LT8302HS8E-3-WPBF 查看數據表（PDF） - Analog Devices

零件编号

产品描述 (功能)

比赛名单

LT8302HS8E-3-WPBF

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

Analog Devices 

LT8302/LT8302-3

APPLICATIONS INFORMATION

Example:

D

=

(5V + 0.3V) • 3

(5V + 0.3V) • 3 + 12V

=

0.57

ISW =

5V • 1.5A • 2

0.8 • 12V • 0.57

fSW = 277kHz

The transformer also needs to be rated for the correct

saturation current level across line and load conditions.

A saturation current rating larger than 7A is necessary to

work with the LT8302/LT8302-3. The 750311564 from

Wurth 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

at the average current requirement for the output diode.

Under output short-circuit condition, the output diode

needs to conduct much higher current. Therefore, a con-

servative metric is 60% of the maximum switch current

limit multiplied by the turns ratio:

IDIODE(MAX) = 0.6 • ISW(MAX) • NPS

Example:

IDIODE(MAX) = 8.1A

Next calculate reverse voltage requirement using maxi-

mum VIN:

VREVERSE

=

VOUT

+

VIN(MAX)

NPS

Example:

VREVERSE

=

5V

+

32V

3

=

15.7V

The PDS835L (8A, 35V diode) from Diodes Inc. is chosen.

Step 4: Choose the output capacitor.

The output capacitor should be chosen to minimize the

output voltage ripple while considering the increase in size

and cost of a larger capacitor. Use the following equation

to calculate the output capacitance:

COUT

=

2

•

LPRI •ISW2

VOUT • ΔVOUT

Example:

Design for output voltage ripple less than ±1% of VOUT,

i.e., 100mV.

COUT

=

9µH• (4.5A)2

2 • 5V • 0.1V

=

182µF

Remember ceramic capacitors lose capacitance with

applied voltage. The capacitance can drop to 40% of

quoted capacitance at the maximum voltage rating. So

a 220µF, 6.3V rating X5R or X7R ceramic capacitor is

chosen.

Step 5: Design snubber circuit.

The snubber circuit protects the power switch from leak-

age inductance voltage spike. A (RC + DZ) snubber is

recommended for this application. A 470pF capacitor in

series with a 39Ω resistor is chosen as the RC snubber.

The maximum Zener breakdown voltage is set according

to the maximum VIN:

VZENNER(MAX) ≤ 60V – VIN(MAX)

Example:

VZENNER(MAX) ≤ 60V – 32V = 28V

A 24V Zener with a maximum of 26V will provide optimal

protection and minimize power loss. So a 24V, 1.5W Zener

from Central Semiconductor (CMZ5934B) is chosen.

Choose a diode that is fast and has sufficient reverse

voltage breakdown:

VREVERSE > VSW(MAX)

VSW(MAX) = VIN(MAX) + VZENNER(MAX)

Example:

VREVERSE > 60V

A 100V, 1A diode from Diodes Inc. (DFLS1100) is chosen.

For more information www.analog.com

Rev. G

19

Share Link: