Publication date: 11 October 2011
Sales of electric vehicles (EV) and hybrid electric vehicles (HEV) have grown strongly over the past decade, and internal-combustion/electric hybrids are increasingly proving themselves as a sustainable and mainstream automotive technology. This high growth is creating robust demand from automotive manufacturers for electronic components, as many of these new cars will have parts or functions that were previously purely mechanical, but now are being replaced by electrical or electronic devices.
Another aspect of growth within the automotive industry is the attention being given to increasing the efficiency of traditional internal combustion powered vehicles. Start-stop, braking energy recovery and mild hybrid systems are some of the initiatives that are currently driving the demand for highly reliable and stable components.
Power management electronics, inverters and DC-DC converters are integral to EV/HEV applications, including the compressor and water pump in the engine cooling system, and will be subjected to the harsh conditions in the engine environment. Additionally, apart from the requirement to maintain high reliability, constraints on components includes minimizing size and weight to ensure that fuel consumption rate is optimized and maximum space is achieved for passengers.
As a fundamental device in electronic circuits, thousands of capacitors are presently used in any car today. Certainly, ceramic capacitors make a strong case for selection over competing technologies for both power electronics and standard circuitry in automotive applications.
The supporting evidence is that ceramic has gradually grown its market share, generally replacing tantalums and aluminum devices. Across all applications, ceramic is now the leading capacitor technology at 1, 10 and 100uF values, while tantalums and aluminum are dominant at 1000uF, and only the latter for values beyond 1000uF. A similar trend is also happening in automotive – ceramic has a market share up to 10uF of more than 90%, and is growing fast for 22uF and 47uF values, in particular.
A key trend for MLCCs, and especially important for EV/HEVs, is the continuous reduction in size and weight. Of course, it is consumer markets – and particularly mobile products – that are leading the demand for low profile and tiny footprint devices: the 0402 size is the leading package, and the 0201 package is growing fast and is expected to take the top spot within the next decade. The automotive industry, however, is more conservative – the leading size is 0603, although 0402 is growing fast, and there is evidence that 0201 will take an increasing share over the next few years.
In addition to the size of the devices, key to the capacitor selection process for automotive applications is reliability and electrical characteristics such as high-voltage and current handling capabilities. The first step is to examine these characteristics (see Fig. 1). Overall, ceramic devices make a strong case as the best choice in terms of reliability, high voltage resistance, impedance and ESR (Equivalent Series Resistance). Although offering a much inferior capacitance range, a case can also be made for the use of film capacitors, which are necessary for specific applications in automotive such as DC link capacitors, rated at 600uF/600V, for power converter circuitry.
Comparing the performance of a 1uF MLCC (Multi-Layer Ceramic Chip) versus 10uF tantalum and aluminum devices, respectively, ceramic delivers at least comparable or even significantly better voltage handling performance, meaning that automotive customers can replace a 10uF tantalum with a 1uF MLCC. MLCCs also have lower impedance than tantalum and aluminum capacitors at the higher frequencies (and at a lower capacitance). Another factor is better internal self-heating performance, as MLCCs have a lower ESR than tantalums or aluminum types at higher frequency ranges. Additionally, the high breakdown voltage of MLCCs means higher reliability.
Additionally, ceramic chip capacitors offer a number of other advantages over tantalum and aluminum types, including excellent noise absorption performance, longer MTTF (Mean-Time-To-Failure), and low or no change in capacitance over wide frequency ranges (although, MLCC capacitance will change in very high temperature environments, or when DC Bias is applied).
A critical issue for ceramic capacitors in automotive is the prevention of short circuits. Typically, when subjected to high mechanical stress, induced by board flexing or a wide variation of temperature, the capacitor body can crack, and may cause a short circuit. Since the failure mode of a ceramic capacitor can be a short circuit, and in a worst-case scenario where a capacitor is directly connected to the battery, board burnout could occur.
Consequently, Murata has dedicated significant R&D resources to deliver solutions that will avoid this condition. For example, the company’s recently announced GCJ and GCE series of MLCCs have been specifically designed to accommodate the bending, vibration and thermal conditions experienced in under-the-bonnet automotive applications to provide a fail-safe against short circuit. These innovative devices use a ‘soft termination’ for the ceramic body (see Fig 2.). The conductive resin on the outer electrodes acts like a cushion and absorbs any excessive mechanical stress resulting from board flexing or temperature cycles, meaning that cracks in the ceramic body will not occur. In addition, the multi-layer serial capacitor (MLSC) approach implemented in the GCE family uses an internal ‘floating’ plate, which creates an equivalent circuit of two capacitors in series. In this way, a short-circuit condition is highly unlikely to happen due to component failure.
Normally, the maximum operating temperature for ceramic capacitors is 125°C, but the automotive market sometimes requires 150°C or even higher temperature-rated components. A growing trend in traditional, non EV/HEV, vehicles is that manufacturers, usually for reasons of overall car size, want to pack in electrical circuits under the bonnet, which naturally means high temperature conditions. Murata has already developed 150°C rated devices, such as its GCM X8R series of MLCC or RH series of leaded MLCCs for use in ECU (Engine Control Unit) sensor noise filtering.
A key development for high reliability has been the introduction of metal-terminated ceramic capacitors. These new metal-pin devices are ideally suited to the requirements of EV/HEV, start stop systems and energy recovery applications, although they were originally required for various markets, including automotive, telecom basestations and LED lighting applications, to deal with issues including solder crack, board-bending crack and also acoustic noise. Of these, perhaps the major concern for automotive applications is solder crack.
This phenomenon has become more common with the use of lead-free solder, which is usually much harder than previous solutions. In heat-cycle shock tests, after 2000 cycles say, cracks between the ceramic and solder fillet can occur. However, metal terminated capacitors will perform better than standard MLCCs, due to the elastic action of the metal pins, which help to absorb the amount of stress generated by thermal and mechanical impact, making the capacitors very reliable.
Additionally, high capacitance values can also be achieved by stacking two MLCCs, which means the use of a large size capacitor. Typical metal-termination case sizes are often as large as 6.1 x 5.3mm.
The EV/HEV requirement in DC-DC converters and inverters includes high-voltage, high-capacitance devices. In the example diagram (see Fig. 3), the filter capacitors, C1 and C5, need to be high-voltage rated, for example, at 25V 47uF and 35V 33uF. So for high-power electronic applications in EV/HEVs, only metal-termination devices can support these requirements. However, C2, C3 and C4 are standard chip capacitors for automotive. Again, in the inverter circuit (also Fig 3), the C1 smoothing and C2 snubber (noise suppressor) capacitors, should also be metal-termination ceramic devices.
But for EV/HEV home-plug-in battery-charging applications, a new requirement for automotive, an application-specific component such as a Y capacitor (rated at 250V AC, 100-4700pF) will be required for safety reasons, because of the direct connection to the mains.
In addition to all these different ceramic capacitors, demands are growing for even more application-specific types, and devices with higher withstand voltages, rated to 250V, 630V and 1kV, and, of course, in ever-smaller packages, while also delivering high reliability. And all the indications are that the demands from the automotive and EV/HEV market are only likely to increase for innovative and advanced ceramic capacitor technologies.
