Reliability testing of switching power devices requires the balance of several competing challenges. The first is to understand the intrinsic reliability under application conditions without destroying the device as failures are induced. To properly analyze this, a “soft-switching” methodology is useful. This approach applies a high-voltage signal when the device is OFF, followed by a high current surge when the device is ON. In the OFF state, there is minimal current (leakage), and in the ON state, there is very little voltage (assuming RON is small); hence, power dissipation in the device is minimized in either state.
A competing requirement for reliability evaluation is realizing a circuit that emulates a real-life application in what is referred to as a “hard-switching” operation. In a real application, due to reactive loading, there is a small interval of time during each cycle in which high current and high voltage can exist simultaneously in the device. For example, in a 1-MHz switching regime (i.e., 500ns ON and 500ns OFF), there might be 1- to 10-ns in which high current and high voltage exist simultaneously. Depending on the duty factor of such a condition, this can significantly increase the power dissipation in a device. While this is very application dependent, it is nevertheless a reality and can dramatically affect reliability.
Soft-switching is a specific mode of operation where power devices discreetly switch from a high voltage, low current state to a low voltage, high current without any overlap. This operation, paired with elevated temperature, provides the opportunity to look for a change in the Drain Source ON Resistance, RDS-ON. As resistance increases and a change in RDS-ON is detected over the part's lifetime, that can result in performance degradation of the device. So this Soft-switching test functions as an accelerated life test to make the change in RDS-ON occur in a reasonable amount of time. This allows power device manufacturers to extrapolate the product lifetime and perform analysis for process improvement.
The concept of “soft-switching” implies avoidance of the condition in which high-voltage and high-current may exist simultaneously. As noted earlier, this can result in excessive power dissipation, which can lead to extremely fast device destruction in a time frame for which it is impossible to protect – after which failure analysis is unavailable.
Because the requirement to pulse the devices on and off very quickly is challenging, the test system needs to have precise digital controls to operate the DUT in a controlled manner. Precise biasing of the device and measurement of the RDS-ON value are also required for successful testing since that measurement needs to be captured and tracked on a microsecond scale throughout a 1000-hour test.
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