AN2783 datasheet(24/47 Pages) STMICROELECTRONICS

24 / 47 page

Power converter

AN2783

24/47

The maximum duty cycle can be obtained as:

Equation 7

10.2

Main switch current and current sensing

The current shape in the primary power switch is different when the flyback converter is

working in DCM or CCM. In DCM the shape is triangular with the current starting from zero,

while in CCM the shape is trapezoidal.

For CCM operations the peak current can be computed as:

Equation 8

which can be expressed as:

Equation 9

The RMS current can be:

Equation 10

The above formula can be used to calculate the right current sense resistor, taking into

account that the first level of OCP is for PM8800 at 500 mV:

Equation 11

The associated power dissipation on the sense resistor is:

Equation 12

10.3

Main switch power dissipation

The power dissipation on the main power MOSFET is the sum of two terms:

Ton

max

Tsw

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

Vout

()

⋅

Vin

min Vds

–

() N Vout Vd

()

⋅

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

Ipeak

Ipave

∆I

--------

Iout

η N1 D

max

–

()

⋅⋅

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

Vin

max

Tsw

⋅⋅

2Lp

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

Ipeak

Pout

η D

max

Vin

Vds

–

()

⋅⋅

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

Vin

max

Tsw

⋅⋅

2Lp

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

Iprms

max

Ipeak

∆I

--------

∆I

-----------

⋅

⎝⎠

⎜⎟

⎛⎞

–

⋅

Rcs

500mV

1.3 Ipeak

⋅

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

Pcs

Rcs Iprms

⋅

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