Silicon carbide (SiC) technology has surpassed traditional silicon (Si) insulated-gate bipolar transistor (IGBT) applications due to major thermal and electrical advantages for high-power systems. These benefits include higher switching frequencies, greater power density, improved operating temperatures, higher current/voltage capabilities, and overall better reliability and efficiency. SiC devices are rapidly replacing Si-based components and modules as both system upgrades and as new options for system design.
With the WolfPACK family of press-fit-pin, baseplate-less modules, designers have an industry-standard SiC upgrade readily available for designs that increase system reliability while bringing down BOM and maintenance costs.
The market for SiC devices spans a wide spectrum of power and applications (EV charging, energy harvesting such as solar and wind, power inverters, industrial power supplies, data centers). On the low end (1–50 kW), discrete MOSFETs help maximize topology flexibility, enable multi-source capabilities, and minimize total BOM cost. For the medium-power range (20–150 kW), the WolfPACK family of power modules offers an industry-standard footprint that can be scaled up as needed (Figure 1 shows the FM3 power module) and still provide flexible solutions and topologies. And on the high end (150–600 kW+), high-power modules exist, such as the XM3 series, that can be scaled to meet high-power demands with various DC busbar configurations.
As with most applications, there is a need to dynamically characterize and test the system during development. By measuring the key signals and parameters and then tweaking the layout and components, a designer can take full advantage of these high-performance SiC benefits and produce a more reliable, efficient system. Measuring and optimizing certain characteristics such as gate drive performance, switching energy and loss, deadtime, ringing/oscillation, and efficiency can be challenging for engineers coming from lower voltage and lower speed Si devices. It is important that the equipment and measurement methods used for verification/validation to both quantify and qualify the data is accurate, as well as fast enough to keep up with the high switching speeds.
This article will focus on methods for measuring SiC-related signals and recommend associated equipment while demonstrating examples with the CAB011MI2FM3 half-bridge power module by Wolfspeed.
SiC validation with gate and drain voltage measurements
There are three main areas of dynamic measurement for MOSFET switching that enable proper validation of the device: gate-to-source voltage (VGS), drain-to-source voltage (VDS), and current. From these parameters, the engineer can determine (and optimize) energy and switching loss, gate characteristics and stability, timing/switching speeds, and overshoot/ringing.
Measuring the gate voltage of a SiC power device is challenging because it is a low-voltage signal that is generally referenced to a node that may have a high DC offset and high dV/dt relative to the oscilloscope ground. Rapid transients can introduce erroneous readings due to parasitic impedances between circuit ground and scope/probe ground, requiring a device that is both decoupled from ground and features a large common-mode rejection ratio. Newer optically isolated probes (shown in Figure 2) can capture gate voltage measurement much more accurately than existing differential probes.
When comparing the optically isolated probe with the more traditional, differential probe, there is typically much more ringing on the differential probe at the gate due to the changing reference voltage inducing common-mode currents within the probe. This can make it difficult for designers when trying to discern behavior that can be attributed to the device under test versus artifacts from the probe itself. If misinterpreted, the measurement artifacts may cause the designer to increase the gate resistance to slow down switching and reduce ringing. False measurements on the gate often require significant reduction in dV/dt to eliminate, thereby dramatically increasing losses with no actual benefits. Understandably, it is critical to ensure that the voltage probe accurately reflects the actual dynamics of the system.
Figure 3 shows a comparison of the differential probe and the optically isolated probe measuring gate voltage. From the waveforms, the standard differential probe introduces additional ringing and oscillations that are not actually present. The optically isolated probe show much cleaner behavior, enabling a much better understanding of the device dynamics.
There are some benefits, however, to using the differential probe, such as measuring across arbitrary nodes of a circuit. When using the probe for the drain-to-source voltage, it’s important to reference your system in a way that prevents multiple grounding points. Figure 4 shows a ground-referenced probe that connects its shield to the oscilloscope’s earth ground. This can lead to small ground currents on the probe reference and reduce the accuracy of the measurement. With high dV/dt, as seen in SiC designs, this effect can be exacerbated due to parasitic currents flowing in the scope probe ground reference.
When using a current-viewing resistor (CVR), setting up grounding becomes even more critical. If measuring directly across VDS without considering the CVR, it can introduce unintended behavior due to multiple reference points relative to the oscilloscope. If the CVR is re-positioned so that its reference point aligns with the same one for the VDS measurement (providing an inverted signal, which can be adjusted via the oscilloscope), the ground loop will be eliminated resulting in increased accuracy.
See Figure 5 for a diagram of what is considered poor probe grounding. In this figure, two ground-referenced probes are attached to references at different voltage levels. This results in the device current bypassing the CVR and flowing through the ground leads and oscilloscope causing erroneous readings and possible damage to the equipment. In general, differential probes are recommended for measurements of drain-to-source voltage as they eliminate most grounding issues.
As previously referenced in the above figure, CVRs can be used to perform current measurements for a MOSFET. Another approach to measuring current is by using a Rogowski coil as it can easily be added to a circuit in a non-invasive manner. The drawback to the Rogowski coil is that they often have significant bandwidth limitations that usually make them unsuitable for SiC-based measurements due to the fast switching.
Figure 6 demonstrates the limitations of the Rogowski coil’s bandwidth and how it impacts the measured switching energy. CVRs offer high bandwidth and accurate current measurements, but they have their own set of challenges as well. This includes the need for additional components in series, which may require careful planning during PCB layout and the addition of unwanted parasitic inductance from the PCB. It is important to understand probe limitations when characterizing switching behavior of SiC MOSFETS. Knowing the appropriate frequency range and use cases of the probes utilized will help ensure that the right probe is utilized for the right situation. As demonstrated before, a Rogowski coil will under-report switching losses when inserted into the high-frequency power loop, but they make a great probe for measuring the phase/inductor current. A CVR, on the other hand, is not the best choice to use when trying to characterize the output current of an inverter due to its difficulty of insertion, steady state current limits, and lack of isolation.
Importance of probe de-skewing and connectivity techniques
In addition to using probes with sufficient bandwidth and noise rejection, probes must be deskewed in time to ensure that the voltage and current signals have matched delays. Without de-skewing, erroneous measurements can lead to switching energy calculation errors of more than 30%. Before de-skewing, it’s important to auto-zero and calibrate the probes as necessary.
De-skewing can be performed on voltage probes for VDS and VGS by connecting both probes to a function generator and using a square wave to check that both the rising and falling edges of the current, gate voltage, and drain voltage signals are aligned. This process can also be performed alongside a current probe using a test circuit board (test fixture) to compensate for timing differences between the two probes. Figure 7 shows an example of such a board.
When performing gate measurements, it’s important to consider the connectivity and cleanliness of the signal captured from the power-conversion module. The MMCX provides a modular, pre-fabricated approach to device connectivity, while the square-pin approach has a connector that is adaptable to different PCB implementations (see Figure 8).
The MMCX connectors are industry-standard and offer high fidelity and a well-shielded ground path. For maximum performance, the connector should be inserted as close as possible to the voltage nodes under test. When an MMCX connector is unavailable, the next best approach is a tip cable that can be adapted to fit onto industry-standard square pins. The adapters shown in the figure above have an MMCX jack for connection to an IsoVu probe tip, and the best performance is achieved when the probe tip adapter is located as close as possible to the circuit board.
The TIVP series of probes also provide a square-pin-style, 0.100”-pitch adapter, which is capable of measuring large differential voltages as well as provides an easy, secure connection for hands-free operation.
Ideally, test points are predetermined and integrated into the PCB layout of the gate driver or evaluation board, such as the Wolfspeed KIT-CRD-CIL12N-FMC. When there aren’t enough test points available but a connection to a signal is necessary for testing, unplanned test points can be added via the following procedures/guidelines:
- Use an MMCX connector when voltage ratings allow.
- Position the connector as close to the IC or component as is safely possible.
- Keep any required flying leads short or non-existent.
- Mechanically reinforce the connector using non-conductive hot glue, Kapton tape, or similar
Figure 9 shows how a square-pin header can be soldered across VGS nodes to measure a high-side gate signal.
To summarize, SiC technology has pushed power electronics systems to be smaller, faster, and more efficient over traditional silicon components. With a wide variety of applications spanning from eco-conscious industries like EV charging and solar to high power inverters and data centers, it becomes important to properly test and characterize for not only reliability but also to take full advantage of the efficiency and power density enabled with SiC.
IsoVu isolated probing systems provide a floating, non-grounded differential probing experience well-suited for gate measurements, while the 5 Series MSO oscilloscope is a high-resolution scope ideal for testing small voltages in the presence of much higher voltages. When measuring drain-to-source voltages, make sure that grounding doesn’t lead to parasitic currents flowing back into the probe.
Consider the differences between CVRs and Rogowski coils for measuring current and how the switching conditions can lead to false test results. Verify that probes are properly de-skewed to ensure that both voltage and current signals have matched delays. Reference designs and evaluation kits are provided to help guide engineers through the design process, while software suites such as 5-PWR are designed to run automated, accurate, and repeatable power measurements on the 5-Series MSO oscilloscope.