Economical Soft Ferrite Core Selection for Power Transformers

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ECONOMICAL SOFT FERRITE CORESELECTION FOR POWER TRANSFORMERSGeorge G. Orenchak

Ferrite International CompanyWadsworth, IL

ABSTRACTFerrite core material perfor-mance, core configuration and sizevs. inherent costs are examined intypical high frequency tranformer ap-plications.

THE CORE SEJsECTION PROCESSA power transformers core selec-tion is dependent on the power thecore will need to handle. The appar-ent power (Pt) is dependent on trans-former configuration.

Full-wave Bridge

LOAD

Pt = PO 6 + 1)t = PO 6 + 1)Full-wave Center tappedull-wave Center tapped

NI010 jI

NlLOADOAD

Push-Pull Full-wave Center tapped

: LOAD

Pt = PO (5 + v2)Consider a full-wave bridge config-uration with the following criteria:Input voltage Vin = 200VOutput voltagesVoutl = 12V at 10 amps (120 watts)Vout2 = 48V at 8 amps (384 watts)vout3 = 5V at 15 amps (75 watts)

Total 580 wattsFrequency F = 1OOKHzEfficiency = 0.90Calculate the apparent power (Pt).Pt = PO (3; + 1)Pt = (580)(& + 1) = 1224

A core is needed with sufficientcross-sectional area (Ae) to carrythe flux and a window area (Wa) largeenough to physically support theneeded turns. The following re-lationship is derived from Faraday'sLaw.Ap = WaAe = Pt x lo4Ku Kf Bm f J

Pt = PO (A + uz)

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Ae Effective core area cm2Bm Flux Density Teslaf Frequency HZJ Current density A/cm2Kf Wave form coefficientKu Window utilization factorPt Apparent power wa tsWa Window area cm 5

Before calculating the area pro-duct, choose a material grade and de-termine the flux density according tothe maximym permissible core loss.128 mw/cm typically results in 35 to40 C temperature rise. Figure #2shows core loss vs. flux density.

Choose TSF-05 material, 0.1 Tesla.1224 x 104Ap=wa&e= (4)(.4)(.1)(100000)(120)

Ap = Wa As = 5.313 cm4Select one of the followingcores pith area product greater than6.3 cm.

ConfigurationE core ETD coreP/N 05-42-21-20 05-49-00-00Le 9.723 cm2 11.562 cm2Ae 2.304 cm3 2.114 cm3Ve 22.404 cm2 24.467 cm2Wa 2.769 cm46.381 cm 3.744 cm4WaAe 7.914 cmMaximum dimensignsLxWxH 34.42 cm 39.21 cm3

PQ core05-40-40-0010.19 cm22.01 cm20.45 cm33.26 cm26.55 cm446.12 cm3

CT OF MATERIALS WITHLOWER CORE LOSS PROPERTIES

If a lower core loss material ischosen, the gauss level can be in-creased by 20% and smaller less ex-pensive cores may be used.Choose TSF-55 material, 0.12 Tesla.2 4* = w&U2 = (4)(.4;(~~2) ~10~000)(120)

= 5.313 cm4The new area product is 16.7%smaller. One of the following corescan be used.

ConfigurationE core ETD coreP/N 55-43-21-15 55-44-00-00Le 9.834 Cm2 10.475 Cm2Ae 1.838 Cm3 1.732 Cm3Ve 18.075 cm 18.112 cm2Wa 2.888 cm25.308 cm4 3.053 cmWaAe 5.288 cm4Maximum dimensiogsLxWxH 27.94 cm 29.06 cm3

The TSF-55-43-21-15-0000 is 18%less expensive than the TSF-05-42-21-20-0000 for a number of reasons:2': 20% less material (thinner part).17% increase in sintering rate(more pieces/kiln tile and fas-ter sintering cycle).3. 33% increase in grinding rate(thinner part yields more pieces/

hour ground at same throughputconveyor speed).Additional cost savings forsmaller bobbins and less wire arealso realized.

COST CONSIDERATIONS OFaNFIGURATIONSConsider a Push-Pull Full-waveCenter tapped configuration with thefollowing criteria:

Input voltage Vin = 110Output voltage Vout = 12V at 10 amps(120 watts)Frequency F = 1OOKHzEfficiency n = 0.90

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Pt = 120 (5 + V?!)pt = 120 $9 + Vz) = 358Choose TSF-55 material, .12 Tesla.Ap = WaAe = 358 x lo4(4)(.4)6.12)(100000)(245)= . 761 cm

ConfigurationE core Pot PQP/N 55-30-15-07 55-30-19-00 55-26-25-00Le Cm 6.607Ae cm2 .573

4.52 5.55Ve cm3 3.785

1.36 1.18Wa cm2 1.344

6.15 6.53WaAe cm4 .770

.80 .851.09 1.00

12.81

Toroid55-22-14-13

5.4185.2202.8291.478

.772

6.200The toroid is the least expen-sive core, however, it is the mostexpensive to wind because coilform cannot be used. The TSFT55-30-15-07-0000 E core is nearly as inex-pensive as the toroid and less expen-sive to wind. The PQ core is 136%more expensive than the E core andthe pot core is 218% more expensivethan the E core. The reasons areprimarily as follows:

1. Configurations with round centerposts must be "end molded" with mul-tiple level tooling rather than "sidemolded" with single layer tooling.This causes too1 cost, tool main-tenance costs and set-up times to in-crease while pressing speeds de-crease.2. Shapes other than rectangularwaste area on kiln tile. Many morepieces of an E core fit on a kilntile than do pot cores.3. Pot cores and PQ cores must beground on their back sides and theirmating surfaces, while E cores needto be ground only on their matingsurfaces.4. E cores can be packaged in ship-ping containers more densely than potcores.

5. Pot cores must be gapped indivi-dually, while E cores can be gappedin line while grinding mating sur-faces on a high speed throughputgrinder.

There are many other issues suchas VDE and U requirements that needto be considered when designing apower transformer. Such issues wouldaffect the core size and cost simi-larly, regardless of material gradeor core configuration.Other issues such as radiationaffects may dictate the core configu-ration, but if cost is the most im-portant parameter, an E core in apremium low core loss material ismost often the best choice.

REFERENCESGeorge Chryssis, Hiah Freo encvSwitchina Power Sunolies McGrawUHillInc. 1984C. Wm. McLyman, Maanetic Core Selec-tion for Tranformers and InductorsMarcel Dekker, Inc. 1982E. C. Snelling and A. D. Giles, mrites for Inductors and TransformersResearch Studies Press Ltd., 1983Nathan R. Grossner, Transformers forElectronic Circuits McGraw-HillInc., 1967Peter Wood, Switchina Power Conver-ters Litton Educational Publishing,Inc. 1981George G. Orenchak, "ApplicationConsiderations of the Non-Linear Fer-rite Characteristics" Coil WindingProceedings 1990, pp. 121 - 126.

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Economical Soft Ferrite Core Selection for Power Transformers