**Filter inductor design constraints**

Objective:

Design inductor having a given inductance L,

which carries worst-case current Imax without saturating,

and which has a given winding resistance R, or,

equivalently

**Index**

- Assumed filter inductor geometry

- Constraint: maximum flux density

- Constraint: Inductance

- Constraint: Winding area

- The window utilization factor Ku

- also called the “fill factor”

- Winding resistance

- The core geometrical constant Kg

- Core geometrical constant Kg

- A step-by-step procedure

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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.

**INDUCTOR DESIGN in SWITCHING REGULATORS**

Technical BulletinBetter 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. 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,

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, 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,

**index**

- COMPONENT SELECTION

- INDUCTOR DESIGN

- CORE SELECTION PROCEDURE

- DESIGN EXAMPLE

**Switching Regulator Inductor Design**

COILTRONICS Application Notes Magnetics

In switching regulator applications the inductor is used as

an energy storage device, when the semiconductor

switch is on the current in the inductor ramps up and

energy is stored. When the switch turns off this energy is

released into the load, the amount of energy stored is

given by;

Energy = 1/2L.I2 (Joules) (1)

Where L is the inductance in Henrys and I is the peak

value of inductor current.

The amount by which the current changes during a

switching cycle is known as the ripple current and is

defined by the equation;

V1 = L.di/dt (2)

Where V1 is the voltage across the inductor, di is the

ripple current and dt is the duration for which the voltage

is applied. From this we can see that the value of ripple

current is dependent upon the value of inductance.

Choosing the correct value of inductance is important in

order to obtain acceptable inductor and output capacitor

sizes and sufficiently low output voltage ripple.

**Index**

- Inductor Selection for Buck Converters

- Inductor Selection for Boost Converters

- Inductor Selection for Buck-Boost Converters