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LT3748E

100V Isolated Flyback Controller

Linear Technology 

LT3748

Applications Information

the output voltage multiplied by the windings ratio plus

some amount of overshoot caused by leakage inductance.

Second, increasing the turns ratio will increase the peak

current seen on the output diode generally increasing the

RMS diode current thereby lowering the efficiency. This

efficiency limitation is worse at lower output voltages when

the diode forward voltage is significant compared to the

output voltage. In a typical application such as the 5V, 2A

output shown on the back page, the diode losses dominate

all the other losses, as shown in Figure 4. To calculate

RMS diode current, two equations are needed—the first

for calculating duty cycle, D, and the second to calculate

the RMS current of a triangle waveform:

( ) D =

VOUT + VF(DODE) • NPS

( ) VIN + VOUT + VF(DIODE) • NPS

( ) IDIODE(RMS) =

ILIM • NPS 2 • (1– D)

3

25

NPS = 2:1

ILIM = 3A

20

15

NPS = 3:1

ILIM = 2A

10

NPS = 6:1

ILIM = 1A

5

0

0

20

40

60

80 100

INPUT VOLTAGE (V)

3748 F03

Figure 3. Maximum Output Power at 12V Out Using Three

Transformers with Equal Peak Output Current and Secondary

Inductance

100

VIN = 12V

95

90

DOUT

For a more general analysis, Figure 5 illustrates a sweep

of windings ratio on the x-axis while comparing output

power and estimated efficiency for a 5V output using a

48V input. If the desired application required 20W, the

maximum power curve indicates that a winding ratio of

12:1 would be sufficient at a current limit of 2A (RSENSE =

0.05Ω), while a winding ratio of 5:1 would deliver the same

power at 3A. However, when examining the corresponding

efficiency at max load for those two windings ratios and

current limits, the 5:1, 3A selection is clearly the superior

solution with an estimated efficiency of 85% compared to

78% for the 12:1, 2A application.

There are several caveats to this evaluation. First, as the

diode forward voltage becomes a smaller percentage of

total loss at higher output voltages (>12V) the RMS current

becomes less of a concern and minimizing it will have a

much smaller impact on efficiency. More significantly, if

a lower turns ratio forces the use of a diode with a larger

forward drop to obtain a higher reverse voltage rating,

any gains from minimizing current might be lost. For low

output voltages (3.3V or 5V) or high input voltages (>48V),

a turns ratio greater than one can be used with multiple

primary windings relative to the secondary to maximize

the transformer’s current gain.

85

fSW • QG + IQ

80

FET RDS(ON)

75

70

0.2A MIN

TRANSFORMER I • R + LEAKAGE

IOUT (A)

2A MAX

3748 F03

Figure 4. Sources of Loss In 5V, 2A Out Typical Application

100

ILIM = 3A

95

ILIM = 2A

90

85

32

28

OUTPUT

POWER

24

20

80

16

75

12

70

8

EFFICIENCY

65

4

60

0

0

3

6

9 12 15 18

NPS

3748 F05

Figure 5. Estimated Efficiency and Output Power at 5VOUT from

48VIN vs Windings Ratio, NPS, at 2A and 3A Current Limits

3748fb

14

For more information www.linear.com/LT3748

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