9/42

AN1262 APPLICATION NOTE

Once this information has been found, it is possible to evaluate the power dissipation of the IC and check for

thermal limitations. Table 8 summarizes the relationships that can be used for this evaluation. In those formulae:

the primary winding and, in case, some external capacitance.

capacitor is added on the drain.

With the worst-case total losses in the IC it is possible to find the maximum junction-to-ambient thermal resis-

tance allowed for safe operation at maximum ambient temperature.

The operating temperature range of the devices extends to 150 °C, however designing for such high tempera-

ture is not recommended. A reasonable target can be to design for 125 °C maximum die temperature:

(3)

Table 8. IC's power losses estimate

With the aid of the diagrams shown in fig. 20 it is possible to estimate whether the required thermal resistance

is feasible or not and, in the positive case, how large the on-board copper area is supposed to be. Consider that

copper areas larger than 4 cm

outing to become a serious issue.

If the thermal check does not give positive results, a different heatsinking strategy may be considered, otherwise

Symbol

Description

Definition

Total RMS Secondary Current

RMS Secondary Current (AC component only)

Symbol

Description

Definition

Conduction losses

Switching losses

Capacitive losses

Quiescent losses

Assume:

Is

RMS

Is

pk

D'

3

-----

⋅

=

Is

AC

Is

RMS

2

Is

DC

2

–

=

R

thm ax

125

T

am b

–

P

Q

P

c ond

P

sw

P

ca p

++

+

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

=

P

cond

Ip

RMS

2

R

DS o n

()max

⋅

=

P

sw

1

3

---

V

in

V

R

+

() Ip

pk

T

c

⋅⋅

⋅

≈

f

sw

⋅

P

CA P

1

2

---

C

dr ain

V

in

V

R

+

()

2

⋅⋅

≈

f

sw

⋅

Table 7. (continued)