Tackling power conversion in auto electronics

As the electronic component count in automotive systems increases, the amount of available space continues to shrink, greatly increasing the density of each system. All of these systems require power conversion ICs, usually with multiple voltage rails for each subsystem. Traditionally, linear regulators provided the majority of these power conversion needs as efficiency and small size were not of significant importance.

But as the power density has increased by orders of magnitude, and many applications require relatively high ambient temperature operation, any practical heat-sinking is too large to be accommodated. Thus power conversion efficiency has become critical in order to minimize the power lost as heat, driving step-down switching regulators to replace linear regulators.

However emerging automotive designs require the switching regulators to deliver very high efficiencies even with a wide variation of supply voltages, very low quiescent current and fast switching frequencies, all in a very compact, cost effective solution footprint.

Electronic Transient Challenges: Stop/Start, Cold Crank & Load Dump Conditions

To maximize fuel mileage while minimizing carbon emissions, alternative drive technologies continue to evolve. Whether these new technologies incorporate an electric hybrid, clean diesel or a more conventional combustion design, chances are they will also incorporate a stop/start motor design. Already prevalent in virtually all hybrid designs around the world, many European and Asian and car manufacturers have also been incorporating stop-start systems into conventional gas and diesel vehicles as well. In the USA, Ford recently announced that it will incorporate such systems into many of its 2012 domestic models.

Stop-Start Systems

Stop/start systems create yet another challenge for power management systems. First, the battery must be capable of powering the vehicles lights, environmental control and other electronics, while the engine/alternator is off. Additionally, it must be capable of powering the starter when the engine is once again re-started. This extreme loading of the battery during start-up introduces yet another design challenge, this time electrical, as the large draw of current required to restart the engine can temporarily pull the battery voltage as low as 4V, quite similar to the cold crank voltage profile in Figure 1. The challenge for electronics arises in supplying a well regulated output just a few hundred millivolts below the input to keep critical systems running uninterrupted when the battery bus voltage is briefly below its nominal 13.8V when the charger returns to steady state.




Cold Crank
“Cold Crank” is a condition that occurs when a car’s engine is subjected to cold or freezing temperatures for a period of time. The engine oil becomes extremely viscous and requires the starter motor to deliver more torque, which in turn, draws more current from the battery.  This large current load can pull the battery/primary bus voltage below 4.0V upon ignition, after which it typically returns to a nominal 13.8V. The electrical result to the vehicles power bus can be seen in Figure 1, but for different reasons. It is imperative for some applications such as engine control, safety and navigation systems to require a well regulated output voltage (at least 3V) through a cold-crank scenario, so as to continually operate while the vehicle starts.