ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 33

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Winding Resistance Effects
Primary or secondary winding resistance acts to reduce
overall effi ciency (P
increases effective output impedance, degrading load regu-
lation. Load compensation can mitigate this to some extent
but a good design keeps parasitic resistances low.
Bifi lar Winding
A bifi lar or similar winding is a good way to minimize
troublesome leakage inductances. Bifi lar windings also
improve coupling coeffi cients and thus improve cross
regulation in multiple winding transformers. However,
tight coupling usually increases primary-to-secondary
capacitance and limits the primary-to-secondary
breakdown voltage, so it isn’t always practical.
Primary Inductance
The transformer primary inductance, L
based on the peak-to-peak ripple current ratio (X) in the
transformer relative to its maximum value.
As a general rule, keep X in the range of 20% to 40%
(i.e., X = 0.2 to 0.4). Higher values of ripple will increase
conduction losses, while lower values will require larger
cores.
Ripple current and percentage ripple is largest at minimum
duty cycle; in other words, at the highest input voltage.
L
L
P
P
is calculated from:
=
(
V
f
IN MAX
OSC
(
•
X
)
MAX
•
DC
OUT
MIN
•
P
IN
/P
)
IN
2
). Secondary winding resistance
=
(
V
IN MAX
f
OSC
(
•
)
X
•
MAX
D D C
P
MIN
,
•
P
is selected
OUT
)
2
•
Eff
where:
Using common high power PoE values a 48V (41V < V
< 57V) to 5V/5.3A Converter with 90% effi ciency, P
26.5W and P
= 200kHz:
Optimization might show that a more effi cient solution
is obtained at higher peak current but lower inductance
and the associated winding series resistance. A simple
spreadsheet program is useful for looking at tradeoffs.
Transformer Core Selection
Once L
effi ciency converters use ferrite cores to minimize core
loss. Actual core loss is independent of core size for a fi xed
inductance, but decreases as inductance increases. Since
increased inductance is accomplished through more turns
of wire, copper losses increase. Thus transformer design
balances core and copper losses. Remember that increased
winding resistance will degrade cross regulation and
increase the amount of load compensation required.
f
DC
X
DC
L
OSC
MAX
P
MIN
MIN
=
P
is the oscillator frequency
200
is known, the type of transformer is selected. High
is ripple current ratio at maximum input voltage
is the DC at maximum input voltage
=
(
1
5
kHz
IN
+
7 7
V
N V
= 29.5W Using X = 0.4 N = 1/8 and f
• . •
• .
•
0 4 26 5
0 412
V
1
IN MAX
OUT
(
)
2
.
)
W
=
1
=
+
260
8
1
1
LTC4268-1
•
μ
57
5
H
=
41 2
. %
33
42681fa
OUT
OSC
IN
=