Ruggedness and Reliability – A Journey Beyond the Datasheet
Xuning Zhang, Tomas Krecek and Nitesh Satheesh
Transportation touches lives every day, moving people and goods from points A to B. An interruption in such a system would have a cascading effect. Trains especially are subject to a variety of weather systems that may influence the electronics used within. It is therefore important for transportation system developers to consider parameters that are not typically presented in datasheets. This is even more important in the case of wide bandgap power electronics such as Silicon Carbide (SiC), which is a novel material in such an application.
Microchip Technology’s SiC power devices are rugged, robust and apt for demanding applications within the transportation segment. A strong portfolio of standard, custom packaging options provide customers with flexibility in design. Digital programmable gate drivers, available as printed circuit board (PCB) plug-and-play or core drivers, provide engineers with tools to optimize system performance and tune the system to the application with minimal hardware modifications.
The toughness of SiC MOSFETs across wide-ranging conditions is essential for auxiliary power units (APUs) that power both conventional and emergency loads. The following must be verified:
- the stability and lifetime of the MOSFET’s gate oxide;
- the stability of the MOSFET’s body diode; and
- failure toughness measures such as avalanche ruggedness.
Stability and Lifetime of MOSFET Gate Oxide
To ensure the stable operation of the power converter, the power devices must have minimum threshold voltage shift and reliable device performance throughout the converter lifetime. Figure 1 shows how Vth data for production-grade SiC MOSFETs should exhibit no meaningful change after 1000 h of stress at 175°C.
The gate oxide lifetime can be predicted by accelerating samples to failure using elevated temperature and electric field. A production-grade SiC MOSFET gate oxide can last well beyond 100 years at high stress, ensuring confidence in routine, reliable APU operation beyond the designed service lifetime.
Body Diode Stability
A SiC MOSFET can conduct reverse current using its intrinsic body diode. Compared to an Insulated Gate Bipolar Transistor (IGBT) solution, using a SiC MOSFET with stable body diode enhances reliability and cuts cost by eliminating the antiparallel diode. However, the body diode reliability varies greatly across different suppliers. In some devices, this diode degrades over time, leading to an increased RDSon and more heat than designed. Figure 2 (left) shows body diode I-V curves and MOSFET ON-state drain-source resistance (RDSon) after many hours of constant forward current stress. Microchip devices under test shows no perceptible shift.
Transportation APUs are susceptible to a variety of fault conditions, demanding SiC MOSFETs designed to safely and reliably perform through these events, and to maintain consistent performance before and after faults. Avalanche ruggedness is one of the key demands. The cause of avalanche of a power device can be very often unclamped induction switching. The load current is suddenly dumped into the MOSFET, forcing the drain-source voltage to rise to breakdown. Unlike short circuit, the MOS channels are not enhanced; avalanche current crowds the die edge, rapidly taking the device to its thermal limitations.
Avalanche phenomenon is serious for power semiconductors due to possible lifetime degradation due to the electrical and overheat stressing. Repetitive unclamped inductive switching (R-UIS) is used to evaluate a device’s avalanche ruggedness. Figure 2 (right) shows time-dependent dielectric breakdown (TDDB) for commercial SiC MOSFETs before and after 100 000 cycles of R-UIS. Many suppliers maintain oxide strength but the ability of Microchip SiC MOSFETs with up to four times the toughness alongside stability in RDSon and drain-source leakage reinforces the SiC MOSFETs’ ability to safely ride through the most demanding electric overstress conditions.
Smart/Intelligent Gate Drivers Demand
As a gate driver represents an interface (very often galvanically isolated) between high and low voltage sides and, in addition, reliable gate control, monitoring and many other safety features, under any condition and/or circumstances it is one of the most important sub-systems from performance and reliability points of view. Under normal operational conditions the gate driver follows commands from the host controller to turn-on/off a power semiconductor. The converters require gate drivers with adjustable dead time, like that the gate driver provides enough time (dead time) to recover blocking capability of the device being turn-off. The voltage is applied to the gate to turn-on the power semiconductor switch affects the RDSon and hence is another important parameter to minimize conduction losses.
Finally, gate resistors define the switching transients speed and hence the time taken for the power semiconductor to turn-on or off. Designers often optimize these parameters according to various requirements. Reliability also means protecting the converter from faults, which can be, in the worst case, destructive. Simply, many parameters and features can be assigned to gate drives that suggest the question, can we have reliable drivers which can be configurable by software like on Figure 3 (right) instead of hardware? Microchip’s family of digital programmable gate drivers, like the one captured in Figure 3 (left), provide designers with full flexibility in adjusting the parameters per their application, load profile or other specific requirements. In addition, they offer fault feedback, which can be useful in fault diagnosis. On top of that, Microchip’s digital gate driver family provides basic DC link voltage and temperature measurement. Short circuit in power converters can become destructive if not properly managed. Protection through Microchip’s patented augmented switching limits fault current by detecting the fault sooner and limits overvoltage by managing the turn-off through a multi-step gate driving voltage.
SiC provides innumerable benefits in rail traction. Microchip’s SiC goes well beyond the datasheet in fulfilling demanding application requirements for rail traction.
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