SiC Power Device｜Application
Points to be Noted for Bridge Configurations – Probe CMRR
2024.08.29
Points of this article
・In measurements of a HS MOSFET in a bridge configuration, the common mode rejection ratio (CMRR) of the probe being used may decline at higher frequencies, resulting in larger waveform fluctuations.
・In measurements of VGS in particular, surges several volts in amplitude are measured, and so it is necessary to determine whether an observed waveform is the actual waveform or is a fluctuation due to an inadequate CMRR.
・Optically isolated differential probes have outstanding CMRR frequency characteristics, and enable observation of the actual waveform.
Points to Note When Measuring SiC MOSFET Gate-Source Voltages: Points to be Noted for Bridge Configurations – Probe CMRR
In measurements of a high side (HS) MOSFET in a bridge configuration, high-voltage differential probes or differential pair probes (*4) are used to observe waveforms. However, the common mode rejection ratio (CMRR) of the probe that is used may decline in the high-frequency region, so that waveform fluctuations become pronounced. Particularly when measuring the gate-source voltage VGS, surges of amplitude in the several-volt range are measured, and so it is necessary to determine whether the observed waveform is the actual waveform or is instead a fluctuation originating in an insufficient CMRR.
Fig. 13 compares waveforms during HS switching and during LS switching in a bridge configuration. The differential voltage probe used was a YOKOGAWA model 701297 (150 MHz, 1400 V). When the waveforms are compared, we see that during LS switching the commutation side (HS) VGS fluctuates considerably. This is caused by a decline in the probe CMRR when the commutation side changes with a high dV/dt such as between 20 and 50 V/ns.
Fig. 13. Comparison of VGS waveforms during HS switching and during LS switching
To confirm the reason for this behavior, Fig. 14 shows the results of measurement of the CMRR performance of the differential voltage probe. The positive and negative voltage probe heads were connected to the driver source terminals on the HS and LS respectively to perform measurements. The measurement method used is described in detail in the Tektronix “ABCs of Probes” Application Note (*5), and should be referenced as necessary.
Fig. 14. CMRR performance of isolated voltage probe
In the waveforms at turn-on and turn-off of Fig. 14, fluctuations occur in the potentials at the driver source pins on both the HS and the LS during switching. However, after the end of switching operation, the LS returns to the state before switching, whereas the HS remains at a constant potential. This is the error in the CMRR. The residual potential vanishes after some time (several microseconds) has elapsed. In these measurements, when VDS rises, the potential at the driver source terminal swings negative, and swings positive when VDS falls; but due to the characteristics of the differential probe, the changes may be in the opposite direction. Recently optically isolated differential probes have been commercialized by measurement equipment manufacturers as measurement devices that are not affected by CMRR, and are attracting attention as effective solutions to the problem of accurately measuring waveforms.
Below we compare the performance of an optically isolated type differential probe and a generic high-voltage differential probe. The optically isolated type differential probe is an optically isolated differential probe (TIVH08, MMCX50X) manufactured by Tektronix that adopts the company’s IsoVu® technology.
The board (P02SCT3040KR-EVK-001) used in measurements was provided with a pattern for mounting MMXC connectors, to which the optically isolated probe was connected. As indicated in Fig. 15, the optically isolated probe and the generic high-voltage differential probe were connected simultaneously to perform measurements. As has already been explained, in order to eliminate to the extent possible the effect on the measured waveform of the measurement location and the installation position of the differential voltage probes, short extension wires were soldered directly below the SiC MOSFETs as the voltage probe measurement locations, and 100 Ω damping resistors were connected. Fig. 16 shows the gate-source voltage VGS waveforms for each of the probes.
Fig. 15. Generic high-voltage differential probe (above) and optically isolated differential probe (below)
Fig. 16. Comparison of the CMRR performance and observed VGS waveforms in HS switching of the generic high-voltage differential probe and the optically isolated differential probe
Because switching is on the HS, the HS gate-source voltage VGS as observed using the generic high-voltage differential probe (green line) exceeds the driving voltage of 18 V after turn-on, and falls below 0V after turn-off, due to the decline in CMRR. On the other hand, the waveform observed using the optically isolated probe showed no signs of fluctuations at 18 V and 0 V attributable to the CMRR, and it is thought that the probe enabled accurate observation of the waveform during switching operation.
These results are also clear from the CMRR frequency characteristics shown in Fig. 17 (*4, *6). The CMRR frequency characteristic of the optically isolated probe is far superior to that of the high-voltage differential probe, and we see that common mode noise is adequately rejected even in the frequency region approaching 100 MHz.
Fig. 17. Comparison of the CMRR characteristics of a generic high-voltage differential probe and an optically isolated differential probe
IsoVu® is a registered trademark of Tektronix.
*4. Reference Material: “Inverter Circuit Evaluation Methods” Application Note (v. 1.3), Iwatsu Electric, December 2018
*5. Reference Material: “ABCs of Probes” Application Note (No. EA 60W-6053-14), Tektronix, January 2016
*6. Reference Material: “Complete ISOLATION Extreme COMMON MODE REJECTION” White Paper (0/16 51W-60485-1), Tektronix, 2016
SiC Power Device
Basic
- What are SiC Schottky barrier diodes? ? Introduction
- What are SiC-MOSFETs? – SiC-MOSFET Features
- What are Full-SiC Power Modules?
- Summary
- Introduction
- What is silicon carbide?
Application
-
Introduction
- SiC MOSFET Bridge Configuration
- SiC MOSFET Gate Driving Circuit and Turn-On/Turn-Off Operation
- Currents and Voltages Occurring Due to Switching in Bridge Circuits
- Behavior of the Gate-Source Voltage During Low-side Switch Turn-on
- Behavior of the Gate-Source Voltage During Low-side Switch Turn-off
- Summary
- SiC MOSFETs: Method for Determining Losses from Switching Waveforms
-
SiC MOSFETs： Snubber Circuit Designs ーIntroductionー
- Non-Discharge RCD Snubber Circuit Design
- Surges Occurring between Drain and Source
- Types and Selection of Snubber Circuits
- C Snubber Circuit Design
- RC Snubber Circuit Design
- Discharge RCD Snubber Circuit Design
- Non-Discharge RCD Snubber Circuit Design
- Differences in Surge Occurrence Depending on Package
- SiC MOSFETs： Snubber Circuit Designs ーSummaryー
- Points to Note When Measuring SiC MOSFET Gate-Source Voltages: General Measurement Methods
-
Conventional MOSFET Driving Method
- Packages Provided with Driver Source Terminals
- Differences Made by and Benefits of a Driver Source Pin
- Benefits of a Driver Source Terminal: Comparisons Using Double Pulse Tests
- Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-on
- Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-off
- Points to be Noted Relating to Board Wiring Layout Key Points of This Article
- Verification of Loss Reduction Using Latest-Generation SiC MOSFETs
- About Surges in Gate-Source Voltages
Product Information
- SiC Schottky Barrier Diodes
- SiC MOSFET
- SiC Power Modules
- SiC Schottky barrier diode Bare Die
- SiC MOSFET Bare Die
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