INDUCTOR DESIGN

1 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 presents DESIGN information. In particular, MAGNETICS Ferrite and Molypermalloy Powder cores used for the power INDUCTOR are highlighted. DESCRIPTION. A typical circuit consists of three parts: transistor switch, diode clamp, and an LC filter.

2 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, Ein , is applied to the LC filter, thus causing current through the INDUCTOR to increase; 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, ton , of the transistor switch, using a feedback system from the output. The result is a regulated DC output, expressed as: E out = E in ton f COMPONENT INDUCTOR DESIGN .

3 SELECTION Two different types of core materials are com- monly used for the INDUCTOR in a switching regula- The switching system consists of a transistor tor Powder Cores and Ferrite Cores. It is diffi- and a feedback from the output of the regulator. cult to recommend one material over the other Transistor selection involves two factors (1) since the designer must take into consideration voltage ratings should be greater than the maxi- factors such as cost, volume, size and space limi- mum input voltage, and (2) the frequency cutoff tations, and winding capabilities.

4 Each material characteristics must be high compared to the ac- type has advantages as described below. tual switching frequency to insure efficient opera- tion. The feedback circuits usually include opera- MAGNETICS POWDER CORES have a dis- tional amplifiers and comparators. Requirements tributed air gap structure, making them ideal for for the diode clamp are identical to those of the switching regulator applications. This structure transistor. The DESIGN of the LC filter stage is eas- gives a soft saturation characteristic that has many ily achieved.

5 Given (1) maximum and minimum DESIGN benefits, including an overall smaller core input voltage, (2) required output, (3) maximum size, and overcurrent protection. It also alleviates allowable ripple voltage, (4) maximum and mini- the fringing flux difficulties which occur if a dis- mum load currents, and (5) the desired switching crete gap DESIGN is used. The DC bias character- frequency, the values for the inductance and ca- istics of Powder Cores allow them to be used at pacitance can be obtained. First, off-time (t off) high drive levels without saturating.

6 They are avail- of the transistor is calculated. able as toroids in three different materials: Molypermalloy (catalog MPP-400), High Flux t off = (1 - E out /E in max) / f (2) (catalog ), and Kool Mu (catalog KMC. ). E-core shapes are also available in Kool-Mu When Ein decreases to its minimum value, material (bulletin KMC E1). A wide range of sizes and permeabilities exists to meet the needs of di- f min = (1 - Eout/E in min ) / t off (3) verse applications. With these values, the required L and C can be FERRITE CORES offer the advantages of de- calculated.

7 Creased cost and low cores losses at high frequen- Allowing the peak to peak ripple current ( i) cies. For switching regulators, power ferrite mate- through the INDUCTOR to be given by rials (F, P, R, and K) are recommended because of their core loss and DC bias characteristics. By i = 2 l o min (4). adding discrete air gaps to these ferrite shapes, the cores can be used efficiently while avoiding the inductance is calculated using saturation. Magnetics produces many sizes and shapes to suit a variety of needs. Hardware is also L = E out t off / i (5) available for most parts.

8 Detailed descriptions are covered in the Magnetics Ferrite Cores Catalog The value calculated for i is somewhat arbitrary FC-601. and can be adjusted to obtain a practical value for the inductance. The minimum capacitance is given by CORE SELECTION. C = i/8f min e o (6) PROCEDURE. These core selection procedures simplify the Finally, the maximum ESR of the capacitor is DESIGN of inductors for switching regulator appli- cations. ESR max = e o / i (7). For Powder Cores: One can determine the smallest core size, as- suming a maximum decrease in inductance of 50%.

9 And wire current carrying capacity of 500 circular mils per ampere. MAGNETICS BUTLER, PA. 2. Only two parameters of the DESIGN application 2. Locate the LI value on the Ferrite Core Selec- must be known: tion charts on pages 6 and 7 in this bulletin (also (1) Inductance required with DC bias, located in the Magnetics Ferrite Cores Catalog (2) DC current. FC-601). Follow this coordinate to the intersec- tion with the first core size curve for the ferrite In this bulletin, Molypermalloy Powder Cores shape of choice. Read the maximum nominal are featured in the examples.

10 However, this de- inductance (A L) on the Y-axis. This represents sign procedure can be used for any of the Powder the smallest core size and maximum A L at Core types, including the Kool Mu E-cores. Sim- which saturation will be avoided. ply refer to the DESIGN charts and data within those catalogs for the material and shape of choice. 3. Any core size line that intersects the LI coordi- nate represents a workable core for the induc- 1. Compute the product of LI where: tor if the core's AL value is less than the maxi- L= minimum inductance required with DC bias mum value obtained from the chart.