LT3757EDD-PBF Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

fabricante

LT3757EDD-PBF

Boost, Flyback, SEPIC and Inverting Controller

Linear Technology 

LT3757/LT3757A

Applications Information

where VSN is the snubber capacitor voltage. A smaller

VSN results in a larger snubber loss. A reasonable VSN is

2 to 2.5 times of:

VOUT • NP

NS

LLK is the leakage inductance of the primary winding, which

is usually specified in the transformer characteristics. LLK

can be obtained by measuring the primary inductance with

the secondary windings shorted. The snubber capacitor

value (CCN) can be determined using the following equation:

CCN

=

VSN

∆VSN • RCN

•

f

where ∆VSN is the voltage ripple across CCN. A reasonable

∆VSN is 5% to 10% of VSN. The reverse voltage rating of

DSN should be higher than the sum of VSN and VIN(MAX).

Flyback Converter: Sense Resistor Selection

In a flyback converter, when the power switch is turned

on, the current flowing through the sense resistor

(ISENSE) is:

ISENSE = ILP

Set the sense voltage at ILP(PEAK) to be the minimum of

the SENSE current limit threshold with a 20% margin. The

sense resistor value can then be calculated to be:

RSENSE

=

80mV

ILP(PEAK )

Flyback Converter: Power MOSFET Selection

For the flyback configuration, the MOSFET is selected with

a VDC rating high enough to handle the maximum VIN, the

reflected secondary voltage and the voltage spike due to

the leakage inductance. Approximate the required MOSFET

VDC rating using:

BVDSS > VDS(PEAK)

where:

VDS(PEAK) = VIN(MAX) + VSN

The power dissipated by the MOSFET in a flyback con-

verter is:

PFET = I2M(RMS) • RDS(ON) + 2 • V2DS(PEAK) • IL(MAX) •

CRSS • f /1A

The first term in this equation represents the conduction

losses in the device, and the second term, the switching

loss. CRSS is the reverse transfer capacitance, which is

usually specified in the MOSFET characteristics.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that absolute

maximum ratings are not exceeded.

3757afd

20

Share Link: