Revving Up Film Capacitors for 42V Vehicles

The new 42V standard for automotive electrical systems will be based on a 36V battery, with an alternator generating 42V to fully charge the battery. In contrast, today's 12V standard battery is paired with a 14V alternator (the voltage required to keep the battery fully charged).

The objective of the higher voltage is to boost the vehicle's generating power to raise the alternator output from its current low of 1,800W to more than 8,000W. This will require higher voltage components — one of which is the polymer film capacitor.

Present vs. Future

To understand the need for new components, let's examine the details behind this step-function change in automotive electrical systems. The present electrical system is stretched to its power limits to run navigation systems, lighting, engine electronics, and dozens more applications. Today's typical luxury vehicle has more than 3 mi of wiring, 2,000 terminals, 50 connectors, 1,500 electronic circuits, and 100 electric motors. Such a vehicle demands up to 2.8kW, while the 14V system only produces about 3kW.

Tomorrow's vehicle will demand even more power due to five basic requirements: improved reliability, enhanced vehicle performance, improved fuel economy, emissions reduction, and “intelligent” electronic systems.

These basics include a myriad of proposed enhancements, including:

Electric power steering.
Electromechanical brakes.
Entertainment systems.
Electrical air-conditioning and heating systems.
Variable valve timing.
Dynamic suspensions.
Multiple computers.
Improved steering and traction control.
Satellite tracking.
Start-stop technology.

Add to this overall weight savings with smaller diameter wires (wiring bundle size could be 20% smaller). In addition, more electronics and “creature comforts” will be standard, such as onboard televisions and DVD players, electrical outlets, heated seats, heated steering wheels, heated windshields, heads-up displays, and navigation systems.

Power consumption of tomorrow's vehicle will continue to grow, as shown in the figure. Less than 30% of the energy in gasoline actually runs cars; the remainder is waste heat burned off during idling and squandered by inefficient components. A more powerful electrical system will allow efficient electronic components to replace the comparatively inefficient systems found today, which are driven by belts connected to the engine.

To reduce fuel consumption and emissions, components such as air conditioning compressors, water pumps, and power steering systems will be operated on demand instead of remaining a continuous parasitic load on the engine when they are in the “off” part of their operating cycle. Inverters will be needed to efficiently drive high load ac motors, creating a whole new industry for these specialized devices. For example, reconversion back to dc will be required to recharge the system during braking.

The 42V standard will update today's 12V systems — much like the change from 6V to 12V did in the 1950s. The 42V standard was established in 1995 with automotive industry and academic consensus at the Massachusetts Institute of Technology (MIT) Consortium on Advanced Automotive Electrical/Electric Components and Systems. It allows three times the voltage of the system now in use (3×14), and holds the voltage under the 50V threshold — which can stop a human heart.

Although the bus will change to 42V, many subsystems, such as computers, sensors, and cell phones, will still use 2.5V to 12V. This requires regulated dc-dc converters throughout a car's electrical system. Motors, relays, and actuators that were once “quiet” at 14V can be prone to arcing, sparking, and generating EMI at 42V, if they're in the higher voltage power circuits.

Polymeric Film Capacitors

Many components used in telecom's power distribution technology will be directly applicable to the automotive industry's new 42V requirement. Foremost among these components will be polymer film capacitors. To maintain its mandated 99.999+% uptime, the telecom industry standardized on the use of polymer film capacitors in all its critical power distribution systems. With their ability to handle high energy surges, have a relative high capacitance-to-volume ratio, be low in mass, and have a benign failure mode, polymer film capacitors are a natural fit for dc-dc converters in the critical high-end part of every car's new 42V electrical system. Other capacitor technologies, such as ceramic and tantalum, will still be used in the low-voltage subsystems. However, their use in the 42V power distribution system will be greatly curtailed, due to their increasing instability at higher voltages and reverse bias limitations.

In the present 14V charging system, electrolytic and electrostatic capacitors are used in the power delivery, control circuits, and analog actuators and motors. Motors turning on and off create large back-EMF issues, potentially damaging EMI on the automobile bus. At 42V, the higher energy (proportional to the square of the voltage increase at a constant resistance, or nine times higher) will make switching device (motors and actuators) suppression mandatory. The choice of suppression and bypass components will favor multilayer polymer film (or conventional metallized plastic film types), because of their stability at the elevated voltage and inherent high current handling capabilities.

Polymer film capacitors are neither voltage nor current sensitive and, as such, offer the best reliability at higher voltages. The lesson learned over the years on the 48V telecom bus is applicable to the 42V automotive platform: Avoid the short-circuit failure modes of MLCs (ceramic capacitor cracking and aging) and potential fires in systems. Polymer film capacitors, with their “open” failure mode, eliminate this important reliability issue.

The MIT Consortium wants to standardize on a 60V overvoltage limit for the 42V bus. However, this will be difficult to maintain because relays, actuators, motors, fans and such may produce excessive bus noise. The durability of polymer film capacitors under these conditions is well documented. The automotive industry has been using polymer film capacitors for decades for EMI suppression in both the 6V and 12V systems. Polymer film capacitors have been considered so important to vehicle production that, back during the days of total vertical integration in manufacturing, some automakers even produced their own.

Further Considerations

The 42V systems are still under design, along with the problems associated with redesigning a car's electrical system. To understand the scope of this redesign, it took more than 10 years to fully implement the change from 6V to 12V. The conversion to 42V could take even longer.

The increase in bus voltage will necessitate using more polymer film based capacitors, whether wound or stacked (multilayer polymer film). Stacked-type capacitors are generally smaller and have better attenuation properties than the wound type at the high end of the radio frequency range. For this reason, they're preferred in EMI suppression (motor and relay noise harmonics) and in the input filter circuit of high-frequency dc-dc converters.

Polymer capacitors rated at 400V and 500V are available for the expected 370V output of the motor driving inverters. Ceramic and tantalum capacitors aren't suitable for these high-voltage applications. With the energy level increasing with the square of the voltage rise, greater noise suppression is necessary close to the noise sources.

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