The days of a simple DC charging subsystem in a car consisting of an alternator, rectifier diodes, basic regulator and battery are almost over, there’s no doubt about it. Now the 12V automotive regulator has to do more than this basic function. It must provide better regulation (perhaps using closed-loop control), cope with adverse operating scenarios, work with different alternator arrangements, and even provide basic grid capability. (Keep in mind that legacy 12V subsystems are present in many vehicles, even pure electric vehicles, for low power and accessory functions.)
If you have any doubts, take a look at the L9918 from STMicroelectronics. Within the unit’s simple five-wire housing is an alternator voltage regulator for 12V systems with enhanced functionality to ensure stability of vehicle electrical systems, as well as a LIN protocol interface (Fig.1). It also includes non-volatile memory (NVM) cells for device parameter programmability, making it suitable for a wide range of charging applications.
The regulator contains a MOSFET to supply the alternator excitation current and a free-wheeling diode to manage the rotor current when the excitation is off. Closed-loop operation with load response control (LRC) and LRC feedback maintains a constant voltage when the vehicle’s overall power demand is constantly changing, an essential regulator function.
The excitation MOSFET can supply up to 13 A to the coil, which is more than typical alternator regulators can supply, improving response to fluctuating demands. With its built-in NVM, the L9918 allows customers to program key parameters including voltage set point and alternator characteristics. It can be used with alternators that have up to nine pole pairs, providing more flexibility to meet a wide variety of load applications than offered by other regulators (Fig.2).
The Local Interconnect Network (LIN) bus interface allows easy connection to the vehicle’s electrical/electronic “infrastructure”, which is enabled by the external Electronic Control Unit (ECU). ECU control parameters include target voltage, status and diagnostic information.
Safety and fail-safe features of the L9918 include auto-start activation by phase signal, which independently maintains the alternator voltage set point if the LIN connection is lost for any reason. The controller returns control to the ECU when communication is restored. Current limiting, overcurrent protection, and thermal protection are also built in, as is thermal compensation that automatically adjusts the target voltage as system temperature changes.
A state diagram manages the operation of the device (Fig.3). Clearly, these are orders of magnitude more complex and sophisticated than the crude relay-based regulators of the early automotive days (admittedly effective for their time and place) and even the solid-state ones of recent vehicles.
If you have any doubts about the functional complexity and sophistication of this device, check out its 109-page spec sheet. In addition to being AEC-Q100 qualified, the L9918 is ISO26262 compliant, ASIL-B ready for use in functional safety systems and LIN VDA (Verband der Automobilindustrie) alternator regulator specification compliant. . It is offered as a bare die and housed in the inline TO220-5 package (Fig.4). Prices start at $6.12 (1,000 coins).
STMicroelectronics, “Battery Management in Electric Vehicles” (a 6-minute video, scroll down).