What is Avalanche Test of Power MOSFETs? (Part 2)

Hello everyone.
I’m Nakamatsu from the Power Device Design Section.

This is a sequel to my last blog that explained the avalanche capability tests for power MOSFETs.

In the previous article, I showed that the avalanche energy EAS can be obtained by the following equation:

 

 

This time, we will use a MOSFET with certain parameters as an example for our calculations. Assuming that the avalanche voltage BVDSS = 600 V and the supply voltage VDD = 300 V, the case (1) with L load value L= 100 µH and avalanche current IAS = 20 A, and the case (2) with L= 400 µH and IAS = 10 A, will result in the same energy EAS = 40 mJ.

 

Now, can you compare avalanche capability of the two when their EAS is the same?

 

Let us consider the junction temperature of the chip. Naturally, Tj must not exceed the maximum junction temperature Tjmax of the chip even during the avalanche operation period. Let us assume Tjmax=150 °C and the case temperature Tc=25 °C.

Avalanche period tAV is:

 

 

Thus, tAV=6.67 µs for case (1), and tAV=13.3 µs for case (2).

 

Transient thermal resistance between the junction and its case Zth(t) is hard to manually calculate as it is, because of the For this reason, we assume Zth(t) to be as shown in Figure 1. In this case, the avalanche operation period is up to the order of tens of µs. Within this range, the slope can be approximated as a straight line with 1/2 slope on double logarithmic graphs. In this case, Zth(t) is expressed by the following equation.

 

 

Where a=10 KW-1s-1/2

 

Fig. 1 Transient thermal resistance of a device

 

Now that we are ready, let’s calculate Tj . During the avalanche period, the voltage is constant because it is clamped by BVDSS, and the current decreases linearly. Therefore, the loss during the avalanche operation period decreases linearly with time, resulting in the right angled triangle shown in Figure 2.

Here, let me explain only the main points about the figure. If the change in loss versus time has this shape and the transient thermal resistance can be approximated as shown in Fig. 1, the temperature will reach its maximum at half the time of the pulse, and the temperature change at that time will be 2/3 that of the case where the loss is constant.

 

Fig.2 Loss vs. time in avalanche operation

 

Therefore, in case (1)

 

 

In case (2)

 

 

You can see that Tj is above 150 °C in case (1), but below 150 °C in case (2).

Figure 3 shows a graph of Tj vs. time.

 

Fig. 3 Graph of Tj vs. time

 

Thus, even if the energy EAS is the same, the avalanche capability values differ depending on the L load value and transient thermal resistance, so care must be taken.

Also, when selecting a device, comparing devices measured under different conditions is not very useful. It is recommended that you not only refer to the data sheet values, but also make actual measurements in an environment as close to actual operation as possible.

 

WTI has a special test environment for power devices and engineers with rich expertise for special tests, so please contact us if you need evaluation of various power devices, including avalanche capability tests.

 

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