AN4337

6/14

DocID025012 Rev 1

3

The following section provides step-by-step guidelines on how to achieve the right trade-off

MOSFET operates in a flyback converter. We essentially explain how to approach the

avalanche phenomenon in order to understand if a MOSFET device can function safely.

3.1

device can sustain an avalanche if the starting case temperature is 25 °C and if a specific

value of ID is set.

The first step is to evaluate the maximum energy that the MOSFET must dissipate during

the single avalanche phenomenon. As previously mentioned, in the flyback converter, the

reached. In the absence of an external clamping network, we estimate the amount of energy

Equation 1

Where

Equation 2

Equation 3

For example, consider the 650 V/5.4 A device shown below:

Table 1. Electrical rating, absolute maximum rating

Symbol

Parameter

Value

Unit

Drain source voltage

650

V

5.4

A

Avalanche current repetitive or not repetitive (pulsed width

5.4

A

100

mJ

t

BVD

I

dt

t

Vds

t

Id

E

ss

P

t

lk

∆

⋅

⋅

=

=

∫

∆

0

2

1

)

(

)

(

N

V

BVD

I

L

t

out

ss

P

lk

−

⋅

=

∆

N

V

BVD

BVD

L

I

E

out

ss

ss

lk

P

lk

−

⋅

=

2

2

1