AN4337 Datasheet, PDF (9/14 Pages) STMicroelectronics – power MOSFETs are considered rugged with respect to the avalanche

AN4337
I_AR and E_AS electrical thermal approach
Equation 12
Zth_j−a (τ
)
=
20us _{}125°C
−
(1 −
100ns _{) ⋅ 0.005°C}
20us
2 *123uJ
/W
⋅
2
⋅1230W


= 13.38°C
/W
So, in conclusion:
Equation 13
k(τ ) = ^{Zthj−a (τ )} = ^{13.38°C /W} = 0.214
Rth_{j−a(max)} 62.5°C /W
Figure 5. Z_thj-a thermal impedance
.
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=WKMD . 5WKMD
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Therefore, the MOSFET device is safe if the duration of repetitive avalanche energy pulses
is less than Æ®=7 ms.
Below, a specific example illustrates how to estimate the quantification of the safety margin
for a MOSFET in terms of E_AS 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 I_AR parameter) and
maximum leakage inductance, taking into account the E_AS=100 mJ fixed value data
specification.
From Equation 3 we have:
Equation 14
I_P(max) =
2E_as
⋅ (BVD_ss − V_out N )
L_lk ⋅ BVD_ss
⇒
I _P(max)
= 114A
With E_AS=100 mJ and L_lk=13 µH.
Equation 15
L_{lk (max)}
=
2E_as
⋅ (BVD_ss − V_out N )
I
2
P
BVD_ss
⇒
L_{lk (max)}
=
5.8mH
With E_AS=100 mJ and Ip=5.4 A (data specifications).
Note that specifying E_AS=100 mJ results in I_p(max) current and L_lk(max) leakage inductance
being much higher than typical values for real flyback converters. This means that, apart
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