POWER & ENERGY EFFICIENCY HANDBOOK
Why it’s tough to characterize SiC power MOSFETs Levi Gant, Xuning Zhang, Ph.D. | Littelfuse, Inc.
Switching transients and parasitics can combine to thwart the accurate measurement of important MOSFET operating parameters.
Silicon carbide (SiC) power MOSFETs get a lot of attention because they can switch fast while maintaining high blocking voltages. But their superior switching qualities also have potential drawbacks. Parasitic inductances caused by less-thanoptimal board layouts, along with the SiC MOSFET’s fast dv/dt and di/dt qualities, can create voltage and current overshoot, switching losses, and system instability problems. To head off such difficulties, designers must understand SiC MOSFET switching qualities in depth. Additionally, the extremely fast switching speeds of SiC MOSFETs also present challenges when characterizing the devices. For example, equipment selection can affect test and measurement accuracy. The highly sensitive design and integration schemes of the driving and power stages also play a role in minimizing voltage spikes, EMI, and switching losses.
ENSURING TEST AND MEASUREMENT ACCURACY Circuit and package parasitics and the high-speed switching of SiC MOSFETs all complicate characterization tasks. The fast dv/dt and di/dt amplify measurement inaccuracies, voltage/current ringing, etc. High dv/dt can produce large transient voltage spikes, as well as common-mode noise that can appear as damped oscillations. High di/dt generates noise that can couple with nearby magnetic fields. These effects can be difficult to measure and diagnose. It
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DESIGN WORLD — EE NETWORK
Littelfuse — P&EE HB 10-19.indd 28
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takes special tools and test methods to uncover hidden problems before they emerge during product qualification stages or as infant mortality failures. And it takes tools with exceptional bandwidth and dynamic range to characterize SiC power devices that are switching high levels of power at high speeds. Differential probes are commonly used for high-voltage measurements of this type. But though they offer built-in galvanic isolation, they have relatively limited bandwidth. In contrast, passive voltage probes have enough bandwidth but lack galvanic isolation. Additionally, many passive voltage probes are not rated for high voltages. If this is the case, a traditional voltage divider must be designed into the circuit as well, introducing another resistive load. All things considered, the best option for these voltage measurements is a passive voltage probe with a voltage rating high enough to capture high dv/dt transients. Four methods are commonly used for measuring current: a Rogowski coil, an active current probe, a current transformer, or a coaxial current shunt. Each method presents both pros and cons. For example, an active current probe and a Rogowski coil are unobtrusive in terms of incorporating them into the test circuit. However, they typically lack the bandwidth to measure current ringing effects. A current transformer likely has enough bandwidth to capture ringing frequencies. But it requires that current pass through its aperture – sometimes a tight bottleneck -- and can’t make dc measurements, a drawback it shares with the Rogowski coil. eeworldonline.com | designworldonline.com
10/10/19 9:11 AM
