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LT8302HS8E-3-WPBF

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

Analog Devices 

LT8302/LT8302-3

APPLICATIONS INFORMATION

Design Example

Use the following design example as a guide to design-

ing applications for the LT8302/LT8302-3. The design

example involves designing a 5V output with a 1.5A load

current and an input range from 8V to 32V.

VIN(MIN) = 8V, VIN(NOM) = 12V, VIN(MAX) = 32V,

VOUT = 5V, IOUT = 1.5A

Step 1: Select the transformer turns ratio.

NPS

<

65V

–

VIN(MAX) – VLEAKAGE

VOUT + VF

VLEAKAGE = Margin for transformer leakage spike = 15V

VF = Output diode forward voltage = ~0.3V

Example:

NPS

<

65V – 32V – 15V

5V + 0.3V

=

3.4

The choice of transformer turns ratio is critical in deter-

mining output current capability of the converter. Table 2

shows the switch voltage stress and output current capa-

bility at different transformer turns ratio.

Table 2. Switch Voltage Stress and Output Current Capability vs

Turns Ratio

VSW(MAX) at

NPS

VIN(MAX) (V)

1:1

37.3

IOUT(MAX) at

VIN(MIN) (A)

0.92

DUTY CYCLE (%)

14-40

2:1

42.6

1.31

25-57

3:1

47.9

1.53

33-67

Clearly, only NPS = 3 can meet the 1.5A output current

requirement, so NPS = 3 is chosen as the turns ratio in

this example.

Step 2: Determine the primary inductance.

Primary inductance for the transformer must be set above

a minimum value to satisfy the minimum switch-off and

switch-on time requirements:

LPRI

≥

tOFF(MIN)

•NPS • (VOUT

ISW(MIN)

+

VF

)

LPRI

≥

tON(MIN) • VIN(MAX)

ISW(MIN)

tOFF(MIN) = 350ns

tON(MIN) = 160ns

ISW(MIN) = 0.87A

Example:

LPRI

≥

350ns

•

3 • (5V

0.87A

+

0.3V)

=

6.4µH

LPRI

≥

160ns • 32V

0.87A

=

5.9µH

Most transformers specify primary inductance with a tol-

erance of ±20%. With other component tolerance consid-

ered, choose a transformer with its primary inductance

40% to 60% larger than the minimum values calculated

above. LPRI = 9µH is then chosen in this example.

Once the primary inductance has been determined, the

maximum load switching frequency can be calculated as:

fSW

=

tON

1

+ tOFF

=

LPRI •ISW

VIN

1

+

LPRI •ISW

NPS • (VOUT +

VF

)

ISW

=

VOUT

η•

•IOUT •

VIN •D

2

Rev. G

18

For more information www.analog.com

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