Suzana Jankuloska, Product Manager, Electrolytic BU Americas, KEMET
Automakers have spent years and billions of dollars building their reputations. Yet, the systemic failure of a single electronic component could ruin those hard-won reputations very rapidly. With ever more electronics being added to vehicles with every new generation platform, and to meet the needs for autonomous cars, there is an increasing number of potential points of failure for designers to consider. Automakers need to have complete confidence that device-level technology is robust and reliable before it can be included within systems in their vehicles.
Vehicles rely on capacitors
Capacitors are, in principle, relatively simple devices, yet they are used in most vehicle applications ranging from cooling fans, pumps and electric power-assisted steering (EPAS) through to systems such as injection, clutch, transmission, start-stop, and charging. Generally speaking, the primary characteristics required from a capacitor to be used in automotive applications are the ability to withstand high ripple currents, high-temperature operation, resistance to vibration, low ESR, long life / minimal or no de-rating, and, of course, low cost.
Capacitor manufacturers have recognized the importance of their products in automotive systems and spent time and resources in advanced research and development, bringing to market new technologies that will better meet the needs of the automotive industry – now, and in the future.
Electrolytic capacitors have been the mainstay of many applications, including automotive. Over the years they have shown that they can be reliable and, in particular, they are able to tolerate the harsh transients found in vehicles. Although they are generally competitively priced, their ESR is high, due to the low conductivity of the wet electrolyte. This is more pronounced at low temperatures which can lead to intermittent issues. Similarly, higher ESR leads to higher self-heat up which accelerates wear-out.
The ideal technical solution to capacitors for automotive
KEMET’s investment in this field has led to the introduction of solid material for use as a dielectric in future electrolytic capacitors. The solid material has much-improved conductivity which delivers a 10x improvement in ESR, as well as being stable over temperature. An additional benefit is that the self-heating of the capacitor is significantly reduced, thereby improving the temperature profile of the design. This extends the operating life of the capacitor as it has been shown that a 20°C reduction in operating temperature equates to approximately a ten-fold increase in life expectancy.
However, while the solution is technically excellent there is one important drawback, namely cost. Conductive polymer electrolytic capacitors typically struggle to achieve the price point required to make them successful in automotive applications.
A compromise solution meets all the needs of automotive applications. By combining the solid electrolyte from conductive polymer electrolytics with the established ‘wet’ electrolyte from conventional electrolytic capacitors then a hybrid solution is achieved that has performance comparable to solid electrolyte electrolytics, with a price point that is acceptable within the automotive arena.
Price point: Ripple performance is key to comparisons
If one were to simply compare the unit cost of a ‘wet’ electrolytic with a hybrid electrolytic, then the hybrid device would continue to have a higher price tag. However, the important measure is related to the ability to handle the high levels of ripple current found in automotive applications – and the cost related to doing so.
Hybrid capacitors can handle higher levels of ripple current in the same size package than their traditional electrolytic counterparts. As can be seen from the chart, a 20×29 electrolytic capacitor has slightly lower performance than a 10×29 hybrid capacitor, with both being able to handle around 15A of ripple current at 125°C @ 100kHz. However, the volume of the hybrid solution would be around 30% of the electrolytic solution.
Capacitance forms an integral part of most systems deployed in modern vehicles and is present in systems that pump, cool, drive and charge the vehicle – as well as many safety-critical systems. While ‘wet’ aluminum electrolytics have served the automotive industry well, the need to deliver ever more compact and reliable solutions has necessitated innovation in technology.
By combining ‘wet’ electrolytic technology with the benefits of solid conductive polymers, a hybrid electrolytic capacitor is created.
Given the improved performance of hybrid capacitors with regard to ripple current, solutions can be created using fewer capacitors than their ‘wet’ electrolytic counterparts. This, combined with their smaller unit size, allows for significantly smaller solutions to be created. Not only do these designs weigh less, but they are far better suited to the compact dimensions of modern automotive applications. And, as fewer components are used – the cost is lower. They show huge promise as the capacitor technology for current and future technology laded vehicles.
For more information on KEMET’s automotive electrolytics have a read of the “New Axial Leaded Conductive Polymer Hybrid Aluminum Electrolytic” presentation.