9034 Datasheet, PDF (5/7 Pages) Fairchild Semiconductor – Power MOSFET Avalanche Guideline

For example, the calculation of the temperature rise resulting from single 2kW power pulse
applied to FQA11N90C during 1µs can be expressed as follows:
T = P_DZ_ΘJC(1µs)
= 2000 × 1.49 × 10^–3 ≈ 3°C
The applied power is substantial but the temperature rise is only 3 degrees. Note that a power
dissipation rating specified in the datasheet is a steady state power rating, and in a relatively
short time the Power MOSFET can handle even greater power pulse.
In the above example, however, transient thermal resistance of 1µs is not available in Figure
6. In cases where the given time is too short and out of the graph range, the single pulse tran-
sient thermal resistance is known to be proportionate to the square root of time. So Z_ΘJC(1µs)
becomes
Z_ΘJC(1µs) = Z_ΘJC(10µs) × ₁--¹--₀--^µ--_µ--^s--_s-
= 4.72 × 10^–3 × 0.1 = 1.49 × 10^–3
where
Z_ΘJC(10µs): taken from Figure 6
The above thermal response is based on a rectangular power pulse. It is possible to obtain a
response for arbitrary shapes. However, since the mathematical solution would be very com-
plex, it would be easiest to convert it to an equivalent rectangular pulse. Some examples for
triangular and sine wave power pulses are shown in Figure 7.
P
0.7P
0.71t
t
P
0.7P
0.91t
t
Figure 7. Conversion of Power Pulses
The equation (3) can also be applied to applications that have repetitive pulses. The transient
thermal resistance for repetitive pulses can be approximated as follows^[3]
Z_ΘJC(t) = _t-^t--¹²- + _1 – ^tt---¹²-_ r(t₁ + t₂) + r(t₁) – r(t₂) R_ΘJC
=
t^{-t--12- RΘ J C}
+


1
–
t^{-t--12-}
Z_ΘJC(t₁
+
t₂)
+
Z_ΘJC(t₁)
–
Z_ΘJC(t₂)
(4)
where
t₁: pulse width of the power pulse
t₂: period of the power pulse
©2004 Fairchild Semiconductor Corporation
5
Rev. A, March 2004