Performance and reliability testing of modern IGBT devices under typical operating conditions of aeronautic applications

Microelectronics Reliability 48 (2008) 1453–1458

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Performance and reliability testing of modern IGBT devices under typical operating conditions of aeronautic applications J.L. Fock-Sui-Too a,b,c,*, B. Chauchat b, P. Austin a,b, P. Tounsi a,b, M. Mermet-Guyennet b, R. Meuret c a

LAAS-CNRS Laboratory, 7 Avenue du Colonel Roche, 31077 Toulouse, France ALSTOM Transportation – Power Electronics Associated Research Laboratory, Avenue du Docteur Guinier, 65600 Séméac, France c Hispano-Suiza Company, Rond Point René Ravaud, 77551 Moissy-Cramayel, France b

a r t i c l e

i n f o

Article history: Received 2 July 2008 Available online 13 August 2008

a b s t r a c t As for railway traction applications, aeronautical power electronics implies high power density handling. Moreover typical aeronautical applications impose a harsh thermal environment. SiC technology has recently emerged for high power and high temperature application, but is not yet mature enough. Consequently it is still important to push the silicon devices temperature limits in order to increase the amount of switched power. Device ageing is accelerated and there exists the risk of catastrophic failure by thermal runaway. In order to design correctly high temperature power systems, knowing the IGBT characteristics at extended temperature ranges becomes essential. This paper describes an experimental setup and test procedure conceived to experiment with different available IGBT technologies at temperatures beyond the limits rated by manufacturers ( 55 °C, +175 °C). The aim is to characterize the devices for a better understanding and optimized safe application. This will ease prototyping for future development of IGBT modules in aircraft. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The aeronautics industry is more and more interested in power electronics due to the more electrical aircraft (MEA) vision. As electrical energy becomes more and more present in aircraft, the study of power electronics system under harsh thermal environment related to aeronautics applications is essential. Nowadays, power insulated gate bipolar transistor (IGBT) has been largely used for medium power applications range thanks to its good trade-off between switching speed, on-state voltage drop and ruggedness. Researches in the field of power semiconductor technology are continuously improving. Increasing the switching frequencies, on-state characteristics [1] and searching to decrease switching energy losses without degrading on-state performance [2] are the main topics. Increasing the switched power implies an increase of the silicon junction temperature which leads to an accelerated ageing of the components. The degradation of the device performances can give rise to thermal instabilities during switching operation that can lead the IGBT to catastrophic failure. In literature many experimental studies explore the possibility of increasing the junction temperature limit, beyond the limits

* Corresponding author. Address: LAAS-CNRS Laboratory, 7 Avenue du Colonel Roche, 31077 Toulouse, France. Tel.: +33 (0) 5 6253 4349; fax: +33 (0) 5 6253 4481. E-mail address: [email protected] (J.L. Fock-Sui-Too). 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.07.051

established by manufacturers, in order to increase power capabilities. Comparative studies between NPT and PT IGBT technologies [3–5] show that NPT IGBT temperature limit is found to be around 230 °C while thermal runaway can be observed for PT IGBT even below 150 °C when operated at high frequency. Trench IGBT has been also explored at high and low temperature [6,7]. In this context, the knowledge of IGBT performances and detailed electrical characteristics at extended temperature becomes essential to system designers. It is necessary to dispose of an experimental facility, adapted for the thermal characterization of power devices at high and low temperature. The aim of the test bench is to obtain accurate current and voltage waveforms from power die components. It is important to be able to extract the intrinsic electrical characteristic of the component instead of using power modules bought commercially. Built around the die and adaptable to any IGBT component, the experimental facility is relative low cost and allows a large temperature exploration without changing the system. The thermal characterization shows the efficiency of such test bench. Exploration of the power system behaviour at large range temperature will serve to obtain in one hand, a large database of the components performances, and in the other hand the possibility of optimizing the cooling system or the increase of the switched power for the same package characteristics, depending on the results. The most rugged devices are going to be used in the power module prototypes for aeronautics applications.

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2. Experimental facility

100.0 T [ºC]

2.1. Electric setup

80.0

The test device (Fig. 1) is built around the die and according to a chopper cell circuit configuration [8]. This simplified well known circuit allows to reenact the switching conditions encountered during the different applications. The main asset provided by using IGBT chips instead of commercial power modules, is that it is possible to extract the electrical intrinsic behaviour of the components knowing each elements that compose our test device. Indeed, this method allows us a precise PEEC modelling of the different elements as well as thermal modelling knowing the exact composition of the test device stacking. Thus an extraction work of the inductance parasitic values permits to highlight the impact on the chips of the surrounding elements. Also designing our test device offers the possibility to setup the exact desired circuit configuration. Devices under test are mounted on small metallised ceramic substrates. Both substrates are sold on a Cu baseplate. The ‘‘internal” layout is realised by wire bonding. Dielectric aspect is taken into account by addition of a silicon gel encapsulated by a case in PBT material. This constitutes a simplified power module using only two components instead of realising 1-phase inverter composed of two IGBT with two anti-parallel diodes associated. Two different test devices and test circuits are used for the electrical characterization of the IGBT chips and the PiN diodes. Only the IGBT testing is presented here for brevity. For the characterization we use PT trench IGBT as well as SPT IGBT. For this design, no particular restriction concerning current and voltage ratings for testing power IGBT components exists. The matter will be the dielectric aspect in the case of high voltage testing. Indeed, the designed busbar actually is not robust enough for high voltage tests.

60.0

2.2. Thermal setup In aeronautical power electronics applications, the most constraining thermal consideration is the harsh passive thermal cycling. A typical thermal profile (Fig. 2) shows a large temperature range going from 63 °C to +90 °C. In our case, power components must be functional when operating at 55 °C to +110 °C. Also considering the important high temperature aspect in power electronics, we decide to push the higher limit to +175 °C. This will also provide large security margin. Active power devices under test must be maintained at controlled temperature. Also, self-heating due to operation during tests must be minimized by giving the heat an easy thermal path, with minimum thermal resistance to evacuate. Although, one shot

Fig. 1. Test device for general characterization.

ISA

ISAmin

ISAmax

40.0

20.0 Time [s]

0.0 0

5000

10000

15000

20000

25000

30000

-20.0

-40.0

-60.0

-80.0

Fig. 2. Thermal mission profile.

test configuration normally does not provoke self-heating phenomenon. The DUT (device under test) is mounted on a specific device designed in order to collect the airflow and share it homogeneously at the bottom of the baseplate, optimizing the airflow path. The whole assembly is placed into a thermally isolated box, where the flow will recirculate in order to keep surrounding air at the desired temperature. The principle is shown schematically in Fig. 3. Exit paths are placed judiciously in order to ease the evacuation of the airflow without degrading the convective exchange coefficient of the system. The temperature controlled air is injected by a device for thermal characterization of components: ThermoStreamÒ. An ensemble of 3 T thermocouples type is placed on the baseplate, silicone gel side, in order to keep the actual substrate temperature monitored. The center thermocouple will give feedback information to the ThermoStreamÒ, which will vary the airflow temperature and pressure to reach the correct value. In this passive system, the thermal setting up of the components at a temperature level will depend on the thermal time constant of each element. However, it is obvious to notice that the most constraining thermal time constant is the ThermoStreamÒ one. In fact, time constants of the chip and of the AlN substrate are smaller than the baseplate one. Yet the airflow regulation is done by the thermocouple placed on the superior surface of the copper baseplate. So the time we achieve the stabilization of the measured temperature, the whole system will be at the desired temperature value.

Fig. 3. Principle of the thermal setup.

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J.L. Fock-Sui-Too et al. / Microelectronics Reliability 48 (2008) 1453–1458 Table 1 Static test conditions Positive temperature (°C) Negative temperature (°C)

Trench3 : Ice (@150A) = f(T°C) at turn-off 145

27; 50; 75; 100; 125; 150; 175 0; 15; 25; 35; 45; 55

T=-55°C

T=-25°C

T=0°C

T=27°C

T=125°C

T=150°C

130 115

Table 2 Dynamic test conditions 27; 100; 125; 150; 175 0; 25; 40; 55 5 540 50; 100; 150 300

85

Ice [A]

Positive temperature (°C) Negative temperature (°C) Gate resistance: on, off (X) Voltage (V) Current levels (A) Loop inductance (lH)

100

T=175°C

70 55 40

T˚

25 10 t [s] -5 -5.000E-07

5.000E-07

Fig. 4. T3: Ice curve @ turn-off (@ Ice = 150 A; Vce = 540 V).

Trench3 : Vce(@540V) = f(T°C) at turn-off 900 800

T˚ 700 600 500

Vce [V]

Computational fluid dynamic simulations have been performed with FLOTHERMÒ software in order to validate the design of the whole thermal assembly. This shows that temperature is homogeneous on the baseplate surface and on the devices, and equal to the desired temperature. Fluidics simulation shows a good behaviour of the whole thermal system, that impacts on the optimization of the convective exchange coefficient, and consequently on the thermal response time for the electrical tests. The temperature limitation of the test bench is determined on the one hand by the cooling capability of the equipment ( 80 °C to +225 °C), and on the other hand by the choice of each element material. Obviously, the insulation capability of the whole system must be taking into account. In our case, the main limiting considerations are the case material, regarding the fusion temperature of the PBT, as well as the material of the linking device between the ThermoStreamÒ and the thermal box. In a second time, for temperature above +220 °C, one will have to consider the upper limit temperature imposed by the solder type. This system has been conceived to bring power components at a desired ambient temperature and does not constitute a cooling system.

T=-55°C

T=-25°C

300

T= 0°C

T=27°C

200

T=125°C

T=150°C

400

3. Experimental tests

100 T=175°C

We performed static and dynamic characterizations for different temperature steps as shown in Tables 1 and 2. The power components are tested in order to extract the electrical intrinsic behaviour. On-state curve highlights the existing cross point when varying the temperature. This data is important when paralleling components taking into account the positive/negative temperature coefficient impacting on possible thermal runaway. The cross point level depends on the density and level of the recombination centers introduced by ion irradiation during the technology process. Dynamic curves provide valuable details about the device switching and the effects of temperature; especially interesting is turn-off and turn-on losses variation with temperature. Fig. 4 shows the turn-off switching for various temperatures at a load Ice current of 150 A. The most important parameter here is the current tail at turn-off phases that impacts directly on the switching losses. At turn-off due to an injection of minority carrier by the P+ emitter in the storage region of the component, in order to modulate the drift region resistivity, the IGBT needs to totally evacuate the charges before being able to block the voltage again. Thus the carrier lifetime is an essential parameter. Depending on the generation/recombination mechanisms and on the injection level, the carrier lifetime is a sensible parameter versus the temperature. The curve shows that the current tail increases when the temperature increases. However, it can be seen that at low temperature,

0 -4.0000E-07

1.0000E-07

6.0000E-07

t [s]

Fig. 5. T3: Vce curve @ turn-off (@ Ice = 150 A; Vce = 540 V).

the carrier lifetime is not as strongly influenced as for high temperature. Figs. 5 and 8 show the decrease with positive temperature of the Vce overshoot at turn-off, depending on the Ldi/dt quantity, what is important information regarding the RBSOA. Fig. 9 shows the evolution of the Ice current (blue color1) fall time at turn-off correlates the fact that the carrier lifetime increases with the positive temperature. One can observe that the fall time Ice varies much more than the rise time (Fig. 10) of the current related to the turnon phase (Fig. 6). That shows the component behaviour and points out the increase of the Ice current with the high temperature. As we saw, the current fall time is related to the evolution of the current at turn-off phase wherein it is the bipolar part of the component that is contributed. Concerning the Ice rise time during turn-on, it is the MOS part of the component that is pulled out. Going from one extreme to the other of the curve tf can hugely increase by about 303.14% when tr only rise up of about 20.7%. 1 For interpretation of color in Figs. 1, 2, 4–13, the reader is referred to the Web version of this article.

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J.L. Fock-Sui-Too et al. / Microelectronics Reliability 48 (2008) 1453–1458 Ice [A]

Overshoot Vce @ Turn-off (I=150A)

SPT: Ice(150A) = f(T°C) @ Turn-on

75.00

375 350

T=-55°C

T=-40°C

T=-25°C

T=0°C

T=27°C

T=100°C

T=125°C

T=150°C

70.00

T˚

325

T3

SPT

300 250 225 200

65.00 60.00 Overshoot [%]

275

T=175°C

175 150 125

55.00 50.00

100

45.00

75 50

40.00

25 0 0.00E+00 -25

5.00E-07

1.00E-06

t [s] 2.00E-06

1.50E-06

35.00

Fig. 6. SPT: Ice curve @ turn-on (@ Ice = 150 A; Vce = 540 V).

-75.00

30.00 -25.00

25.00

75.00

125.00

175.00

T° [°C]

Fig. 8. Vce overshoot versus temperature at turn-off (@ Ice = 150 A; Vce = 540 V).

500.00 450.00

Trench3 : Fall time = f(T°) Ice @ Turn-off Vce @Turn-on

400.00 350.00

tf [ns]

Indeed, the MOS part is less affected by the temperature than the bipolar part of the IGBT. The rise time and fall time (Figs. 9 and 10) of the Ice current (blue color) and the Vce (red color) voltage provide important data considering the design and the optimization of the driver and/or the snubbers for soft and hard switching. So it can be seen that the switching times can be more than doubled (four times for the tf of Ice at turn-off) when going from one extreme to the other of the curve. This information is paramount to optimize device and system performance at all operating conditions. The reverse recovery current of the PiN diode is shown in Fig. 7. One can see that the recovered charge is growing with the high temperature. Contributing to the evolution of the Ice current during turn-on phase, the Qrr also impacts on the power switching losses. Finally the last figures show the evolution of the energy of the IGBT components as well as for the PiN diodes during switching phases (Figs. 11–13). Comparing the performances of the SPT and Trench IGBT, one can observe that TIGBT is dissipating more Eoff than the SPT component. The two curves grow relatively linearly and can present a difference of about 50% at low temperature when at high temperature the gap can reach about 27%. On the contrary,

300.00 250.00 200.00 150.00 100.00 50.00

-80.00 -55.00 -30.00 -5.00 20.00 45.00 70.00 95.00 120.00 145.00 170.00 195.00 T° [°C] Emcon3: If(t) = f(T°C) 125

Fig. 9. T3: Fall time versus temperature (@ Ice = 150 A; Vce = 540 V).

100 75

T˚

50 25

If [A]

0 0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 -25

t [s] 1.00E-06 1.20E-06 1.40E-06 T=-55°C

T=-40°C

-75

T=-25°C

T=0°C

-100

T=27°C

T=100°C

T=125°C

T=150°C

-50

-125 -150

T=175°C -175

Fig. 7. Emcon3: IRR curve versus temperature (@ Ice = 150 A; Vce = 540 V).

TIGBT is best performer than the SPT component concerning the Eon. Indeed the SPT IGBT presents a 66.5–76% Eon losses more important than the TIGBT when going all along the curve. For these four curves, the general tendency is that from the 100 °C temperature point, the slope increases slightly and may become relative non linear (case of the Eon curve of the SPT IGBT). Concerning the Erec of the PiN diodes (Fig. 13), at low temperature (until reaching the 0 °C) slopes of the curves are almost linear for the two components. It changes with the higher positive temperature. We can observe a slight increase of the slope from the ambient temperature point until reaching 100 °C. Like previously for the IGBT curves, passing this temperature implies a strong increase of the Erec values. This is clearly remarkable on the MPS curve. However, EMCON diode seems to be dissipating less energy than the MPS one at this operating point (I = 150 A; Vce = 540 V). The MPS diode energy curve can present an increase of 104–30.4% going from 55 °C to 27 °C comparing with the EMCON curve. This gap is maintained until the 100 °C point before increasing again to reach a difference of about 56.4% at 175 °C. The energy dissipated by the components

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22.00

160.00 150.00

Ice @ Turn-on

20.00

SPT: Eon.off = f(T°C)at Ice = 150A

Eon

Eoff

140.00

18.00

130.00

16.00

120.00

14.00 E [mJ]

tr [ns ]

Vce @ Turn-off

110.00

12.00

100.00

10.00

90.00

8.00

80.00

6.00 4.00

70.00

2.00

60.00 -80.00

-30.00

20.00

70.00 T° [°C]

120.00

170.00

-75.00

75.00

125.00

175.00

Fig. 12. SPT: Eon,off versus temperature (@ Ice = 150 A; Vce = 540 V).

Erec = f(T°C)

IGBT3:Eon.off = f(T°C)at Ice = 150A

20.00

26.00 Eoff

25.00

T° [°C]

Fig. 10. T3: Rise time versus temperature (@ Ice = 150 A; Vce = 540 V).

28.00

-25.00

18.00

Eon

Emcon3

MPS

24.00 16.00

22.00 20.00

14.00

E [mJ]

18.00 12.00 Erec [mJ]

16.00 14.00 12.00

10.00 8.00

10.00 6.00

8.00 6.00

4.00

4.00

-75.00

2.00 -25.00

2.00 25.00

75.00

125.00

175.00

T° [C°] -75.00

0.00 -25.00

Fig. 11. T3: Eon,off versus temperature (@ Ice = 150 A; Vce = 540 V).

25.00

75.00 T°[°C]

125.00

175.00

Fig. 13. Erec versus temperature (@ Ice = 150 A; Vce = 540 V).

can vary 4–6 times when going from one extreme to the other of the curves. 4. Conclusion This paper presents the setting up of an experimental test bench that permits with a simple electrical test circuit to obtain essential characteristics of IGBT and diodes components. Fluidics and thermal simulation permit to design a facility that eases the electrical test versus temperature by optimizing the convective exchange coefficient and the thermal response time. The characterization work shows the efficiency of such a test bench. Static and dynamic characterizations have been performed on two of the latest IGBT technology: Trench and SPT IGBT, and show its dependencies with temperature on a large range ( 55 °C; +175 °C), beyond manufacturers’ data for a better understanding.

Acknowledgement This work has been carried out in the framework of the ModErNe (Modular Electrical Network) project. The authors would like to acknowledge S. Nicolau from PEARL laboratory for his support regarding the thermal setup. References [1] Rahimo M, Schneider D, Schnell R, Eicher S, Schlapbach U. HiPak Modules with SPT+ technology rated up to 3.6 kA. In: PCIM06’, Nürnberg, Germany. [2] Bäßler M, Kanschat P, Umbach F, Schaeffer C. 1200 V IGBT4 – high power – a new technology generation with optimized characteristics for high current modules. In: PCIM06’, Nürnberg, Germany. [3] Sheng K, Williams BW, Finney SJ. Maximum operating junction temperature of PT and NPT IGBTs. Electron Lett 1998;34(23):2276–7. [4] Sheng K, Finney SJ, Williams BW. Thermal stability of IGBT high-frequency operation. IEEE Trans Ind Electron 2000;47(1).

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