LT8301 datasheet(20/38 Pages) AD

20 / 38 page

LT3757/LT3757A

Rev. G

For more information www.analog.com

APPLICATIONS INFORMATION

The snubber resistor value (RSN) can be calculated by the

following equation:

RSN = 2 •

V2 SN − VSN • VOUT •

I2 SW(PEAK) •LLK • f

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

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:

CCN =

VSN

ΔVSN •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 abso-

lute maximum ratings are not exceeded.

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