Electronic Design

MLCC Advance Powers Electric Vehicles

By Shoji Tsubota

 

Until recently, it was necessary to use film or aluminum electrolytic capacitors in the power circuitry needed for electric and hybrid-electric vehicle types of applications.

These applications typically require tens or thousands of microfarads in capacitance for use as smoothing capacitors.

Such capacitors had to able to withstand the high ripple currents and harsh environments experienced in high-performance automotive environments. It simply wasn’t possible to make a multilayer ceramic capacitor (MLCC) big enough for high-voltage applications. MLCCs also didn’t have the maximum allowable ripple current needed for high-current applications.

Scientists at Murata were convinced they could make an MLCC that could take on film capacitors and aluminum electrolytic capacitors in power electronics applications. In fact, a new low-loss material developed for this project allows properties different from any other type of MLCC. The result was the biggest capacitor Murata has ever made—the EVC series—featuring a footprint 50 times larger than anything else in its capacitor range.

CONSTRUCTION AND PROPERTIES
The EVC series consists mainly of Murata’s low-loss ceramic material (Fig. 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 situ in the circuit is a problem. The capacitor’s terminals therefore had to be specially designed to prevent cracking when the printed circuit board it’s mounted on is subject to mechanical stress.

On a graph of rated voltage versus capacitance (Fig. 2), the EVC series falls outside the area traditionally occupied by MLCCs into a part of the matrix normally occupied only by film capacitors; as such, the EVC series represents a totally new class of MLCC. Its specially developed low-loss highcapacitance material enables an allowable ripple current per unit volume of 1.56A/cm3, an order of magnitude higher than film capacitors, and two orders of magnitude higher than aluminum electrolytic types (Fig. 3).

Because the permissible ripple current is much higher, designers can replace film or aluminum capacitors with lower-capacitancevalue MLCCs, which can be mounted closer to other components because of their lower heat-generation characteristics. In some applications, the EVC series can contribute to reducing the requirements for system cooling and potentially a simpler overall cooling system.

The EVC MLCCs
feature a capacitance per unit volume of 2.4μF/ cm3, compared to 1.2μF/cm3 for film capacitors and 1.89μF/cm3 for aluminum electrolytic types. This means that despite the relatively “huge” footprint for an MLCC (32 x 40 x 4 mm), capacitors in the EVC series are still smaller than their film/aluminum counterparts.

Murata’s new technology lends itself in particular to electric and hybrid electric vehicles. These vehicles require high rated voltage components with high rated current, small size, and excellent thermal properties.

Figure 4 shows the powertrain 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 the other driving the airconditioning unit.

Each of these inverters are dealing with voltages up to 400V. Typically, film capacitors would be used as snubber capacitors in such inverter circuits. However, film and aluminum capacitors also tend to suffer from low heat resistance. That’s because they both contain organic material.

The EVC series, made entirely from inorganic materials, 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 the EVC series is available with rated voltage up to several hundred volts, they’re finding homes in a number of electric vehicle applications.

As an example, EVC series capacitors have been designed into an electric scooter. Powered entirely by batteries, the scooter has a top speed of 100 kilometres per hour, acceleration of 0 to 80 kilometres per hour in 6.8 seconds, and can travel 68 miles on a charge of two hours. Its emissions are zero.

The electric scooter’s powerconversion system utilises five Murata EVC series capacitors, on a board designed for smoothing the inverter. The EVC capacitors help to diminish the surge that’s being generated by the IGBT (insulated gate bipolar transistor).

The properties of MLCCs substantially lower the voltage surge experienced when the IGBT switches, which might be enough to allow a lower working voltage of the IGBT. This could potentially allow the IGBT to be downsized. Downsizing the IGBT along with the downsizing of the smoothing capacitor can lead to shrinking the inverter system as a whole.

KINETIC ENERGY RECOVERY SYSTEM
Another application involves a kinetic energy recovery system (KERS) for Formula 1 cars from a leading Italian auto-racing designer. The small size and light weight of this KERS system relies on components like the EVC series.

Due to changes in the technical regulations of Formula 1, KERS systems are allowed to be used during 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, which is to be released at at a maximum of 60kW. This is equivalent to a boost in speed of 6.7 seconds for each lap, and it’s hoped the addition of such systems will add a new dimension to the sport, particularly regarding overtaking.

Designers of this KERS system selected Murata’s EVC series for the power-conversion electronics in their KERS design due to the MLCC’s good ripple performance in small and, importantly, lightweight package sizes. The weight of the KERS is particularly important since weight distribution inside the vehicle’s body is critical for 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 the EVC series’ capacity to perform reliably in extreme and harsh environments. The parts have a high intrinsic resistance to high temperatures. EVC series capacitors maintain their ripple performance over the full automotive operating temperature range, up to 125ºC.

Formula 1 teams often will develop ideas and technology that subsequently find their way into a number of commercial vehicles. Consequently, the KERS initiative has become partly an effort to encourage Formula 1 teams to develop “greener” technology.

TAGS: Automotive
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