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Selection of Measurement Points

2024.08.07

Points of this article

・Depending on the waveform measurement point, the observed waveform may differ from the actual one.

・Ideally, the measurement point should be as close to the DUT as possible; the best points are at the bases of the DUT terminals.

Points to Note When Measuring SiC MOSFET Gate-Source Voltages: Selection of Measurement Points

The next thing that must be considered is the DUT measurement point.

When actually observing the waveform of a DUT, often a probe or other part is connected at points where measurement is easy. These are cases in which typically a heat sink or the like is mounted on the DUT and direct measurement at the DUT terminals is difficult, or when the DUT is mounted on the PCB via through-holes and the protruding DUT leads on the rear face of the board are used as though they are test pins. As a result, the points at which measurement is possible are strongly influenced by the construction of the product, and it may not be possible to perform measurements at the optimal places. To demonstrate this influence, we compare measurements made at the following three points (the points for c and d being the same). Please refer to Fig. 9, which shows the several measurement points.

(a) DUT terminals, very close to the package (terminal bases)
(b) DUT terminals where they are soldered to the PCB
(c) Test pins soldered to the PCB
(d) Test pins soldered to the PCB, plus twisted wires and 100Ω

Fig. 9. Measurement points

Here (a) is the parts of the DUT terminals adjacent to the package, (b) is a case where the terminal soldering area on the board solder surface is measured, and (c) is at test pins, prepared in advance close to the DUT. (d) is the same point as (c), but twisted wires soldered in place so that the probe head is distant from the main circuit.

Fig. 10 shows the observed waveforms. We see that there are large differences in surge waveforms due to the different measurement points.

Fig. 10. Comparison of observed waveforms for different measurement points

In (a), measurement is performed at the places closest to the DUT, and so stable waveforms are observed. In the case of (b), because there is some distance between the bases of the DUT terminals and the measurement points, the observed waveforms include emfs that are induced as a result. As a result, the surge polarity is exactly the opposite of that in (a). In (c) and (d), closed paths are formed by the thin film wiring from the DUT to the test pins, and so the waveforms include noise. From these results, we see that measurement points should be as close to the DUT as possible.

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