AN1262 APPLICATION NOTE

8/42

All of the above mentioned calculation steps are summarized in table 6.

Table 6. Preliminary Calculations (step 2)

8

From the thermal point of view the heaviest operating conditions for the IC, and for most of the other parts of

the converter as well, are usually encountered at minimum input voltage.

relationships in table 7.

Symbol

Parameter

Definition

Transformer Input Power

Maximum Duty Cycle

Maximum drain Voltage in OFF-state

Max. Peak Primary Current

Symbol

Description

Definition

D

Duty Cycle (switch ON-time to switching period ratio)

Peak Primary Current

DC Primary Current

Total RMS Primary Current

RMS Primary Current (AC component only)

D‘

Secondary diode conduction time to switching period ratio

Peak Secondary Current

DC Secondary Current

P

inT

V

out

V

F

+

() I

ou t

⋅

η

T

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

=

V

DS on

()

V

inmin

V

R

+

1

V

inmin

V

R

⋅

P

in

R

DS on

()

⋅

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

+

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

≈

D

X

V

R

V

inm i n

V

DS on

()x

–

() V

R

+

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

=

Ip

pkx

P

inT

V

inm i n

V

DS on

()x

–

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

2

D

X

-------

⋅

=

D

V

inm i n

V

DS on

()x

–

V

DCmin

V

DS on

()x

–

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

=

D

X

⋅

Ip

DC

DIp

pk

⋅

2

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

=

Ip

RM S

Ip

pk

D

3

----

⋅

=

Ip

AC

Ip

RMS

2

Ip

DC

2

–

=

D'

V

DCmin

V

DS on

()x

–

V

R

------------------------------------------------- D

⋅

=

Is

pk

2I

out

⋅

D'

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

=