2 Short-Circuit Mechanism

To analyze the mechanisms of an SCT 1, the half-bridge model is shown in Figure 2-1. The two SiC MOSFETs S_HS and S_LS are controlled by a gate driver unit (GDU), respectively. Furthermore, two capacitors provide energy for the converter: the capacitance C_C provides the needed energy for the current commutation between the two SiC MOSFETs S_HS and S_LS, while the capacitance C_B is used as bulk filter at the output of the converter. The inductances L_σ,C and L_σ,B represent the sum of parasitic inductances of capacitors, MOSFETs and PCB traces. A common circuit breaker S_B connects the high-voltage battery U_batt to various power electronic devices.

Figure 2-1 Simplified Schematic to Analyze an SCT 1

In the event of a short circuit, it is crucial to isolate the fault to avoid further damage, such as fire hazards. Using the breaker S_B to disconnect the fault disrupts the operation of other functioning devices connected to the battery. One idea is to incorporate an additional mechanical or electronic fuse in each converter; however, this approach increases costs, which is not great in the price-sensitive automotive application. Consequently, there is a pressing need for a reliable and cost-effective short-circuit protection scheme to prevent thermal incidents.

Two of the short-circuit detection methods utilize an additional current sensor. As discussed in the Section 1.3, the current sensor is placed between the two capacitors C_C and C_B.

For the measurement, two SiC MOSFETs S_HS and S_LS form the half-bridge topology. The drain and source pins of the MOSFET S_LS are soldered together to make sure a low impedance short circuit to simulate SCT 1 on MOSFET S_HS. At time t₀, the GDU of MOSFET S_HS gets the turn-on command and the gate-source voltage of high-side SiC MOSFETs u_GS starts to increase.

Figure 2-2 Waveforms of a Failing MOSFET During an SCT 1

The current slope di_SC/dt is dependent on various parameters such as stray inductances and capacitances of the SiC MOSFETs [16]. The current i_SC reaches the peak at the time t₁ and decreases afterwords. The decreasing current can be explained due to the self-heating of the SiC MOSFET die, caused by losses and therefore increasing drain-source resistance [16].

Even though the current continues to decrease, more and more energy is dissipated in the SiC MOSFET as the drain-source voltage of high-side SiC MOSFETs u_DS stays high. Hence, the MOSFET keeps continuously increasing the temperature. At time t₂, the SiC MOSFET reaches the critical junction temperature and gets damaged in a low impedance state. As the current i_SC is not limited by the drain-source resistance anymore, i_SC starts to increase again. From now on, the short circuit cannot be isolated by the MOSFETs and as long as the battery provides energy, this short circuit is a potential risk of fire and smoke hazards. Therefore, there is a need for short-circuit protection to prevent damage to the MOSFETs. Current sensing methods such as shunt-based detection and hall-effect sensor- based detection, along with the voltage sensing method like the desaturation method, are selected as protection methods for analysis.