Online,Junction,Temperature,Measurement,of,Double-sided,Cooling,IGBT,Power,Module,through,On-state,Voltage,with,High,Current*

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(1.Institute of Electrical Engineering,Chinese Academy of Sciences,Beijing 100190,China;2.University of Chinese Academy of Sciences,Beijing 100190,China;3.Wuxi Power Supply Company of State Grid,Wuxi 214026,China)

Abstract: The aim of this study is to achieve online monitoring of the junction temperature of double-sided-cooling insulated gate bipolar transistor (IGBT) power modules by using the on-state voltage under a high current to maximize the utilization of IGBT power chips.Online junction temperature measurement plays an important role in improving the reliability of the inverter with IGBT,increasing the power density of the motor controller of electric vehicles,and reducing the cost of electric vehicles.

Keywords: Double-sided-cooling IGBT,junction temperature monitoring,on-state voltage with high current

The motor controller is one of the core components of electric vehicles (EVs) and is used to determine the cost and performance of EVs.Power modules account for 46% of motor controller costs.In addition,power modules are crucial and delicate components of motor controllers.More than 30% of motor controller failures are caused by the power module[1],and the thermal cycle caused by power fluctuations is the key factor affecting the reliability and usefulness of power modules[2].

Thermal management of the power module is required to improve the reliability of the power converter and reduce the intensity of thermal fluctuations.Thermal management can be both external and internal.External thermal management refers to a heat dissipation design that reduces thermal resistance and improves heat dissipation performance because of the structural design of the power module.Many manufacturers and institutions have developed double-sided-cooling (DSC) package structures.The heat generated by the power chip in the DSC power module can be transferred in the upper and lower directions,thus significantly improving thermal performance[3-6].The Hitachi Automotive Systems Laboratory produced direct-water-and doublesided-cooled power modules[5].Compared with single-sided direct-cooling power modules,the thermal resistance was reduced by 50%,and the power density was increased by 70%.Fig.1 shows a motor controller based on a DSC power module with a system voltage of 800 V and a maximum power density of 94 kV·A/L[6].

Fig.1 Motor controller for Taycan,a pure electric car

Planar DSC modules are more compact and have lower parasitic parameters and thermal resistance than their wire-bonded counterparts.Owing to the DSC module’s high working temperature and low thermal capacitance,its junction temperature monitoring system is required to have a high measurement accuracy and response speed.More importantly,the high power-density packaging of DSC modules causes challenges in the design of the measurement system.

In internal thermal management,the thermal shock is reduced by adjusting the control strategy,that is,active thermal control[7].The prerequisite for active thermal control is the online measurement of the junction temperature of the power device.Junction temperature monitoring techniques are of four types:thermal resistance networks[8],physical methods[9],optical methods[10],and temperature-sensitive electrical parameters (TSEPs)[11-14].A comparison of the accuracy,sensitivity,and invasiveness of these four methods reveals that TSEPs have the highest potential for industrial applications.

The TSEPs currently used include the on-state voltage with a small current,on-state voltage with a large current,threshold voltage,turn-off delay time,and short-circuit current.These TSEPs differ in terms of linearity,sensitivity,and implementation difficulty.The turn-on voltage with a small current requires a test current and cannot be used for online monitoring[15].The threshold voltage is difficult to measure synchronously[16].The turn-off delay time depends on many parameters,and online measurements are difficult[17].The short-circuit current causes accelerated aging and damage to the power modules[18].The turn-on voltage with a high current is influenced by the operating current[19].

The aim of this study is to achieve online monitoring of the junction temperature of DSC insulated gate bipolar transistor (IGBT) power modules by using the on-state voltage with a large current.First,the relationship between the on-state voltage and junction temperature and the temperature calibration curves of the on-state voltage under a high current of the studied power module is described.Subsequently,an online on-state voltage measurement circuit is presented.Finally,the accuracy of the junction temperature measurement under the largecurrent is verified by performing offline measurements for the on-state voltage with a small current.

2.1 Studied DSC IGBT power module

The DSC Si IGBT power module investigated in this study is GD800HFT65N3S,which is a half-bridge DSC power module produced by Starpower.The external structure and internal chip are shown in Fig.2a and Fig.2b,respectively.Each bridge comprises two Si IGBT chips rated at 650 V and 300 A and two Si fast recovery diodes connected in reverse parallel.The power rating of the module is 650 V and 600 A.Spacer1/Gasket connects the upper bridge arm’s emitter and the lower bridge arm’s collector,and Spacer2 connects the emitter of the lower bridge arm and the negative power terminal.The power terminals and the control signal terminals are present on both sides of the module,and the specific configuration of the terminals is shown in Fig.2c.

Fig.2 Commercial DSC power module

Because the on-state voltage of the IGBT module is selected as the TSEP for junction temperature monitoring,the error caused by the parasitic resistance of the terminals needs to be considered.The parasitic resistance of the proposed DSC module is extracted by using Q3D,and the results for both terminals are 0.021 5×2 mΩ,as shown in Fig.3.For a 600 A IGBT power module,the voltage measurement error introduced by the power terminals is 12.9 mV,which is negligible.

Fig.3 Q3D extraction results of DSC IGBT module

2.2 Experimental platform for calibration

To suppress the influence of self-heating caused by power loss of the power chip on calibration,a short-pulse current is used to calibrate the temperature of the on-state voltage with a high current.The traditional on-state voltage junction temperature calibration circuit of the IGBT directly switches the IGBT under testing to control a large current.Therefore,the accuracy of the calibration result is affected by the electronic interference generated by the parasitic parameters during the transient switching process.To avoid electronic interference and excessive self-heating,the H-bridge circuit used in the power cycle experiment[20]is used to calibrate the junction temperature of the on-state voltage with a high current,as shown in Fig.4.The H-bridge calibration circuit comprises two half-bridge power modules: Leg1 and Leg2.The IGBT bridge to be calibrated remains on,and the bridge of the other leg is controlled to control the load current.Therefore,changing the circuit structure of the H-bridge calibration circuit is unnecessary.The upper (lower) bridge of one leg is always on,the lower (upper) bridge of the other leg is controlled to generate a single-pulse high current,and the four IGBTs can be calibrated separately.

Fig.4 Schematic of the platform

In the temperature calibration experiment,the power module is typically heated by using an electric hot plate to control the junction temperature.Because the upper and lower planes of the DSC power module have good heat dissipation capacity,to ensure that the internal junction temperature of the power module is consistent with the set value during temperature calibration,a heating and cooling water bath and two double-sided water-cooling plates are used in this platform.The experimental platform is illustrated in Fig.5.Tab.1 lists the parameters of the crucial components and the type of instrument.Leg1 is an IGBT power module with a higher power level to turn a large current on and off,and Leg2 is the studied DSC IGBT power module.

Fig.5 Experimental platform

2.3 Calibration results

According to the experimental platform described in Section 2.2,the following bridge,IGBT4,is used as the test device.Fig.6 shows the switching sequence.IGBT2and IGBT3remain turned off,IGBT4remains on,and a single pulse signal is given to IGBT1to control the width of the calibration current.The amplitude of the calibration current is controlled by adjusting the bus voltage source.

Fig.6 Switch sequence

The junction temperature variation during IGBT calibration is simulated based on the thermal resistance network to select an appropriate width for the single pulse signal.Based on the Foster thermal network model parameters of the junction-to-case transient thermal resistance given in the datasheet,the transient thermal resistance network model is built in Matlab/Simulink,as shown in Fig.7.The power loss is taken as the input,and the transient response curve of the junction temperature can be obtained as shown in Fig.8.Here,the calibration width is selected as 100 μs,and the junction temperature change is 2.1 ℃.Therefore,the influence of the increase inTjon the calibration result is negligible.

Fig.7 Transient thermal resistance model of IGBT

Fig.8 Curve of IGBT junction temperature vs.calibration pulse

The DSC power module has small thermal resistance,double-sided heating,and fast temperature response.The temperatures of the water inlet and outlet are monitored by using a thermistor,and calibration sampling is performed after stable heating for 10 min.The calibration curves between the junction temperature and on-state voltage with high current are displayed in Fig.9.When the collector current value is small,the on-state voltage has a negative temperature coefficient.When the collector current value is large,the on-state voltage has a positive temperature coefficient.The turning point“knee” of the positive and negative temperature coefficients of the studied DSC Si IGBT is approximately 200-300 A.Therefore,when the collector current is approximately 200-300 A,the sensitivity of the junction temperature measurement based on the large current on-state voltage is low.Accordingly,the on-state voltage with a high current can be used for online measurement of the junction temperature under large current conditions.

Fig.9 Calibration curve

3.1 Online monitoring of on-state voltage

An on-state voltage measurement circuit (OVMC)similar to a desaturation protection circuit was proposed for the on-state voltage[21].A diode blocked the high voltage.The voltage from the collector to the emitter of the IGBT was obtained through the flow path of the constant current source.The same method is used in this study.The OVMC with a large current is shown in Fig.10,whereD1andD2are PiN high-voltage diodes of the same type,R1andR2are high-precision resistors with the same resistance,and the constant current source,Im,is composed of an LM317 regulator.The output of the operational amplifier is the on-state voltage of the IGBT,and the output terminal is connected to the emitter of the IGBT under testing through a signal metal-oxidesemiconductor field-effect transistor (MOSFET).The control signal of the IGBT under test is opposite to that of the MOSFET.The switching condition of the IGBT under testing can be divided into two situations.

Fig.10 Switch sequence

(1) When the IGBT under testing is turned on,the MOSFET is turned off.The constant current source,Im,flows throughD2,D1,and the IGBT,and the voltages of the three points,a,b,and c,to the ground areVa,Vb,andVc,respectively.VoltageVais the on-state voltage,Vce(on),of the device under test;D1andD2are the same power diodes with the same on-state voltage(Vc-Vb=Vb-Va) under the same current,Im,and the operational amplifier has negative feedback.

In Eq.(1),Vopis the output of the operational amplifier.Solving Eq.(1) yields

(2) When the IGBT under testing is turned off,the MOSFET of the OVMC is turned on.

3.2 Online extraction of collector current

The packaging structure of the power module for vehicles limits the collector current sampling of a single IGBT bridge arm.Therefore,the collector current of a single bridge arm is extracted from the phase current.When the lower arm of the half-bridge power module is taken as an example,the switching condition of the arm to be tested and the direction of the load current can be divided into four situations.

(1) The lower-arm IGBT2turns on,and the load current flows into the power module.As shown in Fig.11a,the load current at this time is the collector current of IGBT2.At this time,the phase current is the collector current flowing through IGBT2,that is,IC2=iL.

(2) The lower-arm IGBT2turns off,and the load current flows into the power module.As shown in Fig.11b,diode D1freewheels.Therefore,ID1=iL.

(3) The lower-arm IGBT2turns on,and the load current flows out of the power module.As shown in Fig.11c,the load current,iL,freewheels through D2,ID2=iL.

(4) The lower-arm IGBT2turns off,and the load current flows out of the power module.As shown in Fig.11d,the load current is the collector current flowing through IGBT2,that is,IC1=iL.

Fig.11 Switch sequence

In summary,regardless of the current commutation time,if the upper bridge arm is turned on and the corresponding phase current flows out of the power module,the phase current is the collector current of the upper bridge arm.If the lower bridge arm is turned on and the corresponding phase current flows in the power module,the phase current at this time is the collector current of the lower bridge arm.

When the output of the operational amplifier in the on-state voltage measurement circuit is greater than zero,the load current is the collector current of the corresponding bridge.

3.3 Simulation

A double-pulse test circuit and an on-state voltage online measurement circuit were built in the SIMetrix based on the SPICE language to verify the on-state voltage online measurement circuit.As shown in Fig.12,the on-state voltage of the lower bridge arm is measured.

Fig.12 Simulation circuit in SIMetrix

Fig.13 displays the drive signal (Vge_IGBT),collector-emitter voltage (Vce),and collector current(Ic) of the bridge arm to be tested and the output signal (Vce(on)) of the on-state voltage online measurement circuit.The simulation results are consistent with the analysis results presented in Section 3.1 and Section 3.2.

Fig.13 Simulation results

To demonstrate the OVMC and the method discussed above,the on-state voltage and the collector current of a single bridge arm were extracted online,and the junction temperature was estimated in the hardware experimental circuit.Fig.14 shows the driving circuit board integrated with on-state voltage measurements.The OVMC measures the parallel voltage of multiple chips in a bridge arm,and the output of this circuit reflects the global temperature distribution of a bridge arm.OVMC is an additional measurement circuit whose accuracy is determined by its hardware design.The error of the proposed circuit is mainly due to the mismatch of the voltage drops betweenD1andD2caused by the temperature drift.Based on the assumption that the difference betweenVD1andVD2is ΔV,the output equation of OVMC can be revised by using

Fig.14 Drive and on-state voltage measurement circuit board

To measure the temperature drift of these two diodes,an IR camera was used to measure the surface of the OVMC in its working state,and the results are shown in Fig.15.The temperature ofD1,D2,andD3is 31.2 ℃,31.0 ℃,and 29.1 ℃ respectively.This means that the error caused by the diode temperature drift is almost negligible.

Fig.15 Temperature distribution of OVMC

The power experiment platform and measurement equipment are shown in Fig.16.The experimental circuit is a buck circuit based on the DSC Si IGBT power module,where the upper bridge arm is kept off,and the lower bridge arm is to be tested with a switching frequency of 2 kHz.The on-state voltage and current sampling waveforms captured by employing the oscilloscope are shown in Fig.17.

Fig.16 Circuit of power experiment

Fig.17 Sampling waveform of power experiment

According to the previous calibration data,a function can fit the relationship between the junction temperature and on-state voltage,as shown in Eq.(5).Polynomials can be derived continuously and easily converge.Thus,polynomial functions are used,as shown in Eq.(6).Because the temperature sensitivity of the on-state voltage is low when the current is low,the model parameters are solved for the calibration data of the three current levels of 400 A,500 A,and 600 A.Each parameter value is solved according to the calibration data.

The data sampled by using the oscilloscope are imported into Matlab,the junction temperature is estimated by applying a polynomial model,and the junction temperature curve of the IGBT is calculated,as depicted in Fig.18.

Fig.18 Junction temperature curve calculated by using mathematical model

In this study,the online measurement of the junction temperature of the DSC IGBT power module based on the on-state voltage under a large current was investigated.Temperature calibration,on-state voltage drop,and collector current extraction by using the on-state voltage with a large current in the DSC IGBT module were discussed in detail.Finally,an attempt was made to predict the junction temperature in a buck power experiment by using a mathematical model.

Because the DSC IGBT module cannot be unpacked,an optical method was not used to verify the junction temperature measurement results.In future work,the on-state voltage with a low current will be used to measure the junction temperature offline to verify the results.