The widely varying voltage of the 12V automotive board net can present a challenge for automotive electronic application design engineers. With DC voltages ranging from 3.5 to 28V (and for transients it can be as high as 45V during a clamped load dump (disconnected battery) pulse), simple power supplies such as linear voltage regulators and buck or boost DC/DC converters are not always applicable to provide the desired electronic control unit (ECU) supply voltages. A buck-boost converter topology can solve this dilemma without the need for magnetically coupled coils (transformers such as a SEPIC (single-ended primary inductance converter) or a flyback converter) and offer a cost-effective, flexible system.
For many applications and ECUs in the automotive environment, the voltage supplied by the battery and alternator is not adequate and first needs to be converted to the correct voltage level. DC/DC switching voltage regulators and linear voltage regulators are a widely used solution to achieve this goal. Because linear solutions can not be used to generate output voltages higher than the supplied input voltage, this article focuses on switching voltage regulators only.
The most common topology is the buck converter (see below). By requiring only a single inductance, a diode, and a switch, it's one of the simplest and most cost-effective switching DC/DC solutions. There is a drawback, however, which is the limit in generating only output voltages lower than the input voltage.
If a higher output versus input voltage is required, the "inverse" topology or boost converter (below) can be used. This topology requires similar components as the buck converter, but provides output voltages greater than the input voltage.
As the automotive board-net voltage can vary within a wide range (low as 3.5V during cranking and up to 45V during a clamped load dump), a cross over of input and output voltage levels in some ECU applications is inevitable. The loss of functionality during cranking (starting the engine) is not acceptable, especially for power train applications or some navigation and infotainment systems during boot-up. This problem could be solved with solutions like a flyback converter or SEPIC topology, but the additional cost and space for the required transformer type inductances makes them less attractive to customers.
A solution that can provide both constant output voltage, even if the input voltage crosses the output voltage value, as well as simple design with only a single coil is the buck-boost topology. It combines the buck and boost converter in one topology. A seamless transition between the two different modes allows a stable, uninterrupted output voltage under all input voltage conditions.
As two different topologies are combined, two switches and two diodes are required for the non-synchronous buck-boost converter (see above), compared to one switch and one diode for the simple buck or simple boost. To increase overall system efficiency, the two diodes can be replaced by switches. The topology now looks similar to a full H-bridge with inductor (below).