Introduction/Basic Operation

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 energy

is released into the load. The amount of energy stored is calculated by the

= ½

• L is the inductance in Henrys

• 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. Ripple current is defined as V

Vll =

= LL..ddii//ddtt::

•Vl is the voltage across the inductor

• di is the ripple current

• dt is the duration for which the voltage is applied

Inductor current is made up of AC and DC components (Figure 1). The AC

component is high frequency and will flow through the output capacitor

because it has a low HF impedance. A ripple voltage is produced due to

the capacitor ‘equivalent series resistance’ (ESR) that will appear at the

output of the switching regulator. This ripple voltage needs to be

sufficiently low as not to effect the operation of the circuit the regulator is

supplying, normally in the order of 10-500mVpk-pk.

Selecting the correct ripple current impacts the size of the inductor and

output capacitor. The capacitor needs to have a sufficiently high ripple

current rating or it will overheat and dry out. To achieve a good

compromise between inductor and capacitor size a ripple current value of

10-30% of maximum inductor current should be chosen. The current in

the inductor will be continuous for output currents greater that 5-15% of

full load.

The following parameters need to be defined or calculated to select an

inductor:

• Maximum input voltage

• Output voltage

• Switching frequency

• Maximum ripple current

• Duty cycle

0

1

2

I load

I inductor

dI

V out

ESR

Figure 1:

Buck Inductor

Inductor Selection for Switching Regulators

SD3814