Foreword: Zhendrive Technology (Shanghai) Co., Ltd. (hereinafter referred to as “Zhendrive Technology”) is a high-tech company with R&D, production and sales of new energy vehicle powertrain and its power semiconductor modules as its core business. At the end of 2019, Zhendrive Technology and Japan Roma Semiconductor Corporation established a joint laboratory and signed a strategic cooperation agreement. The cooperation content includes the development of power semiconductor modules based on Roma silicon carbide chips and corresponding motors based on the needs of certain customers. Development of the controller. This article introduces the development, testing and system evaluation of SiC power modules by ZhenDrive.
Introduction
Silicon carbide power semiconductors are becoming a hot topic in energy conversion applications in recent years: due to material properties, it has a higher maximum junction temperature, lower losses, and lower material thermal resistance than silicon-based semiconductor devices Wait.
Therefore, many people claim that when silicon carbide power devices are used in energy conversion, the inverter system will have higher power density, smaller volume, higher allowable operating temperature, and lower losses, thus giving the application system bring greater advantages.
Zhendrive Technology plans to encapsulate the silicon carbide chip into the power module and apply it to the motor driver of the new energy vehicle (hereinafter referred to as “Electronic control”) to replace its existing silicon-based IGBT power module (peak power is about 150 kW).
Before proceeding with development, the user needs to evaluate which SiC properties will bring the most value to the main driver application. For example, for this type of DC-AC topology, the introduction of silicon carbide technology has no significant effect on the reduction of the volume of the electronic control, because the volume of the electronic control mainly depends on the packaging technology of its sub-components and the power module only occupies A small percentage of them; others claim to be able to take advantage of the higher operating junction temperature of silicon carbide, reduce the number of mounted chips and allow it to operate at high temperatures, thereby reducing costs. Perhaps, this feature is suitable for applications with high ambient temperature such as underground drilling, but for new energy vehicles, is it necessary to push up the junction temperature at the expense of efficiency (Note: the loss of silicon carbide will increase significantly at high temperatures) , and whether system cost can be saved by saving the number of chips needs to be questioned.
In Zhendrive’s view, the main system advantage of silicon carbide technology applied to the main drive electronic control lies in the improvement of efficiency and the increase of peak output power. The former can increase the cruising range or reduce the number of battery installations, and the latter can bring a greater acceleration of 100 kilometers to the whole vehicle. The first one developed by Zhenqu is a 750V silicon carbide module, which is aimed at A-class and above passenger models; the second one is a 1200V silicon carbide module, which is used in passenger cars or commercial vehicles with an 800V system. Among the silicon carbide modules developed by Zhendrive, the latest fourth-generation 750V and 1200V chips from Roma are used in Zhendrive. Taking the 1200V chip as an example, its comprehensive performance is significantly improved compared with the previous generation products, see Table 1.
This paper introduces the research and development process of the project: including system performance evaluation (top-down flow), which is used to select the number of chips in parallel; the body design of the silicon carbide module, including packaging form, electromagnetic, thermal, structure, manufacturability, etc.; module The performance test is based on a well-known IGBT power module; the system performance evaluation is iteratively based on the calibration results of the module, including the maximum output power, high-efficiency area, supplemented by the actual measurement results of the bench, and the analysis of its impact on the cruising range is carried out. Based on the above results, this paper will conclude with a summary of the methodology for the application of SiC modules to the main drive design.
System analysis
According to the fourth-generation SiC chip specification provided by ROHM, the author imported its relevant parameters into the system analysis tool of ZHENQU – ScanTool. ScanTool is a time-domain-frequency-domain hybrid steady-state simulation tool. It is mainly used for the preliminary design of power electronic systems. It can be used to calculate the power, efficiency, output waveform distortion, and bus capacitor voltage of the system under different hardware and software configurations. ripple and current stress, etc. The calculation principle of ScanTool is to convert the time-domain excitation waveform into the frequency-domain spectrum, and at the same time express the load in the form of a frequency-domain matrix, multiply the two to obtain the frequency-domain response, and then inversely transform the frequency-domain response into domain waveform. In this way, the output waveform of the tool has extremely high steady-state accuracy, and at the same time, it avoids the waiting time of general time-domain simulation tools from the initial state to the final steady state, so that the simulation time can be changed from dozens of times per simulation. Minutes are reduced to 1-2 seconds. Therefore, ScanTool is particularly suitable for the early design of power electronic systems with high degrees of freedom, which often need to simulate hundreds of hardware and software design combinations. An introduction to the principle of visualization is shown in Figure 1.
Generally speaking, when people design a power module based on an IGBT chip, the type of chip and the number of parallel connections are mostly selected based on the junction temperature of the chip (or the peak power that can be output at the maximum junction temperature). This project uses silicon carbide chips, which have a small single area and are suitable for parallel connection of multiple chips, but their price is much higher than that of IGBTs. On the other hand, SiC is a unipolar device, so the more SiC chips are connected in parallel, the lower the total conduction loss, and thus the efficiency of electrical control can be improved. Therefore, when choosing the number of chips in parallel, in addition to the maximum junction temperature limiting the maximum output power, it must also consider its advantages at the system level – as mentioned before, the overall efficiency improvement must be considered, especially as in NEDC, The improvement of the cruising range under cyclic road conditions such as WLTC and CLTC is combined with a comprehensive analysis of the financial return model. A simplified financial model could incorporate the cost difference due to modules using SiC (compared to IGBT modules), a reduction in battery installation costs, and a subsequent reduction in the cost of charging usage. The first two are the initial investment expenditure (CAPEX), and the latter is the operating expenditure (OPEX), which can finally be converted to the point in time when the financial return is obtained. The break-even point can be between 1-4 years depending on the model and the frequency of use by the user. Since the system-level measurement model involves assumptions of many variables, this article will not repeat them.
After a series of system analysis, we have verified that too many chips in parallel will not help to further improve the cruising range, but can only improve the maximum acceleration of the car; too few chips, it seems that the cost of the module is reduced, but it is also Efficiency/economic advantages may be lost – especially considering the positive temperature coefficient of SiC chips.
Based on this result, the author optimizes the number of chips selected according to the financial model, which can not only avoid the cost increase caused by unnecessary installation of multiple chips, but also avoid the economic advantage caused by too few chips in parallel. At the same time, Zhendrive silicon carbide module also introduces the concept of platform design, that is, when customers have higher requirements for vehicle acceleration (for example, for some high-end models), more chips can be connected in parallel inside the module according to customer needs. , thereby increasing the maximum instantaneous output power and providing a greater push-back experience for the vehicle users.
Module body design
When the chip selection and the number of parallel connections are determined, we enter the body design stage of the power semiconductor module, which generally includes electromagnetic, thermal, structural and manufacturability. It should be noted that the switching speed of silicon carbide is much higher than that of silicon-based IGBTs. Therefore, some indicators that are usually not strict in IGBT modules will become very critical in the design of silicon carbide modules. These indicators include the synchronization of switching timing between the parallel silicon carbide chips, the balance of transient current and voltage stress of the chip, and the interference of the power link to the gate. Among them, the first two indicators are reflected in the external characteristics of the module, and they will determine the limit voltage and current output capability of the module; the interference of the power link to the gate is the moment when the device is turned on and off, the electromagnetic energy is coupled to the space through space. On the control link, the consequences may be excessive gate transient voltage stress, resulting in accelerated gate aging and reduced lifespan, and in severe cases, false triggering of power and damage to modules and systems.
In addition, in the design project of the silicon carbide power module before Zhendrive, it was found that there is a relatively obvious oscillation phenomenon in the silicon carbide module, which is the LC resonance composed of the leakage inductance of the power module and the junction capacitance of the silicon carbide chip, usually its frequency at tens of megahertz. This oscillation will affect the electromagnetic compatibility performance of the electronic control system and reduce the efficiency advantage of the silicon carbide module, and even in some extreme conditions, the resonance will further deteriorate, making the voltage and current amplitude exceed the safe operating area of the device (SOA ). In order to solve this problem, Zhenqu developed a series of design aids, and optimized the design of the module ontology based on this, and finally basically solved the problem. Figure 2 is a comparison of the two output waveforms. It can be seen that under the same working conditions, the optimized module design no longer has obvious oscillation phenomenon.
In the end, the silicon carbide power module designed by ZhenDrive has been iterated for many times, and the transient stress imbalance between the multiple chips inside the module has been reduced to less than 10%. According to the benchmarking evaluation of competing products conducted within the team, it is believed that this performance alone has achieved the top level in the industry. At the same time, the voltage glitch interference of the power link to the gate is also greatly reduced; the high frequency oscillation problem at the moment of module switching has also been well resolved.
Silicon carbide module performance benchmarking test
Power module testing includes performance and reliability testing, and performance testing can be divided into static testing for conduction loss evaluation and dynamic testing for switching loss evaluation. The latter method is usually implemented by a method called “double-pulse test”, which requires applying different voltages, currents, device temperatures, and even different gate drive resistances to the device under test for a full test evaluation. A complete test DoE form (Design of Experiment) can contain thousands of work points. Considering that a large amount of post-processing work of test data is required, dynamic testing of power devices is obviously a time-consuming and labor-intensive task. Therefore, in many cases, users have to choose to reduce the density of test points, that is, to reduce the length of the DoE table to shorten the test time.
Zhendrive Technology has developed a set of high-precision, high-speed power module dynamic test and calibration platform, which can basically complete the automatic test of thousands of working points with “one key”, and do data post-processing after automation. , and semi-automatically generate standardized module test reports. All the user needs to do is to configure the hardware before the test, generate a scientific DoE table, and add the content of subjective evaluation to the final test report. For a module calibration task with more than 3,000 test points, compared to the general manual/semi-manual test system, this automated calibration platform can compress the work from 2 months to 2 days, and includes data post-processing and report generation . Figure 3 presents the core functionality of the testbed.
In this project, the reference object of dynamic performance is a well-known IGBT power module. The test results show that the SiC power module developed by Zhendrive surpasses the reference IGBT power module in terms of dynamic performance, including turn-on loss, turn-off loss and reverse recovery loss of the body diode. At the same time, the silicon carbide module did not show significant oscillations at extreme temperatures.
Efficiency benchmarking test of silicon carbide electronic control
Next, the SiC-based power module and its matching gate driver are loaded into the motor controller, and a permanent magnet motor is matched to calibrate the efficiency map, and the result is used to benchmark the electronic control based on the IGBT power module. The electronic control and drive motor test system is shown in Figure 4.
The measured efficiency diagrams and key parameters of the IGBT electronic control and the silicon carbide electronic control are shown in Figure 5 and Table 2, respectively. It can be seen that the electronic control using silicon carbide power modules has been significantly improved in terms of the highest efficiency, the lowest efficiency, and the high-efficiency area. Especially at light loads with low torque, the efficiency advantage of silicon carbide is very obvious. This is mainly due to the low conduction loss of unipolar power devices at light loads and low switching losses in the entire region.
Efficiency Simulation Verification of Silicon Carbide Electronic Control
In addition, we also imported the data of the double-pulse test into the system evaluation tool ScanTool, and simulated the efficiency map. It should be pointed out that due to the obvious positive temperature coefficient characteristics of silicon carbide devices (that is, the loss increases with the increase of temperature), the temperature iteration function is set in ScanTool, that is, the device is calculated based on the junction temperature of the previous simulation result. For the loss at this junction temperature, recalculate the junction temperature until the temperature deviation between the two calculation results before and after is less than 1 degree. It is conceivable that when the number of chips in parallel is too small, the loss of the device will increase due to the increase of the junction temperature; on the contrary, when the number of chips in parallel is large, the loss of a single device is lower, so that the operating junction temperature is also lower. At this lower junction temperature, the losses of SiC chips will be further reduced. It can be seen that the ScanTool with the iterative function of temperature-loss is a key to ensure the modeling accuracy.
The simulation results are shown in Figure 6 and Table 3. Comparing the measured results in Figure 5 and Table 2, we can see that the analysis tools are in good agreement with the measured results. The remaining difference between the two is mainly reflected in the low-speed area, where the output power of the electronic control is very low, so the residual loss in the electronic control is obvious, such as the loss on the copper bar and the busbar capacitor. In addition, the pulse width modulation scheme and the accuracy of the test equipment are also possible reasons, but these small differences do not affect the subsequent system-level cruising range analysis.
Analysis of Maximum Output Capability of Silicon Carbide Electronic Control
The greater the number of parallel chips inside the SiC module, the greater the output capability of its electronic control. In this analysis, we assume that SiC and IGBT are allowed to operate at the same maximum junction temperature of 150°C. The simulation results of ScanTool show that when the module uses 6 chips in parallel, the maximum output power increases by 12.4%; when 8 chips are used in parallel, the power increases by 31%.
In the experiment, due to the capacity limitation of the powertrain bench, we used the Inductor as the load to test the maximum output capacity. Compared to using a real motor load, this compromise solution is acceptable for evaluating SiC modules, because the bidirectional conduction characteristics of SiC chips make their losses insensitive to the power factor of the load.
Figure 9 shows that the SiC electronically controlled output reaches 600 Arms and has reached the maximum capability of the test equipment. It should be pointed out that in the electronic control application scenario, we maintain the switching frequency of 10kHz, and the percentage of switching loss of the SiC module is still low (about 20%). Therefore, by upgrading the control frequency of the software and the power capability of the drive circuit, the switching frequency of the electronic control can be significantly increased without causing significant power derating. At high switching frequency, the fundamental frequency of the load can also be significantly increased, that is, electronic control is used in application scenarios such as high-speed air compressors and aerospace.
Evaluation of system advantages brought by SiC electronic control
The system evaluation here mainly refers to the cruising range at the vehicle level. To this end, Zhendrive Technology has developed a set of calculation tools for the entire vehicle based on the specified road spectrum: after the user selects a model and specifies the road condition template, the tool will output the output corresponding to the powertrain (motor + electronic control) According to the torque and speed commands, the cruising range of the whole vehicle is calculated according to the efficiency map of the silicon carbide electronic control and motor obtained by ScanTool calculation or actual calibration.
Here we choose a car model with low wind resistance and match the measured efficiency of IGBT/SiC electronic control and its corresponding drive motor as shown in Figure 5, and place it in CLTC-P (China Light-duty Vehicle Test Cycle – passenger car , China’s light vehicle driving conditions-passenger vehicle) simulation analysis is carried out under the road spectrum, and the energy consumption comparison of the whole vehicle system is shown in Figure 11. Compared with the original IGBT electronic control solution, the energy consumption of the vehicle equipped with Zhendrive silicon carbide electronic control is reduced by 4.4%, that is, the cruising range can be increased by 4.4% with the same battery capacity! This exciting result proves the significant advantages of silicon carbide technology in the main drive application of new energy vehicles. Based on this result, the user can further analyze the economic aspects of the vehicle.
Project summary
This paper introduces the development, testing and system evaluation of SiC power modules and electronic controls by Zhendrive Technology. The actual measurement results prove that the silicon carbide power module works stably, and the loss is significantly lower than that of the IGBT module; There are significant advantages. This project also proves that the application of silicon carbide technology to the main drive of new energy vehicles is the general trend.
The silicon carbide power module developed in this paper is compatible with a mainstream IGBT power module in the power terminal part, and the gate position has been optimized and modified to optimize the electrical performance inside the module. The silicon carbide electronic control developed in this paper is fully compatible with the functions of the IGBT electronic control and has obvious performance advantages, and can be mass-produced on the existing electronic control automation production line of Zhendrive Technology.
Zhendrive Technology has independently developed a set of automated production lines (see Figure 12). Its planned production capacity is 150,000 units per year. The automation rate of the assembly line is about 85%, and the automation rate of the test line is 100%. The factory has passed the IATF16949 quality system certification of TUEV (German Technical Supervision Association).
Towards the end, the author’s discussion on the experience of silicon carbide electronic control is as follows:
1. The main advantage of silicon carbide for electronic control is efficiency, and the economic advantage brought by higher efficiency lies in the reduction of battery installation costs and charging costs;
2. When designing a silicon carbide module, the number of parallel chips needs to be designed to achieve the best economy; more chips in parallel will reduce the economy, but it can help the vehicle achieve greater acceleration;
3. The difficulty in the design of the silicon carbide module body lies in the electromagnetic part, and it is necessary to develop accurate modeling and design aids;
4. When silicon carbide technology is used in models with small wind resistance, the cruising range can be increased by more than 4%.
In general, silicon carbide electronic control is suitable for high-end models with long cruising range and low wind resistance, and has higher economic value for users who use the whole vehicle more frequently.
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