SC4809B datasheet(10/16 Pages) SEMTECH

10 / 16 page

 2005 Semtech Corp.

www.semtech.com

SC4809A/B/C

POWER MANAGEMENT

The primary inductance can be calculated given an

acceptable current ripple,

∆I

L. ∆IL was set to equal one-

half the peak primary current. For a CCM flyback design,

the peak primary current is calculated:

max

(max)

OUT

PEAK

∆









−

•









Because the converter is operating in the continuous

mode, the maximum peak flux density B

MAX, is limited by

the saturation flux density, B

SAT. Taking all this into

consideration, the maximum core size is determined by.

MAX

RMS

PEAK

420









•

where AP = the core area product in cm4,

k = winding factor,

MAX ≈ BSAT,

The result is compared to the product of the winding area,

Aw (cm2), and effective core area, Ae (cm2), listed in the

core manufacturer’s data sheet.

The minimum number of primary turns is determined by:

MAX

PEAK

•

Based upon this result and the predetermined turns ratio,

the number of secondary turns is established.

The energy stored in the flyback transformer is actually

stored in an air gap in the core. This is because the high

permeability of the ferrite material can’t store much

energy without saturating first. By adding an air gap, the

hysteresis curve of the magnetic material is actually tilted,

requiring a much higher field strength to saturate the

core. The length of the air gap is calculated by:

)

(

−

•

Application Information (Cont.)

MOSFET Selection

The switching element in a flyback converter must have

a voltage rating high enough to handle the maximum input

voltage and the reflected secondary voltage, not to

mention any leakage inductance induced spike that is

inevitably present. Approximate the required voltage

rating of the MOSFET using.

()

(max)

•













•









where V

ds = the required drain to source voltage rating of

the MOSFET,

L = the voltage spike due to the leakage inductance of

the transformer, estimated to be thirty percent of V

IN(MAX),

and the additions 1.3 factor includes an overall thirty

percent margin.

This FET will experience both switching and conduction

losses. The conduction losses will be equal to the I2R

losses, as shown by:

()

)

(

RMS

COND

•

Switching losses are the result of overlapping drain

current and drain to source voltage at turn on and turn

off.

The total switching losses are estimated based on

equation:

PEAK

OSS

)

(

•

where t

ch:

)

(

VDD

−

•

Diode Selection

Schottky rectifiers have a lower forward voltage drop than

typical PN devices, making it the rectifier of choice when

considering reducing converter losses and improving

overall efficiency. Selecting the appropriate Schottky for

a specific application depends mainly on the working

peak reverse voltage rating and peak repetitive forward

current.

SC4809B Datasheet(PDF) 10 Page - Semtech Corporation

SC4809B Datasheet(HTML) 10 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL