AN296 Silicon_Laboratories, AN296 Datasheet - Page 9

• Manufacturer Part Number: AN296
• Description: Using THE Si3400 AND Si3401 POE PD Controllers IN Isolated AND Non-isolated Designs
• Manufacturer: Silicon_Laboratories

4. DC-DC Converter Operation
There are two basic configurations for the dc-dc converter: buck and flyback. Additionally, the converter may be
designed so that its power output is electrically isolated from the power input. Isolation is required per IEEE 802.3af
when the PD does not provide the isolation.
In the non-isolated case, the buck topology is generally used, and, in the isolated case, the flyback topology is
generally used. It is possible to use the flyback topology in the non-isolated case, although this is not described in
detail in this application note.
The equations used for determining all of the components surrounding the switching converter are briefly described
below. A spreadsheet utility has also been developed to enable easy calculations of component values.
4.1. Non-Isolated Buck Design
Under most conditions, the current through the inductor (L1 in Figure 3) is continuous, and the voltage across the
inductor switches from positive to negative as shown in Figure 7.
The average voltage across the inductor must be zero; so, the duty cycle is:
Where V
PD input voltage, which is generally 42–55 V. Solving:
The ripple current that has to be supported in the output filter is:
D
I
D
ripple
×
=
(
V
(
-------------------------- -
out
(
V
in
=
V
out
is the desired output voltage; V
–
in
(
---------------------------------------------------------------
V
V
+
+
in
out
(
V
(
V
–
V
f
)
f
)
)
V
in
=
out
+
(
V
)
1 D
f
×
)
–
(
×
V
I
L
)
out
V
×
×
F
(
+
V
D/f
)
V
out
f
)
Figure 7. Voltage Polarity
+
f
is the forward drop of the diode (D1 in Figure 3), and V
Vin- Vout
(1- D)/f
V
f
)
Rev. 0.8
Iripple
Iout
Vout+Vf
t
in
AN296
varies with the
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