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LT8300ES5

100VIN Micropower Isolated Flyback Converter with 150V/260mA Switch

Linear Technology 

LT8300

Applications Information

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) = 52mA

Example:

LPRI

≥

350ns

•

2 • (12V

52mA

+

0.3V)

=

166µH

LPRI

≥

160ns • 72V

52mA

=

222µH

Most transformers specify primary inductance with a toler-

ance of ±20%. With other component tolerance considered,

choose a transformer with its primary inductance 20% to

40% larger than the minimum values calculated above.

LPRI = 300µH is then chosen in this example.

Once the primary inductance has been determined, the

maximum load switching frequency can be calculated as:

fSW

=

1

tON + tOFF

=

LPRI • ISW

VIN

1

+

LPRI • ISW

NPS • (VOUT +

VF

)

ISW

=

VOUT

η•

•IOUT • 2

VIN • D

Example:

D

=

(12V + 0.3V) • 2

(12V + 0.3V) • 2 + 48V

=

0.34

ISW

=

12V • 0.12A • 2

0.85 • 48V • 0.34

=

0.21A

fSW = 260kHz

The transformer also needs to be rated for the correct

saturation current level across line and load conditions. A

saturation current rating larger than 400mA is necessary

to work with the LT8300. The 10396-T022 from Sumida

is chosen as the flyback transformer.

Step 3: Choose the Output Diode.

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

as the average current requirement for the output diode.

A conservative metric is the maximum switch current limit

multiplied by the turns ratio,

IDIODE(MAX) = ISW(MAX) • NPS

Example:

IDIODE(MAX) = 0.52A

Next calculate reverse voltage requirement using maxi-

mum VIN:

VREVERSE

=

VOUT

+

VIN(MAX )

NPS

Example:

VREVERSE

=

12V +

72V

2

=

48V

The SBR0560S1 (0.5A, 60V diode) from Diodes Inc. is

chosen.

8300f

17

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