Technical Library

Recent Technology Trends in Vibration-Resistant
Aluminum Electrolytic Capacitors for Automotive Use

NICHICON CORPORATION

■Introduction

The increase in electric vehicles/hybrid electric vehicles (EVs/HEVs) is driving an increase in the number of electronic components in automobiles. The Japanese government recently established an automated driving systems promotion committee as one of its science and technology policies, and the government is now firmly behind efforts to establish systems related to self-driving cars. As a first step, car manufacturers have implemented the Advanced Driver Assistance System (ADAS) with the goal of improving driving safety. The main ADAS technologies cover collision avoidance cameras and sensors, automatic braking and pedestrian detection systems, as well as lane departure alert systems. Electronic control units (ECUs) make these ADAS technologies possible and also ensure that cars operate efficiently. ECUs in cars are increasing in number and in the roles they perform. Table 1 shows some of the examples of ECUs applications for automotive use, aluminum electrolytic capacitors are used in all of the examples.

Expected applications for ultrafine electric double layer capacitors

Table 1 Applications for aluminum electrolytic capacitors for automotive use

The aluminum electrolytic capacitors used in ECUs are primarily chip-type and radial lead-type capacitors, and their specifications depend on the particular applications in which they are used. For example, the aluminum electrolytic capacitors in battery monitoring units of EVs/HEVs require high withstand voltages of as much as 400V. Meanwhile, aluminum electrolytic capacitors for airbag ECUs, used to trigger the ignition of the inflator (the gas generation device), require a high capacitance and rated voltages of 25–35V since the airbag is inflated while the battery is disconnected. In addition, aluminum electrolytic capacitors undergo repeated charge and discharge when the car starts up in order to ensure there is sufficient capacitance to inflate the airbag. Aluminum electrolytic capacitors used in ECUs must be able to withstand repeated charge and discharge cycles.

The aluminum electrolytic capacitors used in ECUs require different levels of performance depending on the application. For example, powertrain capacitors are usually installed near the engine, so they must be able to withstand high temperatures, be guaranteed to operate in low-temperature environments for cold regions, as well as have a vibration-resistant structure so they can withstand engine vibration.

■Requirements for Aluminum Electrolytic Capacitors for Powertrain ECUs
ECUs used to control engines, engine-cooling fans, transmissions, and pumps are called powertrain ECUs. These tend to be integrated directly into as the engines, transmissions, and motors. Aluminum electrolytic capacitors installed in such environments are exposed to high temperatures of between 125°C and 150°C, and need to continue to perform in temperatures as low as -40°C. They withstand high temperatures by using a low-transpiration, stable electrolyte and sealed parts that are highly resistant to heat. They handle low temperatures by having a low ESR. Generally, the higher the viscosity of the electrolyte, the lower its transpiration; however, a high viscosity tends to mean a higher ESR. The key, therefore, is to strike a balance of these factors in order to satisfy user needs.

Different types of aluminum electrolytic capacitors use different methods of vibration-resistant construction. In chip-type capacitors, vibration is curbed by having long base plate side walls to control the vibration of the capacitors themselves. In addition, auxiliary pads are added around the main pad to strengthen the bond between the board and capacitor so that the capacitor does not peel off the board. Chip-type capacitors have the following vibration-resistant performance: a vibration frequency range of 10 to 2,000Hz, total amplitude of 1.5mm or 294m/s2 (30G), whichever is looser, a sweep rate of 1.0 octave/minute, and a vibration direction and time of X,Y,Z in each direction for two hours, totaling six hours. Nichicon offers vibration-resistant chip-type aluminum electrolytic capacitors in the UCZ Series (1,000 to 4,000 hours at 125°C guaranteed), the UCX Series (2,000 hours at 135°C guaranteed), the UUE Series (2,000 to 5,000 hours at 125°C guaranteed), and the UBC Series (1,000 hours at 150°C guaranteed). We can also provide the same vibration-resistant performance in other series as well. However, because reflow soldering is performed on chip-type capacitors, there is a limit on the height of the product (if a product is too high, the heat from reflow will not reach the circuit). Therefore, when high capacitances are required, multiple capacitors in parallel must be used.

Radial lead-type capacitors use a different method of soldering than chip-type products that allows them to be made larger and to have fewer parts. However, their larger size and weight means they are more easily affected by vibration. If the capacitor is secured in place by a holder to prevent vibration, the internal elements still vibrate and therefore may experience internal disconnections. To solve these problems, Nichicon developed the UXY Series.

■UXY Series of Vibration-Resistant Radial Lead Aluminum Electrolytic Capacitors
The number of ECUs in cars has been steadily growing, and ECUs are increasingly being installed in the engine compartment or directly integrated into parts. This means that the aluminum electrolytic capacitors inside ECUs must be able to withstand high temperatures and have durable vibration resistance. The UXY Series was developed in response to these needs. Photo 1 shows the exterior of the UXY Series.

Expected applications for ultrafine electric double layer capacitors

Photo 1 UXY Series exterior

The UXY Series employs our proprietary slit case, which holds internal elements in place more securely than our previous products and thus achieves greater vibration resistance.

Figure 1 shows the vibration-resistant construction. In our previous products, there were gaps between the elements, which allowed them to shake and increased the risk of disconnection of the lead circuit. The slits in the case of the UXY Series fill in these gaps between the case and element, holding them in place and preventing them from shaking under extreme vibration, this makes for a highly vibration-resistant construction.

Expected applications for ultrafine electric double layer capacitors

Figure 1 Comparison of construction of previous product and UXY Series

Generally, three contact points are used to secure items to each other, but we considered that doubling or tripling this number—to six or nine contact points—would improve the securing effectiveness. However, increasing the number of contact points involves a trade-off with stability of the case shape. Using simulations we confirmed the relationship between the number of slits and the stress on the tab bases of the elements. We were able to determine that six was the right number of slits to give the optimal effect. It was decided that nine slits would not be used for the reason given above: a trade off with case shape stability. Table 2 shows the results of the simulations. Table 3 shows vibration test results using a prototype slit case that we built based on the simulation results. As you can see, the existing product not using a slit case failed the test for vibration in the X-axis direction after two hours. However, the UXY Series passed the vibrations tests in all directions.

Expected applications for ultrafine electric double layer capacitors

Table 2 Results of slit simulations (φ18×40L-equivalent element subjected to acceleration of 392m/s2 )
*Note: Tab stress is 100% when there are three slits and acceleration is against the direction of the slits.

Expected applications for ultrafine electric double layer capacitors

Table 3 Vibration test results

The UXY Series specifications: case size ofφ16×25L to φ18×40L (mm), category temperature range of -40°C to 135°C, a life of 3,000 hours, a rated voltage range of 25 to 50VDC, and a rated capacitance range of 1,300 to 12,000μF. The vibration frequency range is 10 to 2,000Hz, amplitude or acceleration is either 1.5mm or 392m/s2 (40G), whichever is looser, the sweep rate is 1.5 octaves/minute, and the vibration direction and time is X,Y,Z in each direction for two hours, totaling six hours (product is secured with fixtures).

■Near Future of Vibration-Resistant Aluminum Electrolytic Capacitors for Automotive Use
The UXY Series was developed to have 392m/s2 (40G) vibration-resistant performance, and we developed this product because users demanded this level of performance. Users have a range of requirements for vibration resistance; for example, they may require maximum acceleration, or relatively loose acceleration but for longer periods. But they all require some level of vibration resistance. We believe that powertrain ECUs will need to be not only vibration resistant but also have increasingly higher performance in all aspects (i.e. be compact, and have high temperature resistance, high ripple-current resistance, and low ESR).

To continue meeting user expectations, Nichicon will pursue further technological innovations that will lead to new products.

 
 
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