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Analysis of the Influence of Time Offset on Measurement Results in Switching Loss Measurement
The power loss of switching devices is a critical factor in evaluating their performance, and it's also an advanced feature found in many modern oscilloscopes. Although many laboratories have the necessary equipment to measure power loss, the tools and probes required can be quite costly. However, if the time offset between voltage and current channels is not properly accounted for, all the test results may become unreliable or even meaningless.
What should you consider when measuring switching losses?
In practical testing, one channel of the oscilloscope is typically used to measure voltage, while another measures current. The software then multiplies the two signals to generate a power curve, and by integrating this over the relevant time interval, the total switching loss is calculated.
Two key factors must be taken into account:
First, ensure that the oscilloscope and probes have sufficient bandwidth to accurately capture the voltage and current waveforms during the turn-on and turn-off transitions of the device. Second, make sure that the phase relationship between the voltage and current is precisely aligned.
While the importance of bandwidth is relatively straightforward, ensuring accurate phase alignment is more complex. How can we guarantee that the voltage and current are measured in sync?
The impact of time offset on measurement accuracy
When there is a time delay between the voltage and current channels, the resulting power loss measurement can be significantly off—either too high or too low. The faster the switching speed of the device, the more pronounced this error becomes. Figure 1 shows a schematic of MOSFET turn-off loss measurement. It clearly illustrates that only after correcting the time offset can accurate results be obtained. This offset is common due to differences in probe design and cable length between the voltage and current channels.
[Image: Impact of channel delay on power loss testing]
Figure 1: The effect of channel offset on power loss measurement results
How to correct the channel offset?
As shown in Figure 2, a dedicated offset correction fixture can directly address time delays between the voltage and current probes. The principle is simple: the fixture generates synchronized pulse signals on both voltage and current channels. By observing the time difference between these pulses on the oscilloscope, the offset can be corrected manually or automatically.
This compact fixture is powered via USB, making it easy to use. Before and after correction, the waveforms are shown in Figures 3 and 4 respectively.
[Image: ZDF1000 offset correction fixture]
Figure 2: ZDF1000 offset correction fixture
[Image: Waveform before offset correction]
Figure 3: Waveform before offset correction
[Image: Offset corrected waveform]
Figure 4: Offset corrected waveform
Effect of extension cables on transmission delay
In addition to correcting the inherent offset between the probes, it's also important to consider the delay introduced by extension cables. A typical setup is shown in Figure 5. Since current clamps often cannot measure current directly on the PCB, the current signal is usually routed through an extension cable. These cables introduce a transmission delay, which varies depending on the material and length. For example, a standard copper extension cable introduces about 33.5 ps/cm of delay. This delay can be compensated using the oscilloscope’s built-in delay correction settings.
Similarly, the voltage probe’s extension cable can also cause a delay, which should be adjusted based on actual measurements. For instance, if the extension cable is 100 cm long, the current channel delay would be 3.35 ns. After using the offset correction fixture, the current channel should be set to lead the voltage channel by this amount.
[Image: Typical test schematic]
Figure 5: A typical test setup with extension cables
From the above discussion, it’s clear that when measuring the switching loss of high-speed devices using an oscilloscope, it's essential not only to ensure accurate waveform capture but also to account for time offsets between the channels. These offsets, often introduced by the probes or extension cables, can lead to significant errors in the final result. Therefore, always use an offset correction fixture to ensure precise power loss calculations.