Choosing a High-voltage Oscilloscope Probe for SiC/GaN Power Semiconductor Device Measurements

Figure 1: Wide-bandgap (GaN) power semiconductor device
waveforms captured using two, different probe topologies.
Click on any image to expand.

In our last post, we introduced you to a new tool on the Teledyne LeCroy website: The High-voltage Probe Selection Guide. To demonstrate the benefits of the guide, let’s explore further what must be considered when choosing an HV oscilloscope probe for power semiconductor device measurements.

Why are power semiconductor device measurements challenging?

Device Analysis in Switch-Mode Power Supplies

Figure 1: Setup for analysis of switching losses

in a switch-mode power supply's MOSFET

Our survey of testing switch-mode AC-DC power supplies started by looking at the variety of measurements one might make on these devices and why differential probes and amplifiers are often the best choice over passive probes. Subsequently, we examined the key sources of error in power-supply measurements and how to minimize them. Now it's time to start taking some measurements with an eye toward device analysis, particularly the switching transistor in a switch-mode supply.

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.

Testing Techniques For Switch-Mode Power Supplies

Figure 1: A simplified schematic of
a switch-mode power supply circuit

On its journey from wall socket to the device being powered, power typically passes through a switch-mode power supply, where the AC signal is rectified into DC before it reaches the device. After that, the DC signal (often 5 V) is passed on to DC-DC converters on the device's PC board for feeding various voltages to branches of the device's power-delivery network. Let's look at some of the measurement techniques and considerations relative to testing switch-mode power supplies.