AN2344 Datasheet, PDF (15/27 Pages) STMicroelectronics – Power MOSFET avalanche characteristics and ratings

AN2344
Datasheet avalanche ratings
The total power includes the power conduction (P_COND) and switching (P_SW) losses:
Equation 6
P_TOT = P_COND + P_SW + P_AV
The STW9NK80Z is used again as an example (see section 3.3 ), however, in this case, the
avalanche is not a single event, but is repetitive, with a frequency, f=50kHz.
For the single event example, when the current is below I_AR and the T_J is below 150°C, the
device works in a safe operating mode. When the device is subjected to repetitive avalanche
events, it needs to be checked to see if it maintains the T_J below 150°C. Since power
dissipation for each avalanche pulse is 0.24mJ, the average avalanche power dissipation is
expressed as:
Equation 7
P_AV = E • f = 0.24mJ • 50kHz = 12W
If the device has a junction-to-ambient thermal resistance (R_thJA),
Equation 8
R_thJA = R_thJC + R_thCS + R_thSA= 10° C ⁄ W
and the average switching and conduction losses equal to 2W,
Equation 9
P_TOT = P_COND + P_SW + P_AV= 2W + 12W= 14W
then the average temperature is calculated as follows:
Equation 10
T_J = P_TOT • Rth_JA + T_A= 140° C + T_A
Given that the average temperature and peak temperature during the avalanche should be
higher, it is clear that the avalanche phenomenon is generating power so high that the
system thermal behavior cannot dissipate in order to maintain the T_J below T_J(max).
The only solution is to reduce the thermal resistance of the system by changing the heat
sink or redesigning the application to avoid the avalanche failure.
In order to obtain a more accurate computation, the only way to evaluate the maximum
temperature during repetitive avalanche is to calculate it by the Zth of the system. Despite all
the concerns already discussed about adopting the published Zth for the avalanche (see
Equation 1), the computation results for repetitive avalanche as given by manufacturers are
sufficiently guardbanded for real world applications.
Several methods can be used to find the steady-state maximum temperature during a
steady-state or after a finite number of avalanche occurrences.
The most frequently and sufficiently conservative equation used in order to find the
maximum temperature for periodic rectangular power pulses at steady-state is:
Equation 11
∆T _{J C ( m a x )} =
P_{0}
-^t-^p-
T
R_thJ
C
+
¹
–
t_{-T-p- }
Z t h_{J C} ( t_p )
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