High-temperature characteristics of SiC module and 100 kW SiC AC-DC converter at a junction temperature of 180 °C

1 Introduction

At present,high-temperature,high-power converters play an important role in many industrial fields,such as petroleum exploration,multi-electric aircrafts,and electric vehicles [1-4].High-temperature semiconductor devices constitute the key component of these converters.Compared with Si insulated-gate bipolar transistor (IGBT) devices of the same grade,the SiC metal-oxide-semiconductor fieldeffect transistor (MOSFET) serves as a new,wide bandgap,high-temperature device,and exhibits a higher breakdown voltage and an enhanced high-temperature operation capacity that can overcome the temperature limitation of Si devices [5-9].High-temperature operations of the high-power SiC MOSFET module can simplify thermal management,reduce the complexity of the system’s heat dissipation design,and thus leads to considerable reductions in the volume of the heat sink,costs,and power losses [10-13].Therefore,the investigation of the characteristics and application of SiC MOSFETs at high temperatures are necessary and valuable.

The current investigations of high-temperature characteristics of SiC MOSFETs are mainly focused on discrete devices.Previously published studies [14-17] discussed the static and switching characteristics of these devices at different temperatures,and verified the superior electrical performance of the SiC MOSFETs for hightemperature operations.However,the focus of these investigations has been and continues to be on discrete devices and not on high-power modules.Meanwhile,only a few research studies have been conducted on the shortcircuited and heat dissipation characteristics of these devices at high temperatures.In the past few years,numerous efforts have been expended to achieve the successful operation of the SiC converter in high-temperature environments [18-20].In [18],a high-temperature 1.4 kW AC-DC converter was built with a SiC junction field-effect transistor (JFET) and seven SiC diodes,and it was operated at ambient temperatures above 100 °C.In [19],a 1.25 kW buck converter whose operation was based on the multifunctional power stage was used to conduct a continuous operation test at the junction temperature of 161 °C.In [20],a 30 kVA,three-phase inverter was developed without customized components,and was evaluated in a high-temperature environment up to 180.However,the above prototypes had small-power capabilities and were unsuitable for highpower applications.

To study the characteristics and application of the highpower SiC MOSFET module at high temperatures,this study considers the 1200 V 300 A SiC MOSFET module (CAS300M12BM2) produced by Cree as the research object.First,its static and dynamic characteristics are tested at the junction temperature of 180 °C,and the transient electrical stress is analyzed.Owing to the high risk and cost of the direct power operation test at high temperature,this study proposes a simple and economical method to perform thermal equivalent power operations on the module based on the use of the thermal resistance test platform.Furthermore,the module’s heat dissipation capacity is verified at the junction temperature of 180 °C.Based on the above research,a 100 kW converter is built to conduct a long-term operation test study at the junction temperature of 180 °C to provide the experimental basis for hightemperature,high-power applications.

2 Static and dynamic characteristics

2.1 Static characteristic test

An Agilent B1505A power device analyzer and a hot plate were used to evaluate the high-temperature static characteristics of the SiC MOSFET module.To achieve this,the module was placed on a hot plate and its static characteristics were quantified at different temperatures based on the adjustment of the hot plate’s temperature.In this study,the temperatures of 25 °C,80 °C,160 °C,180 °C,220 °C,250 °C,280 °C,and 310 °C,were selected as the test temperature points,as illustrated in Fig.1(a).The I/V output characteristic and on-state resistance were tested when the gate-source voltage Vgs was fixed at 20 V,which is the recommended gate-source voltage to ensure that the MOSFET is fully turned on.The results are shown in Fig.1.

Fig.1 Static characteristic parameters

As the temperature increases,the I/V curve shifts to the right,thus exhibiting a negative temperature characteristic.For the same drain-source voltage Vds,the drain current Ids decreases at increasing temperatures.

When the module is fully turned on,the effect of temperature on the I/V curve is the same as that on the onstate resistance Rds(on). The relationship between Rds(on) and the module’s junction temperature Tj is shown in Fig.1(b).It can be fitted to the following function:

As the temperature rises,Rds(on) increases from 4.45 mΩ at 25 °C to 5.2 mΩ at 100 °C,at a slow rising rate.Additionally,its value increases from 5.2 mΩ at 100 °C to 7.1 mΩ at 180 °C at a fast rising rate.

2.2 Dynamic characteristics test

2.2.1 Double-pulse test

To evaluate the high-temperature dynamic characteristics of the SiC MOSFET module,a double-pulse test platform is built.The test principle and platform are shown in Fig.2.To show changes in the dynamic characteristics,experiments were carried out at junction temperatures of 25 °C,70 °C,100 °C,130 °C,150 °C,and 180 °C.In the test,the bus voltage,load current,load inductance,gate drive voltage,turn-on resistance of drive,turn-off resistance of drive,first pulse width,and the second pulse width,were all kept constant at 800 V,216 A,47 uH,-5 V/20 V,2.5 Ω,5 Ω,12.7 us,and 3 us,respectively.

Fig.2 Double-pulse test

Owing to the existence of stray inductance in the loop,the module will cause voltage spikes when it is turned off [21-23].Therefore,the voltage spike is compared and analyzed at different temperatures.The results are shown in Fig.3.We can see that the voltage spike decreases slightly as the temperature increases at 800 V/216 A.The voltage spike caused by the module’s shutdown at the junction temperature of 180 °C is 932 V,which is within its maximum allowable voltage range of 1200 V.

Fig.3 Shutdown voltage spike at different temperatures

According to the double-pulse test,the current and voltage value can be obtained during the module’s switch on and off processes.The switching loss of the module can then be obtained by calculating its instantaneous power and by integration.The switching loss at different temperatures is shown in Fig.4.As observed,the turn-on loss Eon decreases as the temperature increases,and the turn-off loss Eoff is reversed.Therefore,the total switching loss Etotal is almost constant.

Fig.4 Switching losses at different temperatures

2.2.2 Short-circuit test

To evaluate the high-temperature transient electrical stress when the half-bridge module of the SiC MOSFET is shorted,a monopulse test platform is built to conduct relevant tests at the junction temperature of 180 °C.The platform and short-circuit test setup are shown in Fig.5.In the test,the bus voltage,gate drive voltage,and pulse width,are kept constant at 800 V,-5 V/20 V,and 1.5 us,respectively.

Fig.5 Short-circuit test

When the module is short-circuited,the rate of change of the short-circuit current di/dt is large and causes a higher voltage spike when the module is turned off [24-26].Fig.6 shows the short-circuit test waveform at the junction temperature of 180 °C.It can be observed that the voltage spike caused by the module’s shutdown is 1075 V,which is within its maximum allowable voltage range of 1200 V.

Fig.6 Short-circuit test waveform

According to the above analysis,the high-power SiC MOSFET module has excellent transient electrical characteristics at the junction temperature of 180 °C.

3 Thermal resistance test and heat dissipation characteristics

Direct and high-temperature operations of the SiC MOSFET module are costly and extremely risky.Therefore,this study proposes a simplified method to study the heat dissipation characteristics and equivalent high-temperature current running capacity based on the thermal resistance test platform.

The thermal resistance is the ratio between the temperature difference along the thermal flow path of the device given a thermal balance,and given that the dissipated power generated by the temperature difference [27] is given by:

where Rth(j-c) is the thermal resistance,Tj is the junction temperature,Tc is the case temperature,and P is the dissipated power.

Given that the chip is packaged inside the module,it is difficult to establish direct contact and conduct measurements.The thermal resistance testing platform uses the device under test (DUT) as a temperature sensing component,and maps its chip temperature information to the external electrical characteristic parameters (the junction voltage drop Vf),which are affected by the junction temperature.The junction temperature of the chip is obtained by the inverse values of Vf ,thereby allowing the estimation of the thermal resistance value [28].

The entire testing process is divided into two parts.Firstly,the relationship between Vf and Tj is obtained by a constant temperature method based on the K curve.This is used to guide the acquisition of the junction temperature in the subsequent thermal resistance measurement.The electrical method is used to measure the thermal resistance.In this process,the thermal resistance value of the module can be obtained by (2).In addition,the thermal equivalent power operation test can be conducted to verify the module’s heat dissipation capacity in normal operating conditions at the junction temperature of 180 °C.

3.1 K curve test

3.1.1 Test principle

The constant temperature method is used to test the K curve,and the test principle is shown in Fig.7.The module is placed in a thermostat.When the junction temperature and case temperature reach thermal balance (the junction temperature at this time is equal to the case temperature),a small test current Im is applied to the module to obtain the junction voltage drop Vf at this junction temperature.By changing the temperature of the thermostat,the relationship between Vf and the known junction temperature Tj ccan be obtained.This can be expressed by (3).

Fig.7 K curve test principle

3.1.2 Test results

The test current Im of the K curve test is 20 mA,and the test result is shown in Fig.8.It can be observed that there is a good linear relationship between the module’s junction temperature Tj and junction pressure drop Vf,as expressed by (4).

Fig.8 K curve test result

3.2 Thermal resistance and thermal equivalent power operation test

3.2.1 Test principle

The electrical method was used to measure the thermal resistance of the module.Based on this,the thermal equivalent power operation test was carried out.The test device circuit is shown in Fig.9.It can be observed from the K curve test that the junction voltage drop Vf is obtained using a small test current Im.Given that the module cannot directly measure the junction temperature at normal operating conditions,it is necessary to switch to the test circuit with the test current Im for measurements.When the module reaches the thermal balance with the heating current Ih in normal operating conditions,the junction voltage drop Vf can be obtained by switching to the test circuit.By substituting the value of Vf in (4),the junction temperature value of the module at the thermal balance state can be deduced reversibly,and the steady-state thermal resistance value can be calculated according to (2).

During the module’s normal operation with the heating current Ih,the effective value of the heating current can be equivalent to the effective value of the power operating current.The module can be cooled by the circulating watercooling machine to emulate the heat dissipation condition of the converter.Accordingly,the thermal equivalent hightemperature power operation test is completed.

Fig.9 Test circuit

3.2.2 Test

In this study,the Phase11 thermal resistance tester is used to conduct the relevant test.The test platform is shown in Fig.10.The power amplifier can provide the SiC MOSFET module with a heating current Ih during normal operation.Additionally,the cooling device is composed of a circulating water cooler,which is used to dissipate heat for the module.To reduce the measurement error of the case temperature Tc caused by the uneven distribution of the heat dissipation of the MOSFET chip in the substrate,the thermocouple should be placed directly below the case at a position,which corresponds to the position of the MOSFET chip.For the sake of cost and safety,the test was carried out five times before the converter was built,to ensure that the SiC module could achieve power operations at high temperatures.In the test,the test current Im,circulating water cooler temperature,and the heating current Ih were set up to 20 mA,50 °C and 280 A,respectively.

Fig.10 Experimental test platform

The Phase11 thermal resistance tester automatically collected data,such as the voltage,current,and temperature,and recorded these data on the host computer.In addition,the test results are listed in Table 1.It can be observed that the steady-state thermal resistance at the junction temperature of 186 °C was 0.0807 °C/W,which was larger than the steady-state thermal resistance value of 0.07 °C/W at 100 °C listed on the datasheet.Given that the steadystate thermal resistance value of the module is negatively correlated with the thermal conductivity of the material,the thermal conductivity is mainly determined by the movement of free electrons.As the temperature increases,molecular vibration increases and the mean free path of the molecule decreases.This results in the inhibition of free electron flow and the decline of heat transfer capacity,thereby reducing the thermal conductivity [29].Therefore,the higher the temperature,the greater the thermal resistance.Additionally,the measured steady-state thermal resistance value was slightly larger than that specified in the datasheet.

At the same time,it can be observed that the high-power SiC MOSFET module can realize power operation at the junction temperature of 186 °C,that is,the module has a good heat dissipation capacity at this junction temperature.This shows that the module can operate at junction temperatures above 180 °C based on a reasonable thermal design.More importantly,the power operation associated with the experimental method based on the use of the thermal resistance test platform is convenient to verify the heat dissipation capacity of the device.

Table1 Test results

Parameters Value Slope and Offset of the K Curve -273.0,430.1 Voltage across the Module V (V) 4.18 Heating Current Measured I (A) 277.83 Junction Temperature Tj (°C) 186.0 Junction Voltage Vf (V) 0.912 Case Temperature Tc (°C) 92.3 Steady-State Thermal Resistance Rth(j-c) (°C/W) 0.0807

4 Long-term operation test of the SiC converter at the junction temperature of 180 °C

4.1 Analysis of high-temperature converter loss

In the case of the 380 V AC 100 kW converter,the most common topologies are two-level and three-level.The advantage of a two-level topology compared to a threelevel topology is that it is simpler,while the complexity of the control is also reduced [30],but the limitation of switching frequency and power loss needs to be analyzed and calculated.Given that the Si IGBT has a large switching loss,and the equivalent switching frequency of the threelevel converter is twice as that of the two-level converter,the three-level topology is used to achieve the purpose of improving the efficiency in project applications.The SiC MOSFET loss itself is small,and it is usually used to form two-level topology.The temperature limit of Si IGBT is 150 °C,but the high-heat resistance potential of the SiC MOSFET has not been discovered yet.Therefore,this study takes the three-phase converter with a rated power of 100 kW,an input voltage of 800 V,and an output voltage of 380 V as an example,and calculates a) the efficiency of the two-level or three-level converters composed of Si IGBT at the junction temperatures of 25 °C and 150 °C,and b) the two-level converter composed of the SiC MOSFET module at the junction temperature of 180 °C,in singlestage frequency doubling cases,for a modulation degree (M) of 0.9,and for full-power operations (cosθ =1).

Fig.11 shows the schematic of a single module of the two-level topology.The calculation formula of the loss of the Si IGBT two-level converter is as follows.The total loss Ptotal includes the on-state loss Pcon and the switching loss Psw of the antiparallel diode.

Fig.11 Schematic of the single module for a two-level topology

Pcon composed of on-state loss of the Si IGBT and the on-state loss of the antiparallel diode.Furthermore,the calculation formulas are as follows:

where IM is the peak current,VCEO and VDO are the conducting threshold voltages of the Si IGBT and antiparallel diode,respectively,and RT and RD are the on-state resistance of Si IGBT and antiparallel diode,respectively.

Psw is composed of the switching loss PTsw of the Si IGBT and the switching loss PDsw of the antiparallel diode.Furthermore,the calculation formulas are as follows,

where f is the switching frequency,Erec is the loss of the antiparallel diode, Eon and Eoff are the turn-on and turn-off losses of IGBT,respectively.

Fig.12 shows the schematic of the single module of the three-level topology.The calculation formula of the lossy three-level converter is described next [31].The on-state losses of the module are as follows,

where VQWO is the conducting threshold voltage of the clamping diode,and the RQW is the on-state resistance of the clamping diode.Switching loss can be referred to (8) and (9).These parameters are available from the datasheet of IGBT.

Fig.12 Schematic of the single module for a three-level topology

The on-state loss of a two-level converter composed of a SiC MOSFET module at the junction temperature of 180 °C includes the on-state loss PCM of the SiC MOSFET and the on-state loss of the antiparallel diode.The calculation formula of PCM is,

where Rds(on) is the on-state resistance which can be obtained as described in section 2.1.The on-state loss of the antiparallel diode can be referred to (7).The switching loss can be referred to (8),and the values of Eon and Eoff can be obtained as described in subsection 2.2.1.

The calculation results are listed in Table 2.For a converter with a class of 100 kW,the efficiency of a threelevel converter composed of Si IGBT at 25 °C is nearly equivalent to that of a two-level converter composed of SiC MOSFET at 180 °C.Si IGBTs are not suitable for hightemperature environments owing to the physical properties of Si materials.SiC MOSFETs can be used in high-temperature applications because of their wide bandgaps.In addition,high-temperature operations can reduce the size of the heat sink and increase the power density.Therefore,the two-level structure composed of the SiC MOSFET can replace the three-level structure composed of the Si IGBT in the hightemperature converter.This simplifies the topology,while concurrently ensures efficiency,reduces the complexity of the control,and meets the application requirements.

Table2 Efficiency comparison

Parameters Devices Si IGBT SiC MOSFET Topology twolevel Si IGBT Si IGBT Si IGBT twolevel threelevel threelevel twolevel Junction Temperature (°C)25 150 25 150 180

Continue

Parameters Devices Si IGBT Si IGBT Si IGBT Si IGBT SiC MOSFET Nominal voltage (V) /current (A)1200/300 1200/300 650/300 650/300 1200/300 Frequency (kHz) 20 20 10 10 20 Total on-state loss (W) 642 714 1248 1296 533 Total switching loss (W)3396 3780 276 429 720 Efficiency 96% 95.5% 98.5% 98.3% 98.7%

4.2 Test design

Based on the excellent high-temperature characteristics of the SiC MOSFET module and the advantages of twolevel high-temperature SiC converter,this study used CAS300M12BM2 to build a two-level,high-temperature converter for long-term operations at the junction temperature of 180 °C.

Given that the junction temperature Tj of the chip could not be directly measured,Tj was calculated based on the measurement of the thermal resistance,loss and case temperature.First,according to the previously measured thermal resistance Rth(j-c) of 0.0807 °C/W and loss P of 417.67 W of SiC MOSFET module at the junction temperature of 180 °C,the case temperature Tc at the junction temperature Tj of 180 °C was 146.3 °C according to (2).A long-term operation test of the converter was then conducted.During the test,the case temperature increased and exceeded 146.3 °C based on the adjustment of the heat dissipation air quantity.It finally reaches the steady state.The case temperature curve was recorded,the junction temperature of the module was calculated,and the current waveforms were obtained at the same time.

4.3 Test platform

Figure 13 shows the converter circuit diagram and test platform.To reduce the measurement error of the case temperature Tc,the thermocouple should be placed in the same position as that used when for the measurements of the thermal resistance.The temperature value of the thermocouple was recorded during the test.When the case temperature was close to 150 °C,the dissipated air quantity was adjusted to make the temperature relatively stable.In the test,the filter inductor was 1 mH,and the DC side capacitor was 4200 uF.

Fig.13 High-temperature converter operation test

4.4 Test results

The variation curve of the case temperature recorded by the thermocouple as a function of temperature is shown in Fig.14.As observed,the case temperature of the SiC MOSFET module is stabilized at 152.1 °C after the converter operates for 110 min.A thermal imager is then used to measure the case temperature at this time.The test result is shown in Fig.15.Because of the angle problem,the thermal imager cannot display the temperature of the thermocouple position.Thus,the temperature measured by the thermal imager is 146 °C,which is slightly lower than that recorded by the thermocouple.

Fig.14 Case temperature recorded by the thermocouple

Fig.15 Case temperature measured by the thermal imager

The case temperature of the SiC MOSFET module reaches 152.1 °C and remains stable (Fig.14).According to the previously measured results of the thermal resistance and loss,it can be inferred that the junction temperature of the module at this time is approximately 185.5 °C.The current waveform of the converter at this junction temperature is shown in Fig.16.It can be observed that there is no abnormal current waveform.In addition,no breakdown occurs during the test,no thermal runaway is observed,and the device temperature remains stable.The test results show that the SiC converter can achieve stable power operation at the junction temperature of 180 °C.

Fig.16 Current waveform at junction temperature of 180 °C

5 Conclusion

The investigation of the characteristics of the highpower SiC MOSFET module and the SiC AC-DC converter at the junction temperature of 180 °C can provide a test basis to high-temperature,high-power fields.The following inferences have been drawn based on the conducted study:

(1) When the module is fully turned on,its on-state resistance increases as temperature rises,and the higher the temperature is,the higher the speed becomes.

(2) The module has an excellent high-temperature transient electrical stress response.With the use of a test voltage of 800 V and a junction temperature of 180 °C,the voltage spike was 932 V when the module was normally turned off,while the voltage spike was 1075 V when the short-circuit fault occurred.All the operations were conducted within the maximum voltage range of 1200 V that the module could withstand.

(3) The turn-on loss of the module will decrease as the temperature increases,and the turn-off loss will exhibit the opposite trend so that the total switching loss is hardly affected by temperature.

(4) The thermal resistance test platform can be used to conduct a high-temperature power-operating test,which is convenient for the study of the heat dissipation characteristics of the module.

(5) The three-phase,two-level,high-power 100 kW converter composed of the CAS300M12BM2 can operate in a stable manner for a long time at the junction temperature of 185.5 °C with a maximum efficiency of 98.7%.

In summary,the 100 kW SiC AC-DC converter is capable of operating at the junction temperature of 180 °C in compliance with the requirements of high-temperature engineering applications.

Acknowledgements

This work was supported by the National Key R＆D Program of China (grant no.2017YFB0903303).