Datasheet

LT3512
17
3512fb
applicaTions inForMaTion
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 • 0.2A • 2
0.83 • 48V • 0.39
= 0.39A
Let’s choose L
PRI
= 200µH. Remember, most transform-
ers specify primary inductance with a tolerance of ±20%.
f
SW
= 240kHz
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 • 0.2A • 2
0.83 • 36V • 0.46
= 0.44A
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 700mA and 800mA is chosen.
Table 1 presents a list of pre-designed flyback transformers.
For this application, the Sumida 10396-T023 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
= 0.44 • 2 •
1– 0.46
3
= 0.37A
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 1.0A, 60V diode from Diodes Inc. (DFLS160) 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
∆V
OUT
• f
SW
Example:
Design for ripple levels below 50mV.
C =
0.2A • 0.39
0.05V • 240kHz
= 6.5µF
A 22µF, 25V output capacitor is chosen. Remember ce-
ramic capacitors lose capacitance with applied voltage.
The capacitance can drop to 40% of quoted capacitance
at the max voltage rating.