Collapse effect of field-effect transistors in power applications 4

4. Analysis of Joint Surface Temperature

Generally speaking, if the voltage of Drain and Source exceeds the maximum rated voltage specified in the specifications by a slightly larger amount, it is not common to cause the field-effect transistor to collapse. The curve shown in Figure 4 represents the rated maximum operating voltage (BVDSS) vs junction temperature (Tj) of the field-effect transistor, and its characteristic is represented by a positive temperature variation. When the junction temperature reaches 120 ℃, BVDSS can reach nearly 990V. Therefore, it can be inferred that under higher junction temperature conditions, field-effect transistor require a higher voltage from Drain to Source to achieve the necessary conditions for the collapse effect to occur.

But it must be reminded that the BVDSS indicated in Figure 4 is defined based on the ID current of 250uA as a condition. When the actual crash occurs, the ID current is much greater than the range of uA. Therefore, the desired breakdown voltage will also be much higher than the voltage derived in Figure 4 above.

In practical considerations, the actual breakdown voltage of field-effect transistor is generally 1.3 times the rated low current breakdown voltage. Taking Figure 5 as an example, it represents a waveform that exceeds the maximum rated voltage but does not enter the collapse phenomenon. The voltage of the Drain Source has reached 668V>600V but is not embedded in the collapse effect.