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RC4391
PRODUCT SPECIFICATION
17
3.
Pot cores can be easily air-gapped.
The length of the
gap is simply adjusted using different washer
thicknesses. cores are also available with predetermined
air gaps.
4.
Electromagnetic interference (EMI) is kept to a
minimum.
the completely enclosed design of a pot core
reduces stray electromagnetic radiation—an important
consideration if the regulator circuit is built on a PC card
with other circuitry.
Not quite. Core size is dependent on the amount of energy
stored, not on load power. Raising the operating frequency
allows smaller cores and windings. Reduction of the size of
the magnetics is the main reason switching regulator design
tends toward higher operating frequency. Designs with the
RC4391 should use 75 kHz as a maximum running fre-
quency, because the turn off delay of the power transistor
and stray capacitive coupling begin to interfere. Most appli-
cations are in the 10 to 50 kHz range, for efficiency and EMI
reasons.
The peak inductor current (I
MAX
) must reach a high enough
value to meet the load current and simultaneously the induc-
tor value is decreased, then the core can be made smaller.
For a given core size and winding, an increase in air gap
spacing (an air gap is a break in the material in the magnetic
path, like a section broken off a doughnut) will cause the
inductance to decrease and I
MAX
(the usable peak current
before saturation )to increase.
The curves shown are typical of the ferrite manufacturer's
power HF material, such as Siemens N27 or Stackpole 24B,
which are usually offered in standard millimeter sizes
including the sizes shown.
Use of the Design Aid Graph
1. From the application requirement, determine the
inductor value (L) and the required peak current (I
MAX
).
2. Observe the curves of the design aid graph and determine
the smallest core that meets both the L and I
requirements.
3. Note the approximate air gap at IMAX for the selected
core, and order the core with the gap. (If the gapping is
done by the user, remember that a washer lspacer results
in an air gap of twice the washer thickness, because two
gaps will be created, one at the center post and one at the
rim, like taking two bites from a doughnut.)
4. If the required inductance is equal to the indicated value
on the graph, then wind the core with the number of turns
shown in the table of sizes. The turns given are the
maximum number for that gauge of wire that can be
easily wound in cores winding area.
5. If the required inductance is less than the value indicated
on the graph, a simple calculation must be done to find
the adjusted number of turns. Find A
L
(inductance index)
for a specific air gap.
Then divide the required inductance value by A
L
to give the
actual turns squared, and take the square root to find the
actual turns needed.
Turns
)
L indicated
A
L
Turn
inHenries
è
=
ActualTurns
A
L
)
L required
=
Figure 22. Inductor Design Aid
6
Air Gap = 0.012"
Air Gap = 0.006"
Air Gap = 0.02"
No Air Gap
#1
#2
#3
#4
1 mH
2 mH
3 mH
3A
2A
1A
0
I
M
Inductor Value (Henries)
*Includes safety margin (25%) to ensure nonsaturation
#1
#2
#3
#4
22X 13 mm
24 Gauge
70 Turns
DC
W
= 0.5
W
18X 11 mm
26 Gauge
70 Turns
DC
W
= 0.7
W
14X 8 mm
28 Gauge
60 Turns
DC
W
= 0.6
W
11X 7 mm
30 Gauge
50 Turns
DC
W
= 1
W