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LT8302MPS8E

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Linear Technology 

LT8302

Applications Information

Output Power

A flyback converter has a complicated relationship between

the input and output currents compared to a buck or a

boost converter. A boost converter has a relatively constant

maximum input current regardless of input voltage and a

buck converter has a relatively constant maximum output

current regardless of input voltage. This is due to the

continuous non-switching behavior of the two currents. A

flyback converter has both discontinuous input and output

currents which make it similar to a nonisolated buck-boost

converter. The duty cycle will affect the input and output

currents, making it hard to predict output power. In ad-

dition, the winding ratio can be changed to multiply the

output current at the expense of a higher switch voltage.

The graphs in Figures 1 to 4 show the typical maximum

output power possible for the output voltages 3.3V, 5V,

12V, and 24V. The maximum output power curve is the

calculated output power if the switch voltage is 50V dur-

ing the switch-off time. 15V of margin is left for leakage

inductance voltage spike. To achieve this power level at

a given input, a winding ratio value must be calculated

to stress the switch to 50V, resulting in some odd ratio

values. The curves below the maximum output power

curve 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 8V and a maximum input volt-

age of 32V. A three-to-one winding ratio fits this design

example perfectly and outputs equal to 15.3W at 32V but

lowers to 7.7W at 8V.

20

MAXIMUM

OUTPUT POWER

15

10

5

N = 6:1

N = 4:1

N = 3:1

N = 2:1

20

MAXIMUM

OUTPUT POWER

N = 2:1

15

N = 3:2

N = 1:1

10

N = 1:2

5

0

0

10

20

30

40

INPUT VOLTAGE (V)

8302 F01

Figure 1. Output Power for 3.3V Output

20

MAXIMUM

OUTPUT POWER

15

10

N = 4:1

N = 3:1

N = 2:1

N = 1:1

5

0

0

10

20

30

40

INPUT VOLTAGE (V)

8302 F03

Figure 3. Output Power for 12V Output

20

MAXIMUM

OUTPUT POWER

15

N = 1:1

N = 2:3

N = 1:2

10

N = 1:3

5

0

0

10

20

30

40

INPUT VOLTAGE (V)

8302 F02

Figure 2. Output Power for 5V Output

0

0

10

20

30

40

INPUT VOLTAGE (V)

8302 F04

Figure 4. Output Power for 24V Output

For more information www.linear.com/LT8302

8302fa

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