AN2783 datasheet(25/47 Pages) STMICROELECTRONICS

25 / 47 page

AN2783

Power converter

25/47

Equation 13

where

Equation 14

Vspike is due to the leakage inductance of the power transformer and can be assumed to be

40 V max, as the snubber network on the primary side is built with a TVS with breakdown

voltage of 40 V.

Tm is the time to charge the Miller capacitor of the power MOSFET and can be estimated

as:

Equation 15

Note that if the major contribution to the MOSFET losses comes from the second term of the

switching power losses, the right choice is for a low gate charge power MOSFET.

10.4

Rectifier diode dissipation

The secondary output current is:

Equation 16

The reverse voltage across the rectification diode is:

Equation 17

The power dissipated in the secondary diode, neglecting the reverse leakage losses, can be

estimated as:

Equation 18

Pcond

Ron Iprms

max

⋅⋅

Pswitching

Coss Vds

Fsw

⋅⋅

-------------------------------------------------

Vds

max

Ipeak Tm Fsw

⋅⋅

⋅

Vds

max

1.2

Vin

max

Vspike

() NVout Vd

()

⋅

[]

⋅

Qgd Rg

⋅

Vcc

Vgsth

–

---------------------------------

Ispeak

Isave

∆I

-----------

Iout

max

–

-----------------------

Vout

() Tsw Ton

–

()

⋅

[]

() N

⁄

---------------------------------------------------------------------------

Isrms

max

–

() Ispeak

Ispeak

∆I

-----------

⋅

–

⋅

diode

Vin

max

-------------------

Vout

diode

Isrms Vd

max

–

()

⋅⋅

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