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Properties of a GaAs Power Rectifier Diode Module for Ultra-Fast Electric Vehicle Battery Charging Systems Thomas Blank1

, Volker Dudek2, Bao An1, Helge Wurst1, Matthias Luh1, Dai Ishikawa3, Benjamin Leyrer1, Marc Weber1

1 Karlsruhe Institute of Technology, Germany 2 3-5 Power Electronics GmbH, Germany 3 Hitachi Chemical Co., Japan Thomas Blank, [email protected] Abstract New GaAs power diodes with an electron mobility of 8000 cm2/V•s, a breakdown voltage 750 V, rated at a forward current of 20 A are evaluated in a rectifier module for ultrafast battery charging. Thermal simulations reveal a thermal module resistivity of 0.73 K/W. The high resistivity is caused by the thickness of the die (570 µm) and the low thermal conductivity of GaAs (33 W/m•K at 100 °C). Hence, the DCB was sintered in a large-area, low-temperature, copper-sinter process to the baseplate to reduce the thermal resistivity. A thinned, 70 µm thick diode yields a resistivity of 0.33 K/W.

1 Introduction GaAs is well-established in ultra-fast signal processing applications and in infrared sensor devices (e.g. in mobiles). Its bandgap is 1.42 eV and slightly higher than silicon’s (1.12 eV). GaAs’s electron mobility is approximately six times higher compared to silicon and measures 8500 cm2/(V•s). The high electron mobility is beneficial for low switching losses, as the stored charge in the diode’s drift zone is inversely proportional to the sum of the mobility of the charge carriers µ𝑒𝑒 + µℎ. and the drift voltage over the intrinsic zone wB (see (Eq. 1) [1].

𝑄𝑄𝑓𝑓 =𝐼𝐼𝑓𝑓 ∙ 𝑤𝑤𝐵𝐵2

𝑉𝑉𝑑𝑑𝑑𝑑𝑑𝑑𝑓𝑓𝑑𝑑 ∙ �𝜇𝜇𝑛𝑛 + 𝜇𝜇𝑝𝑝� (Eq. 1)

Due to the higher bandgap-voltage GaAs diodes can block higher voltages compared to silicon. Simulations show that the blocking voltage (Vbr) of a GaAs diode exceeds silicon’s by 450 V, if they have the same base width and doping profile [2]. Large GaAs wafers can be easily produced with a high yield and high growth rates at moderate temperatures [3]. The low elastic modulus of 85 GPa and comparably high coefficient of thermal expansion of 5.7 ppm [4] lessens thermomechanical stress in high temperature applications. Hence, GaAs has outstanding properties for power devices. However, as the

thermal conductivity of GaAs is 50 W/m•K and much lower compared to silicon (150 W/m•K) or SiC (490 W/m•K) [5] the package concept must be carefully considered. Recently, a new prototype of a bipolar GaAs PIN freewheeling diode with a blocking voltage of 750 V and a current carrying capability of 20 A has been proposed for applications in power electronic devices [2]. This paper investigates the properties of GaAs power diodes in an LLC-converter application for electric vehicle battery chargers. Ultra-fast battery chargers provide an output power of up to 400 kW at a voltage level of 800 V and a charge current of 500 amperes. In a prior, conceptual study a highly integrated, LLC converter has been proposed [6]. The LLC converter of this study utilizes SiC components only and comprises two half-bridges and one diode rectifier stage. The SiC MOSFETs of the half-bridges are rated at 1200 V and 95 A (SiC MOSFET type S4103), the SiC diodes are rated for 650 V and 20 A (SiC diode type S6203). In cases where an output power of 800 kW is required, several LLC modules have to be operated in parallel and the diode rectifier stage on the secondary (output) side of the LLC has to be modified. With the availability of new GaAs power diodes, we investigated the LLC converter with GaAs power diodes as a drop-in replacement for the SiC diodes in the SiC LLC-converter.

PCIM Europe 2019, 7 – 9 May 2019, Nuremberg, Germany

ISBN 978-3-8007-4938-6 © VDE VERLAG GMBH · Berlin · Offenbach1472

2 System and Module Design

2.1 System Design The LLC converter belongs to the class of resonant converters and uses a half- or full-bridges to excite an L-L-C series resonant circuit on the input side, with one inductance being the primary winding of a transformer. The bridge topology converts a DC voltage into a sinusoidal AC voltage. Therefore, the DC voltage is converted to deliver a square-wave voltage. The square-wave voltage is fed to a resonant tank tuned to the fundamental frequency component of the square wave. The resonant tank filter acts as band pass, so the voltage passed to the load is essentially sinusoidal (see Fig. 1). In case of an LLC resonant converter the tank is made up of a series connection of two inductors (L-L) and one capacitor (C). Both, the switch network and the rectifier reside in the power module, which is shaded green in Fig. 1.

Fig. 1: Block diagram illustrating the principal

components of a resonant converter

The LLC converter for charging lithium ion batteries is schematically depicted in Fig. 2. The switch network is controlled by a NCP-1395B from ON Semiconductor. The NCP outputs a digital control signal, which is amplified by the drivers and determines the frequency of switch network and hence, the square-wave voltage. By changing the frequency of the square wave voltage the output current Iout or voltage level Vout are set. Next to the NCP the LLC converter requires a microcontroller (TC275-Infineon Aurix Family) and analog circuitry to implement typical constant current (CC) or constant voltage (CV) charging profiles. According to [11] the resonant frequency fr can be calculated to

𝑓𝑓𝑑𝑑 = 1(2 ∙ 𝜋𝜋 ∙ �𝐿𝐿𝑑𝑑 ∙ 𝐶𝐶𝑑𝑑)�

In our actual implementation the converter is operated at a resonance frequency of fr = 184 kHz. Due to a lack of high power SiC MOSFET availability S4101 SiC MOSFETS (also ROHM) were used to build up the switch network. These MOSFETs can be operated at 1200 V and 55 A at room temperature. The body diode, also rated for 55 A, has a forward voltage of 3.2 V at 20 A and room temperature.

The rectifier network comprises two power diodes D5 and D6 (Fig. 2) forming a center-tap rectifier. Hence, the diodes must be able to block twice the output voltage. Two different modules have been assembled. One module uses the GaAs MEA-18 diodes from 3-5 PE GmbH [12]. The GaAs diodes can block a voltage of 750 V. The second module is used for comparison and comprises two ROHM S6203 SiC diodes with a blocking voltage of 650 V and a rated forward current of 20 A.

Fig. 2: Schematic of the LLC-power module.

Fig. 3 shows the complete board with the transformer as part of the resonant tank, the input- and output filters, gate drivers and the microcontroller board on the test bench.

Fig. 3: LLC Converter module

2.2 Power Module Design The power module comprises the diode rectifier module and the switching network made of two identical half-bridges (Fig. 4). Two half-bridges allow the module to be operated in full-bridge topology. The power module measures 108 mm x 45 mm x 17 mm (L x W x H), which is compliant to integrated in an EconoPACK2 package. Both half-bridge and rectifier sections are implemented with zirconia-toughened alumina



(ZTA) substrates - Ceramtec 708HP [13], measuring 32 mm x 24 mm x 0.38 mm. ZTA has an improved bending strength, fracture toughness and a higher thermal conductivity than 96% alumina. Cost-competitiveness and acceptable thermal conductivity make ZTA an attractive substrate material for power modules. However, silicon nitride (Si3N4) shows even better features, making it an ideal substrate material for sintered WBG power modules. Table 1 lists the properties of several ceramic substrate materials. Si3N4 has a high bending strength, fracture toughness and thermal conductivity. Hence, it shows outstanding reliability in thermal shock cycles from -55 °C to 150 °C. More than 2300 cycles are reported without delamination in dimple free substrates [13]. Alternatively, thick printed copper layers on alumina or ZTA can be used. Gundel et al. report more than 3000 thermal shock cycles for copper thick film substrates from -40 °C to 150 °C [15].

TABLE 1: OVERVIEW OF VARIOUS SUBSTRATE MATERIALS AND THEIR PROPERTIES Material Unit Al2O3-

96% ZTA AlN Si3N4

Bending strength MPa 400 700 450 800

Young’s Modulus GPa 330 310 320 310

Fracture Toughness MPa √𝑚𝑚 3.0 3.5 3.0 6.5

CTE 10-6/K 6.7 7.1 4.6 2.6

Thermal conductivity at 25 °C

W/mK 24 27 180 85

Due to the low cost and good availability, we decided to use thick printed copper on ZTA. The high bending strength and fracture toughness also mitigate the risk of substrate cracks caused by pressure assisted sintering processes. Fig. 4 shows the internal arrangements of the power module with the two half-bridge modules on the right side and the diode module on the left side, in this case the GaAs-rectifier.

Fig. 4. Assembled power module in EconoPACK2

housing

3 Experimental

3.1 Module production and Large Area Copper Sintering

The thick printed copper substrates are built up by sequentially printing a copper film onto the substrate, drying it at 100 °C to evaporate the solvents and firing it under nitrogen containing 2 ppm to 10 ppm of oxygen at 920 °C at a peak time of 8 minutes. Heraeus copper paste C7403A was utilized as first layer. The C7403A paste was printed with a 165 mesh, resulting in a fired thickness of 40 µm. The Paste C7403A contains a glass which further promotes the adhesion to the ZTA substrate. On top of the C7403A a glass-free paste C7404A is printed with a 105 mesh, resulting in fired copper thickness of 60 µm. Altogether, five printing steps are required to achieve a fired copper thickness of 300 µm. The printing on a fully automated stencil printer machine (EKRA) yields well defined copper outlines comparable to edges of etched copper on DCB substrates (see Fig. 5). The printed substrates were fired in a one-hour cycle in a PEO 604 sinter oven. Both, the SiC MOSFETs for the switching half-bride tank and the power diodes for the rectifier modules are pressure sintered by low temperature silver sinter paste (ASP 338 - Heraeus), which is compatible to back-side semiconductor metallization made of silver or gold. 200 µm thick aluminum wires connect the top side contacts of the semiconductors. The wire thickness results from the limited gate size pad of the S4101 SiC MOSFETs, as shown in Fig. 5.

Fig. 5. S4101 MOSFET wire bonded with 200 µm Al wire on thick film copper layer

Alternatively, pressureless copper sintering was used to connect the GaAs diodes to the substrate.



Pressureless sintering mitigates cracking process of the GaAs dies, as GaAs is more brittle and e.g. easily damaged in packaging processes like ultra-sound wire bonding [10]. Fig. 6 shows a picture of the GaAs devices sintered pressureless with copper paste onto the substrate.

Fig. 6: GaAs power diodes sintered onto the thick

printed ZTA substrate

Optimizations of the pressureless copper sintering process have been performed. The pressureless copper sinter process, including the initial heating and the final cooling ramp, is extremely fast and can be completed in less than 25 minutes. At a peak temperature of 260 °C and a peak sinter duration time of five minutes the average shear values are close to 50 MPa (see Fig. 7).

Fig. 7: Die-shear strength over different sinter

durations at various peak temperatures

After wire bonding, the substrates are soldered onto the base plate. Hence, we printed a 140 µm thick layer of SAC solder onto a flat baseplate and soldered three ZTA test substrates on the baseplate. As a substitute to soldering, we investigated the attachment of the substrates to the baseplate by large area pressure copper sintering. Therefore, we printed a 140 µm thick Cu sinter paste with copper particles varying from 100 nm to 500 nm provided by Hitachi Chemicals Ltd. onto the baseplate, dried the paste at 100 °C

for 30 minutes and sintered the ZTA substrates at different sintering conditions under nitrogen. Due to the mismatch of the thermal coefficients as well as the different young’s modulus of the ceramic substrate material of 7.1 ppm/K and the copper baseplate (16 ppm/K) the baseplate warps roughly by 430 µm in case of soldering (Fig. 8-right side). Hence, commercially available base plates are usually pre-bent.

Fig. 8. Left: solder paste printed on baseplate and

soldered ZTA substrates. Right: warpage of the base plate

Large area pressure copper sintering of the ceramic substrate to the base plate can significantly reduce the warpage of the base plate. We evaluated three different sinter parameter sets: A) 5 minutes at 260 °C and 5 MPa, B) 5 minutes at 260 °C and 3 MPa and C) 5 minutes at 220 °C and 12 MPa. The warpage of the base plate can be reduced by approximately 25 % from 430 µm to 300 µm if the ZTA substrate is copper sintered to the base plate instead of soldered (Fig. 9).

Fig. 9. Set C, max. warpage ~290 µm

However, due to the pressure used in the copper sinter process the substrate might be damaged. Fig. 10 shows a scanning acoustic microscopic (SAM) image revealing cracks in a standard Al2O3 substrate sintered onto the base plate. Hence, ZTA substrates are favored for the large area copper sintering process.



Fig. 10. SAM image of cracks in a large-area copper

sintered standard Al2O3 substrate on the base plate.

3.2 Thermal Evaluation The thermal resistance of the module has been evaluated by FE methods. With a set base plate temperature of 75 °C, 30 W electrical losses, a GaAs die thickness of 580 µm and a thermal conductivity of 𝜆𝜆𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺 = 33 𝑊𝑊/(𝑚𝑚 ∙ 𝐾𝐾) the junction temperature rises to Tj=97 °C (Fig. 11). The thermal resistance of 0.73 K/W can be significantly reduced by thinning of the diode from 580 µm down to 70 µm, resulting in a thermal resistivity of 0.33 K/W, which is close to the 0.27 K/W of the SiC diode rectifier module. The strong impact of thinning is correlated to the low thermal conductivity of GaAs of 50 W/mK at room temperature. Hence, the major temperature drop is limited almost completely to the 580 µm thick GaAs diode (Fig. 11).

Fig. 11. FE simulation of the thermal behavior of a

GaAs power diode

3.3 Electrical Evaluation Fig. 12 shows a photograph of the GaAs diode on the left side. It measures 3.3 x 3.3 mm and can handle 20 A in forward conduction mode. At 100 °C the diode blocks a voltage up to 750 V. The right side of the Fig. 12 reveals a cross section of the diode. Notably, the cathode is located on the top side unlike other power diodes. The particular arrangement of the cathode is doping process related and will be changed in future releases of the diode.

Fig. 12. Left side: Photography of the diode. Right: cross section of the diode.

The forward voltage drop of the MEA-18 GaAs diode at 20 A is 1.38 V and 1.29 V at room temperature and 100 °C, respectively (see Fig. 13).

Fig. 13. GaAs power diode forward characteristic.

The blue lines indicate the forward voltage drop at room temperature, the orange line at 100 °C.

The reverse bias breakdown characteristics of the diode at 100 °C are shown in Fig. 14. Even at an elevated temperature of 100 °C the leakage current at a reverse voltage of 750 V is less than 50 µA.

Fig. 14. GaAs power diode forward characteristic.

The blue lines indicate the forward voltage drop at room temperature, the orange line at 100 °C.

First electrical data has been derived for the rectifier diodes. The LLC charger was operated in a forced air-cooled environment. The operational conditions and loads are summarized in Table 2.



TABLE 2: OVERVIEW OF OPERATION CONFIGURATIONS AND OUTPUT LOADS Power Module Configuration Load

[kW]

A SiC full-bride switching network and SiC diode rectifier 0.85

B SiC full-bride switching network and GaAs diode rectifier 0.85

C SiC full-bride switching network and hybrid GaAs SiC diode rectifier 0.85

Configuration (A) serves as reference configuration. With an input voltage of 260 V and a measured current of 3.35 A, the input power was calculated to 0.871 kW. The output voltage was set to 101 V. The measured output voltage was 101.01 V with a peak-peak voltage of 3.2 V. The measured output current was 8.423 A with a peak-peak current of 0.265 A. The output power was 850.8 W, implying an efficiency of 97.7 %. Configuration (B) aims at a direct comparison of the Full-SiC rectifier with the Full-GaAs rectifier. Unfortunately, no measurement results are available, as one GaAs diode burned out at high load tests prior to recording the characteristic electrical values. Configuration (C) comprises a hybrid rectifier module with one SiC and one GaAs diode. It is used to compare the behavior of the new GaAs diode in the same circuitry in order to derive optimal layouts recommendations for systems comprising GaAs rectifier modules and is operated with the same setting as configurations (A) and (B). The input voltage was set to 260V, the input current was measured to 3.4 A, translating to an input power of 884 W. The measured mean output voltage was 100.96 V, but the observed output peak-peak voltages and current was higher due to reverse recovery effects of the PIN GaAs diode. The measured output current was 8.423 A with peak-peak values of 2.3 A. The output power was 850.4 W, resulting in a slightly reduced efficiency of 96.2 %. Fig. 15 depicts the voltage over the SiC diodes in the Full-SiC rectifier module.

Fig. 15. Voltage drop of the two rectifier SiC diodes

of reference configuration A

Fig. 16 shows the efficiency values of the LLC converter in configuration (C) with the hybrid

rectifier stage comprising one SiC and one GaAs diode. At loads over 200 W the efficiency is about 96.5 %.

Fig. 16. Efficiency of the hybrid SiC-GaAs rectifier

module

Fig. 17 shows a thermal image of the board in operation. The power module indicates a temperature of 60 °C on the housing at an output load of 850 W. Additionally, the transformer and inductor exhibit an elevated temperature.

Fig. 17. Thermal image of the board in operation.

4 Conclusion An LLC converter for ultra-fast battery charging applications has been proposed utilizing GaAs power diodes in the rectifier stage of the converter. Additionally, new packaging concepts have been proposed, which handle the thermal losses of the power module in operation. The efficiency of the converter is very high and meets the expectations for the Full-SiC power module. The efficiency of the module comprising one GaAs in the power rectifier output stage is 96,5 % and only 1,2 % less than the Full-SiC module. This is remarkable, as the prototype diode shows higher reverse recovery losses compared to the SiC diodes due to the high reverse recovery charge Qrr of about 550 nC. The high Qrr can be accounted for by doping, the layer structure but also just by thinning the 580 µm thick bulk material, which is beneficial also for the thermal performance of the diode. The innovative packaging solutions utilizing large area copper sintering and silver sintering for the dies results in an excellent static thermal and reliable thermal cycling performance of the power module. The



output current shows a low ripple for the Full-SiC module, making it ideal for charging batteries without stressing them. Future devices of a thinner GaAs diode are already in production. They are expected to resolve the effects of the pronounced reverse recovery behavior, as simulation indicate the reduction of Qrr to a value of about 50 nC. Additionally, the junction-to-case thermal resistance of modules comprising these new diodes will be reduced, allowing for the operation at higher output power. Future investigation will characterize the behavior of the diode in hard and soft voltage commutation applications

5 References: [1] J. Lutz et al., “GaAs pin Diodes as Possible

Freewheeling Diodes,”, CSManTech2014 [2] V. Dudek, J. Kowalsky, and J. Lutz, “GaAs pin

Diode Devices and Technology for High Power applications at 600V and above, CS MANTECH International Conference on Compound Semiconductor MANufacturing TECHnology, Denver CO(USA), p. 397-400. - GaAs Mantech, Incorporated, 2014,”

[3] S. Steinhoff and M. Redding, GaAs Schottky Diodes Allow Higher Power Density, Power Systems Design Europe, Dec. 2004, Online Available: https://www.ixys.com/Documents/AppNotes/IXAN0040.pdf. [Online] Available: https://www.ixys.com/Documents/AppNotes/IXAN0040.pdf. Accessed on: Nov. 02 2018.

[4] Physical properties of Gallium Arsenide (GaAs), Online: Physical properties of Gallium Arsenide (GaAs), Accessed: 1. Nov. 2018. [Online] Available: http://www.matprop.ru/GaAs. Accessed on: Nov. 02 2018.

[5] L. ROHM CO., “SiC Application Note :SiC Power Devices; Online:

http://rohmfs.rohm.com/en/products/databook/applinote/discrete/sic/common/sic_appli-e.pdf,”

[6] T. Blank and et al., Highly Integrated SiC-power Modules for Ultra-Fast Lithium Ion Battery Chargers in LLC-topology, PCIM Europe 2018, Nuremberg, Germany: International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management : 5-7 June 2018. Frankfurt am Main: VDE, 2018.

[7] Ceramtec GmbH,”Rubalit®708 HP –High-Performance”, https://www.ceramtec.com/files/el_rubalit_708_hp

[8] Bao Ngoc An et al.:”A highly integrated copper sintered SiC power module for fast switching operation”, ICEP-IAAC 2018 Proceedings, 17-21 April 2018, Mie, Japan

[9] W. Schmitt, L. Chew,” Silver Sinter Paste for SiC Bonding with Improved Mechanical Properties”, ECTC 2017, Volume: 1, Pages: 1560-1565

[10] S.T. Riches, G.L. White, "Wire Bonding to GaAs Electronic Devices", Microelectronics International, Vol. 4 Issue: 3,pp.75-79, 1987 https://doi.org/10.1108/eb044295

[11] H. Choi, “Analysis and Design of LLC Resonant Converter with Integrated Transformer“, APEC 2007, Anaheim, USA, DOI: 10.1109/APEX.2007.357736

[12] www.3-5pe.com [13] www.ceramtec.com [14] Ulrich Voeller, Bernd Lehmeier, “Silicon Nitride

Substrates for Power Electronics”, http://blog.rogerscorp.com/wp-content/uploads/2013/03/Rogers_Si3N4_DCB_AMB_20130117.pdf, last accessed: 15. Feb. 2019

[15] P. Gundel et al., “Thermal Performance and Reliability of Copper Thick Film Substrates”, CIPS 2016, Nuremberg, Germany, ISBN: 978-3-8007-4171-7



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