Analysis of issues to pay attention to and influencing factors in switching loss measurement

Power loss is an important part of the performance evaluation of switching devices and is also an advanced function that many oscilloscopes offer as optional extras. Although many laboratories are equipped with a power loss measurement environment and have invested a lot in equipment and probes, if engineers ignore the time offset between probes, the test results may become meaningless.

1. What issues should be considered in switching loss measurement

In the actual measurement and evaluation, we use one channel to measure voltage and another channel to measure current. The software then multiplies them to obtain the power curve and then integrates them over the time interval to obtain the final result.

Two points to note:

Ensure that the bandwidth of the oscilloscope and probe is sufficient to accurately obtain the waveform of the switching device during the opening and closing process;

Accurately measure the phase to ensure the correspondence between voltage and current.

It is easy to understand that the bandwidth is sufficient, but how can we ensure the correspondence between voltage and current?

2. Impact of time offset on measurement results

When there is a time offset between the voltage channel and the current channel, the measurement result is obviously too high or too low, and the faster the switching speed of the device, the more obvious the impact of the offset. Figure 1 is a schematic diagram of the MOS tube turn-off loss measurement. It can be seen that only after correction can the correct measurement result be obtained. It is worth noting that this offset is common due to the differences in the implementation principles of voltage probes and current probes and the length of the probe transmission cable.

Figure 1. The impact of channel offset on measurement results

3. How to correct channel offset

As shown in Figure 2, the offset correction fixture can directly correct the time offset between the voltage probe and the current probe. The basic principle is that the fixture generates a set of voltage and current pulse signals with zero phase difference and acts on the voltage and current probes at the same time. The time offset of the pulse signal after passing through the probe is observed through an oscilloscope, and the offset time is corrected on the oscilloscope.

The fixture is powered by a USB port and is easy to use. The time offset correction can be done manually or automatically. Using the offset correction fixture, the waveforms before and after correction are shown in Figures 3 and 4, respectively.

Figure 2 ZDF1000 offset correction fixture

Figure 3 Waveform before offset correction

Figure 4 Waveform after offset correction

4. The impact of extension lines on transmission delay

In addition to performing offset correction, in actual measurements, attention should also be paid to the influence of extension cords. A typical test diagram is shown in Figure 5. Since current clamps usually cannot measure current directly on the PCB board, the current needs to be led out through extension cords. Extension cords will introduce transmission delays. Ordinary copper extension cords can be calculated at 33.5ps/cm and compensated for by the delay correction parameters of the oscilloscope.

Similarly, the extension line of the voltage probe will also cause transmission delay, which can be compensated by adjusting the delay correction parameters according to the lead-lag relationship in the actual measurement. Taking Figure 5 as an example, assuming that the length of the extension line is 100cm, the delay time of the current channel is: 33.5ps/cm×100cm=3.35ns. After the offset correction fixture is used to complete the correction, the delay correction time of the current channel should be adjusted to be 3.35ns ahead of the voltage channel.

Figure 5 Typical test diagram

V. Conclusion

From the above analysis, we can see that when using an oscilloscope to measure switching loss, in addition to ensuring accurate waveform measurement, we must also pay attention to the time offset between channels. This time offset introduced by the probe will introduce a large error to the measurement itself. Therefore, when accurately evaluating power loss, it is necessary to use an offset correction fixture to correct the channel delay.