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PRODUCT SPECIFICATION
RC4391
16
Pot Core Inductor Design
Figure 20. Electricity vs. Magnetism
E
I
R
E = I * R
65-3464-07
North
Electrical Circuit
Magentic Circuit
South
Flux
H =B
1
U
Electricity Versus Magnetism
Electrically the inductor must meet just one requirement, but
that requirement can be hard to satisfy. The inductor must
exhibit the correct value of inductance (L, in Henrys) as the
inductor current rises to its highest operating value (I
MAX
).
This requirement can be met most simply by choosing a very
large core and winding it until it reaches the correct induc-
tance value, but that brute force technique wastes size,
weight and money. A more efficient design technique must
be used.
Question:
What happens if too small a core is used
First, one must understand how the inductor's magnetic field
works. The magnetic circuit in the inductor is very similar to
a simple resistive electrical circuit. There is a magnetizing
force (H, in oersteds), a flow of magnetism, or flux density
(B, in Gauss), and a resistance to the flux, called permeability
(U, in Gauss per oersted). H is equivalent to voltage in the
electrical model, flux density is like current flow, and perme-
ability is like resistance (except for two important differences
discussed to the right).
First Difference:
Permeability instead of being analogous to
resistance, is actually more like conductance (1/R). As per-
meability increases, flux increases.
Second Difference:
Resistance is a linear function. As volt-
age increases, current increases proportionally, and the resis-
tance value stays the same. In a magnetic circuit the value of
permeability varies as the applied magnetic force varies. This
nonlinear characteristic is usually shown in graph form in
ferrite core manufacturer's data sheet.
As the applied magnetizing force increases, at some point the
permeability will start decreasing, and therefore the amount
of magnetic flux will not increase any further, even as the
magnetizing force increases. The physical reality is that, at
the point where the permeability decreases, the magnetic
field has realigned all of the magnetic domains in the core
material. Once all of the domains have been aligned the core
will then carry no more flux than just air, it becomes as if
there were no core at all. This phenomenon is called satura-
tion. Because the inductance value, L, is dependent on the
amount of flux, core saturation will cause the value of L to
decrease dramatically, in turn causing excessive and possibly
destructive inductor current.
Figure 21. Typical Manufacturer’s Curve Showing
Saturation Effects
Pot Cores for RC4391
Pot core inductors are best suited for the RC4391 switching
regulator for several reasons:
1.
They are available in a wide range of sizes.
RC4391
applications are usually low power with relatively low
peak currents (less than 500mA). A small inexpensive pot
core can be chosen to meet the circuit requirements.
2.
Pot cores are easily mounted.
They can be bolted
directly to the PC card adjacent to the regulator IC.
6
6000
5000
4000
3000
2000
1000
0
-0.5 0 0.5 1
2 2.5 3
5
7
9
B
H Oersteds
+25
C
+85
C
+125
C
Stackpole Ceramag 24B
Hysteresis Loop vs. Temperature