Wurth Elelctronics Product Training Module about coupled inductors ...

Welcome to Wurth Elelctronics Product Training Module about coupled inductors for the
use in SEPIC converters. The PTM will explain the functionality of a SEPIC converter
and how to design coupled inductors into such a circuit, as well as which coupled
inductor families are available from Wurth Electronics.
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Before going in detail about couple inductors for SEPIC converters, a SEPIC
converter must be defined. SEPIC stands for Single Ended Primary Inductance
Converter, and this circuit offers the possibility to use the function of a buck and
boost regulator in one circuit
A buck converter regulates a high voltage into a smaller voltage, so input voltage is
greater than output voltage. For example, like a car charger for a cell phone, 12V is
converted down to 5V.
A boost converter regulates a small voltage into a higher voltage, so input is smaller than
output voltage. As an example: an LED driver with a 3V input converts to a 9V output.
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Typical SEPIC applications are battery operated equipments and handheld devices
powered by a battery. In addition, SEPIC is being used in NiMH charger applications, as
well as, LED Drivers & DC Power Supplies with a wide range of input voltages.
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On this slide, you can see a typical layout of a SEPIC converter from Linear Technology
which uses two 22µH inductors, or L1 and L2, in the circuit. The capacitor, C6, basically
splits the windings in a primary and a secondary. This design can now be done like
shown in this example by using 2 individual inductors with 22µH, but there are clear
advantages when using one of Wurth Electronics coupled inductors in this design. The
construction and the advantages of a coupled inductor will be shown on the next slides.
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A coupled inductor basically consists of two identiical windings wound onto one core.
The important criteria is that the windings are exactly identical to generate the coupling
effect in a SEPIC converter.
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From an economical standpoint, the use of coupled inductors saves cost and reduces
the size of the BOM. It also helps to save actual PCB real estate, due to the usage of
one component with 2 integrated windings. From a technical standpoint the advantages
of a coupled inductor are as follows:
If L1 and L2 are closely coupled, the ripple current is divided between them , and the
required inductance is halved. In some applications, there is even close to zero ripple
current due to the use of a coupled inductor. This also results in a simpler EMI filtering
and smaller input capacitor. In addition, the DCR of the inductors can also be reduced.
For example, two 47µH single inductors can be replaced with one coupled inductor
of 22µH per winding, potentially allowing for the selection of a smaller inductor, or one
the same size that has lower DCR and higher current handling.
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Wurth Electronics‘s WE-DD series is one of the industry‘s most used coupled inductor
families. The great advantages of this family are the flexibility in size. The WE-DD series
offers two standard footprints: 7x7 and 12x12mm, each with different heights to find the
right part by taking maximum heights, max currents or targeted DCR resistances in
measurement. The WE-DD series is available with a crossed, as well as a straight, pin
layout. An overview is shown on the next slide.Wurth Electronics‘s WE-DD series has
maximum current handling capabilities of 8.6A DC current and up to 18A in saturation
current. Besides in SEPIC converters, the WE-DDs can also be used in CUK topologies,
non-isolated flyback applications and switching regulators with a second, unregulated
output voltage.
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As mentioned previously, the WE-DD series is available in 7x7mm and 12x12mm, with a
staight-pin layout, and in the size 12x12mm with a crossed layout. The parts with a
crossed layout have a higher leakage inductance due to the winding structure, where the
straight-pin layouts have a lower leakage. The need of low or high leakage is defined by
the IC used in the SEPIC convterter. It is recommended to follow the winding layout
suggested on the reference design of the IC manufacturer.
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In addition to the WE-DD, Wurth Electronics is offering a new generation of low-profile
coupled inductors made for applications where board space or height is restricted.
Besides the low profile, other key features of the WE-TDC series are low self losses, low
leakage inductances, and high mechanical stability due to the 4 large landing pads
which get soldered to the board. The WE-TDC has current handling capabilities of 4.5A
DC current and up to 14A in saturation current. In addition to SEPIC converters, the WE-
DDs can also be used in CUK or ZETA topologies, non-isolated flyback applications and
switching regulators with a second unregulated output voltage.
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The WE-DCT series is Wurth Electronics‘s series for larger-current SEPIC applications
and finds its usage often also as a 1:1 transformer. The very low RDC and core losses
go back to the specially trimmed WE-PERM core material, which has proven its
outstanding performance in other high current inductor families of Wurth Electronics.
Due to the WE-PERM material, the WE-DCT has a very stable (soft) saturation behavior,
which guarantees, even at max conditions, a great performance. The WE-DCT has
current handling capabilities of 14.5A DC currents and up to 120A in saturation current.
Besides in SEPIC converters, the WE-DDs can also be used in CUK topologies, solar
inverters and high-power switching regulators.
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In order to design an effecient SEPIC converter, it is crucial to design the right inductor
into your circuit. It is best to do it with a practical example:
The SEPIC converter, in our case, has a flexible input of 5-24V and needs to deliver a
stable 12V output. The current output is 0.7A with a calculated ripple of 20%.
Of course, there are always 2 ways to calculate the inductor for the SEPIC converter.
First is to calculate the formula of an inductor for switching regulators.
Wurth Electronics recommends calculating the SEPIC converter in the Inductor Selector
Calculation Tool for DC/DC Converters. That process is shown in the next slides.
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The Wurth Electronics Component Selector is an all-in-one magnetics design tool for
calculating ceramic inductors for RF circuits, chip bead ferrites for filter applications,
transformers for DC/DC flyback topologies, and inductors for DC/DC converters. You
can easily access the tool at www.we-online.com/toolbox or by downloading it straight
from the ‚tools & support‘ tab of Wurth Electronics supplier portal on
www.digikey.com/wurth.
After opening the software tool just click on Inductor Selector.
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A new design window opens up, and by clicking on ‚settings‘, a new DC/DC converter
topology opens. Simply select the correct topology (SEPIC Converter, in this example),
and enter values of the SEPIC converter. Before clicking on ‚apply‘, select upfront if the
calculation should be done with single inductors or one coupled inductor. In this example
we use ‚Coupled L‘. Then click ‚apply‘.
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After calculating the SEPIC converter topology, the program suggests the inductors
which fit best electronically into the circuit. The list can then be further optimized by
taking DCR or mechanical dimensions into consideration. Below, the example with a
coupled inductor pre-setting shows an example using two individual inductors. The
inductance value of the coupled inductor is exactly half of the individual inductor. In this
example, it is clearly shown that by choosing a coupled inductor, the DCR can be
reduced significantly (67mOhm vs. 2x72mOhm). All products shown in the inductor
selector tool are catalog products. For Wurth Electronics, this means available in stock,
short lead times for production volume and outstanding quality. All products are also
stocked at Digi-Key.
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In Summary, a SEPIC converter is used in applications with a flexible input voltage
range, which needs a stable output of a certain voltage. Such applications are battery
operated devices, NiMH chargers and LED driver applications. Wurth Electronics is
offering a full range of coupled inductors for such applications: WE-DD, WE-TDC and
WE-DCT series. All products shown are available in stock at digikey.com.
Additionally, Wurth Electronics offers an easy-to-use design software for magnetics
design, which allows calculation of a coupled inductor for a SEPIC converter within
seconds.
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