Applied Power Electronics Conference (APEC) being held March 4-8 in San Antonio, Texas.

Coilcraft CST2020The LPD8035V provides 1500 Vrms, one-minute isolation (hipot) between windings from a package that measures just 7.92 X 6.4 X 3.5 mm, providing users with significant size and cost reductions over conventional bobbin-wound alternatives. It is ideal for Flyback, SEPIC and isolated-Buck converter designs.

The LPD8035V Series is currently offered in six inductance values ranging from 4.7 to 150 µH. It provides peak current ratings up to 2.7 Amps, which represents a 40% increase over previous generation products. It also has a tight coupling coefficient (≥0.97).

LPD8035V coupled inductors are qualified to AEC-Q200 Grade 3 standards (-40° to +85°C ambient), making them suitable for automotive and other high-temperature applications. They feature RoHS compliant matte tin over silver-platinum-glass frit terminations and are halogen free.

Like all Coilcraft products, complete technical specifications and free evaluation samples of the LPD8035V Series are available online at www.coilcraft.com.

For more information, contact Len Crane, +1-847-639-6400, lcrane@coilcraft.com or visit Coilcraft in booth 719 at the Applied Power Electronics Conference.

About Coilcraft

Headquartered outside of Chicago in Cary, Illinois, Coilcraft is a leading global supplier of magnetic components including high performance RF chip inductors, power magnetics and filters. In addition to a large selection of standard components, Coilcraft also designs and builds custom magnetics to fit a customer’s exact electrical requirements.

Engineers and buyers consider Coilcraft a preferred supplier because of its reputation for quality, reliable delivery, engineering support and the superior performance of our products. In independent surveys, engineers consistently rank Coilcraft the number one magnetics company they would recommend to a friend.

Coilcraft
1102 Silver Lake Road
Cary, IL 60013 USA
+1-847-639-6400
Fax +1-847-639-1469

New 0402-sized Ceramic Chip Inductors Provide Q Factors up to 162 at 2.4 GHz

Courtesy of Coilcraft
Cary, IL, USA – Coilcraft’s new 0402DC Series ceramic wirewound chip inductors offer the industry’s highest Q factors in an 0402 (1005) size – up to 162 at 2.4 GHz – for super low loss in high frequency circuits. It is offered in 26 standard inductance values ranging from 3.0 to 120 nH. An additional 73 values are available upon request, including 0.1 nH increments from 2.8 nH to 10 nH. Most values are available with 2% tolerance.Coilcraft CST2020The 0402DC Series features a wirewound construction for extremely high self resonance – up to 16 GHz. It also offers DCR as low as 37 mΩ, significantly lower than other 0402-sized chip inductors.

0402DC Series chip inductors are ideal for impedance matching circuits for low band (700 – 960MHz) and high band (1710 – 2700MHz) antennas. Its 0402-sized package is perfect for smartphones and tablets.

0402DC Series inductors feature RoHS compliant, matte tin over nickel over silver-platinum-glass frit terminations and offer a maximum reflow temperature of 260°C.

Like all Coilcraft products, complete technical specifications and free evaluation samples of the 0402DC Series are available at www.coilcraft.com. Parts are available from stock and can be ordered on-line at buy.coilcraft.com or by calling a local Coilcraft sales office.

For more information, contact Len Crane, +1-847-639-6400, lcrane@coilcraft.com.

About Coilcraft

Headquartered outside of Chicago in Cary, Illinois, Coilcraft is a leading global supplier of magnetic components including high performance RF chip inductors, power magnetics and filters. In addition to a large selection of standard components, Coilcraft also designs and builds custom magnetics to fit a customer’s exact electrical requirements.

Engineers and buyers consider Coilcraft a preferred supplier because of its reputation for quality, reliable delivery, engineering support and the superior performance of our products. In independent surveys, engineers consistently rank Coilcraft the number one magnetics company they would recommend to a friend.

Coilcraft
1102 Silver Lake Road
Cary, IL 60013 USA
+1-847-639-6400
Fax +1-847-639-1469

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Coilcraft Press Room

Current Sense Transformers Sense Current up to 40 Amps

Courtesy of Coilcraft

For Editorial Use:
English Word Document
Image
Coilcraft Press Room

Cary, IL, USA — Coilcraft’s new CST2020 Series current sense transformers sense current up to 40 Amps over a frequency range of 400 Hz to 1 MHz and offer 4000 Vrms isolation voltage between the sense and output windings. They provide Reinforced Insulation per UL 60950-1.

Coilcraft CST2020CST2020 Series current sense transformers provide output feedback for load current measurement and control in switching power supplies and overload/short-circuit protection. They are qualified to AEC-Q200 Grade 1 (-40°C to +125°C) standards, making them ideal for automotive applications like current measurement in traction motor and battery management systems. They are also well suited for use in 48V vehicle systems. Other applications include aerospace power management systems, three-phase solar inverters, industrial motor controls and other applications requiring high isolation between the sense and output windings.

The CST2020 Series is available with four turns ratios ranging from 1:70 to 1:300, and offers very low sense resistance (0.00084 Ohms).

As with all Coilcraft parts, free evaluation samples are available online at www.coilcraft.com. Parts are available from stock and can be ordered online at buy.coilcraft.com or by calling Coilcraft sales.

For more information about the CST2020 Series, contact Len Crane, +1-847-639-6400, lcrane@coilcraft.com.

About Coilcraft

Headquartered outside of Chicago in Cary, Illinois, Coilcraft is a leading global supplier of magnetic components including high performance RF chip inductors, power magnetics and filters. In addition to a large selection of standard components, Coilcraft also designs and builds custom magnetics to fit a customer’s exact electrical requirements.

Engineers and buyers consider Coilcraft a preferred supplier because of its reputation for quality, reliable delivery, engineering support and the superior performance of our products. In independent surveys, engineers consistently rank Coilcraft the number one magnetics company they would recommend to a friend.

Coilcraft
1102 Silver Lake Road
Cary, IL 60013 USA
+1-847-639-6400
Fax +1-847-639-1469

Selecting the best inductor for a DC/DC converter

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

Courtesy of www.electronicproducts.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.

Learn more about Coilcraft