Datasheet

LT3825
15
3825fe
Ripple current and percentage ripple is largest at minimum
duty cycle; in other words, at the highest input voltage.
L
P
is calculated from:
L
P
=
V
IN(MAX)
• DC
MIN
( )
2
f
OSC
• X
MAX
• P
IN
=
V
IN(MAX)
• DC
MIN
( )
2
• Eff
f
OSC
• X
MAX
• P
OUT
where:
f
OSC
is the oscillator frequency
DC
MIN
is the DC at maximum input voltage
X
MAX
is ripple current ratio at maximum input voltage
For a 48V (V
IN
= 36V to 72V) to 5V/8A converter with 90%
efficiency, P
OUT
= 40W and P
IN
= 44.44W. Using X = 0.4
and f
OSC
= 200kHz:
DC
MIN
=
1
1+
N • V
IN(MAX)
V
OUT
=
1
1+
1
8
•
72
5
= 35.7%
L
P
=
72V • 0.357
( )
2
200kHz • 0.4 • 44.44W
= 186µH
Optimization might show that a more efficient 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 trade-offs.
Transformer Core Selection
Once L
P
is known, the type of transformer is selected.
High efficiency converters use ferrite cores to minimize
core loss. Actual core loss is independent of core size for
a
fixed 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.
The main design goals for core selection are reducing
copper losses and preventing saturation. Ferrite core
material saturates hard, rapidly reducing inductance
when the peak design current is exceeded. This results
in an abrupt increase in inductor ripple current and, con-
sequently, output voltage ripple. Do not allow the core
to saturate! The maximum peak primary current occurs
at minimum V
IN
:
I
PK
=
P
IN
V
IN(MIN)
• DC
MAX
• 1+
X
MIN
2






now :
DC
MAX
=
1
1+
N • V
IN(MIN)
V
OUT
=
1
1+
1
8
•
36
5
= 52.6%
X
MIN
=
V
IN(MIN)
• DC
MAX
( )
2
f
OSC
• L
P
• P
IN
=
36 • 52.6%
( )
2
200kHz • 186µH • 44.44
=
0.202
Using the example numbers leads to:
I
PK
=
44.44W
36 • 0.526
• 1+
0.202
2






= 2.58A
Multiple Outputs
One advantage that the flyback topology offers is that ad-
ditional output voltages can be obtained simply by adding
windings. Designing a transformer for such a situation is
beyond the scope of this document. For multiple windings,
realize that the flyback winding signal is a combination of
activity on all the secondary windings. Thus load regulation
is affected by each windings load. Take care to minimize
cross regulation effects.
APPLICATIONS INFORMATION