AN4337 datasheet(9/14 Pages) STMICROELECTRONICS

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AN4337

IAR and EAS electrical thermal approach

Equation 12

So, in conclusion:

Equation 13

Figure 5. Zthj-a thermal impedance

Therefore, the MOSFET device is safe if the duration of repetitive avalanche energy pulses

is less than Ʈ=7 ms.

Below, a specific example illustrates how to estimate the quantification of the safety margin

for a MOSFET in terms of EAS single pulse avalanche energy.

Using the same previous conditions derived from 30 W flyback converter, we can calculate

the theoretical maximum current (not accounting for the instant IAR parameter) and

maximum leakage inductance, taking into account the EAS=100 mJ fixed value data

specification.

From Equation 3 we have:

Equation 14

With EAS=100 mJ and Llk=13 µH.

Equation 15

With EAS=100 mJ and Ip=5.4 A (data specifications).

Note that specifying EAS=100 mJ results in Ip(max) current and Llk(max) leakage inductance

being much higher than typical values for real flyback converters. This means that, apart

Zth

123

1230

005

)

100

(

125

)

(









⋅

−

− τ

214

)

(

)

(

(max)

−

Rth

Zth

BVD

out

114

)

(

(max)

⇒

⋅

−

⋅

BVD

out

)

(

(max)

⇒

−

⋅

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