SP6132 Datasheet, PDF (7/14 Pages) Sipex Corporation – Wide Input, 300KHz Synchronous PWM Controller

Inductor Selection
There are many factors to consider in selecting
the inductor including cost, efficiency, size and
EMI. In a typical SP6132 circuit, the inductor is
chosen primarily for value, saturation current
and DC resistance. Increasing the inductor value
will decrease output voltage ripple, but degrade
transient response. Low inductor values provide
the smallest size, but cause large ripple currents,
poor efficiency and more output capacitance to
smooth out the larger ripple current. The induc-
tor must also be able to handle the peak current
at the switching frequency without saturating,
and the copper resistance in the winding should
be kept as low as possible to minimize resistive
power loss. A good compromise between size,
loss and cost is to set the inductor ripple current
to be within 20% to 40% of the maximum output
current.
The switching frequency and the inductor oper-
ating point determine the inductor value as fol-
lows:
L = ^{VOUT (V IN (max) −VOUT )}
V F K I _{IN (max) S r OUT ( max)}
where:
Fs = switching frequency
Kr = ratio of the ac inductor ripple current to the
maximum output current
The peak to peak inductor ripple current is:
I _PP
= ^{VOUT (VIN (max) −VOUT )}
V_IN(max) F_S L
Once the required inductor value is selected, the
proper selection of core material is based on
peak inductor current and efficiency require-
ments. The core must be large enough not to
saturate at the peak inductor current
APPLICATIONS INFORMATION
I _PEAK
= I _{OUT (max)}
+
I_PP
2
and provide low core loss at the high switching
frequency. Low cost powdered iron cores have
a gradual saturation characteristic but can intro-
duce considerable ac core loss, especially when
the inductor value is relatively low and the
ripple current is high. Ferrite materials, on the
other hand, are more expensive and have an
abrupt saturation characteristic with the induc-
tance dropping sharply when the peak design
current is exceeded. Nevertheless, they are pre-
ferred at high switching frequencies because
they present very low core loss and the design
only needs to prevent saturation. In general,
ferrite or molypermalloy materials are better
choice for all but the most cost sensitive appli-
cations.
The power dissipated in the inductor is equal to
the sum of the core and copper losses. To mini-
mize copper losses, the winding resistance needs
to be minimized, but this usually comes at the
expense of a larger inductor. Core losses have a
more significant contribution at low output cur-
rent where the copper losses are at a minimum,
and can typically be neglected at higher output
currents where the copper losses dominate. Core
loss information is usually available from the
magnetic vendor.
The copper loss in the inductor can be calculated
using the following equation:
P = I R _L(Cu)
2
L ( RMS ) WINDING
where I_L(RMS) is the RMS inductor current that
can be calculated as follows:
( ) ^{IL(RMS) = IOUT(max)}
1+1
3
I_PP
2
I_OUT(max)
Date: 8/4/04
SP6132 Wide Input, 300KHz Synchronous PWM Controller
7
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