introduction
Powered by high power density, reliability, and reduced system cost requirements, power module technology must find new ways to meet market demand. In order to achieve significant performance improvements, existing design methods must be reviewed. In the electric and renewable energy markets, connection and assembly technologies play a key role. The demand for compact systems, high reliability, and low cost mean that new technological methods are needed, and the classic modules used in power electronics - copper substrates, soldering, module housings and bonding wires - will gradually disappear from the market. Today's 6 MW wind turbines use approximately 3000 cm2 of silicon area (IGBTs and diodes). In order to realize the high efficiency of power electronics, a new inverter concept must be implemented. This means higher silicon utilization, fewer component counts, and very importantly fewer mechanical and electrical interfaces. Today, MW-class high-power inverters are based on the parallel connection of modules and/or inverters. This not only increases the cost but also reduces overall reliability. Redundancy is an alternative method of achieving the required high efficiency system, the only drawback being the high initial investment.
Figure 1: SKiN technology replaces bond wires based on flexible foils. The driver interface uses a spring to contact the surface of the flexible foil.
Power module design
SKiN packaging technology is based on the use of sintered layers instead of welding [1]. In this structure, the bond wire is replaced by a flexible board that is sintered on the surface of the chip. Unlike systems using bond wires, the top and bottom of the chip have the same metallization layer (eg, silver layer), which means that the highly reliable sinter layers at the top and bottom of the chip are widely connected to the current path. The bond wire can only contact about 20% of the potential chip contact area. Figure 1 shows the use of SKiN technology to connect IGBTs and diodes. Flexible printed circuit boards with copper layers on both sides are attached to the chip by Ag diffusion sintering. The spring provides an auxiliary electrical connection to ensure that the driver interface is less welded and very compact.
The maximum allowable power dissipation of a power semiconductor is limited by the maximum allowable junction temperature, the temperature of the cooling medium, and the thermal resistance between the chip and the cooling medium. In a thermal model of a power electronics system with a high performance water-cooled heat sink, thermal grease is a key influencing variable, accounting for approximately 30% of the thermal resistance of the entire system. This drawback can be eliminated by sintering the DCB with the heat sink. Bring the backing material into the liquid cooling circuit with great care and long-term corrosion. The cooling medium and substrate material/coating must match. Therefore, aluminum is preferred because when the liquid contains a small amount of oxygen, aluminum will self-passivate (natural alumina). However, for the substrate, aluminum is not preferred because of its high coefficient of thermal expansion and poor compatibility with soldering. However, there are ways to solve these problems: use a silver diffusion sintering to sinter a small area pin-fin heat sink of pure aluminum to a DBC substrate. A comparison of the thermal resistance of the layered system with the substrate and SKiN technology shows that the thermal resistance between the IGBT junction temperature and the coolant temperature is reduced by 30%. Figure 2 shows that the main terminal is also sintered onto the DCB substrate, providing a large current contact to the DC bus. Welding with capacitors or DC busbars enables a cost-effective, compact and reliable interface. High current densities can be utilized in the MW class range to produce highly compact and reliable systems.
Fig. 2: Module based on SKiN technology with sintered main terminals and auxiliary contacts
Inductance
For modules that are suitable for high and medium switching frequencies, the stray inductance between the IGBT and the diode must be designed to be small. This has high di/dt and can support fast and low switching losses. In order to prevent high noise levels on the switching signal, low coupling must be provided between the main circuit and the auxiliary circuit. The use of a soft copper layer allows a new design approach to improve and simplify the layout of the half-bridge circuit. Symmetrical layout makes the commutation channel short and simplifies parallel operation, thereby simplifying current sharing between IGBTs. Based on the module simulation model with the main parasitic inductance, the overall commutation inductance can be calculated. The commutation inductance values ​​of the TOP IGBT and the free-wheeling diode in both terminal and terminalless cases have been calculated.
No terminal
With terminal
TOP IGBT1
L = 4,66nH
L = 15,0nH
TOP IGBT2
L = 5,08nH
L = 15,2nH
IGBT1 || IGBT2
L = 4,62nH
L = 14,8nH
Table 1: Comparison of stray inductances when there are no terminals and terminals
Figure 3: Module layout in power building blocks
Simulation results show that the stray inductance mainly comes from the main terminal. Parallel operation of the two SKiN units increases the commutation inductance to 25.7nH. The stray inductance (typical) is reduced by 10% compared to the classic module using bondline technology. An analysis of the current flow during current commutation indicates that this improvement has no effect on eliminating the bond wire loop, but it is also somewhat stimulated by the smaller closed side area of ​​the current path on the SKiN flex circuit layer. Compared to the modular design using busbars, the two modules can reduce the stray inductance by at least 50% in parallel at close range, which is a good indication for the good current sharing that is expected. The main inductance brought by the main terminal can be further improved by different layouts. This shows that the suitability of SKiN technology will become the next packaging platform for wide bandgap materials.
Multi-MW module assembly concept
Based on the thermal performance and the realized power density, a new design method has been completed to lay out power electronic devices and heat sinks in a different way. For liquid-cooled applications, the highest available power density is the biggest benefit. Today, most heat sinks are arranged at the same level as the main assembly direction of the module and busbars. Smaller SKiN cells allow other design methods. The SKiN module is arranged opposite the water-cooled radiator to form a power module building block (Figure 3). The main stream uses the third dimension.
The water flows into the four pin fin radiator through the distribution pipe. Multiple building blocks are arranged on a water distribution path. Compared to the SKiiP4 design, the thermal performance Rth(ja) per square centimeter of chip area decreased from 0.36 K/W to 0.26 K/W. With an optimized pin-fin design, pressure drop is minimized. The layered water flow ensures that the entire radiator surface is close to perfect cooling performance. The first result shows that for 4 paralleled SKiN modules, the difference in thermal resistance between the chip semiconductor junction and water is less than 20%. A pressure drop of less than 100 mbar is achieved for each building block. Each building block contains four SKiN base modules that provide 700 kW of output power. Low inductance DC and AC connections are achieved by soldering the busbar structure on the opposite side of the SKiN cell. The gate driver is mounted on the top of the SKiN module for good switching control. The drive plate and the power supply section are connected by springs (see Figure 4). The spring interface of the auxiliary contacts supports fast and reliable solderless assembly. Due to the excellent thermal performance achieved for the first time in the power module design, the rated current of 600 A is also an effective RMS current at 50[deg.] C. coolant. By arranging 4 such building blocks on a pallet and distribution rail, ultra-compact power modules up to 3 MW can be achieved. The distribution guides provide the drive interface and coolant connection. This concept doubles the power density of today's known power modules and prevents all the difficulties of a single module in parallel. The environmental level is upgraded to 3K4 with pollution level 3 and supports installation in harsh environments. Compared with the SKiiP3 (towards the market in 2000), the area used for the same output power is reduced by 70% and the inverter size can be reduced by 35%.
Figure 4: Detailed view of the spring contacting the PCB and the compact power block with 4 SKiN units and driver board
Dynamic performance
In order to verify dynamic performance, the module was tested for switching. The four building blocks are mounted on a distribution rail and configured as a shunt half bridge. Figure 6 shows the switching characteristics of a 1700V unit. At a DC bus voltage of 1300V, it is possible to switch 2.000A until the blocking voltage limit is reached. A total of 16 SKiN rated at 150A Icnom were used. Close-range and low-inductance connections between adjacent modules reduce inductance, allowing superior current sharing between parallel units. Each power block also has excellent overload capability and does not require any absorption capacitance. For 1 power block, the 2400A pulse switch test has been completed.
in conclusion
SKiN technology is a revolutionary technological advancement. It enhances reliability, reduces thermal resistance, and improves internal parasitic inductance through a bondless-wire packaging technology platform. However, in order to take advantage of this new technology, the module's appearance and system configuration are different for this new packaging platform.
Due to the removal of thermal interface materials and the integration of a high-performance pin fin heat sink, it is possible to double the power dissipation compared to conventional designs. Removing only thermal grease can reduce the total thermal resistance of the chip junction to water by 25%. SKiN technology-based units are building blocks that support compact assembly. This is ensured by the screwless connection of the main terminal and the connection of the drive plate with spring contacts. A new design method using a 3 MW water-cooled module for wind power converters shows that the current density can be doubled compared to a standard module-based solution. Regulatory requirements for wind power inverter applications impose stringent requirements on grid support such as voltage and low frequency ride-through conditions. The power module must support these conditions and have the ability to operate under overload or high pressure conditions. With this modular concept, wind turbine inverters with a capacity of more than 3 MW can be easily implemented with limited cabin space. Compared to medium voltage systems, the use of these known advantages will open up a market of more than 6MW for low voltage inverters due to fewer restrictions and lower costs. With SKiN technology, packaging technologies that include new wide bandgap material power devices have emerged and allow them to be used in the mid-to-high power field.
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