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LT3758

3758f

APPLICATIONS INFORMATION

LLK is the leakage inductance of the primary winding,

which is usually speciﬁed in the transformer character-

istics. 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:

••

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:

SENSE

LP 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.

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:

RRM

IN MAX

OUT

•

()

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

RθJCforthedevice,plusthethermalresistancefromtheboard

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.

The RMS ripple current rating of the output capacitors

in discontinuous operation can be determined using the

following equation:

RMS COUT DISCONTINUOUS

O MAX

(),

(

) •

(•

)

•

≥

−

3 D

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