Datasheet

LT3511
17
3511fc
Example:
L
PRI
≥
400ns • 2 • 15+ 0.5
()
0.055
L
PRI
≥ 225µH
L
PRI
≥
100ns • 72
0.055
L
PRI
≥ 131µH
In addition, primary inductance will determine switching
frequency.
f
SW
=
1
t
ON
+ t
OFF
=
1
L
PRI
•I
PEAK
V
IN
+
L
PRI
•I
PEAK
N
PS
•V
OUT
+ V
F
()
I
PEAK
=
V
OUT
•I
OUT
•2
η •V
IN
•D
Example:
Let’s calculate switching frequency at our nominal V
IN
of 48V.
D =
15+ 0.5
()
•2
15+ 0.5
()
•2+ 48
= 0.39
I
PEAK
=
15V • 0.1A • 2
0.75 • 48V • 0.39
= 0.21A
Let’s choose L
PRI
= 350H. Remember, most transform-
ers specify primary inductance with a tolerance of ±20%.
f
SW
= 256kHz
Finally, the transformer needs to be rated for the correct
saturation current level across line and load conditions.
In the given example, the worst-case condition for switch
current is at minimum V
IN
and maximum load.
I
PEAK
=
V
OUT
•I
OUT
•2
η •V
IN
•D
I
PEAK
=
15V • 0.1A • 2
0.75 • 36V • 0.46
= 0.24A
Ensure that the saturation current covers steady-state
operation, start-up and transient conditions. To satisfy
these conditions, choose a saturation current 50% or more
higher than the steady-state calculation. In this example, a
saturation current between 400mA and 500mA is chosen.
Table 1 presents a list of pre-designed flyback transform-
ers. For this application, the Würth 750311660 transformer
will be used.
Step 4: Choose the correct output diode.
The two main criteria for choosing the output diode include
forward current rating and reverse voltage rating. The
maximum load requirement is a good first-order guess
at the average current requirement for the output diode.
A better metric is RMS current.
I
RMS
=I
PEAK(VIN(MIN))
•N
PS
•
1–D
VIN(MIN)
3
Example:
I
RMS
= 0.24 • 2 •
1– 0.46
3
= 0.2A
Next calculate reverse voltage requirement using maxi-
mum V
IN
:
V
REVERSE
= V
OUT
+
V
IN(MAX)
N
PS
Example:
V
REVERSE
= 15V +
72V
2
= 51V
A 0.5A, 60V diode from Diodes Inc. (SBR0560S1) will
be used.
Step 5: Choose an output capacitor.
The output capacitor choice should minimize output voltage
ripple and balance the trade-off between size and cost for
a larger capacitor. Use the equation below at nominal V
IN
:
C =
I
OUT
•D
ΔV
OUT
•f
SW
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