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11 June 2015

Reducing Errors in Switch-Mode Power Supply Measurements

Figure 1: Skew between voltage and current probes
results in power measurement errors
Almost all portable electronic devices, and lots of non-portables, come with switch-mode power supplies. These range from common "wall warts" to the larger brick-sized supplies that power a laptop. We've taken a look at the typical measurements one might make on a switching power supply and at why single-ended measurement techniques should take a back seat to differential approaches. Now, let's see what steps we can take to ensure that our measurements on power supplies are accurate.

There are two primary sources of measurement error: DC offset errors and probe deskew. To counteract DC offset, most probes have an autozero adjustment that enables users to simply enter their oscilloscope's menu and press an autozero button. However, for DC power measurement, that isn't good enough. You'll need finer adjustment of the DC offset to really get it right, but we'll return to that shortly.

Then there's probe deskew, which concerns propagation delay differences from voltage and current probes. When we're measuring power, we need to apply both a standard probe for voltage and a current probe at the same time. If there is any difference in skew between the voltage and current signals to the oscilloscope, resulting perhaps from differences in probe cable lengths, there will be error in the power waveform resulting from multiplication of voltage and current waveforms.

Figure 2: A probe deskew calibration source
is the easiest way to cure deskew issues
between current and voltage probes
Skew between the voltage and current probes can bring about a good amount of measurement error. In Figure 1, both images show a current signal (falling edge) and voltage signal (rising edge) at top. Multiplying these together results in the power waveforms at bottom. On the left are the traces obtained using a properly deskewed voltage and current probe, while the traces on the right were acquired using non-deskewed probes. Circled at bottom are the device turn-off transition losses, and the difference is large: 7.88 nJ of energy for the deskewed probes and 13.43 nJ for the non-deskewed probes. This clearly shows why it is so important to ensure proper deskewing before taking measurements.

So if you find yourself having skew issues between your voltage and current probes, how do you set them right? How do you determine the proper deskew value? Deskewing voltage and current probes is most easily achieved with a probe deskew calibration source, which creates time-aligned voltage and current signals (Figure 2). This makes it an easy matter to dial in the proper deskew value.

Figure 3: Using the Math integral function during the
switching supply's off state to adjust for zero slope to
find the proper DC offset value
Returning to the subject of fine adjustment of DC offset for current probes, the same question applies as it does for deskew: What value should be plugged into the oscilloscope for DC offset? One method is to use your oscilloscope's Math integral function to adjust for zero slope. In the screen capture in Figure 3, the top trace in yellow is the voltage waveform, the middle trace in pink is the current, and the trace at bottom is power.

Note that during the off state, the voltage waveform rises in amplitude to several hundred volts, drops down to near zero, and then back up to the off state. While in the off state, we should not be conducting any current or losing any energy. If we apply the oscilloscope's Math integral function to the power waveform, we get the integrated waveform (in orange and circled). The integral of power is energy. The statistics column at bottom center is for the off state and the Power Analyzer software available in Teledyne LeCroy oscilloscopes and Motor Drive Analyzers measures energy consumption automatically, and we want to see the energy usage as flat as possible because we should not be using any energy at all in that off state. Now the dc offset adjustment can be fine-tuned to minimize energy loss in the off state.

Now that we've covered the bases regarding minimizing measurement errors, next we'll turn our attention to analysis of the switching transistor in our switch-mode power supplies.

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