NSEC Symposium (Presentation III 07 THA PhraNakhonSiAyutthaya)
Hung Tha Chee 136 Presentation
-
Upload
johnvarghesee113 -
Category
Documents
-
view
219 -
download
0
Transcript of Hung Tha Chee 136 Presentation
-
7/29/2019 Hung Tha Chee 136 Presentation
1/43
Inductors and Chokes In Switch
mode Supplies
Thach, Hung
12/06/03EE136
-
7/29/2019 Hung Tha Chee 136 Presentation
2/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
3/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
4/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
5/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
6/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
7/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
8/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
9/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
10/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
11/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
12/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
13/43
Core Area Product Graph
-
7/29/2019 Hung Tha Chee 136 Presentation
14/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
15/43
From this permitted maximum resistance,the wire gauge, turns, and inductance can
be established.
-
7/29/2019 Hung Tha Chee 136 Presentation
16/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
17/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
18/43
Graphical Design of ACommon-Mode-Line-Filter
Inductor (using a Ferrite Ecore)
-
7/29/2019 Hung Tha Chee 136 Presentation
19/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
20/43
Nomogram for establishing wire size
for chokes in ferrite material, as a
function of turns and core size
-
7/29/2019 Hung Tha Chee 136 Presentation
21/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
22/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
23/43
Calculating Inductance(for common-mode
inductors wound on Ferrite
E cores)
-
7/29/2019 Hung Tha Chee 136 Presentation
24/43
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:
-
7/29/2019 Hung Tha Chee 136 Presentation
25/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
26/43
CHOKES
Inductors with DC Bias Current
-
7/29/2019 Hung Tha Chee 136 Presentation
27/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
28/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
29/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
30/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
31/43
Area Product
It is the product of the winding windowarea and the core center pole area.
In general, AP = Aw * Ac cm ^ 4
-
7/29/2019 Hung Tha Chee 136 Presentation
32/43
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 :
-
7/29/2019 Hung Tha Chee 136 Presentation
33/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
34/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
35/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
36/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
37/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
38/43
Design Examples of aGapped Ferrite E-core
ChokeUsing an Empirical Method
-
7/29/2019 Hung Tha Chee 136 Presentation
39/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
40/43
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
-
7/29/2019 Hung Tha Chee 136 Presentation
41/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
42/43
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.
-
7/29/2019 Hung Tha Chee 136 Presentation
43/43
References
Switchmode Power Supply Handbook 2nd
edition authored by Keith Billings published by
McGraw Hall. (chapter 3.1)