LT3512_11 Datasheet, PDF (9/24 Pages) Linear Technology – Monolithic High Voltage Isolated Flyback Converter

LT3512
APPLICATIONS INFORMATION
similar to a nonisolated buck-boost. The duty cycle will
affect the input and output currents, making it hard to
predict output power. In addition, the winding ratio can
be changed to multiply the output current at the expense
of a higher switch voltage.
The graphs in Figures 1-4 show the typical maximum
output power possible for the output voltages 3.3V, 5V,
12V and 24V. The maximum power output curve is the
calculated output power if the switch voltage is 100V
during the off-time. 50V of margin is left for leakage volt-
age spike. To achieve this power level at a given input, a
winding ratio value must be calculated to stress the switch
to 100V, resulting in some odd ratio values. The following
curves are examples of common winding ratio values
and the amount of output power at given input voltages.
One design example would be a 5V output converter with
a minimum input voltage of 36V and a maximum input
voltage of 72V. A four-to-one winding ratio fits this design
example perfectly and outputs close to 3.0W at 72V but
lowers to 2.5W at 36V.
The equations below calculate output power:
Power = η • V_IN • D • I_PEAK • 0.5
Efficiency = η = ~83%
( ) _{Duty cycle = D =} ^V_OUT ^{+ V}_F ^•N_PS
( ) ^V_OUT ^{+ V}_F ^•N_PS ^{+ V}_IN
Peak switch current = I_PEAK = 0.44A
5.0
N = 15
4.0 _{N = NPS(MAX)}
N = 12
N = 10
3.0
N=8
N=6
2.0
N=4
1.0
N=2
0
0
20
40
60
80
100
INPUT VOLTAGE (V)
3512 F01
Figure 1. Output Power for 3.3V Output
5.0
N=5
4.0 ^{N = NPS(MAX)}
N=4
N=3
3.0
N=2
2.0
N=1
1.0
0
0
20
40
60
80
100
INPUT VOLTAGE (V)
3512 F03
Figure 3. Output Power for 12V Output
5.0
4.0 ^{N = NPS(MAX)}
3.0
2.0
1.0
N=8
N=7
N=6
N=5
N=4
N=3
N=2
N=1
5.0
N = N_PS(MAX)
N=2
4.0
3.0
N=1
2.0
1.0
0
0
20
40
60
80
100
INPUT VOLTAGE (V)
3512 F02
Figure 2. Output Power for 5V Output
0
0
20
40
60
80
100
INPUT VOLTAGE (V)
3512 F04
Figure 4. Output Power for 24V Output
3512f
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