A Digital-Controlled SiC-Based Solid State Circuit Breaker with Soft Switch-Off Method for DC Power System

Abstract

Due to the lower on-state resistance, direct current (DC) solid state circuit breakers (SSCBs) based on silicon-carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) can reduce on-state losses and the investment of the cooling system when compared to breakers based on silicon (Si) MOSFETs. However, SiC MOSFETs, with smaller die area and higher current density, lead to weaker short-circuit ability, shorter short-circuit withstand time and higher protection requirements. To improve the reliability and short-circuit capability of SiC-based DC solid state circuit breakers, the short-circuit fault mechanisms of Si MOSFETs and SiC MOSFETs are revealed. Combined with the desaturation detection (DESAT), a “soft turn-off” short-circuit protection method based on source parasitic inductor is proposed. When the DESAT protection is activated, the “soft turn-off” method can protect the MOSFET against short-circuit and overcurrent. The proposed SSCB, combined with the flexibility of the DSP, has the μs-scale ultrafast response time to overcurrent detection. Finally, the effectiveness of the proposed method is validated by the experimental platform. The method can reduce the voltage stress of the power device, and it can also suppress the short-circuit current.

1. Introduction

The direct current (DC) power system has been playing an increasing role over the past decade due to the high efficiency and high reliability. The DC power distribution system is commonly used in many applications, such as ships, tanks and aviation. The DC circuit breaker is one of the key components in low voltage direct current (LVDC) and high voltage direct current (HVDC) systems [1,2], and a breaker is usually used to isolate the circuit when it is overload or short circuit. In the field of multiterminal DC compact node feeding system, such as electric vehicles, secondary distribution networks, and photovoltaic systems, the fault protection is extremely required [3,4,5]. Traditional electromechanical circuit breakers, with low reliability and high cost, are too slow to respond to a DC fault current. This makes them hard to meet the high requirements of the DC power system [6]. To tackle these issues, solid state circuit breakers (SSCBs) have been investigated and developed in recent years, but these SSCBs are mainly based on silicon (Si) devices, such as Si isolated gate bipolar transistor (IGBT) or Si gate turn-off thyristor (GTO), which causes significant conduction losses due to the large on-state resistance [7]. Another drawback is that Si devices cannot operate at high-temperature conditions due to insufficient thermal capability, resulting in additional radiators with the increase of both volume and weight. Due to the low on-state resistance, high switching speed [8] and excellent thermal conductor of silicon carbide (SiC) devices [9,10,11], SiC-based SSCBs are expected to be widely employed in future DC power system to achieve high power density and to withstand the high-temperature environment.

However, compared with Si metal-oxide-semiconductor field-effect transistors (MOSFETs), SiC MOSFETs tend to have a lower short-circuit withstand time due to the smaller chip package and higher current density [12,13]. In addition, the stability of the SiC MOSFETs gate oxide interface is lower owing to the physical structure of the device and the characteristics of the SiC material. This means SiC MOSFETs cannot operate stably and reliably under extremely harsh working conditions, such as an overcurrent or short circuit [14]. One of the most important features of SiC SSCBs is that the system can respond to the fault in a very short time interval. When an overcurrent or short circuit occurs, the current flowing through the device rises rapidly and the power loss will also increase. This will result in the increase of junction temperature, and the performance of SiC MOSFETs will be decreased [15]. Therefore, the short-circuit mechanism of SiC MOSFETs should be fully investigated and the effective protection method is also needed.

Several studies have undertaken into SiC-based SSCBs. In [16], a bidirectional solid-state circuit breaker with bipolar current actuated gate driver is studied, and [17] introduces a new self-powered SSCB concept with the utilization of a normally-on SiC JFET as the main static switch. However, the fault protection of the switch is not provided and analyzed. To address the overvoltage and the resonant current, a novel control method with the slope transition in the turn-off period is proposed in [18], in which the transient overvoltage and the resonant current during the interruption process are suppressed. To minimize SSCB losses, a package method is proposed in [19] with the aim of high-temperature operation by simplifying the cooling system. Meanwhile, [20] proposes a single isolated gate driver to minimize conduction losses, and a photovoltaic driver is used to control SSBCs based on SiC MOSFET [21]. Among them, the inrush current is suppressed, but the short-circuit protection is not considered. To study the short-circuit characteristics, the output current of a SSCB under overcurrent and soft-start conditions are discussed in [22], and the design method and operation principle of the SSCB under overload current condition are analyzed in [23]. However, within the academic literature, studies of power devices in SSCBs under overload and short-circuit conditions are rare and not comprehensive, and device-level issues, such as damaged conditions, are not addressed.

In this paper, the schematic diagram and the operating principle of SSCB under over current and short-circuit conditions are introduced. The device deterioration mechanism of SiC MOSFETs is also revealed based on the internal electric field distribution and the characteristics of SiC material. Then, the gate-source voltage clamp method is discussed. With the combination of the desaturation current detection, a “soft turn-off” protection method based on the common source parasitic inductor is investigated which can effectively inhibit the voltage overshoot and short circuit peak current. Also, the optimization of the detection circuit is discussed to shorten the response time to the fault. With the “soft turn-off” method, the SSCB reliability during the short circuit and overcurrent will be further increased. As a case study, a SiC-based SSCB is tested to validate the effectiveness of the proposed method.

The remainder of this paper is organized as follows. In Section 2, the basic operation principle of SSCB is given. In Section 3, the overall short-circuit behavior of SiC MOSFET under hard-switching fault (HSF) condition is presented from a physical standpoint. In Section 4, the implementation approach of the short-circuit protection is analyzed. Experimental results are presented and analyzed in part V, and, finally, conclusions are drawn in the last section.

2. Circuit Topology and Operation Principle

It is required that the circuit breaker should work with a high level of safety and reliability to either switch fast or reduce the fault current. Several breaker topologies are analyzed and discussed in [24]; however, both of them use more than two switches, this will increase the cost and complexity of the circuit. The topology of the SSCB with SiC MOSFET is proposed in [25], the fault detection circuit is composed of two resistors to detect variations in gate voltage. Due to the losses on the resistors, the current sensor is used in our study.

A simplified block diagram of the SSCB is shown in Figure 1. The power circuit comprises a voltage source (U_DC), a DC-Link capacitor (C_DC), a switch, a snubber circuit and a load. The switch in this study is a SiC MOSFET, which remains at the on state under normal conditions, and turns off when a short-circuit fault is detected. A snubber circuit is used in the topology to suppress the spike voltage of the SiC MOSFET induced by the parasitic inductor in the initial turn-off stage to protect the voltage on the device from exceeding the rated voltage. The control circuit is used to detect the fault and generate the switch signal, and it is composed of auxiliary power, drive and protection circuit and control unit. A DSP is used to control the operation of the SSCB. The current sensor is to detect the load current i_d, and the current signal will be sent to the control unit to drive the MOSFET. The drive circuit is used for the isolation and protection of the MOSFET device. When the overcurrent or short-current occurs, the drive circuit will be disconnected from the power circuit to protect the MOSFET from being damaged.

Figure 2 shows the basic operation principle of the SSCB, and the working states of the SSCB can be divided into three stages, namely, startup, normal conduction and shut down under fault. In Figure 2, U_Fault is used to detect the equipment error, when the short-circuit fault occurs, U_Fault turns to high and it keeps high for a period of time. U_gs is the gate-source voltage of the MOSFET, U_ds is the drain-source voltage, U_load is the load voltage, and I_d is the drain current.

Startup [t₀~t₁]: before t₀, SSCB operates at the shutdown mode. Then, SSCB turns on and the DC source can supply power to the load. For capacitive load, the drain current I_d increases sharply and the SSCB will withstand large current stress.

Normal conduction [t₁~t₂]: SSCB works under a normal load current. Compared with a Si MOSFET, SiC MOSFET can decrease on-state losses and reduce heat stress.

Shutdown state [t₂~t₃]: a short-circuit condition occurs at t₂ and then SSCB switches off to protect the system. The drain current I_d reaches up to the peak instantaneously when a fault is detected and the loss will also increase accordingly. Then, I_d reset to 0 when the MOSFET is completely turned off. When the fault is cleared, SSCB will restart and then repeat the above process.

When SSCB starts with a capacitive load or works under short-circuit conditions, the self-heating of the device would make junction temperature increase rapidly, and the device loss will be further increased. This will deteriorate the working condition of the SiC device. Therefore, the short circuit characteristics of the SiC device for developing the SiC-based SSCBs should be evaluated and investigated.

3. Short-Circuit Mechanism of Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

The short-circuit faults of the power device include fault under load (FUL) and hard-switching fault (HSF). Compared with the FUL condition, the HSF condition will cause more losses, and the additional energy is dissipated with the increased thermal stress of the SiC MOSFETs. So, HSF condition is mainly analyzed in this paper.

3.1. The Short-Circuit Behavior of the Silicon Carbide (SiC) MOSFET

The overall short-circuit behavior of the SiC MOSFET under HSF condition is shown in Figure 3 and can be divided into four stages.

Before t₁, SiC MOSFET is off, and the breaker is closed. Stage 1 [t₁~t₂]: the SiC MOSFET turns on at time t₁. The drain current increases rapidly due to the small inductance of the main power loop. A reverse voltage is introduced by Equation (1):

$$U_{\mathrm{Lloop}}=L_{\mathrm{loop}}\times \frac{\mathrm{d}i}{\mathrm{d}t}$$

(1)

where L_loop is the total inductance of wires, U_loop is the voltage of L_loop, which is applied against U_DC, and di/dt is the current variation. Thus, the drain-source voltage of SiC MOSFET is determined by Equation (2):

$$U_{\mathrm{ds}}=U_{\mathrm{DC}}-U_{\mathrm{Lloop}}=U_{\mathrm{DC}}-L_{\mathrm{loop}}\times \frac{\mathrm{d}i}{\mathrm{d}t}$$

(2)

The power loss of SiC MOSFET increases with the rise of i_d, resulting in the temperature rise and the rise of conduction resistance. The i_d keeps increasing during the short circuit period due to the increase of the channel mobility and the temperature.

Stage 2 [t₂~t₃]: the MOSFET device is in the saturated conduction state as the drain-source voltage of MOSFET is approximated to the DC bus voltage. The semiconductor junction temperature increases rapidly due to the self-heating phenomenon. The temperature rise will result in the reduction of carriers mobility in the channel (and drift region), thus, the short-circuit current tends to a negative slope. The channel carrier mobility of SiC MOSFET shows a negative temperature coefficient and can be estimated as [26]:

$${\mu }_{\mathrm{p}}\left(T\right)={\mu }_{\mathrm{p}0}{\left(\frac{T}{300}\right)}^{-2.2}{\mu }_{\mathrm{n}}\left(T\right)={\mu }_{\mathrm{n}0}{\left(\frac{T}{300}\right)}^{-2.6}$$

(3)

where µ_p is the epitaxial layer hole mobility, which is a function of the temperature T, µ_n is the epitaxial layer electron mobility of 4H-SiC, and also a function of T. And µ_p0 is the hole mobility and µ_n0 is the electron mobility, respectively, when T is 300 K.

Stage 3 [t₃~t₄]: the current variation rate di/dt of the short circuit current changes to be positive with the continuous increase of the junction temperature. This is probably caused by the thermally-assisted impact ionization, and the decrease rate of electrons current in the MOS channel is lower than the increase rate of the leakage current. The leakage current I_{g_ther} can be estimated as [27]:

$$I_{\mathrm{g}\_\mathrm{ther}}=\frac{qS{n}_{\mathrm{i}}}{{\tau }_{\mathrm{g}}}\sqrt{\frac{2{\epsilon }_{\mathrm{s}}}{q}\left(\frac{N_{\mathrm{d}}+N_{\mathrm{a}}}{N_{\mathrm{d}}{N}_{\mathrm{a}}}\right)U_{\mathrm{DC}}}$$

(4)

where q is the total charge, S is the active area of the MOSFET, n_i is the intrinsic carrier concentration, τ_g is the excited state lifetime, ε_s is the dielectric constant, N_a and N_d are the doping concentration in P-well region and N drift region, respectively, and U_DC is the DC bus voltage.

The short-circuit energy E_C can be calculated as [27]:

$$E_{\mathrm{C}}={\displaystyle {\int }_{t_1}^{t_4}{U}_{\mathrm{ds}}{I}_{\mathrm{d}}\mathrm{d}t}$$

(5)

where, U_ds is the drain-source voltage and I_d is the drain current of the MOSFET.

Stage 4 [t₄~]: When the device is switched off at t₄, the drain current decreases to zero. Then, three cases would occur, (a) SiC MOSFET switches off safely and reliably; (b) the gate-source oxide of SiC MOSFET is broken down and the device becomes uncontrolled; (c) the tail leakage current still exists when the MOSFET is turned off, and eventually leads to the thermal runaway and the device failure. What happens after t₄ mainly depends on the conduction time of MOSFET after the fault occurred. Due to the rapid current variation in the stray inductance, there is a spike on the drain-source voltage, which increases the stress of SiC MOSFET.

3.2. Analysis of Short-Circuit Capability

As shown in Figure 4, a pulse test circuit with fast-switching characteristic is presented to measure the short circuit capability of the MOSFET. The input voltage U_DC is a controlled DC voltage source, and R_in is the internal resistance of the power source. A single pulse voltage U_G is applied to drive the MOSFET with a series connected gate resistor R_g. CB is the circuit breaker. C_gs, C_ds, C_gd are parasitic capacitors of the MOSFET. Here, we compare a 1200 V SiC MOSFET (SCH2080KE) and a 1200 V Si MOSFET (IXFH12N120P). Capacitors are noted as C_gs, C_ds, C_gd are 1850 pF, 20 pF, 175 pF in SCH2080KE, while C_gs, C_ds, C_gd are 5400 pF, 40 pF, 290 pF for IXFH12N120P. This shows that the parasitic capacitors of Si MOSFET are much bigger than those of Si MOSFET. L_g, L_d and L_s are parasitic inductances ranging from 10 nH to 50 nH, and they include all the parasitic inductors of the MOSFET as well as the parasitic inductors of the PCB.

During the short-circuit transient, the DC bus voltage U_DC is applied to the MOSFET. Since the gate-source voltage of MOSFET is low, the DC bus voltage is shared by the reverse biased PN junction (the drain-source parasitic capacitor C_ds) and MOSFET capacitor (the source-drain parasitic capacitor C_gd). The internal electric field distribution is built as shown in Figure 5.

The electric field developed in the gate oxide is related to the electric field in the underlying semiconductor by Gauss’s Law [28].

$$E_{\mathrm{oxide}}=\frac{{\epsilon }_{\mathrm{semi}}}{{\epsilon }_{\mathrm{oxide}}}{E}_{\max }$$

(6)

where E_oxide is the electric field in the semiconductor, ε_semi and ε_oxide are the dielectric constants of the semiconductor and the oxide, respectively, E_max is the maximum value of E_oxide.

The ε_semi/ε_oxide is approximately 2.5 for SiC/SiO₂ semiconductor, which means that the electric field strength of the oxide is 2.5 times stronger than that in the semiconductor. The breakdown field strength of SiO₂ is 10 MV/cm. According to Equation (6), the electric field strength of the oxide is usually less than 4 MV/cm to ensure long-term stability of the oxide layer. The electric field in the oxide layer with Si material is always at the limit of reliability due to the maximum electric field with 0.3 MV/cm. However, for SiC devices, the maximum electric field is almost 10 times stronger than that of Si devices. Therefore, the oxide will easily reach its reliability limitation and result in the instability of the oxide layer. The intensity of electric field also increases in high voltage applications, which means suitable schemes for the fault detection is needed for SiC MOSFET such as HVDC microgrids. To achieve the desired threshold voltage, the thickness of the gate oxide is designed to be thinner and the barrier is to be narrower. However, the thin layer will cause the reliability issue on the gate, especially when an extremely harsh operation mode is applied. During the short-circuit operation, the excessive temperature rise in the device will also result in the increase of the gate leakage current. This will lead to the breakdown of the oxide dielectric and result in the deterioration of the device when the leakage current reaches a certain value.

4. Short-Circuit Protection

4.1. Current Detection

The short circuit peak current is related to the device package of the MOSFET on the physical structure; from the electrical characteristics, it is also affected by the applied drive voltage, the drain-source voltage, and short-circuit detection method.

Four different kinds of detection method are considered here, and they are resistor detection, transformer detection, Hall current detection, and desaturation detection [29]. Since the considerable power losses are consumed by the resistor when the current flows through it, the resistor detection is not suitable to measure the high current condition. The transformer detection has lower power losses, but it cannot measure the DC current due to the constant magnetic field of the coil [30]. For the Hall current sensor [31], an auxiliary power supply is needed, and it will increase the size and cost of the system. Therefore, with the fast response time and simple configuration, the desaturation detection method is applied in this paper.

The desaturation detection method is based on the output characteristics of the power device. The drain-source voltage of SiC MOSFET is very low at the conduction state. When the short circuit happens, the drain-source voltage will increase rapidly. The current flowing through the device can be measured by measuring the terminal voltage. Figure 6 shows the peripheral circuit based on the drive IC 1ED020I12-F2 with the function of the desaturation detection [32]. /FLT pin is the fault output, which is used to report the desaturation error of the SiC MOSFET. RDY pin shows the READY status to report the correct operation of the device. DESAT pin is used to monitor the SiC MOSFET voltage (U_ds) to detect desaturation caused by the short-circuit current. R_P1 and R_P2, with the resistance of 10 kΩ, are used to pull up the voltage of RDY and /FLT; C₁, C_VCC2, C_VEE3 are decoupling capacitors of the power supply. The capacitor C_DESAT defines the external blanking time of the DESAT pin. Z₂ is a protector diode to limit the negative voltage of the input of the DESAT pin. R_g is the external gate resistor of SiC MOSFET. R_DESAT, D₁ and Z₁ are required to limit the current flowing in and out of the DESAT pin.

The dynamic behavior of the desaturation detection is shown in Figure 7. When the IN+ pin is high, the OUT pin will jump to high after T_POON delay and the “DESAT” pin is in the state of detection. To ensure safety and to reduce the drain-source voltage of SiC MOSFET during the turn-on transition, a certain blanking time must be considered. The blanking time T_BLANK of 1ED020I12-F2 is calculated as:

$$T_{\mathrm{BLANK}}=T_{\mathrm{DESATleb}}+T_{\mathrm{BLANK}\_\mathrm{EXT}}$$

(7)

where, T_DESATleb is the frontier blanking time determined by the drive chip with a typical value of 400 ns. The capacitor C_DESAT defines the time T_{BLANK_EXT}, and T_{DESAT_EXT} is the external blanking time calculated as:

$$T_{\mathrm{BLANK}\_\mathrm{EXT}}=\frac{C_{\mathrm{DESAT}}\times U_{\mathrm{DESAT}}}{I_{\mathrm{SOURC}\_\mathrm{IN}}}$$

(8)

where U_DESAT is the trigger voltage for desaturation protection and is set to be 9 V. I_{SOURCE_IN} is the high precision current source of the chip and is set to be 500 µA. In addition, to give a sufficient margin for safety, the blanking time T_BLANK must satisfy:

$$T_{\mathrm{BLANK}}>T_{\mathrm{on}}$$

(9)

where, T_on is the turn-on time of SiC MOSFET. During the blanking time T_BLANK, the U_CDESAT can be calculated as:

$$U_{\mathrm{CDESAT}}=U_{\mathrm{D}1}+U_{\mathrm{Z}1}+U_{\mathrm{ds}}$$

(10)

In which, U_D1 is the forward voltage drop of the diode D₁, U_Z1 is the reverse steady-state voltage of Zener diode Z₁, and U_ds is the drain-source voltage of the SiC MOSFET. The protection against the different fault currents can be achieved by selecting different Zener diodes Z₁.

In the normal state, the drain-source voltage of SiC MOSFET is low and U_CDESAT < U_DESAT. When FUL happens, U_CDESAT will increase with the increase of U_ds. When U_CDESAT > U_DESAT, the protection circuit will work. After the T_DESATOUT delay, the OUT pin will output high level, and then SiC MOSFET is turned off. At the same time, U_CDESAT decreases gradually to discharge the capacitor C_DESAT. After a period of delay, both the /FLT pin and the /RST pin turn to low, indicating a fault condition. The chip is then reset and the output is forced to be zero.

Under HSF condition, U_ds approaches to the DC bus voltage, and the diode is reversely cut-off, then U_CDESAT will not follow the Equation (10). After the blanking time T_BLANK, U_CDESAT reaches to U_DESAT, the protection circuit will turn off the SiC MOSFET.

After SiC MOSFET turns off, the reverse voltage of D₁ is similar to the DC bus voltage, so a high voltage diode should be considered here. Meanwhile, to prevent the voltage noise of the DESAT pin, a 1 kΩ decoupling resistor R_DESAT is connected in series to the desaturation detection loop.

4.2. Clamping the Gate-Source Voltage

During the short-circuit condition, the gate-source voltage U_gs will be increased due to the miller current caused by the rapid change of U_ds. Then, the short-circuit current will be further increased. Therefore, U_gs must be clamped in the short-circuit condition to inhibit the peak of the short-circuit current [33].

Several passive schemes have been proposed to clamp the gate-source voltage. One method is to put a diode between the gate and source to clamp the gate-source voltage. When the gate voltage reaches a certain clamping value, the diode is turned on and the gate voltage is clamped to the supply voltage. Another method is to connect a Zener diode in a parallel fashion with the gate and source. The circuit design is simple, while the disadvantage is a time delay of the gate voltage detection and a lack of reliability [34].

In addition, active schemes can be applied to the SiC MOSFET module with Kelvin structure, where the auxiliary source and main power source are classified. The gate drive circuit is usually connected to the auxiliary source to reduce the parasitic inductors of the gate drive circuit and the oscillation of U_gs. As shown in Figure 8, L_AM is the parasitic inductance between the auxiliary source and the main power source of the SiC MOSFET module. The negative feedback of L_AM is introduced to reduce the gate-source voltage and restrain the peak of the short circuit current when the short-circuit fault happens. During the turn-off transition of SiC MOSFET, the current flowing through L_AM is in a reverse direction. This will induce a reverse voltage against the gate-source voltage U_gs, thus, the turn-off speed will slow down and the voltage overshoot will be inhibited. However, when L_AM is too small, the suppression ability will be decreased. Meanwhile, the driver circuit is connected with the auxiliary source, so the high turn-on speed of the SiC MOSFET will not be influenced. This will also shorten the blanking time and speed up the detection speed. In addition, this method can inhibit the voltage overshoot as well.

5. Experiment and Analysis

As shown in Figure 9a, a commercially available 1200 V transistor, SCH2080KE from ROHM, was used for the short-circuit test platform, and compared with a 1200 V Si MOSFET IXFH12N120P from IXYS.

To verify the effectiveness of the proposed “soft turn-off” protection method combined with the desaturation detection method, a 270 V/100 A SSCB experimental platform was built, shown in Figure 9b. The PCB layout of the experiment platform was optimized to reduce the inductance of the parasitic inductors.

5.1. Short-Circuit Capability Test

The short-circuit capabilities of Si MOSFET and SiC MOSFET were investigated under different pulse widths, with DC bus voltage of U_DC = 550 V and drive resistor R_g = 30 Ω. The positive gate of SiC MOSFET bias was set to be 18 V, as recommended by the device manufacturer.

Figure 10 shows the measured U_gs and i_d waveforms of SiC MOSFET and Si MOSFET in 550V DC voltage. When the pulse width was 10 μs, the short circuit current increased immediately and then decreased. With the increase of the pulse width, the short circuit current decreased after the SiC MOSFET turned off, but the short-circuit energy kept increasing. When the pulse width increased to 15 μs, the U_gs of SiC MOSFET decreased slightly at the on-state. This indicates that the performance of the device begins to deteriorate with the increase of the gate leakage current. When the pulse width further increased to 17.5 μs, U_gs decreased to 2 V at the on-state. The U_gs will not drop to 0 V until SiC MOSFET has been turned off for 12 μs. This indicated that the gate-source of SiC MOSFET is short-circuited and the device has been destroyed by the high short-circuit energy E_C with the value of 1.365 J. However, Si MOSFET maintained good performance when the 17.5 μs pulse width was applied. Even though when the pulse width increased to 60 μs with the short-circuit energy E_C of 2.403 J, and showed better short-circuit capability for Si MOSFET.

The parameters of the SiC MOSFET under the normal condition and the damaged condition are shown as

Table 1. The terminal impedance and body diode forward voltage drop SiC MOSFET.

Parameters	SiC MOSFET (SCH2080KE)
	Normal	Damaged
Rgs/Rsg (Ω)	∞/∞	0.8/0.8
Rgd/Rdg (Ω)	∞/∞	∞/∞
Rds/Rsd (Ω)	∞/∞	∞/∞
UF (V)	0.677	0.680

. The date is obtained under the condition with the DC bus voltage of 550 V and the pulse width of 17.5 μs. Under such condition, the gate-source oxide of SiC MOSFET is broken down and the device becomes uncontrolled. However, the drain-source impedance of SiC MOSFET keeps a high value and the device can still block the voltage. Meanwhile, the forward voltage drop of the body diode almost remains unchanged.

5.2. Fault under Load (FUL) Condition Test of SSCB

Figure 11 shows the experimental waveforms of drain-source voltage U_ds, drain current i_d, gate-source voltage U_gs and saturation voltage U_DESAT of SiC-based SSCB under FUL condition. The load current I_d is 100 A. Obviously, the fault response time is only 0.5 µs, benefited from the fault monitor of the DESAT pin on the drain-source voltage of the SiC MOSFET through the diode. When the load falls into a fault, the OUT pin directly reaches a low level after the T_DESATOUT delay without the blanking time. Similar to HSF, U_gs rapidly decreases to a certain voltage value, and then slowly reduces to −5 V due to the negative feedback of the parasitic inductor of the source. It can be seen that the full shutdown time of the short-circuit current is about 2.2 µs.

5.3. Hard-Switching Fault (HSF) Condition Test of SSCB

Figure 12 shows the experimental waveforms of the drain-source voltage U_ds, the drain current i_d, the gate-source voltage U_gs and the saturation voltage U_DESAT of SiC-based SSCB under the HSF condition. The trigger voltage of U_{DESAT_TH} was 9 V. When the gate-source voltage was clamped, the fault response time of SiC-based SSCB was about 1 µs and the full shutdown time of short-circuit current was about 2 µs. Because of the forward voltage drop of clamping diode, the maximum of gate voltage during the turn-off transition was 20.9 V and the short-circuit peak current is limited to 650 A, and the drain peak voltage is 325 V. With the parallel connection between the gate and source of Zener diode, the fault response time was about 1 µs and the full shutdown time of short-circuit current was about 4 µs. Due to the delay of Zener diode clamping, the maximum gate voltage during the turn-off transition was 21.5 V. Then, the short-circuit peak current increases to 650 A, and the drain peak voltage also increases to 358 V. In contrast, the soft switch-off circuit based on L_AM was also tested. The fault response time was about 1 µs and the full shutdown time of short-circuit current was about 4.5 µs. At first, U_gs rapidly decreased to a certain voltage, and then slowly reduced to −5 V due to the negative feedback of the source parasitic inductor. Then, the drain peak voltage was limited to 310 V. When an external Zener diode was connected in series to reduce the trigger voltage for desaturation protection, the peak current of the short-circuit could be further reduced.

The short circuit protection capabilities under different methods are compared in

Table 2. Test data of short circuit protection under different methods.

	Gate-source Voltage Vlamping	Zener Diode Clamping	Soft Turn-Off Circuit
Short-circuit peak current (A)	650	700	600
Overshoot voltage (V)	325	358	310
Short-circuit protection time (µs)	2	2	2.5

. The proposed soft turn-off circuit can protect the SiC MOSFET module from being damaged, although the short circuit protection time is slightly extended. The voltage overshoot is reduced, and the short-circuit peak current is also effectively suppressed, which means this method can greatly improve the reliability of the device as well as the SSCB.

6. Conclusions

The paper analyzes the short-circuit characteristics of SiC MOSFET and the change law of the short-circuit current. The device deterioration mechanism of SiC MOSFET is investigated based on the comparison of the short-circuit capabilities between Si MOSFET and SiC MOSFET. To improve the reliability and durability of SiC MOSFET in SSCB under FUL and HSF conditions, a soft switch-off drive circuit based on desaturation detection was developed, and its average response time (2 µs) was less than the range of short-circuit withstanding capacity (4 µs). The experimental results indicated that the proposed protection scheme has the capabilities to reduce the current detection time and the voltage stress of the SiC MOSFET. Also, the short-circuit current was suppressed to improve the switching speed of SiC MOSFET, and the proposed method was suitable for the SSCB based on SiC MOSFET.