How antivibration capacitors transform the reliability of ECUs
Hybrid electric vehicles (HEVs) also continue to enjoy strong growth. A report by JP Morgan Chase estimates that 30% of all vehicle sales will be either EVs or HEVs by 2025 (Figure 1).
It is not only the proportion of EVs versus internal combustion engines that is fuelling this growth. There is also a steady increase in features like infotainment and Advanced Driver Assistance Systems (ADAS). A typical luxury sedan can now have up to 150 Electronic Control Units installed. All these factors are contributing to an increased demand for power electronics and software solutions.
Up until recently most ECUs have been located in the engine compartment where they are shielded from the extremes of vibration and temperature cycling of the engine and drive train. But advances in technology are placing power electronic components into harsher environments.
Why EV technology is demanding a higher performance from electronic components
The EV industry is characterised by rapid technological advancement. Hybrid vehicles, for example are benefitting from the development of 48V motors. This is compared to the high voltage solutions (up to 800V) that have dominated the market until recently. The lower voltage allows the vehicle to be exempt from onerous high voltage safety requirements. It offers smaller motors with a higher power output as well as better mechanical and electrical integration.
A feature of 48V hybrid systems is the integration of motors and inverters. This makes the whole system far more compact than previous technology. The motors are now being located on the powertrain itself for improved performance, but this exposes the electronics to a higher level of vibration and temperature cycling.
DC-Link capacitors are one of the power electronics components affected by this change. They are located between the inverter and the battery and their role is to smooth out the voltage spikes caused by fluctuating demand. It is important that the capacitor performs consistently in a wide range of operating conditions including vibration and temperature. Because the DC-link capacitor governs the voltage of the entire ECU, its performance affects the performance of the whole unit.
Why standard capacitors fail
Research shows that standard multilayer ceramic capacitors (MLCCs) are vulnerable to flex cracks. When a PC board flexes due to vibration or thermal cycling, the stress passes through the solder joint and into the ceramic body. As this body is inherently inflexible, it cracks leading to an immediate drop off in its performance. The close the MLCC is mounted to a board edge or other stress points like push connectors, the more vulnerable it is to failure.
One of the main parameters that describe a DC-Link capacitor’s performance is its equivalent series resistance (ESR). A low ESR enables a capacitor to smooth out high frequency disturbances from the load.
Flex cracks cause a significant drop off in internal resistance, which undermines the capacitors performance. In a worst-case scenario, the capacitor short circuits completely. For this reason, some capacitors are designed to fail open.
The features of antivibration capacitors
There are a few different approaches to resolving vibration effects on capacitors.
- One solution focuses on the connection between the capacitor and the PC board. By improving the way in which the terminations and the capacitor itself are joined, a barrier can be formed that limits the vibration experienced by the capacitor.
- Polymer capacitors are an alternative to material to ceramic capacitors. They offer lower ESR than MLCCs and better performance over a wide range of operating conditions.
- Hybrid capacitors have a liquid and conductive polymer as the anode and an aluminium cathode. The large surface area of the liquid allows for higher capacitance ratings in a smaller footprint. This is important in the automotive industry where space is at a premium.
Vibration proof capacitors can cope with more severe forces of acceleration. Their specifications fall in the range of 30G as opposed to the 10G of standard components. They are also rated for a vibration frequency of 5 to 2000 Hz and an amplitude of 5 mm peak to peak.
Some leading antivibration solutions
KEMET offers a Flexible Termination (FT-CAP) multilayer ceramic capacitor as a solution to high vibration applications. It has a flexible termination system that reduces the transfer of stress from the board to the capacitor. The flexibility is provided by a conductive silver epoxy layer that steers a potential flex crack away from the ceramic body. These capacitors offer a 5mm flex rating versus 2mm of standard ceramic capacitors.
AVX also offers a flexible termination solution to high vibration applications. Their MLCC with Flexiterm range of capacitors carry the ability to improve flex performance from 2 mm to 5 mm. They also have a soft termination option which offers improved performance in applications with high temperature variations and cycling – like the automotive industry. MLCC with Flexiterm have a temperature cycle of 0% failure up to 3,000 cycles compared to only 1,000 cycles for standard capacitors.
Panasonic have a range of conductive polymer hybrid aluminium catalytic capacitors. Their auxiliary side terminals make the installation more stable and less vulnerable to flex. It offers a superior vibration performance and minimises the dependence of manufacturers on antivibration re-enforcement.
Conclusion
The technological advances in EVs and HEVs are generating a demand for higher performance electronic components. They must offer consistent electrical characteristics in widely changing and adverse operating conditions – especially in terms of vibration and temperature cycling. At the same time, manufacturers of ECUs are under pressure to reduce the footprint so that they fit into the more compact spaces that integrated electromechanical systems require.
Electronic component manufacturers are finding new ways to improve the reliability of capacitors for these harsh conditions. Their solutions offer ECU manufacturers the capability to meet the quality and space demands of the auto industry.
The authoring team is employed at international component distributor TME
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