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Folgendes deckt sich mit der IRF-App-Note, die ich hier schon verlinkt hatte:

Zitat:Second Breakdown

In power MOSFETs, the term ?second breakdown? refers to a sudden reduction in a MOSFET's blocking-voltage capability followed by a loss of current control by MOSFET current. Although in most applications, MOSFETs are typically not subject to second breakdown. This potentially destructive condition can occur as a result of thermal hot spots or ?current focusing? in the silicon, which in turn are caused by the spurious activation of the MOSFET's parasitic BJT.

Normally, when the current attempts to self-constrict to a localized area, the increasing temperature of the spot will raise the resistance of the spot due to a positive temperature coefficient, and will redistribute the current away from the hot spot.[1] This attribute facilitates parallel operation of multiple MOSFETs.

However, applications like programmable resistors and Class A, AB amplifiers cause the power MOSFETs to operate in their linear region, where they must dissipate higher power levels than in the more common on-off switching. In such cases, the current focusing and hot spots may not be self-correcting, which can lead to device failure.

In the linear mode, a power MOSFET is subjected to high thermal stress due to the simultaneous occurrence of high drain voltage and current resulting in high power dissipation. When the thermo-electrical stress exceeds some critical limit, thermal hot spots occur in the silicon causing the device to fail.[2]

Quelle: http://powerelectronics.com/power_semico...nd_stress/