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LT3759

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APPLICATIONS INFORMATION

LLK istheleakageinductanceoftheprimarywinding,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)canbedeterminedusingthefollowingequation:

CCN =

VSN

ΔVSN •RCN •fOSC

where ΔVSN is the voltage ripple across CCN. A reasonable

ΔVSN 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 =

40mV

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 • fOSC/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.

Flyback Converter: Output Diode Selection

The output diode in a flyback converter is subject to large

RMS current and peak reverse voltage stresses. A fast

switching diode with a low forward drop and a low reverse

leakage is desired. Schottky diodes are recommended if

the output voltage is below 100V.

Approximate the required peak repetitive reverse voltage

rating VRRM using:

VRRM >

•VIN(MAX) + VOUT

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

The RθJA to be used in this equation normally includes the

RθJC for the device, plus the thermal resistance from the

board to the ambient temperature in the enclosure. TJ must

not exceed the diode maximum junction temperature rating.

Flyback Converter: Output Capacitor Selection

The output capacitor of the flyback converter has a similar

operation condition as that of the boost converter. Refer to

the Boost Converter: Output Capacitor Selection section

for the calculation of COUT and ESRCOUT.

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