Short-circuit characteristics and protection of SiC MOSFETs

SiC MOSFET的短路保护

1.Short circuit fault isSiC MOSFETOne of the main reasons for failure,尽管SiC MOSFETHas good thermal conductivity,但与Si器件和SiC MOSFETCompared to the short circuit performance,SiC MOSFETShort circuit protection in the following aspects of more challenging.
（1）首先,Under the same rated current capacity,SiC MOSFET芯片面积小,High current density,这就导致SiC MOSFETShort circuit capacity is weak;
（2）其次,Under short circuit conditions,SiC MOSFETWeak interface quality brings grid oxide layer reliability problems,由于SiC MOSFETNeed higher positive gate bias,Grid electric field increases will worsen when short-circuit grid oxide degradation problems;
（3）为了确保SiC MOSFETReliable operation in safety work area,Its weaker short-circuit withstand short-circuit protection circuit requires has faster response speed.然而,与Si器件相比,SiC MOSFET The junction capacitance of smaller、Higher switching speed.SiC MOSFETThe unique positive temperature coefficient transconductance led to the opening of thedI/dt和dV/dt With the increase of junction temperature both increase.在较高的dI/dt和dV/dt 条件下,SiC MOSFET Short circuit protection circuit of quick response and ability to resist noise between.
2.Short circuit fault type

Because of the short circuit loop inductance smaller,A class of short circuit fault current rising fast,Damage to the device,Protection more difficult.

3.Short circuit test method
Two common short-circuit test method

（1）The short-circuit test method based on double pulse test.该方法使用“粗短铜排”Instead of double pulse test load inductance in the circuit to simulate short circuit.

When the pulse generator to drive 器 1 Send high level signal,Open the bridge arm SiC MOSFET,To drive 2 发送高电平信号,就可以实现 HSF;
When the pulse generator to drive 2 Send a signal to make for the SiC MOSFET Normally open,To short circuit control switch S 1 Send closed Combined signal make fault inductance LFault Access to the power circuit,就可以实现FUL.
（2）Nondestructive short-circuit test method based on non-linear element.

不同的 SiC MOSFET Short circuit test methods as shown.The method is tested SiC MOSFETThe short-circuit loop of string into the nonlinear component,如图所示.Nonlinear components in a lower internal resistance at rated current,与SiC MOSFET Compared with the saturation current smaller.When the pulse generator by the drive1When open the non-linear element,Again through the drive2Open the device under test can simulateHSF.When the saturation current of short circuit current to the components,Short circuit current will be 限制.When the short-circuit current continue to increase,The element will“熔断”.

4.Short circuit failure mode
目前,SiC MOSFETShort circuit failure model of the main gate level failure and thermal escape;

Gate level failure：

Thermal runaway failure

Through two kinds of phenomenon and the cause of the failure mode is not hard to see,When short circuit low energy may lead toSiC MOSFETGate the source of failure,The short circuit at high energy may lead to SiC MOSFETThermal escape in failure.SiC MOSFET 栅-The source failure does not necessarily occur heat escape,But failure occurs must be accompanied by a thermal runaway gate-The source of failure.

5.Short circuit protection technology
（1）Surrender and testing
Refund saturation detection principle is simple、成本低,Widely used in the insulated gate bipolar transistor（ Insulated Gate Bipolar Transistor, IGBT）In the short circuit protection,但在SiC MOSFET的短路保护 Using this method there is a huge challenge,Diode type annealing saturation detection as shown.

The graph is diode type annealing saturation detection circuit.在SiC MOSFET导通时,当A 点电压 VA Rise more than threshold Vth1
时,The comparator flip a fault signals off the device.在SiC MOSFET 关断时,晶体管 VT1Conduction willAPoint voltage drop down to low level,Detection circuit are blocked.
The working principle of the detection circuit as shown

PWM为高时, SiC MOSFET 开始导通,Before conducting completely,由于 SiC MOSFETDrain voltage value 较高,二极（ VDS1, …）反向截止,VCC通过 Rblk 对 Cblk 充电,APoint voltage rise.在SiC MOSFET Completely before conducting,Need to set aside enough time to do blind spotTblTo prevent the detection circuit triggered by mistake.当SiC MOSFETShort-circuit exit“饱和” 状态时,VAWill rise more than threshold Vth1 Lead to the comparator flip.
在SiC MOSFET After fully opened, A点电压 VA 的 Size can be expressed as

式中, VD为二极管正向导通压降.
可以看出 A点电位由 SiC MOSFETConduction voltage drop and the pressure drop of diode determine.然而,In the high power SiC MOSFET 应用中, SiC MOSFET Conduction pressure drop is higher,然 While higher busbar voltage requires multiple diode in series to improve the The breakdown pressure,这就导致 ASome potential rise is likely to reach the threshold 值 Vth1,Then to detect circuit triggered by mistake.

此外,SiC MOSFETOpen the instantaneous leakage-The source voltage oscillation also increase the risk of detection circuit triggered by mistake.
（2）Parasitic inductance voltage detection
SiC MOSFET Module power source and secondary source parasitic inductance between,Current changes in the parasitic inductance on induction of a voltage value.Due to the short circuitSiC MOSFET电流变化率 dID/dt 较大,So you can induction voltage value through testing to detect fault,The most typical method is dI/dt 检测,如图所示.图为 dI/dt The working principle of detection technology,在正 Often open process,Rapidly rising current inLSSOn a A negative voltage VSS,The voltage is proportional to the current rate of change.
当发生短路故障时,ID迅速上升,负向 VSS Trigger is protected To protect the threshold Vth3,Short circuit device is shut off. dI/dt Testing time is short、Ease of integration in the drive chip,But the noise caused by the parasitic inductance particularly sensitive.此外,由于 SiC MOSFET When opened higherdID/dt A larger negative is inducedVSS,May also trigger the thresholdVth3 Result in protection circuit triggered by mistake.

（3）电流传感器
Current sensor is widely used in electric power equipment in the current measurement, Such as hall device、Roche coil, etc,The principle of simple and high reliability, Power circuit and measurement circuit with electrical isolation,But low bandwidth、 体积较大,Do not apply to the high frequency、High power density of SiC MOSFET Short circuit protection application.为此, Wang Jun Design a kind of applicable toSiC MOSFET Module of short circuit protection PCB Roche coil type, 如图所示.Not only convenient installation,高达 200MHz 的带宽 可以对 SiC MOSFET Module drain current accurately pick 集,为SiC MOSFET Short-circuit protection module provides reliable guarantee.
然而,In order to improve the measurement of the drain current of the broadband to obtain more accurate, 在 PCB Type of roche's need to increase the coil number of turns in the design of coil.但是 由于 SiC MOSFET Application in the high frequency switch mode,增加 PCBThe coil number of turns can seriously affect its disturbance resistance,May cause short circuit Protection circuit triggered by mistake.此外, PCB Roche coil type signal also The original circuit implementation is more complicated,A serious impediment to the application of the method.

（4）The shunt detection
The shunt detection usually resistor in series with the power circuit、Coaxial shunt for linear components such as short circuit protection. 在SiC
MOSFET The accuracy is often used in short circuit protection、响应速 Faster and higher reliability of coaxial shunt.But with the increase of power circuit current,Brought by the coaxial shunt power consumption and high Leon cost should not be ignored.
此外,通过给 Si MOSFET 栅-Applying different bias voltage source,The flexibility to adjust its saturation current to limit the short circuit current, 防止SiC MOSFET 短路损坏,但是 Si MOSFET Selection is the key,In high current applications,Higher the loss and the cost of making the method application is restricted.

（5）Grid voltage detection
由于HSF 发生时, SiC MOSFET The gate charge of valueQG Is far less than the normal opening gate charge in the process of value,导致HSFWhen grid voltage VGSIs greater than the normal opening process,Grid electricity Pressure detection principle as shown,因此通过检测 SiC MOSFETIn the process of opening the gate voltage can indirectly detect HSF.

This method advantage is no blind area.然而,SiC MOSFETThe miller capacitance is small, HSFNo obvious difference when grid voltage characteristics, Using this method is easy to cause protection circuit triggered by mistake.其次, FUL时SiC MOSFET Grid voltage have maximum forward voltage,Therefore this method can'tFUL进行检测.

（6）Short circuit test method of compare

6.Short-circuit turn off
（1）Big resistance shut off.After the big resistance is shut off in detect short circuit,Using large resistance grid resistance to slow turn-off current decline rate from To achieve shut off overvoltage suppression.然而,Big resistance off overvoltage in curbing off also led off delay time increases,导致 SiC MOSFET Not shut off in time,为此,In the process of shut off with different grid resistance shut off SiC MOSFET 短路电流,Thus both theSiC MOSFET Short-circuit turn off overvoltage and shut off delay time,But the big resistance shutoff may lead to SiC MOSFETDue to shut off too much loss and failure.
（2）Gate voltage drop off.Gate voltage drop are shut off after detect short circuit,First reduce the grid voltage slow,使 SiC MOSFET Keep on state.Under the lower grid voltage,SiC MOSFETDrain current will be limited to a lower level,After a certain delay,Suction shut off the short-circuit current is again.The method through the gate voltage ramp down inhibition of short circuit current,To reduce short-circuit turn off overvoltage,But this method needs a variety of grid voltage,Circuit structure complex.

7.SiC MOSFET和Si IGBT输出特性曲线

8.SiC MOSFETSeries short circuit characteristic

In the load current as0 A时,SiC MOSFET Equalizing occurs,而Si IGBTThere is a device under high voltage.
SiC MOSFETEqualizing evident,T1,T2Short circuit losses are smaller.Si IGBT始终由T2To assume a larger voltage,损耗较大.Reduce the loss as the short circuit current of short circuit,与Si IGBT相比,SiC MOSFETEach device loss smaller,Each device under short circuit impact is low.

参考文献：
【1】SiC_MOSFETShort circuit protection technology in_文阳
【2】SiC_MOSFETPower module research flow_Huang Yiyu