AN4021

An application example

Doc ID 022588 Rev 1

7/10

Step 1: Reverse voltage applied across the diode

When the transistor on the primary side of the transformer is on, the rectification diode is

blocked with a reverse voltage equal to:

Equation 8

The second step is to read the reference point value using the Figure 2 given in each

junction temperature of 125 °C.

In this example:

In order to consider the worst case for the power losses, we use the maximum values for the

leakage current using the ratio given by the Figure 3.

Equation 9

Using Equation 2 and Figure 2, the coefficient “c” can be calculated:

Equation 10

Step 3: Reverse losses expression:

From Equation 5, the maximum reverse losses expression is then:

Equation 11

Finally, one can plot the evolution of reverse losses in the diode versus the junction

temperature (Figure 6).

V

Vout

V

m

V

in

R

70

5

.

14

375

148

.

0=

+

·

=

+

·

=

4

10

40

)

125

,

100

(

)

125

,

100

(

)

(

(max)

=

=

°

°

=

C

V

I

C

V

I

Ratio

typ

R

R

1

6

3

1

Re

2

Re

1

Re

2

Re

069

.

0

)

10

5

10

5

ln(

25

125

1

)

)

,

(

)

,

(

ln(

1

-

-

-

°

=

·

·

·

-

=

·

-

=C

T

V

I

T

V

I

T

T

c

f

j

R

R

f

j

R

R

f

j

f

j

)

(

Re

(max)

Re

)

,

(

)

1

(

)

(

f

j

j T

T

c

f

j

R

R

R

j

REV

e

T

V

I

V

T

P

-

·

·

·

-

=d

)

125

(

069

.

0

3

(max)

10

20

70

)

2

.

0

1

(

)

(

-

-

·

·

·

·

-

=

j

T

j

REV

E

T

P

(max)

12

.

1

)

(

·

=

j

REV

e

T

P

)

125

(

069

.

0-

j

T

with TjRef = 125 °C