Power supplies for automotive applications must perform without failure in the face of harsh conditions—the designer must consider all exigencies, including load dump, cold crank, battery reverse polarity, double battery jump, spikes, and other transients defined in LV 124, ISO 7637-2, ISO 17650-2, and TL82066, as well as mechanical vibration, noise, extremely wide temperature ranges, etc. This article focuses on the critical requirements in automotive power supply specifications and solutions to meeting automotive specifications, including:
- Automotive input transients
- Input voltage range
- Output voltage/current
- Low quiescent current (IQ)
- Electromagnetic interference (EMI)
Several example solutions are shown to illustrate how combinations of high performance devices can easily solve what would otherwise be difficult automotive power supply problems.
Harsh Automotive Environments
Figure 1 illustrates a complete power solution that meets the demanding requirements of automotive applications. At the front end, the LT8672 acts as an ideal diode, protecting the circuit from brutal conditions under the hood and destructive faults, such as reverse polarity. Following the ideal diode is a family of low quiescent current (IQ) buck regulators that feature wide input ranges—working down to 3 V and up to 42 V—to deliver regulated voltages for the cores, I/O, DDR, and other rails required by peripheral devices.
These regulators feature ultralow quiescent current, extending battery run time for always-on systems. Low noise power conversion technology minimizes the need for costly EMI mitigation, as well as design and test cycles to meet stringent automotive EMI standards. For many critical functions that must ride through cold crank events, the LT8603 multichannel low IQ buck regulators with a built-in preregulation boost controller delivers a compact solution with at least three regulated voltage rails. The LT8602 can deliver four regulated voltage rails required for many advanced drive assistance system (ADAS) applications, such as collision warning, mitigation, and blind spot monitoring.
Figure 2 shows a traditional automotive electrical system where the engine drives an alternator. The alternator is essentially a 3-phase generator, with its ac output rectified by a full diode bridge. The output of this rectifier is used to recharge a lead-acid battery and power 12 V circuits and devices. Typical loads include the ECU, fuel pump, brakes, fan, air conditioner, sound systems, and lighting. Increasing numbers of ADAS are added to the 12 V bus, including peripherals, I/Os, DDRs, processors, and their power supplies.
Electric cars change the picture somewhat. The engine is replaced with an electric motor, where a dc-to-dc converter converts a 400 V high voltage lithium-ion (Li-Ion) battery stack to 12 V, instead of an alternator. Nevertheless, traditional 12 V alternator devices are here to stay, along with their transient pulses—including fast pulses.
An engine runs at its peak efficiency in a narrow range of rpms, so the steady state output of the alternator and the battery voltage are relatively stable, say ~13.8 V, under most conditions (more about that below). Every circuit powered directly from the car battery must run reliably over the range of 9 V to 16 V, but robust automobile electronic designs must also operate during outlier conditions that will inevitably occur at the most inconvenient time.
Although output of the alternator is nominally stable, it is not stable enough to avoid the need for conditioning before it powers the vehicle’s other systems. Unwanted voltage spikes or transients are harmful to downstream electronic systems and, if not properly addressed, can cause these systems to malfunction or cause permanent damage. In the past few decades, many automotive standards such as ISO 7637-2, ISO 16750-2, LV 124, TL82066 have been produced to define the spikes and voltage transients that automotive power supplies will face, and set design expectations.
One of the most critical and challenging high voltage transients is load dump. In automotive electronics, load dump refers to the disconnection of the vehicle battery from the alternator while the battery is being charged. During a load dump transient, the excitation field of the alternator remains high given its large time constant—the alternator still outputs high power even without the load. A battery is a big capacitor and will normally absorb the extra energy, but when it is disconnected due to a loose terminal or other issues, it can no longer provide this service. As a result, all the other electronics see the voltage surge and must be able to survive load dump events. An unsuppressed load dump could generate voltages upward of 100 V. Thankfully, modern car alternators use avalanche-rated rectifier diodes, limiting the load dump voltage to 35 V—still a significant diversion from the norm. A load dump event can last up to 400 ms.