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Division of Spang & Company Technical Bulletin BULLETIN SR-1A INDUCTOR DESIGN
IN SWITCHING REGULATORS Better efficiency, reduced size, and lower costs have combined to make the switching regulator a viable method for converting unfiltered DC input voltages into regulated DC outputs. This brochure describes the switching regulator and prents design information. In particular, MAGNETICS®Ferrite and Molypermalloy Powder cores ud for the power inductor are highlighted.
DESCRIPTION
A typical circuit consists of three parts: transistor switch, diode clamp, and an LC filter. An unregulated DC
voltage is applied to the transistor switch which usually operates at a frequency of 1 to 50 kilohertz. When the switch is ON, the input voltage, E in, is applied to the LC filter, thus causing current through the inductor to increa; excess energy is stored in the inductor and capacitor to maintain output power during the OFF time of the switch. Regulation is obtained by adjusting the ON time, t on, of the transistor switch, using a feedback system from the output. The result is a regulated DC output, expresd as:
E out=E in t on f
COMPONENT SELECTION
The switching system consists of a transistor and a feedback from the output of the regulator. Transistor lection involves two factors—(1) voltage ratings should be greater than the maxi-mum input voltage, and (2) the frequency cutoff characteristics must be high compared to the ac-tual switching frequency to insure efficient opera-tion. The feedback circuits usually include opera-tional amplifiers and comparators. Requirements for the diode clamp are identical to tho of the transistor.
The design of the LC filter stage is eas-ily achieved. Given (1) maximum and minimum input voltage, (2) required output, (3) maximum allowable ripple voltage, (4) maximum and mini-mum load currents, and (5) the desired switching frequency, the values for the inductance and ca-pacitance can be obtained. First, off-time (t off) of the transistor is calculated.
t off = (1 - E out/E in max) / f(2)
When E in decreas to its minimum value,
f min= (1 - E out/E in min) / t off(3)
With the values, the required L and C can be calculated.
Allowing the peak to peak ripple current (∆i) through the inductor to be given by
∆i = 2·l o min(4)
the inductance is calculated using
L = E out t off / ∆i(5)
The value calculated for∆i is somewhat arbitrary and can be adjusted to obtain a practical value for the inductance.
The minimum capacitance is given by
C = ∆i/8f min∆ e o(6) Finally, the maximum ESR of the capacitor is
ESR max =∆ e o /∆ i(7)INDUCTOR DESIGN
Two different types of core materials are com-monly ud for the inductor in a switching regula-tor— Powder Cores and Ferrite Cores. It is diffi-cult to recommend one material over the other since the designer must take into consideration factors such as cost, volume, size and space limi-tations, and winding capabilities. Each material type has advantages as described below.
MAGNETICS POWDER CORES have a dis-tributed air gap structure, making them ideal for switching regulator applications. This structure gives a soft saturation characteristic that has many design benefits, including an overall smaller core size, and overcurrent protection. It also alleviates the fringing flux difficulties which occur if a dis-crete gap design is ud. The DC bias character-istics of Powder Cores allow them to be ud at high drive levels without saturating. They are avail-able as
toroids in three different materials: Molypermalloy (catalog MPP-400), High Flux (catalog HFC-1.1), and Kool Mu®(catalog KMC 2.0). E-core shapes are also available in Kool-Mu material (bulletin KMC E1). A wide range of sizes and permeabilities exists to meet the needs of di-ver applications.
FERRITE CORES offer the advantages of de-cread cost and low cores loss at high frequen-cies. For switching regulators, power ferrite mate-rials (F, P, R, and K) are recommended becau of their core loss and DC bias characteristics. By adding discrete air gaps to the ferrite shapes, the cores can be ud efficiently while avoiding saturation. Magnetics produces many sizes and shapes to suit a variety of needs. Hardware is also available for most parts. Detailed descriptions are covered in the Magnetics Ferrite Cores Catalog FC-601.
CORE SELECTION PROCEDURE
The core lection procedures simplify the design of inductors for switching regulator appli-cations.
For Powder Cores:
One can determine the smallest core size, as-suming a maximum decrea in inductance of 50% and wire current carrying capacity of 500 circular mils per ampere.
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Only two parameters of the design application must be known:
(1) Inductance required with DC bias,(2) DC current.In this bulletin, Molypermalloy Powder Cores are featured in the examples. However, this de-sign procedure can be ud for any of the Powder Core types, including the Kool Mu E-cores. Sim-ply refer to the design charts and data within tho catalogs for the material and shape of choice.1.2.Compute the product of LI² where:
L= minimum inductance required with DC bias (millihenries)
I= maximum DC output current = I 0 max + ∆ i.Locate the LI² value on the Core Selection Chart.The Molypermalloy DC bias Core Selector Chart on page 5 will quickly yield the optimum permeability and smallest core size for most switching regulator applications. This chart is bad on a permeability decrea of no more than 50% with DC bias and typical winding fac-tors of 40%. Follow
this coordinate to the inter-ction with the first core size that lies above the solid permeability line. This core size is the smallest that can be ud.
N= 1000
The optimum permeability for that coordinate can also be read from the solid permeability line. For most applications with a given LI²value, the permeability indicated will result in the smallest core size possible. This is due to the tradeoff between the quicker DC bias roll-off of a higher permeability core versus the ad-ditional windings that would be necessary for a lower permeability core.3.4.5.Inductance, core size and permeability are now known. The A L (millihenries per thousand turns)can be obtained from catalog MPP-400. With this information, calculate the number of turns needed to obtain the required inductance. This number will then have to be adjusted for the DC bias roll-off. The procedure for this adjust-ment is described within each of the Magnetics Powder Core catalogs.Choo the correct wire size using the Wire Table located within the catalog.For Ferrite Cores:
1. Compute the product of LI² where:
L= minimum inductance required with DC bias (millihenries)
I= maximum DC output current = l 0 max + ∆ i.
2.3.4.5.Locate the LI² value on the Ferrite Core Selec-tion charts on pages 6 and 7 in this bulletin (also located in the Magnetics Ferrite Cores Catalog FC-601). Follow this coordinate to the interc-tion with the first core size curve for the ferrite shape of choice. Read the maximum nominal inductance (A L ) on the Y-axis. This reprents the smallest core size and maximum A L at which saturation will be avoided.Any core size line that intercts the LI² coordi-nate reprents a workable core for the induc-tor if the core’s A L value is less than the maxi-mum value obtained from the chart. It is impor-tant to remember that the A L value for a gapped ferrite core comes with a tolerance range, and therefore the high end of the tolerance range should be noted to ensure that it is not higher than the A L value obtained from the chart.Required inductance L, core size, and core nominal inductance (A L ) are known.Calculate the number of turns using
Where L is in millihenries.
无错号之虞Choo the wire size from the wire table in Catalog FC-601 using 500 circular mils per amp.Choosing a Powder Core Material
题破山寺后禅院翻译
While Molypermalloy and Ferrite cores are fea-tured in this brochure, Magnetics Kool Mu and High F
lux powder cores are also excellent for inductor applications. Kool Mu cores offer an economical advantage over all three types. The Kool Mu E-core shape may be chon over the traditional toroidal powder core shape for ea of winding considerations. High Flux cores offer the most energy storage capability, resulting in the small-est possible core size when saturation is the limit-ing factor. Molypermalloy cores offer the widest lection and have the lowest core loss of any powder core. Depending on specific circuit require-ments, temperature considerations, etc., size and other advantages of one type over others may be realized. A detailed LI² chart for Molypermalloy cores appears on page 5; the LI² graphs for High Flux and Kool Mu toroid cores are shown in their respective catalogs. For more information, refer to the appropriate catalog.
MAGNETICS • BUTLER, PA
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DESIGN EXAMPLE
Choo a core for a switching regulator with the following requirements:
E o= 5 volts
∆ e o= .5 volts
I o max= 6 amps
I o min= 1 ampare you around
E in min= 25 volts
E in max= 35 volts
f= 20 KHz
fresh1.Calculate the off-time and minimum switching,
f min,of the transistor switch usin
g equations
2 and 3.
千峰培训t off = (1 — 5/35)/20,000 = 4.3 x 10-5
conds and
f min = (1 — 5/25)/4.3 x 10-5 conds =
18,700 Hz.
2.Let the maximum ripple current,∆i, through联系方式的英文
the inductor be
∆i = 2 (1) = 2 amperes
by equation 4.
3.Calculate L using equation 5.
L = 5(4.3 x 10-5) /2 = .107 millihenries
4.Calculate C and ESR max using equations 6
and 7.
C=2/8 (18700) (.5) = 26.7 µ farads and
ESR max = .5/2 = .25 ohms
5.The product of LI² = (.107) (8)² =
6.9 millijoules Molypermalloy Powder Core Design
6.Locate the LI² value of 6.9 millijoules on the
Core Selector Chart. Following this value to the interction with the first core size that lies above the permeability line gives a core size number of 55585. (Note that part numbers in this chart are referenced to 125µ).(a)
(b)
(c)
(d)
(e)
7.The 6.9 coordinate falls in the 60µ ction. The
60µ core that is the same size as the 55585 core is the 55586-A2 core (e the Molypermalloy Powder Cores catalog MPP-400).8.
without considering DC bias is 1000 • (0.107/
34.96)½ = 56.
The 55586-A2 core has a minimum inductance of 34.96 mH/1000T (nominal inductance is 38 +/- 8% mH/1000T). The number of turns needed to achieve a minimum of 0.107 mH
9.Powder cores have the property of soft satu-
ration. This has the effect of gradually decreas-ing the permeability (inductance) with in-cread DC bias current. Therefore, to achieve the minimum inductance of 0.107 mH at the specified DC bias of 8 amperes, additional turns will be required. The procedure for de-termining the additional number of turns is ex-plained in each of the Magnetics Powder Core catalogs. Using this procedure, the final num-ber of turns required when considering the DC bias condition is 68.
10.Using 500 circular mils/amp, this gives a wire
size of AWG 14.
A 55586-A2 core with 68 turns of AWG 14 wire meets the design requirement.
Ferrite Core Design
6.Due to the many shapes available in ferrites,
there can be many choices for the lection.
Any core size that the LI² coordinate intercts can be ud if the maximum A L is not ex-ceeded. Following the LI² coordinate, five of the available choices are:
(a) 43230 (PQ core)A L 270
(b) 43622 (pot core)A L 200
(c) 44229 (solid centerpost core)A L 450
(d) 45015 (E core)A L 350
(e) 45224 (EC52 core)A L 330
7.Given the A L, the number of turns needed for
可可英语听力网the required inductance is:
A L Turns
27020
20024
片断教学
45016
35018
33018
8.Using 500 circular mils/amp, this gives a wire
size of AWG 14.新思维教育
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Molypermalloy Powder Core
DC Bias Core Selector Chart
Ll², mh-amperes²
Molypermalloy Powder Core Permeability vs DC Bias
DC MAGNETIZING FORCE OERSTEDS
MAGNETICS • BUTLER, PA5
Ferrite DC Bias Core Selector Charts
A- 40903
B- 40704
C- 40905
八上英语电子课本
D- 41107
E- 41408
F- 41811
G- 42213
H- 42616
J- 43019
K- 43622
L- 44229
M- 44529
Ll², mh-amperes²
A-42016
42020
B-42614
C-42610
42620
42625
43214
D-43220
43230
E-43535
44040
Ll², mh-amperes²
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