48V Power for Cars: A New Opportunity for Passives

As electrical demands in cars increase beyond the capabilities of the traditional 12V supply, automakers are moving to 48V power rails, which offer some attractive advantages: lower overall fuel consumption, a smaller environmental impact (especially CO2 emissions), improved engine performance and the ability to introduce a new series of advanced driver assistance systems (ADAS).

The preferred 48V configuration consists of two separate branches: a traditional 12V electric branch using a conventional lead-acid battery for customary loads: lighting, infotainment, electric windows, etc. and a new 48V system to support heavier loads like air conditioning compressors, electric superchargers or turbochargers and regenerative braking. The power to start the engine will be supplied by a lithium-ion battery in the 48V electrical system. A high power starter/generator will replace the 12V alternator, reducing noise and vibration during engine starting while allowing regenerative braking to recapture up to four times more of the available kinetic energy.

Passive components such as capacitors and inductors for 48V technology not only must meet the same high quality requirements as components used in 12V automotive electrical systems (such as operating at a range of temperatures between -40 °C and +150 °C and resistance to shock and vibration – up to 60g) but they also must exhibit a high degree of efficiency so that losses can be avoided and the DC/DC buck-boost converters at the heart of the system can reach high efficiency levels.

These bi-directional converters, the central component of this dual-voltage system, insulate the 48V network from the 12V one and are used to transfer energy between the two subsystems. The capacitors and inductors in the DC-DC converter perform key functions: storing electrical energy, smoothing voltages and suppressing electromagnetic interference (EMI) in circuits to ensure EMC compliance.

Aluminum electrolytic capacitors and film capacitors are the principle passives in play in these converters. Overall, 48V systems entail new requirements for these capacitors, particularly in the need for higher voltage capabilities.

Aluminum electrolytic capacitors with axial leads and rated voltages of 63V DC are particularly well-suited to 48V applications. The aluminum electrolytic capacitor consists of several capacitances (anode barrier layer capacitance, anode electrical double layer capacitance, cathode electrical double layer capacitance, cathode protective layer capacitance) and conductors (electrolyte, and electrodes) connected in series. They are characterized by a highly robust design with a high vibration resistance.

In addition to use in 48V-12V system converters, aluminum electrolytic capacitors are already employed in automotive convenience systems like air conditioning, window wipers and motors used for automatic windows, seats and other purposes. They can also be found in key safety and control systems like power steering, airbag controls and braking systems. What is more, aluminum electrolytic capacitors are used in engine control units (ECU) for battery controls, gas- and diesel-engine control and electric motor drives for uses such as fuel pumps and fans.

Film capacitors are used as smoothers, filters and snubbers in the inverter circuits of Electric Vehicles, Hybrid EVs and the newer Mild Hybrid Vehicles (the difference between a mild hybrid and a full hybrid is in the Mild variety the electric motor is not responsible for propelling the vehicle on its own). A power inverter is required in a brushless DC motor (BLDC) running off of a 48V battery supply. In the power inverter, DC link circuits are used primarily to balance the instantaneous power variations between the input source and the output load. A DC link capacitor is placed in parallel with the input to minimize the effects of voltage variations as the load changes. A metallized film capacitor is well-suited for these automotive inverters. The DC link capacitor also provides a low-impedance path for ripple currents generated by power switching circuits.

Each of the buck-boost converters also needs a storage choke, consequently, power inductors are available for the storage and smoothing chokes in converters linking the 12V and 48V power networks. The most important task of power inductors in this application is the short-term storage of energy in the form of magnetic energy. Key requirements of the new high-current chokes include compact dimensions along with current capability. Ferrite cores with flat wire windings can ensure a high copper fill factor to reduce losses.

Automotive OEMs are fully embracing the 48V power subsystem and have signaled their intentions with new car model development now underway. A basic system with starter generator, 48V lithium-ion battery and DC/DC converter will have a rapid market rollout in 2017-2019 and together with a continuing increase in automobile electronics content will ensure a concomitant increase in passive component usage.