For Flyback Transformers

® Division of Spang & Company

Technical Bulletin BULLETIN CG-03

For Flyback Transformers . . . Selecting a Distributed Air-Gap Powder Core

Introduction Flyback converters are based on the storage of energy in an inductor during the “on” charging time period ton, and dis- charge of this energy to the load during the “off” time period, t off , as shown in Figure 1. The operation is unipolar and utilizes the first quadrant of the B-H curve of a magnetic core (Figure 2). The usable flux density is ∆B. The ideal core material should have a maximum available ∆B and low core losses (proportional to the shaded area). For flyback transformers, Magnetics offers: (a) three different materials in toroidal powder cores that have distributed air gaps FIGURE 1 (b) gapped ferrites Gapped ferrites have relatively high losses associated with the discrete air gap, although the material losses are low. Powder cores are made of tiny insulated particles, hence the air gaps are distributed evenly throughout the core structure. The total core losses (air gap plus particle losses) of the three powder core materials are usually much lower FIGURE 2 than those for gapped ferrites. Product details are found in Materials Comparison these MAGNETICS® catalogs: MPP-303, Molypermalloy and MATERIALS COMPARISON CHART High Flux Powder Cores Core Available Total DC Bias Core Relative KMC-01, Kool Mu® Powder Material Permeabil- Core Stability Size Cost Cores ities Losses Advantage FC-601, Ferrite Cores Moly- 14, 26, 60, Lowest Good Highest This brochure, focusing on permalloy 125, 160, the three powder core types, powder 200, 300, serves as a guide to selecting (MPP) 550 core sizes and obtaining an esti- High 14, 26, Higher Best Smallest Medium mate of the number of turns of Flux 60, 125, than size wire in flyback applications. (HF) 160 MPP possible KOOL 26,60,75 Low Good Lowest MU 90,125

(1) Molypermalloy powder (3) KOOL MU powder cores (MPP) cores consist of 79% contain 85% iron, 9% silicon nickel, 17% iron and 4% and 6% aluminum. Although molybdenum. MPP toroids offer KOOL MU cores don’t have the lowest core losses and the core losses quite as low as MPP widest range of permeabilities cores and don’t have the µ vs dc (14µ to 550µ). bias characteristics of the High (2) High Flux (HF) powder Flux cores, they do offer satis- cores consist of 50% nickel and factory performance in many 50% iron. Although HF cores designs at a much lower cost. have higher losses than MPP KOOL MU cores substantially cores, they offer the advantage outperform iron powder cores of sustaining their permeability (100% iron) as their losses are under higher dc bias conditions. much lower than iron powder, This usually results in the particularly at higher frequencies. smallest core size if core losses are not too critical. HF cores are available in permeabilities of 14µ through 160µ.

KOOL MU is a trademark of Magnetics

MAGNETICS • BUTLER, PA Core Selection Selecting Turns Summary The core can be determined and Wire Size The above procedure allows if the peak current (I and pri- the designer to determine the pk The LI 2 core selection mary inductance (Lpri ) are approximate core size and num- known. The requirements should procedure also describes how to ber of turns for a flyback trans- be analyzed to determine the determine the primary number former. Other factors such as following: of turns using Equation 3: continuous or discontinuous Pout = Output power-watts mode of operation can influence Vin(min) = Minimum input (3) core selection. To optimize the voltage—volts where L1000 = inductance per 1000 turns transformer design, the refer- (milihenries) dmax = Maximum duty cycle— enced textbooks can be helpful. ton The number of turns for a ton + t off secondary winding can be determined if the following are f Switching frequency— = known: kHz V = Output voltage-volts Using Equation 1, the peak cur- out VD = Diode voltage drop— rent can be determined: volts (typically 1 volt) Equation 4 calculates the num- 2P out amperes Ipk = δ (1) Vin(min) max ber of turns on the secondary: Once the peak current is deter- (V )(1 – δ )N N out + VD max pri mined, the primary inductance sec = δ turns Vin(min) max (4) can be calculated from: Although the core must be Vin(min) δmax mili- selected based on Ipk due to core L pri = henries (2) Ipk f saturation concerns, wire size selection can be based on the Using the Lpri and Ipk 2 average current. values, the LI core selection Average current is deter- procedure described in catalogs mined by: MPP-303 and KMC-01 can be used to select the correct core. If Pin I ave = amperes (5) the smallest possible core size is V in(min) desired regardless of core loss, (from Reference No. 1) High Flux cores should be con- By using average current to sidered. The permeability vs. dc select the wire size and peak cur- bias graph (catalog MPP-303, rent to select, core size, there High Flux cores) can be used in 2 should be a sufficient window the LI core selection. area for a secondary winding if needed.

Division of Spang & Company References (1) M. Brown, Practical Switching Power Supply Design, Academic Press, San Diego, 1990. (2) G. Chrysis, High Frequency Switching Power Supplies, McGraw- Hill, New York, 1984. (3) A. Pressman, Switching Power Supply Design, McGraw-Hill, New York, 1991. (4) C. McLyman, Magnetic Core Selection for Transformers and Inductors, Marcell Dekker, New York, 1982. (5) C. McLyman, Transformer and Inductor Design Handbook, Mar- cell Dekker, New York, 1988. (6) K. Billings, Switchmode Power Supply Handbook, McGraw-Hill, New York, 1989.

Division of Spang & Company HOME OFFICE AND FACTORY P.O. Box 391 Butler, PA 16003 FAX: 412-282-6955 Phone: 412-282-8282 1-800-245-3984

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