The Integrated Fast Short-Circuit Protection Technique with Soft Turn-off for SiC MOSFET

In this paper, the integrated fast short-circuit protection technique with soft turn-off is proposed for SiC (silicon carbide) MOSFET. Two short-circuit faults, HSF (hard switching fault) and FOF (�ashover fault), can be judged by detecting EMI (electromagnetic interference) noises of SiC MOSFET. When there are short-circuit faults for SiC MOSFET, the soft turn-off is initiated to reduce EMI noises and protect SiC MOSFET. The proposed fast short-circuit protection technique is realized on the chip and is veri�ed in a 0.18µm BCD process. Simulation results show that the proposed short-circuit protection has a fast response speed and then SiC MOSFET is turned off slowly when there are HSF and FOF. Therefore, the proposed fast short-circuit protection technique can be well applied for SiC MOSFETs, making SiC MOSFETs safely used in HV (high-voltage) and high current applications.


Introduction
In high-voltage (HV) and high-power elds, SiC MOSFET is widely used for its excellent performance [1][2][3].The drain-source current of SiC MOSFET has the unsaturated characteristic, which is different from traditional Si MOSFET [4].With the increase of drain-source voltage (V ds ) of SiC MOSFET, its drain-source current rises faster and faster.The SiC MOSFET's V ds could reach several thousand amperes as V ds increase, far exceeding the nominal voltage of SiC MOSFETs.For the several hundred voltages power supply, the several thousand amperes could quickly generate amounts of heat.Therefore, SiC MOSFET cannot withstand a long time under the short-circuit condition, signi cantly increasing the risk of damage for SiC MOSFET and its surrounding components.Because SiC MOSET's parasitic effect is small, high dV/dt and di/dt noises could be generated during SiC MOSFET's turn-off in short-circuit faults.Therefore, there are two requirements for the SiC MOSFET's short-circuit fault protection.On the one hand, su cient response speed is needed.On the other hand, the soft turn-off technique is also needed to reduce EMI noises.Thus, SiC MOSFET can be reasonably shut off in short-circuit faults, avoiding HV and high-current signals damage to components and preventing high EMI noises.
The desaturation technique is the most common design method in existing short-circuit protection techniques for SiC MSOFET.In [5][6][7][8], the short-circuit protection based on SiC MOSFETs adopts desaturation technology.Its primary mechanism is to detect the SiC MOSFET's drain voltage through the charging and discharging of the auxiliary capacitor.When SiC MOSFET's drain voltage is continuously higher than the normal operating voltage range for a while, it is considered that the SiC MOSFET is in the short-circuit fault and should be turned off.These research results show that this design method can shut down SiC MOSFET when the short-circuit fault occurs.However, due to the charging time of the auxiliary capacitor, SiC MOSFET need withstand HV and high-current for a long time, about a few µs [5][6][7][8].This could cause reliability problems for SiC MOSFET's applications.Then, this design method is improved in [9][10], and the short-circuit protection's response time is reduced from several µs to several hundred ns.
In [11][12][13][14][15], the fast short-circuit protection is presented in order to avoid SiC MOSFET in HV and highcurrent condition for a long time.These methods signi cantly improve the short-circuit protection's response speed.However, these design methods are implemented on the PCB board.Due to the inherent defects of circuits on the PCB board, the response speed of these fast short-circuit protection has limitations.At the same time, the existing short-circuit protection does not consider the enormous dV/dt and di/dt noises generated during SiC MOSFET's turn-off.
In order to solve the above problems, this paper proposes the integrated fast short-circuit protection technique with soft turn-off for SiC MOSFET.The proposed protection technique is to determine whether the SiC MOSFET is in the short-circuit fault according to SiC MOSFET's EMI noises.When there is the short-circuit fault, the soft turn-off starts to take effect to turn off SiC MOSFET slowly.Then, SiC MOSFETs does not last long under the HV and high current condition, and the ultra-high di/dt noise is also not produced.Therefore, the proposed fast short-circuit protection can quickly judge SiC MOSFET in a short-circuit fault and slowly turn off SiC MOSFET immediately.SiC MOSFET is always in a safe state to improve the SiC MOSFET's reliability in HV and high-current applications.

Method
As shown in Fig. 1, HV power MOSFETs have two short-circuit faults in practical applications.One is that MOSFET's drain is permanently connected with the HV power supply, so the short-circuit fault could occur during MOSFET's turn-on operation, as shown in Fig. 1(b).This short-circuit fault is named hard switching fault (HSF).The other is that MOSFET's drain suddenly rises to the HV power supply.When HV power MOSFET is fully turned on, its V ds abruptly rise to the HV power supply, as shown in Fig. 1(c).This shortcircuit fault is named ashover fault (FOF).The SiC MOSFET also has the above two short-circuit faults.
In order to prevent SiC MOSFET from being damaged in HSF and FOF, a fast short-circuit protection with soft turn-off is proposed.The proposed short-circuit protection for SiC MOSFET includes EMI noises detection, EMI noises' ag signals generating circuit, HSF protection mechanism for SiC MOSFET, FOF protection mechanism for SiC MOSFET, and the soft turn-off driver.

A. The EMI noise detection circuit
As shown in Fig. 2, SiC MOSFET's drain is connected with HV capacitors C H1 and C H2 to detect V ds 's change, and its source's parasitic inductance is used to sense I ds 's change.When the drain-source current (I ds ) of SiC MOSFET increases from zero, I ds_S is positive.I ds_S is negative when I ds drops.Then, the change of SiC MOSFET's I ds can be detected, and di/dt noises can be detected by I ds_S .Similarly, V ds_S also has the same trend as the drain-source voltage of SiC MOSFET.When V ds increases from low, V ds_S also increases, and when V ds decreases, V ds_S also decreases.

B. EMI noises' ag signals generating circuit
In Fig. 3, SiC MOSFET's EMI noises can be converted into ag signals in order to decide whether SiC MOSFET is in the short-circuit fault.Figure 3  SiC MOSFET in a turn-on state, V ds could quickly rise to HV power supply.V ds_S begins to rise, and D 2_2 could conduct.The high-level narrow pulse V ds_OC is output through INV7_L, SMT4, and INV8.In Fig. 3(b), the falling edge detection circuit, resistors, and diodes have the same function as in Fig. 3(a).During SiC MOSFET's turn-off operation, both V ds and V ds_S rise.V ds_on remains high, while V ds_OC may be triggered by mistake.Therefore, V ds_OC only takes effect when SiC MOSFET is turned on.

C. HSF protection for SiC MOSFET
Figure 4 shows the HSF protection mechanism for SiC MOSFET.During normal turn-on operation of SiC MOSFET, when I ds starts to rise, I ds_on outputs a high-level narrow pulse signal, indicating that the driving current and the di/dt noise are reduced.When there is HSF, SiC MOSFET's drain is connected with the HV power supply during the turn-on switching.When I ds reaches the load current, I ds could rise rapidly beyond the maximum allowed current of SiC MOSFET.At the same time, due to the unsaturation characteristics of SiC MOSFETs, the rising rate of I ds increases with V gs .Thus, INV3_H has a higher trigger in Fig. 3(a) to accurately judge whether SiC MOSFET is in HSF.At the same time, to accurately distinguish the SiC MOSFET's short-circuit fault from the normal operation, I ds_OC does not detect the I ds 's rising rate until Flag1_HS has passed the delay1 circuit.During SiC MOSFET's turn-on operation, when Flag1_HS is high, V gs and I ds rise slowly, and I ds does not immediately reach several hundreds of amperes.Delay1's propagation time is not more than 200ns to avoid high current.Flag1_HS is in the HV region and requires the high-to-low level shifter (LS_down) to be converted to the suitable voltage region.In this paper, LS_down uses the architecture of the HV sub-ns level shifter in [16], which can achieve a fast response protection circuit.When SiC MOSFET has HSF, I ds_OC could output a high-level narrow pulse, and Latch_NOR1 outputs the high-level signal OC_HSF.The high-level OC_HSF indicates that the HSF occurs in SiC MOSFET's applications.Critical signals of the HSF protection circuit are shown in Fig. 4(b).When OC_HSF is high, the high-side power transistor is turned off in the gate driver.So, Flag1_HS becomes low immediately after HSF occurs, and SiC MOSFET starts to be turned off.When the gate driver's input signal IN decreases from high to low, INV9 could output a high-level signal.At this time, Latch_NOR1's node S is low, and OC_HSF can recover to low.This can determine whether the SiC MOSFET is in the HSF during turn-on operation.As analyzed above, the proposed HSF protection mechanism can quickly detect whether the SiC MOSFET is in the HSF and avoid SiC MOSFET under HV and high current condition for a long time.

D. FOF protection for SiC MOSFET
Figure 5 shows the FOF protection mechanism for SiC MOSFET.After SiC MOSFET is turned on, SiC MOSFET's drain is connected to the HV power supply quickly when the FOF occurs.At this time, V gs is equal to the power supply of the gate driver.So, SiC MOSFET's I ds could rise immediately.Therefore, when SiC MOSFET is in the FOF, its I ds and V ds could rapidly increase.When I ds and V ds rise rapidly, both I ds_OC and V ds_OC output a high-level narrow pulse, as shown in Fig. 3.I ds_OC outputs the high-level narrow pulse when IN is high, and Latch_NOR2 immediately outputs the high-level signal.Similarly, V ds_OC outputs the high-level narrow pulse, and Latch_NOR3 immediately outputs the high-level signal.Then, OC_FOF goes high through AND5, which indicates that the SiC MOSFET is in the FOF.The FOF protection circuit's key signals are shown in Fig. 5 (b).When IN is low, Latch_NOR2, and Latch_NOR3's output can be restored low-level signals to ensure the FOF protection's normal operation.The above is the judgment method of whether SiC MOSFET is in the FOF.According to the analysis of the above operation, whether the SiC MOSFET is in FOF can be judged by I ds and V ds 's rising.Unlike the HSF, when the SiC MOSFET is in the FOF, V gs is equal to the gate driver's power supply.So, I ds rises rapidly with V ds increase.Then, the FOF protection circuit needs a fast enough response speed.Therefore, in Fig. 5, there is no delay circuit, and the FOF protection circuit only has several logic gates' delay time.

E. The soft turn-off driver
Figure 6 shows the soft turn-off driver for SiC MOSFET in HSF and FOF.SiC MOSFET uses the negative turn-off voltage [17][18][19][20].Thus, the power supply of the soft turn-off driver is the negative voltage.
OC_HSF or OC_FOF also requires the sub-ns delay leve shifter LS_down to be converted from the conventional to the negative voltage region.When either HSF or FOF occurs for SiC MOSFET, AND5 could output a low-level signal OC_ ag after INV13 or INV14.When OC_ ag outputs a low-level signal, M OC1 turns on, and SiC MOSFET starts to enter the soft turn-off.M OC1 has the smallest current capacity, and SiC MOSFET's V gs drops slowly.The minimum turn-off driving current M OC1 is to reduce the di/dt noise.
After the delay3 circuit, V gs drops for some time so that I ds is greatly reduced, and then M OC2 is turned on.
The soft turn-off driving current is increased, and the turn-off speed of SiC MOSFET is also increased.
After the delay4 circuit, I ds is very low.At this time, M OC3 is turned on, and the maximum driving current is output to pull the SiC MOSFET's gate to the negative turn-off voltage VEE.In the soft turn-off driver, the maximum driving current improves the SiC MOSFET's dV/dt immunity.

Results And Discussion
The proposed fast short-circuit protection with soft turn-off for SiC MOSFET is implemented in a 0.18 µm BCD process.The HV power supply is 600V.In the proposed fast short-circuit protection schematic, LV (low-voltage) 5V devices are widely used except HV LDMOS for power MOSFET M OC1/2/3 and LS_down.
Simulation results are shown in Fig. 7 and Fig. 8.
Figure 7 shows the simulation result of the HSF protection circuit for SiC MOSFET.It can be seen that when SiC MOSFET's drain is connected to the HV power supply, the short-circuit fault occurs during SiC MOSFET's turn-on operation.As shown in Fig. 7, the HSF protection circuit's operation is divided into two parts.One is S 1_OC , which is used to detect whether SiC MOSFET is in the HSF.The other is S 2_OC , SiC MOSFET's soft turn-off when there is HSF.During S 1_OC , Flag1_HS is high, and the smallest driving current could be produced.I ds rises very slowly, resulting in lower di/dt noises.However, when I ds exceeds the SiC MOSFET's maximum allowed current, di/dt noises increases, and I ds_S begins to increase.Then, the high-level narrow pulse I ds_OC is generated.OC_HSF becomes high, and OC_ ag goes low, which indicates that the SiC MOSFET begins to enter the soft turn-off operation.In the S 2_OC , V gs and I ds decrease very slowly.At the same time, Fig. 7 shows the maximum I ds is 125A with a short duration time and small di/dt noises.
FOF protection circuit for SiC MOSFET is simulated in Fig. 8. Figure 8(a) shows that V ds suddenly increases after SiC MOSFET is turned on completely.Then, critical signals such as I ds , I ds_OC , V ds_OC , and OC_FOF begin to turn over and make SiC MOSFET turn off.Figure 8 In S 3_OC , when V ds starts to rise, I ds immediately rises to the load current I L .I ds_OC and V ds_OC output the high-level narrow pulse signal.The FOF protection circuit has a fast response speed, and the short-circuit fault's detection for SiC MOSFETs is completed before V ds rises to the HV power supply.Finally, both OC_FOF and OC_ ag have reversed, indicating that the SiC MOSFET starts to be turned off slowly.In S 4_OC , V gs begin to drop slowly, but I ds rises for some time.This is mainly because the SiC MOSFET has a lot of parasitic inductors and capacitors, which causes the drain voltage of SiC MOSFET to increase slowly.After that, I ds decrease with V gs .In Fig. 8(b), it can be seen that when SiC MOSFET is turned on under a 30A load current, the FOF protection circuit can respond quickly and turn off the SiC MOSFET slowly after the FOF occurs.During this operation, the drain-source current of SiC MOSFET could be up to 94A.In the normal operation, the drain-source current and voltage of SiC MOSFET could not increase simultaneously.Therefore, in the FOF, the protection circuit immediately responds to turn off the SiC MOSFET when both I ds and V ds are detected to rise.Table 1 summarizes the proposed short-circuit protection characteristic and compares it with some previously reported protection circuits for SiC MOSFET.The proposed short-circuit protection technique is realized on the chip and has a fast response speed, no more than 10ns in the HSF or FOF for SiC MOSFET.

Conclusion
In this paper, a fast short-circuit protection with soft turn-off is proposed for SiC MOSFETs, mainly to prevent SiC MOSFETs from being affected or damaged by HSF and FOF.There is a different phenomenon for SiC MOSFET in the short-circuit fault and normal operation, which can judge whether SiC MOSFET is in short-circuit faults.When SiC MOSFET is in short-circuit faults, the protection circuit can respond quickly, and SiC MOSFET starts to be softly turned off.Then, SiC MOSFET can be in a safe state, improving the SiC MOSFET's reliability in HV and high-current applications.

Declarations Figures
HV  The EMI noise detection circuit.
Page 11/14 The soft turn-off driver for SiC MOSFET in the short-circuit fault.
The simulation result HSF protection circuit for SiC MOSFET.
(a) produces the di/dt noises' ag signals.When I ds rises from zero to the load current, I ds_S also rises from zero.Diode D 1_1 starts conduction, and I ds_S is transmitted to the gate of INV1_M and INV3_H.INV1_M is the inverter with medium trigger voltage, and INV3_H is the inverter with high trigger voltage.INV1_M could output a low-level signal with the increase of I ds_S .Then, high-level narrow pulse I ds_on is output after SMT1, INV2, and the falling edge detection circuit.I ds_on is used as the ag signal of multi-level turn-on driving for SiC MOSFET.INV3_H is used to detect whether the SiC MOSFET is in HSF.When SiC MOSFET is in HSF, the rising rate of I ds increases rapidly.So, I ds_OC outputs a high-level narrow pulse through INV3_H and the logic circuit.Both I ds_on and I ds_OC have positive voltage, so the 5V power supply rail is GND to VCC in Fig.3(a).Using R 1_1 , the input of IVN1_M and INV3_H is GND by default.Zener diode Z 1_1 is used to limit logic gate's input voltage to avoid damage to components of internal circuits.When SiC MOSFET's I ds starts to drop, I ds_S drops from GND to a negative voltage, and diode D 1_1 cuts off.So, I ds_on and I ds_OC are always kept low.

Figure 3 (
Figure 3(b) produces the dV/dt noises' ag signals.When SiC MOSFET starts to turn on, the V ds drops from the HV power supply, and V ds_S also drops from GND to a negative voltage.At this time, diode D 2_1 turns on.INV5_M is the inverter with medium trigger voltage, and INV7_L is the inverter with low trigger voltage.When V ds_S is lower than INV5_M's triggering voltage, the low-level narrow pulse V ds_on after SMIT3, INV6, and the rising edge detection circuit.After that, V ds_S restores GND.When there is FOF for (b) shows the key signals' simulation results of SiC MOSFET in the FOF.In the FOF, the key signals' operation is divided into S 3_OC and S 4_OC , respectively, as shown in Fig. 8(b).