Technical news

New MLCC technology finds its way into electric vehicles and Formula 1

New ceramic materials and firing technology have enabled a type of ceramic capacitor never before possible. The new power capacitor is 87 times larger than Murata's next largest offering, but still presents size and weight advantages for Formula 1 cars. Murata's Shoji Tsubota explains.

Until recently, it was necessary to use film or aluminium electrolytic capacitors in the power circuitry needed for such applications as electric and hybrid-electric vehicles. These applications require tens or thousands of µF in capacitance for use as smoothing capacitors. Such capacitors needed to be capable of withstanding the high ripple currents and harsh environments experienced in high performance automotive environments. It simply wasn't possible to make a multi-layer ceramic capacitor (MLCC) big enough for high voltage applications without cracking the ceramic material during firing. MLCCs also did not have the maximum allowable ripple current needed for high current applications.

Scientists at Murata were convinced they could make an MLCC bigger than anything ever seen before that could take on film capacitors and aluminium electrolytic capacitors in power electronics applications. In the process they had to develop a new firing technology for firing large ceramic capacitors without cracking the other type of MLCC. The result was the biggest capacitor Murata has ever made, with a footprint 87x bigger than anything else in Murata's capacitor range: the EVC-series.

Construction and properties

The low-loss high-capacitance ceramic material Murata uses as a dielectric in the series is BaTiO3 (Figure 1). Throughout the ceramic material is a network of inner electrodes made of nickel, connected to a copper outer electrode, which is connected to metal terminals bonded with lead-free material. For any large ceramic component, cracking whilst in site in the circuit is a problem. The terminals therefore had to be specially designed to prevent cracking when the PCB it is mounted on is subject to mechanical stress.

On a graph of rated voltage versus capacitance (Figure 2), the EVC-series fall outside the area traditionally occupied by MLCCs into a part of the matrix normally occupied only by film capacitors; as such, they represents a totally new class of MLCC. Its specially developed low-loss highcapacitance material, BaTiO3, enables an allowable ripple current per unit volume of 1.56A/cm , an order of magnitude higher than film capacitors and two orders of magnitude higher than aluminium electrolytic types (Figure 3).

Since the permissible ripple current is much higher, designers can replace film or aluminium capacitors with lower capacitance value MLCCs, which can be mounted closer to other components because of their inherent lack of self heating effect. In some applications it can contribute to reducing the requirements for system cooling and potentially a simpler overall cooling system.

Crucially, its capacitance per unit volume is just as impressive. The capacitance per unit volume is 2.4µF/cm , compared to 1.2µF/cm for film capacitors and 1.89µF/cm for aluminium electrolytic types. This means that despite its relatively 'huge' footprint for an MLCC(32 x 40 x 4 mm), the EVC-series capacitors are still smaller than their film/aluminium counterparts.

Murata's new technology lends itself in particular to electric and hybrid electric vehicles where high rated voltage components with high rated current, small size and excellent thermal properties are required. Figure 4 shows the power train of a typical hybrid electric vehicle with power coming from both the engine and an electric motor.

The system features two inverter circuits, one driving the electric motor, and one driving the air conditioning unit. Both inverters are dealing with voltages up to 400V. Typically, film capacitors would be used as snubber capacitors in such inverter circuits. However, film and aluminium capacitors also suffer from low heat resistance as they both contain organic material. The EVC capacitor is made entirely from inorganic materials and has a very high intrinsic resistance to high temperatures. MLCCs can also provide better surge suppression ability than film capacitors due to their low equivalent series resistance and inductance (ESR and ESL).

Now that these MLCCs are available with rated current up to 1200V, they are finding uses in electric vehicle applications.

As an example, these power capacitors have been designed into the recently released electric scooter seen in Figure 5. This scooter is entirely powered by batteries. It has a top speed of 100 km per hour, acceleration of 0-80 km per hour in 6.8 seconds and can travel 68 miles on a charge of 2 hours. Its emissions are zero. Four Murata EVC-series capacitors are utilised in the scooter's power conversion system, on a board designed for smoothing the inverter. The capacitors are used to reduce the surge generated by the IGBT (insulated gate bipolar transistor).

The properties of MLCCs substantially reduce the voltage surge experienced when the IGBT switches, enough to allow a lower working voltage of the IGBT. This means the IGBT can be downsized, which along with the downsizing of the smoothing capacitor itself, contribute to the downsizing of the inverter system as a whole.

KERS

KERS Another application to make use of Power's technology is a KERS (Kinetic Energy Recovery System) for Formula 1 cars from leading designer Magneti Marelli. The small size and light weight of this KERS system relies on cutting-edge components like Murata's EVC series capacitors.

Due to changes in the technical regulations of Formula 1, KERS systems are permitted for use in the 2009 season. KERS is a method of storing energy that would otherwise be wasted when braking. This energy can then be released to provide extra power on demand.

The rules permit 400kJ of energy to be stored per lap to be released at maximum 60kW. This is equivalent to a boost in speed for 6.7 seconds each lap and it's hoped the addition of such systems will add a new dimension to the sport, particularly regarding overtaking. Magneti Marelli selected Murata's EVC capacitors for the power conversion electronics in their KERS design due to the MLCC's superior ripple performance in small and, importantly, lightweight package sizes. The weight of the KERS is particularly important as the weight distribution inside the body of the vehicle is critical for the vehicle's performance. The parts' small size is attributed to Murata's ceramic materials technology which allows a very high capacitance per unit volume.

This application also demonstrates EVC's capacity to perform reliably in extreme and harsh environments. The parts have a very high intrinsic resistance to high temperatures and maintain their ripple performance over the full automotive operating temperature range, up to 125 °C.

Since Formula 1 teams often develop ideas and technology that is subsequently found in commercial vehicles, the KERS initiative is partly an effort to encourage Formula 1 teams develop 'greener' technology.

Through KERS and through contributing to the advancement of electric vehicle electronics, Murata hopes to contribute to the protection of the global environment.

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