Hung Tha Chee 136 Presentation

7/29/2019 Hung Tha Chee 136 Presentation

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Inductors and Chokes In Switch

mode Supplies

Thach, Hung

12/06/03EE136


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TERMS

Inductors is reserved for woundcomponents which DO NOT carry DC

current.

Chokes will be used for woundcomponents that carry a large DC biascurrent, with relatively small ac ripplecurrent.


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Design Approach

It will depend on the application.

It is often a compromise, with emphasis

being placed on: 1) Minimum cost

2) Minimum size

3) Minimum loss


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Switch Mode Classification(Inductors)

Inductors will normally be confined to lowpass filters.

Their function is to prevent the conduction

of high frequency noise back into thesupply lines.

For this application, high core permeability

would be an advantage.


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Chokes

They will be found in high frequencypower output filters and continuous-modebuck boost converter transformers.

In these applications, low permeability anda low high-frequency core loss would benormally be considered an advantage.


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Problems

To minimize the number of turns andcopper loss, it might be assumed that ahigh-permeability core material with a low

core loss would be the most desirable. In choke design, the large DC current

component and the limited saturation flux

density of real magnetic materials forcethe selection of a low-permeabilitymaterial or introduction of an air gap inthe core.


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As a result of low effective permeability,more turns are needed to obtain the

required inductance.

So, in choke design, the desired lowcopper and high efficiency are

compromised by the need to support alarge DC current.


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Simple Inductors In power supply applications, pure inductors

(those which do not carry a DC componentor a forced high-current ac component) arerare.

The design of these inductors is relativelyeasy (the inductance may be obtained bythe AL value provided for the core, because

no gap is required.

L = N X AL


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Common-Mode Line Filter Inductors

Common-mode filter inductors have two isolatedwindings with the same number of turns. The 2 windings are connected so that the

magnetic field that results from normal series-mode ac supply currents will cancel to zero.

The only inductance presented will be leakageinductance between the two windings.

The low-frequency line current will not saturatethe core, and a high permeability material maybe used without the need for a core air gap.

Large Inductance can be obtained with a fewturns.


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For common-mode noise (noise currentsor voltages which appear on both lines at

the same time with respect to theground), the 2 windings are in parallel andin phase, and a very high inductance is

presented to common-mode currents.


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Design Example

In this example, it will be assumed that themaximum Common Mode Inductance is requiredfrom a specified core size, using a high-permeability ferrite E core.

The effective DC or Low-frequency ac current inthe core is zero as a result of using 2 equallyopposed and balanced winding.

Core loss is assumed to be negligible becausethe design is to obtain the maximum possibleinductance at the working current from aparticular core size.


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Core Size

We want to select a core size that suits themechanical size requirement.

Then calculate the area product (AP). The areaproduct is the product of the core area and theusable winding window area.

Refer to the core area product graph to obtain

the thermal resistance of the finished inductor.

AP = ACP X AWb cm^4


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Core Area Product Graph


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Winding Dissipation

Now we need to calculate the permittedwinding dissipation W that will give anacceptable temperature rise T.

Then we can obtain the winding resistanceRw at the working (rms) current I.

Assuming zero core loss,

W = T / Rth WRw = W / l W


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From this permitted maximum resistance,the wire gauge, turns, and inductance can

be established.


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Establishing Wire Size, Turns, and

Inductance Many manufacturers provide informationon the resistance and maximum numberof turns of a fully wound bobbin using

various wire gauges.

The AL factors for the core are oftenprovided, from which the inductance can

be calculated.

With balanced windings, there is no needfor an air gap.


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A nomogram is used to, from which thewire gauge, turns, and resistance of the

wound component can be read directly.An inductor wound following the

preceding steps provide the maximum

inductance possible on the selected coresize, at the maximum rated current andselected temperature rise.


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Graphical Design of ACommon-Mode-Line-Filter

Inductor (using a Ferrite Ecore)


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Assumptions

1) EC35 Core is being used to provide the

maximum inductance for a CML filter inductor2) Temperature rise does not exceed 30C

3) Input Current is 5 A rms.

N f t bli hi i i


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Nomogram for establishing wire size

for chokes in ferrite material, as a

function of turns and core size


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The AP of the EC35 is 0.7 (when bobbin is

used).

With AP = 0.7, the thermal resistance is

20C/W.

The dissipation for a temperature rise of 30Cwill be :

Power = T / Rth = 30 / 20 = 1.5 W


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At a Current of 5 A rms, the maximum resistance willbe related to power.

P = IR

R = 1.5 / 25 = 0.06 W

By looking at the 2nd nomogram, you can see that

0.06 W will give you about 56 turns and wire gaugeof about 17 (of AWG).

Note: In common-mode inductor, the winding will besplit into 2 equal parts. Hence, the EC35 bobbin

would be wound with 2 windings of 28 turns of #17AWG


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Calculating Inductance(for common-mode

inductors wound on Ferrite

E cores)


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In dual winding, common-modeinductors, the series-mode line

frequency or DC magnetization forcewill cancel out. High permeabilitycore may be used and a core gap is

not required. For the previous example, AL value

for the EC35 w/o an air gap is

approximately 2000nH. The inductance for each 28-turn

winding can be calculated as follows:


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L = NX AL

For the previous exampleL = 28X 2000E-9 = 1.57 mH

Note: This graphical design approachalso gives the maximum common-mode inductance that can be

obtained from this core at 5 A for atemperature rise of 30C.


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CHOKES

Inductors with DC Bias Current


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Brief Review

Chokes (inductors which carry a large

component of DC current).

They are found in some form in all switch

mode supplies.

Chokes range from small ferrite beads

used, for example, to profile the base drive

currents of switching transistors, up to the

very large high-current chokes used in

power output filters.


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Design Considerations

1) Core Material

2) Core Design

3) Core Size

4) Winding Design

Note: Since this subject is very broad, this

discussion will be confined to those types ofchokes most often used in high-frequency switch

mode applications.


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Core Material

The Core material is chosen to suit thefollowing conditions:

1) The Operating Frequency

2) Ratio of DC to ac Current

3) Inductance

4) The mechanical requirements


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Core Size

Often the most difficult choice is the coresize and configuration.

There are many different core topologiesthat exist, so it may be difficult to decidewhich would be the optimum choice for aparticular application.

The Area Product (AP) tends to be areasonable constant for all core topologiesof the same general power rating, and this

can be used for the core size.


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Area Product

It is the product of the winding windowarea and the core center pole area.

In general, AP = Aw * Ac cm ^ 4


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Temperature Rise

The temperature rise of the woundcomponent in free air cooling conditionswill depend on the total loss in the wound

component and the comment's surfacearea.

The actual temperature rise, DT, that maybe expected from a particular core size APis given by :


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DT = P * Rt

DT temperature rise, C

P the total dissipation, W

Rt thermal Resistance, C/W

Note: In choke design, the loss P will bemainly copper loss. Core Losses are smallin most cases, and may be neglected.


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Core Air Gaps

If considerable DC currents flow in thechoke, the use of gapped E or C coresmay be considered.

Since chokes will normally be required tosupport the DC component w/o saturation,relatively large air gaps are used, and the

effective permeability, irrespective of thematerial chosen, is usually very lowaround 10 and 300


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Ferrite material saturates at lower fluxdensity than iron, even when gapped.

To prevent saturation, a large gap mustbe used in the core, resulting in a lowereffective permeability and giving lower

inductance. The higher saturating flux density of iron

core permits a smaller gap, giving a larger

permeability for the same DC biasconditions; hence inductance is greater,and the ripple current will be smaller.


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A further advantage of the gapped E or Ccore is that the effective permeability can

be optimized for the application byadjusting the gap size for the mosteffective performance.


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Conclusion

The core size depends on the total lossand the permitted temperature rise.

The copper loss depends on the DC

current, turns, and wire size. The core loss, and hence the choice of

material, depends on the ac volt-seconds

that the choke must withstand, that is, theflux density swing DB and the operatingfrequency.


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Design Examples of aGapped Ferrite E-core

ChokeUsing an Empirical Method


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Assumptions

The choke is required to support a large DC currentwith considerable high-frequency ripple current

A low-core loss material is used

An air gap is required The maximum core size is defined by the mechanical

rather than the ideal electrical needs.

Note: Typical applications would be an output filterinductor for a high-frequency forward converter. DCcurrent is 10 A and ripple current does not excced 3 Aat 100Khz


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1) Select core and bobbin size (as defined by

mechanical needs), and completely fill the

bobbin with a gauge of wire that will giveacceptable Power loss and hence acceptable

temperature rise.

2) Assemble core and bobbin, allowing adequateair gap.

3) Fit the choke in the power filter position in

the supply and observe the choke ripple currentwaveform.

4) Adjust the air gap under maximum load and


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4) Adjust the air gap under maximum load and

input voltage conditions until a minimum ripple

current is observed.

Note: By this, maximum dynamic inductance has

now been obtained.


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Presentation Conclusion

This presentation was about mainly designing

basic Inductors and a brief explanations about

how basic chokes are designed.

I would like to thank Dongsheng Zhou, Ph.D.

for giving us the opportunity to learn about

something interesting.


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References

Switchmode Power Supply Handbook 2nd

edition authored by Keith Billings published by

McGraw Hall. (chapter 3.1)

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Hung Tha Chee 136 Presentation