Silicon carbide (SiC) MOSFETs offer great promise for maximizing the efficiency of power converters due to their extremely low switching losses. However, their short circuit robustness has long been a topic of discussion when determining if these devices are practical solutions for real-world power conversion applications.
Higher short-circuit current density and smaller thermal capacitance due to relatively smaller die sizes means that SiC MOSFETs’ short circuit withstand times are shorter than those of similarly rated silicon IGBTs. Fortunately, a recent study on 1200 V, 80 mΩ SiC MOSFETs suggests that proper gate drive design that supports faster response time can protect SiC MOSFETs from short circuit damage. This study will discuss the short circuit capability of 1200 V, 80 mΩ SiC MOSFETs. Results gathered from destructive testing performed under various operating conditions are presented and explained, as well as design trade-offs from both application and device perspectives. Additionally, another segment of this study compared the performance of various off-the-shelf gate drive ICs with de-sat protection functions.
SiC MOSFET Structure and Short Circuit Capability
When comparing Si IGBTs and SiC MOSFETs with similar current ratings, SiC MOSFETs have 5-10 times higher current density under short circuit conditions. Higher instantaneous power density and smaller thermal capacitance results in faster temperature rise and lower short circuit withstand time. MOSFET saturation current is primarily controlled by the design of the channel region. Although a shorter channel and higher on-state gate voltage are desirable for reducing on-resistance, they also increase the saturation current and reduce short circuit withstand time. This trade-off between on-state resistance and short circuit current is inherent in the design of SiC MOSFETs and is best resolved by designing gate drives with a faster response time than traditional IGBT gate drives offer. This allows designs that use SiC MOSFETs to accommodate circuit conditions without compromising the on-resistance or die size of the devices.
Destructive Short Circuit Test Results
A test circuit (Figures 1a and 1b) was designed for evaluating the short circuit capability of 1200 V, 80 mΩ SiC MOSFETs (Littelfuse LSIC1MO120E0080) (Figure 2) under various working conditions. High-bandwidth, high-voltage passive probes were used to measure drain-to-source (VDS) and gate-to-source (VGS) voltages; a Rogowski coil was used for device current (IDS) measurement.
Figure 1a. Short circuit test circuit schematic
Figure 1b. Short circuit test setup. An acrylic enclosure surrounds the test setup to protect equipment and the experimenter in the event of a catastrophic device failure or explosion.
Figure 2. The device under test for this evaluation is the Littelfuse LSIC1MO120E0080 Series Enhancement-mode 1200 V, 80 mOhm N-channel SiC MOSFET
Figure 3 presents the short circuit test results of 10 samples with 600 V drain voltage and 20 V gate voltage at room temperature. Figures 4 and 5 show the short circuit withstand time and critical energy results for destructive failure. The devices’ results have tight distribution with short circuit current of around 250 A and a short circuit withstand time of greater than 7 µs for all devices.
Figure 3. Short circuit test results for different devices
Figure 4. Withstand time under 600V DC drain voltage
Figure 5. Critical energy under 600V DC drain voltage
Figure 6 illustrates the short circuit test results under various drain voltages from 200 V to 800 V at a gate voltage of 20 V. Although the peak current is similar, at around 250A under all conditions, the short circuit withstand time dropped from more than 20 µs for 200 V drain voltage to 3.6 µs for 800 V. As the DC bus voltage increases, the instantaneous power dissipation also increases significantly. Consequently, the temperature rise is much faster, resulting in a lower short circuit withstand time.
Figure 6. Short circuit at different DC drain voltages
Figure 7 shows the test results with gate voltages of 15 V and 20 V for a drain voltage of 600 V. These results indicate that the peak current is strongly dependent on the gate voltage, decreasing from 250 A at a 20 V gate voltage to 100 A at 15 V gate voltage, which supports the design trade-off theory, regarding the driving voltage / on-resistance / short circuit peak current and withstand time relationship, discussed earlier in this article. Additional factors that were studied but proved to have no noticeable effect on the short circuit withstand time of the device were the external gate resistance and ambient temperature of the device.
Figure 7. Short circuit with different gate driving voltages
Short Circuit Protection via Gate Driver IC De-Sat Protection
Protecting a SiC MOSFET from short circuit failure requires the gate drive to detect the overcurrent condition and turn off the MOSFET within its withstand time. Several off-the-shelf driver ICs developed for Si IGBT devices offer an integrated de-saturation (de-sat) protection function that monitors VDS during the on-state and turns the device off if an overcurrent event occurs. The same driver ICs can be used for short circuit protection of SiC MOSFETs if the driver IC can respond quickly enough.
Figure 8 shows the circuit used to implement the de-sat functionality with the various driver ICs. The fast action Si diode (DD) blocks VDS in the off-state, the Zener diode (DC) protects the de-sat pin during the switching transition, and the capacitor (CB) controls the blanking time to avoid false triggering during the switching transient.
Figure 8. De-sat implementation
Figure 9 shows the waveforms of a short circuit event with a de-sat protection-enabled IC.
Figure 9. De-sat protection transient
Due to SiC MOSFET’s fast switching speeds and the required optimized power loop layout, the amount of time that the device voltage and current take to reach steady state after a transient event is much shorter than that of an IGBT. Thus, the required blanking time for the de-sat functions of the driver IC used in a SiC-based design should be much shorter as well. To protect SiC MOSFETs, a CB of less than 100 pF is usually selected and the blanking time can be as short as 200 ns to reduce the total response time of the driver IC.
Table 1 compares the performance of different driver ICs with a 33 pF blanking capacitor. The results indicate that each of the ICs can protect SiC MOSFETs during shoot-through within 1–4 µs. Each of them also has “soft turn-off” features, which turn off the device slowly under short circuit conditions to protect both the MOSFET and the driving ICs.
Table 1. Commercial Driver IC Evaluation
Short Circuit Protection in Half-Bridge Configuration
Finally, the SiC MOSFET and the gate driver IC with the longest response time were tested together under short circuit conditions in both single device and half-bridge configurations. Figure 10 shows the test waveforms during shoot-through transients with an 800 V DC bus voltage. In the half-bridge configuration, the top device was kept on during the whole transient and bottom device was driven by a gate driver with de-sat functions.
Figure 10. Left: Shoot-through protection with a single device. Right: Shoot-through protection in a half-bridge configuration.
The results in Figure 10 show that the SiC MOSFET can be turned off safely under both conditions. The results also show that in the half-bridge configuration, the DC link voltage is shared by two devices and the actual voltage per device is much lower than the DC bus voltage, which provides more margin to the protection circuit reaction time. If both the bottom and top devices have shoot-through protection functions, either one can protect the whole circuit.
This article has presented the short circuit capability of 1200 V SiC MOSFETs and solutions for overcurrent protection using off-the-shelf driver ICs. Several design considerations were discussed to ensure accurate measurements. Test results indicate that the short circuit capability of SiC MOSFETs is highly related with drain voltage and gate voltage, but it is not sensitive to case temperature and switching speed. Longer withstand time can be achieved by reducing gate driving voltage or reducing bus voltage, but these solutions will reduce the performance of SiC MOSFET.
A better solution is to implement overcurrent protection to sense device overcurrents and turn off the device safely. Commercial IGBT drivers with de-sat protection can protect SiC MOSFETs effectively, but the circuit needs to be optimized to ensure that the total response time of de-sat protection is short enough to protect SiC MOSFETs. The performance of various off-the-shelf gate drive ICs with de-sat protection functions was compared and a gate drive design that can protect 1200V SiC MOSFETs under real-life short circuit conditions was demonstrated.
This article was co-authored by Levi Gant, Gin Sheh, and Dr. Xuning Zhang, Littelfuse, Inc.