DocID025012 Rev 1

7/14

AN4337

14

With the following conditions derived from 30 W flyback converter:

•

Maximum avalanche current Ip=4 A.

•

Starting temperature 25 °C.

•

Primary inductance value Lprimary=550 µH.

•

Leakage inductance ~ 13 µH.

•

Transformer ratio N=2.

•

Output voltage Vout=48 V

In these conditions, the energy due to the leakage inductance is 123 µJ. This is the

maximum avalanche energy that the MOSFET device must sustain during breakdown. If we

presume the case temperature to be fixed at 25 °C, we can estimate the temperature

increase due to the avalanche single pulse power dissipation via the following equation:

Equation 4

Where ∆t ~ 100 ns is the avalanche pulse duration (Equation 2).

With:

Equation 5

K thermal transient depends on the duration of the pulse. It can be estimated through the

thermal impedance curve using the following equation:

Equation 6

Table 2. Thermal data

Symbol

Parameter

Value

Unit

Thermal resistance junction-case max

4.17 (TO-220FP)

⁰

C/W

Thermal resistance junction-ambient max

62.5

⁰

C/W

t

E

Zth

T

avalanche

c

j

c

j

∆

⋅

=

∆

−

−

2

c

j

c

j

Rth

t

K

t

Zth

−

−

⋅

∆

=

∆

)

(

)

(

ns

ns

K

K

s

ns

100000

100

)

100

(

)

100

(

⋅

=

µ