LT8331 datasheet(21/34 Pages) AD

21 / 34 page

LT8357

Rev. 0

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The primary and second inductor values of the flyback

converter transformer can be determined using the fol-

lowing equations:

LP =

D2MAX •V2IN(MIN)•η

2•POUT(MAX)•f

LS =

D22•(VOUT +VD)

2•IOUT(MAX)•f

The primary to second turns ratio is:

Flyback Converter: Snubber Design

Transformer leakage inductance (on either the primary

or secondary) causes a voltage spike to occur after the

MOSFET turn-off. This is increasingly prominent at higher

load currents, where more stored energy must be dis-

sipated. In some cases, a snubber circuit will be required

to avoid overvoltage breakdown at the MOSFET’s drain

node. There are different snubber circuits (such as RC

snubber, RCD snubber, etc.), and Application Note 19 is

a good reference on snubber design. An RCD snubber is

shown in Figure 8.

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

following equation:

RSN =2•

V2SN −VSN•VOUT •

I2SW(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 characteristics. LLK

APPLICATIONS INFORMATION

can be obtained by measuring the primary inductance

with the secondary windings shorted. The snubber capac-

itor value (CSN) can be determined using the following

equation:

CSN =

VSN

ΔVSN•RSN•f

where ΔVSN is the voltage ripple across CSN. 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 MOSFET is turned on,

the current flowing through the sense resistor (ISENSE) is:

ISENSE = ILP

Set the sense voltage at ILP(PEAK) to be less than the maxi-

mum SENSE current limit threshold of 60mV (typical).

The sense resistor value can then be calculated as:

RSENSE =

60mV

ILP(PEAK)

The final RSENSE value should be lower than the calculated

RSENSE. A 20% to 30% margin is usually recommended.

Always choose a low ESL current sense resistor.

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

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