Silicon carbide (SiC) has proven to be the ideal material for high power and high voltage devices. However, it is extremely important that devices be reliable, and we are not only referring to short-term, but also long-term reliability. Performance, cost, and manufacturability are other important factors, as well, but reliability and ruggedness are key to the success of SiC. More than thirty companies all around the world have established SiC technology as a basis for the production of their power devices. In addition, several leading manufacturers of power modules and power inverters have laid the foundations in their roadmaps for future SiC-based products. SiC MOSFETs are about to definitively replace Silicon power switches; the industry is demanding new driving and conversion solutions able to address a constantly evolving market.

Performance and reliability

Performance can be assessed by running HTGB (high-temperature gate bias) and HTRB (high-temperature reverse bias) stress tests on a SiC power device. Littelfuse has performed stress tests on a 1200V, 80mΩ SiC MOSFET at a temperature of 175 °C, with different values of VGS and stressing the devices for up to 1000 hours. The results are shown in Figure 1.

Figure 1 Results of HTRB and HTGB stress tests (no relevant shift can be observed). Source: Littelfuse

Despite the excellent results obtained, the duration of HTGB+ test (VGS=+25V, T=175°C) has been extended to 5500 hours, whereas the duration of HTGB- test (VGS=-10V, T=175°C) has been extended to 2700 hours. Even in those cases, minimum deviations have been observed, confirming the performance and reliability of SiC MOSFETs under those conditions.

The gate oxide is a key element for SiC MOSFETs, and its reliability is therefore extremely important. The assessment of the gate oxide reliability has been divided into two parts. The first part was based on a TDDB (time-dependent dielectric breakdown) test. Depending on the electric field applied on the gate oxide (from 6 to 10 MV/cm), the device lifetime changes considerably. Figure 2 shows the results of this test, performed at different temperatures. In the second part, an accelerated gate oxide lifetime test was performed on a common 1200V, 18mΩ silicon MOSFET. The close agreement between the two test results confirms SiC MOSFETs are reliable devices, with a predicted lifetime exceeding 100 years when operating at T=175°C and VGS=25V.

Figure 2 Results of accelerated gate oxide lifetime test
Source: Littelfuse

Short circuit robustness

Another important aspect related to SiC technology is short circuit robustness. To check the short circuit robustness of its SiC power devices, Littelfuse has developed its own specific test board. The circuit, shown in Figure 3, includes a 1200V 80mΩ SiC MOSFET (DUT), an IGBT (Q1) used only for safety reasons, and three capacitors. The results are shown in Figure 4 depending on the applied gate voltage (12V, 15V, 18V, or 20V) the short-circuit withstand time varies significantly.

Figure 3 A short-circuit test circuit
Source: Littelfuse

Figure 4 A short-circuit withstand time at different gate voltages
Source: Littelfuse

The longest time (about 15µs) is obtained with the lowest gate voltage (12V). Moreover, the peak current is strongly dependent on the gate voltage, decreasing from almost 300A at a 20V gate voltage to around 130A at a 12V gate voltage. Even if the short circuit withstand time for a SiC MOSFET is shorter than the one for IGTBs, SiC devices can be protected by the de-sat function integrated into the gate driver IC.

Avalanche ruggedness

Another important parameter in evaluating SiC MOSFETs is the avalanche ruggedness, which is assessed through the unclamped inductive switching (UIS) test. Avalanche energy shows the ability of the MOSFET to survive transients sometimes incurred when driving inductive loads. Instead of performing the typical UIS test (which is a destructive test), Littelfuse’s approach to avalanche ruggedness assessment was based on the thorough characterization of SiC power MOSFETs to better understand their robustness.

A repetitive UIS (R-UIS) test was therefore performed on a 1200V 160mΩ SiC MOSFET, providing only 25% of the maximum energy the device could handle (125mJ instead of the maximum 500mJ). The test was repeated 100,000 times with a UIS period of 20ms. The SiC MOSFET demonstrated excellent R-UIS capabilities: no parametric shift could be observed on key electrical performances such as on-resistance, threshold voltage, breakdown voltage, and drain leakage current after 100,000 cycles of R-UIS stress test. Relevant information can also be obtained running a simulation. Figure 5 shows the results of a simulation performed applying a VDS of 1600V to the MOSFET: the electric field on the gate oxide can reach up to 4 MV/cm, with substantial heat generation. A proper shielding is usually applied to the gate oxide.


Figure 5
Gate oxide shielding is a crucial factor in achieving avalanche ruggedness. Source: Littelfuse

Solutions

The current Littelfuse portfolio includes the following SiC MOSFETs: 1200V 80/120/160mΩ and 1700V 750mΩ, all available in TO247-3L package. Other devices will soon reach production in the same package, plus similar devices which will be available in TO247-4L, TO263-7L (D2PAK) and TO268-2L (D3PAK) packages.

TO247-3L is the most common package, widely used in specific and general-purpose applications. TO-247-4L is a four-pin package which uses a Kelvin connection for the gate-drive source terminal. The inductance of the internal source wiring can thus be reduced, allowing the MOSFET to achieve a high switching frequency. The TO263-7L is a seven-pin package with Kelvin connection and suitable for surface mounting. TO268-2L is a two-lead package, with no pin in the middle, to ensure an optimal creeping distance.

SiC MOSFETs are about to definitively replace Silicon power switches, demanding for new driving and conversion solutions able to address a constantly evolving market. Thanks to their excellent thermal characteristics, SiC devices represent a preferable solution in various applications, such as power driving circuits in the automotive sector. SiC MOSFET transistors must be operated with a higher gate voltage, considering that the latter must have a fast dV/dt to achieve fast switching times. To meet the stringent requirements of next-generation MOSFETs, RECOM introduced various converters, specifically designed for powering SiC MOSFET drivers.

RECOM families RxxP22005D and RKZ-xx2005D have been specifically designed to meet the needs of the SiC MOSFETs increasingly growing market. The two series feature asymmetric outputs for controlling SiC drivers with input voltages ranging from 5V up to 24V. Insulation is indeed an important factor that has carefully been considered throughout the design: both new series offer maximum safety, with an isolation voltage around 4 kVDC. The parasitic capacities are strongly attenuated, thus eliminating oscillatory problems and allowing operation in power sharing mode (asymmetrical current, asymmetrical power). In addition to compliance with the RoHS directives, the new devices have UL-60950-1 certifications (Figure 6).


Figure 6 A typical application circuit with RxxP22005D [Source: RECOM]

The ever-increasing demand for high efficiency and high power density devices is a challenge for silicon-based semiconductors due to the intrinsic and physical characteristics of that material. Those limitations can be overcome by using wide band gap materials, such as SiC. Photovoltaic applications, for instance, require high-efficiency electronic components and SiC-based devices can perfectly adapt to those requirements. The opportunities for DC-DC converter manufacturers are not lacking. Companies are continually pushing the limits with new packaging projects, increasing integration levels.

Maurizio Di Paolo Emilio is a Ph.D. in Physics, telecommunication engineer, and Editor In Chief of Power Electronics News.

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