Ensure test accuracy and effectively measure signals in SiC power electronic systems

SiC is being used in higher power, higher voltage designs such as electric vehicle (EV) motor drives, EV fast chargers, on-board and off-board chargers, wind and solar inverters, and industrial power supplies.

SiC is being used in higher power, higher voltage designs such as electric vehicle (EV) motor drives, EV fast chargers, on-board and off-board chargers, wind and solar inverters, and industrial power supplies.

As power system designers move to SiC, there are several challenges:

• Can the test equipment accurately measure the fast switching dynamics of SiC systems?
• How can I accurately optimize door drive performance and idle time?
• Do common mode transients affect measurement accuracy?
• Is the ringing I see real? Or the probe response result?

For engineers, solving these challenges is very difficult. Another point is that engineers need to see all of these signals accurately in order to make the right design decisions in a timely manner. Increasing design margins and overdesigning will only drive up costs and degrade performance. Using the proper measurement equipment is the key to solving the problem.

The accuracy of time domain measurements and switching loss calculations is affected by the accuracy, bandwidth, and delay of the probe used to acquire the measurement data. Although the focus of this discussion is on the differences between oscilloscope probes, the specific implementation (such as layout, spurious signals, and coupling) also plays a key role in measurement accuracy. Three important parameters, gate voltage, drain voltage, and current, need to be measured in order to properly verify power modules with SiC technology.

Gate voltage measurement

Measuring the gate voltage of a SiC power device is challenging because it is a low voltage signal (~20 Vpp) and the referenced node can have a high DC bias and high dv/dt with respect to the oscilloscope ground. Additionally, the largest dv/dt occurs during switching events, which is the time of greatest concern when measuring gate signals. Even in topologies where the device source is connected to ground, the parasitic impedance between the circuit ground and the oscilloscope ground can still cause erroneous readings due to fast transients. This requires the measurement equipment to be decoupled from ground with a very large common-mode rejection ratio. Such gate voltage measurements have traditionally been performed with standard differential probes (Figure 1a), but the latest opto-isolated probes, such as the IsoVu probing system (Figure 1b), can greatly improve the accuracy of such measurements.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 1. (a) Example of differential voltage probe: Tektronix differential probe THDP0200 probe and accessories;
(b) Tektronix lsoVu TIVP1 optically isolated probe (TIVPMX10X, ±50 V sensor tip).

Figure 2 compares high-side gate voltage measurements with a standard differential probe versus an opto-isolated probe. High frequency ringing can be seen on the gate of the device after it has passed the threshold region, whether it is off or on. Partial ringing is expected due to coupling between the gate and the power loop. However, in differential probes, the magnitude of the ringing is significantly higher than that measured with optically isolated probes. This may be due to reference voltage variations causing common-mode currents inside the probe and aliasing of standard differential probes. While the waveform measured by the differential probe in Figure 2 appears to pass the device’s maximum gate voltage, the opto-isolated probe’s measurement is more accurate, clearly showing that the device is within specification.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 2. Differential probe (blue trace) compared to IsoVu optically isolated probe (yellow trace).

Application engineers using standard differential probes for gate voltage measurements should be aware that they may not be able to distinguish between the probe and measurement system glitches shown here and actual violations of device ratings. This measurement glitch can cause designers to increase gate resistance, slow down switching transients, and reduce ringing. However, this does not necessarily increase losses in SiC devices. To this end, the measurement system used must accurately reflect the actual dynamics of the device in order to properly design the system and optimize performance.

Drain voltage measurement

Differential probes and ground-referenced probes are two commonly used voltage measurement methods in power electronics systems. Differential probes are a popular choice because they can be added to any node of a circuit without any problems. The ground reference probe needs to be implemented carefully, because its shield pin is connected to the ground of the oscilloscope. Improper implementation of reference ground level measurements generally results in small ground currents on the probe reference, significantly reducing the accuracy of the measurement. This effect is more pronounced in SiC designs, where high dv/dt can introduce parasitic currents into the oscilloscope probe’s ground reference, causing measurement errors. In more severe cases (when the ground-referenced shield is connected to a power signal), large currents can flow through the ground, damaging the probe or oscilloscope. In the worst case, a failed connection from the instrument to ground can cause the oscilloscope’s outer metal shell to float to the bus voltage, posing a serious threat to operator safety.

Grounding issues become even more critical when ground-referenced CVRs are used. As shown in Figure 3, when using ground referenced probes with the CVR, it is possible to bypass the CVR through the oscilloscope shield path. This causes current to flow through the oscilloscope through the device, potentially damaging the voltage probe or oscilloscope, and also presents a significant personal safety hazard. In general, differential probes are recommended for device drain-to-source measurements.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 3. When two ground-referenced probes are connected to reference planes of different voltages, device current bypasses the CVR and flows through the ground lead and the oscilloscope. This can lead to measurement errors and possibly damage to the equipment.

Current measurement

In power Electronic systems, current viewing resistors (CVRs) and Rogowski coils (Figure 4a and b) are two commonly used current measurement methods. Rogowski coils are a popular choice because they can be easily added to circuits and are a non-invasive measurement, but such probes typically have significant bandwidth limitations and are not suitable for use with SiC. On the other hand, CVRs have extremely high bandwidth, allowing accurate current measurements. Unfortunately, the addition of extra components when connecting transistors in series requires careful planning of the PCB layout, as adding CVRs generally increases parasitic inductance in the circuit.

Figure 4 compares typical SiC hard switching events measured by Rogowski coil and CVR. The bandwidth of the Rogowski coil is significantly lower, resulting in artificial suppression of the ringing present in the test waveform. More importantly, it artificially suppresses the initial overshoot, giving an early warning of the measured di/dt.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 4. CVR with Rogowski current probe, CAB016M12FM3 (TJ = 25°C, RG = 6.8, Vos= 600 V, Is = 100A).

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 5. CVR with Rogowski current probe, CAB011M12FM3 (TJ= 150°C, RG = 1W), VDS= 600 V, IS = 100A).

Figure 5 compares different probes under more aggressive switching conditions, and the comparison highlights two points of interest. First, when turned off, the Rogowski coil does not adequately capture the shape of the current waveform, missing a slight knee that reduces apparent switching losses. In addition, the predicted di/dt drop at turn-on also leads to a slower predicted switching loss. The cumulative effect of the reduced bandwidth of the Rogowski coil is a reduction in estimated switching losses.

Figure 6 directly compares the estimated switching losses in drain current for the Wolfspeed WolfPACK™ CAB011Ml2FM3. As mentioned above, Rogowski coils consistently underestimate the switching losses of the circuit when predicting, giving the impression that the circuit losses are too optimistic. Since the inconsistency is related to the probe bandwidth limitation, it depends on the edge rate of the Transistor, which increases further with more aggressive gate resistances. For low-speed switching technologies such as IGBTs, the metering differences are negligible.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 6. Estimated switching loss (Eoff + Eon) using different probes (CAB011M12FM3, TJ = 150°C, RG= 1W).

In addition to sufficient bandwidth and noise suppression functions, the probes used for head delay must also perform delay correction to ensure that the delays of voltage signals and current signals match. Voltage probe and current probe delay mismatches of even 1-2ns can result in Eon and Eoff measurement errors of 30% and above. Proper delay correction is critical to the fast switching transients inherent in SiC systems.

Before delay correction, the probe is automatically zeroed and calibrated, if necessary, to remove any offset or scaling errors. The voltage probes VDS and VGS can be corrected for delay by connecting the two probes to a function generator using symmetrical connections. Using the square wave generated by the function generator, check that the ringing and falling edges of the signal are aligned. The function generator and any voltage probe can be easily connected using the board shown in Figure 7. The function generator signal is connected to the center of the board, and there are various options for oscilloscope probe connections around the edge of the board to accommodate various probe interfaces.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 7. Power Measurement Delay Correction and Calibration Fixture (067-1686-00)7 to compensate for timing differences between voltage and current probes.

There are several ways to correct for VDS and ID probe delays to ensure correct measurement of switching losses. The principle behind all methods is the same, which is to have a test circuit, such as the fixture shown in Figure 7, as close as possible to a purely resistive circuit so that the voltage and current waveforms align. This test circuit can then be used to correct the current probe delay to match the voltage probe response.

Probe Attachment Techniques for SiC Circuit-Level Verification

When performing gate measurements, carefully consider connection options to ensure a clean signal is captured from the power conversion module. Since this is an ungrounded measurement at a higher voltage, the connection is critical. There are two main ways to connect: MMCX provides a modular prefab method for device connection, and the goal is to have a connector that can be switched to different PC board implementations.

MMCX style sensor tip cable (high performance, up to 250 V applications)

The IsoVu Gen 2 measurement system achieves its best performance when the MMCX connector is inserted near the test point. Figure 8 a and b show two different applications. These MMCX connectors provide high signal fidelity, and the solid metal body and gold contacts provide a well-shielded signal path. The paired MMCX interface provides a snap-on connection with positive fixation force for stable hands-free connectivity. The separation force provides a safe and stable connection for high pressure applications. MMCX connectors are available in a variety of configurations and can be transitioned to many applications, even if they are not designed into the board.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 8. MMCX Connector (a) Instance 1 (b) Instance 2

Guide to MMCX Adapter

When MMCX connectors cannot be used, tip cables can be adapted to accommodate industry standard guidelines. Tektronix provides probe adapters to connect the sensor tip cables to the pins on the circuit board. Tektronix offers two different pitch adapters: MMCX to 0.1″ (2.54mm) adapters and MMCX to 0.062″ (1.57mm) adapters. The adapter has an MMCX socket for connecting IsoVu tip cables. There is a center pin socket on the other end of the adapter and 4 common (shielded) sockets around the outside of the adapter. The groove on the adapter can be used to hold the shielded socket. Best electrical performance is achieved when the probe tip adapter is close to the board.

Pin-type sensor tip cable

The TIVP series (IsoVu Gen 2) products also include pin-type sensor tip cables that enable higher input differential voltage capabilities. These state-of-the-art interfaces are not only easy to connect, but also secure and safe for hands-free operation in high-pressure environments. There are two types of square sensor tip cables: 0.100˝ (2.54 mm) pitch, which can be used for applications up to 600V, and 0.200˝ (5.08 mm) pitch, which can be used for applications up to 2500V.

Unexpected test point

Ideally, test points are planned in advance and integrated into gate drivers or evaluation boards such as the Wolfspeed KIT-CRDCIL12N-FMC Wolfpack evaluation kit. In this scenario, the MMCX connector will provide the best performance, and if the signal of interest falls within the 300Vpk voltage rating, the MMCX connector is recommended.

Of course, we cannot always predict every possible test point. When specific circumstances require the addition of unintended test points (as shown in Figure 9), the following guidelines should be followed to ensure the highest measurement accuracy:

• Use MMCX connectors when the voltage rating allows.
• Position the connector as safely as possible as close to the IC or component.
• Likewise, any required flying leads should be as short as possible or no flying leads should be used.
• Mechanically strengthen the connector with hot melt adhesive, polyimide tape, or similar.

In the example, a pin header was added to the VGS test point after board assembly. The test points are reinforced with non-conductive hot melt adhesive for added strength.

Ensure test accuracy and effectively measure signals in SiC power electronic systems
Figure 9. Measure the high-side gate drive signal by soldering the pin header through the VGS node.

summary

In conclusion, wide bandgap semiconductor technology will play a huge role in the future development of power conversion and energy efficiency. SiC switches are smaller, faster and more efficient than their silicon equivalents. These technologies are used in a wide variety of applications, from electric vehicles to photovoltaic materials. Therefore, it becomes very important to test these technologies with the right tools so that designers can properly design, develop and integrate them into the final application.

Tektronix solutions play a key role. The IsoVu™ Isolated Probing System provides a floating, non-ground-referenced differential probing experience, ideal for gate measurement needs, with bandwidths from 200 MHz to 1 GHz, and a variety of probing tips that can be attenuated to support higher voltages when needed signal of. Series 5 MSO oscilloscopes are high-resolution (12-bit) oscilloscopes ideal for testing small voltages in the presence of much higher voltages; 8 channels view more timing signals simultaneously, optimize performance, and investigate correlations between large numbers of signals sex. The 5-PWR software is designed to run automated, accurate, repeatable power integrity measurements on 5 Series MSO oscilloscopes, including switching losses, conduction losses, RDS_ON, magnetic losses, SOA, and more under real operating conditions.

About Tektronix

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