Vehicle scale module series: Semikron Danfoss eMPack

The years flow away like the river water, never to return!

The National Day holiday is coming to an end. During this period, we gathered in Yancheng to celebrate the wedding of our classmates. The atmosphere after a long absence made us reluctant to leave. While feeling nostalgic for the passing years, we could see that everyone cherished this hard-won opportunity to get together. Drinking and talking happily, we seemed to have returned to the ease and comfort of that time!

Today we are going to talk about Semikron Denver's automotive module - eMPack. It can be said that it is a relatively mysterious automotive module that has appeared and applied in the market. It can be seen every time at exhibitions, and some of its technologies are well-known even now. But today we are still going to talk about this automotive module based on a related paper published by Semikron last year.

eMPack@2023PCIM

At the just concluded PCIM, Semikron Danfoss exhibited its automotive scale products, including eMPack and DCM series (DCM1000/DCM1000X/DCM500). The so-called strong combination is not without reason.

Power density, reliability and cost have always been the main driving factors for automotive applications. Due to the high cost of batteries and the need to increase driving range, efficiency is particularly important. The development of large electric vehicles will be based on silicon carbide MOSFET inverters with a DC voltage of 800V . The silicon carbide-based eMPack came into being.

Ultra-high peak current, fast switching transient, high reliability and convenient assembly, as well as innovative connection technologies such as chip sintering, laser welding of main power terminals , etc. , are added to meet the low stray inductance after connection with busbar capacitors .

It can accommodate up to 10 SiC chips in parallel , with a total module stray inductance of approximately 2.5nH. All high-power connections from the module internal to the DBC substrate and from the module external to the busbar are laser welded, providing an output current of up to 900A rms.

Based on Peter Beckedahl of Semikron Danfoss, " SiC automotive power module with laser welded, ultra low inductive terminals and up to 900Arms phase current", we will discuss the eMPack automotive scale module in the following three parts.

Module structure, laser welding terminals, and chip layout.

Module Structure

There are two aspects of the power module structure that need to be considered: the inside of the module and the outside of the module.

The internal structure is based on double-sided sintering, with DBC substrate as the bottom connection and two layers of flexible printed circuit board as the top chip connection. Through the flexible circuit board on the top chip surface, an overlapping and absolutely symmetrical power loop can be created, including up to 10 SiC chips (1.4mΩ/1200V, 1mΩ/750V) in parallel with ultra-low stray inductance, and the commutation loop stray inductance inside the module is less than 1nH.

The interface to the external part requires high mechanical robustness , large current carrying capacity and low parasitic parameters such as parasitic resistance and inductance. Low stray inductance from the module to the DC bus requires stacking, and the spacing between the positive and negative potentials must be as small as possible. In addition, simple top-down high-volume production and rugged connections are required for the harsh environment of automobiles. One requirement of the automotive industry is to reduce screw connections as much as possible, while laser welding terminals provide low contact resistance and reliable connections, as well as a more convenient installation process.

After several technical iterations, we now have the eMPack, a three-phase full-bridge module, with DC terminals consisting of two copper bars isolated by a plastic housing, and electrical clearances and creepage distances that meet automotive requirements of LV123 and IEC60664-1, sufficient to meet the maximum DC voltage of 1000V.

The following figure is a schematic diagram of the connection with DC film capacitor .

The film capacitor terminals match the module stack terminals and are connected by laser welding. The negative busbar terminals below are wider than the positive busbar terminals above to allow welding in one assembly step. The omission of screw connections leaves more space for other uses.

Cross-sectional diagram of the connection. The stray inductance on the assembly is mainly related to the thickness of the isolation layer on the capacitor and the module. The overlap of the modules contributes about 2nH to the overall commutation inductance. The overall performance can be further improved by using a thinner isolation layer or completely eliminating the connection between the capacitor and the module with a fully integrated structure.

Schematic diagram of the internal structure of the module.

First, the chip and flexible film are sintered to the substrate, and after the shell is assembled, the power terminals and auxiliary terminals are laser welded to the DBC. There is no solder and binding wire inside the entire module, but it is directly completed by DPD (Direct Pressed Die Technology) technology.

It consists of a central screw, a pressure plate and multiple elastic protrusions at each chip position. Since the module already comes with a flat heat sink, it can be matched with other various heat sinks.

Laser welding connection

Laser welding of copper terminals is already a leading connection technology for battery cell manufacturing. The two interfaces only need to be placed tightly together, and the laser beam acts directly on them to form a welding point. The process is very fast, the material has low thermal shock, and the assembly time is fast. Compared with traditional screw connections, the main advantages of laser welding are:

Fast process time;

Low contact resistance;

No busbar surface treatment is required;

Minimal vertical space;

low cost;

Another benefit is that without the screw, the DC bus does not need additional creepage distance around the screw.

Any process involves many influencing factors. Only by continuously optimizing the process, materials and design parameters can a suitable welding process be obtained .

During the welding process, it is hoped that there will be no gaps between the contacts. A fixing clamp is required to press the busbar onto the module. The assembly chuck should close the welding area so that the residues from the melting process can be absorbed.

The material recommended for the assembly chuck is one with high beam stray reflection and good thermal conductivity .

The thickness of the upper busbar must be the same as or smaller than that of the lower busbar, which will ensure a good process window and avoid the laser beam completely penetrating the lower busbar. A combination of 1mm upper and 1.5mm is a good starting point to try.

For welds of a few millimeters, the laser relief process only takes about 100ms and, in order to limit the thermal shock, short welds are made, jumping from one terminal to another, allowing the joint to cool. This sequence should be repeated several times with a short distance between the previous welds.

We can see that there are multiple parallel welds on each terminal, and this structure results in higher mechanical strength than long straight welds.

The figure below is a schematic diagram of the cross section of a 5mm weld.

The copper busbar is 0.8mm on the top and 1.5mm on the bottom. It can be seen that the material is completely melted in the joint, ensuring ultra-low contact resistance (mainly depends on the characteristics of the material). The contact resistance of this 5mm long and 1mm wide solder joint is less than 4uΩ, and even at 500A, only 1W of additional loss will be generated. And multiple short solder joints are connected in parallel, and we can ignore their contact resistance.

Very symmetrical interior layout

Due to the use of two layers of flexible film, the internal layout of the module can be said to be very symmetrical and the stray inductance is very low. The figure below is the CAD model of the module.

The following shows the average commutation inductance LC, gate inductance LG and gate coupling inductance LCG of each switch in the half-bridge, as well as the deviation (maximum minus minimum) between the paralleled MOSFETs.

It can be seen that the coupling inductance LCG between the commutation loop and the gate loop is very low and relatively uniform, and these values ​​have a significant impact on the dynamic current sharing between parallel MOSFETs.

summary

As you can see, eMPack is also a car module that integrates a lot of "high-tech" and has excellent performance, but at present, only a few customers are trying it in China, and it is not as common as the three mentioned above. As we said at the beginning, automobiles are an application that combines various factors such as power density, reliability and cost. There is no best, only the most suitable.

But there will definitely be more and more developments in the future, not only in cabinet withdrawal modules, I believe the same will be true for electric vehicles.