Power Electronics Europe April/May Issue 2022

20 SILICON CARBIDE www.microchip.com/en-us/product/MOSFET-SIC-1700V Issue 2 2022 Power Electronics Europe www.power-mag.com development time from evaluation through production while enabling designers to change their control parameters throughout the design process. New set of best practices To ensure reliable, safe operation of SiC MOSFET-based power systems, digital gate drivers provide multiple levels of control and a higher level of protection than is possible with analog solutions. The drivers can dampen drain-source voltage (V ds ) overshoots by up to 80 % compared to their analog counterparts while cutting switching losses by as much as 50 %. They can also source/sink up to 20 A of peak current and include an isolated DC/DC converter with low capacitance isolation barrier for pulse width modulation signals and fault feedback. Digital gate drivers can be used to augment switching capabilities in several ways, including providing independent short-circuit response along with robust fault monitoring and detection. Unlike traditional analog gate drivers that control turn-off slope through gate resistors for normal and short-circuit situations, the latest digital gate drivers enable designers to much more precisely control MOSFET turn-on and turn-off, as shown in Figure 1. Furthermore, the second generation takes augmented switching even further to provide up to two steps of control at turn- on compared to the single step of traditional analog drivers, and up to three levels of control at turn-off. This ensures a “soft landing” during turn-off that is analogous to tapping a foot on the brakes of an antilock system. Four levels of short- circuit settings enable digital gate drivers to deliver similar advantages for controlling this secondary effect of SiC switching speeds. Figure 2 shows the benefits that the latest, more granular augmented switching solutions have on overshoot, ringing and turn-off energy, among other variables. It illustrates how the increasing demands of SiC require not only faster switching but also more precise, and dynamic, multi-step turn-on and turn-off. In each of these three examples, the graphical editor toolbar is shown on the top and an associated scope image is shown on the bottom. The scope image on the far left shows a base case with augmented turn-off disabled, while the others depict two turn-off configurations. The one in the center example shows the configuration’s impact on Turn-Off Energy Loss (E off ) and the one on the right shows how augmented switching settings control both voltage overshoot and EMI. Configurable augmented turn-on capabilities are particularly important in applications involving motors, which are highly susceptible to the rate of change of voltage (dV/dt). A dV/dt that is too high deteriorates the motor’s lifetime, increasing warranty costs. Manufacturers are working on higher-frequency motors but, until then, the only way to reduce dV/dt with analog gate drivers is to compromise efficiency by reducing SiC speed. Tuning this to speedily find the most appropriate compromise is only possible with digital gate drivers. Maturing SiC ecosystem Digital gate drivers are tightly integrated digital hybrid mixed-signal ICs that reduce end-product costs through a combination of processing horsepower and low component count. They are entering the market as production-qualified, fully configurable devices that are just one element of a total system solution for implementing SiC MOSFET-based designs. These solutions include the gate driver core, module adapter boards, a SP6LI low- inductance power module, mounting hardware, connectors to the thermistor and DC voltage, and a programming kit for the configurable software. Together, these elements provide a direct path from evaluation to production. Adapter boards are pivotal to maximizing flexibility, enabling designers to configure a gate driver’s turn-on/turn-off voltage and then use it across many different suppliers of SiC MOSFETs with different positive or negative voltage ranges. These SiC devices can be used without any redesign, even if they previously were used with an analog gate driver – the digital gate driver is simply reconfigured. Once a module has been chosen and the digital gate driver circuitry has been completed, the solution can immediately move into production. Because the core driver board will work across multiple adapter boards, designers can continue mixing and matching gate driver cores and adapter boards with a similarly fast path to production. Figure 3 illustrates how today’s maturing SiC ecosystem is transforming the design experience while accelerating time to market. Increasing a designer’s module options has important benefits. Some SiC MOSFETs have more robust intrinsic body diodes, and designers should select those that show no perceptible shift in tests of pre- to post-stress ON-state drain–source resistance (R DSon ). They do not degrade after many hours of constant forward current stress when conducting reverse current and commutating whatever energy remains after a switching cycle. Other MOSFET options show some level of degradation, and some actually become unstable, so reviewing SiC MOSFET test results is critical. Choosing correctly enables designers to eliminate the die cost and power module real estate that comes with adding an external antiparallel diode to solve the degradation problem. These savings, however, also come at the cost of potentially choppy body diode performance (some more choppy than Figure 2: Benefits that the latest, more granular augmented switching solutions have on overshoot, ringing and turn-off energy, among other variables