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).

Buck-mode operation
For operation in buck mode, the input voltage always must be above the output voltage. The function is similar to the basic buck topology. The converters boost switches (B1 and B2) do not switch during this mode. Switch B1 is always closed. This allows current to flow from the inductor to the output capacitor. Switch B2 must be open to avoid a short from the output to ground.

During the "ON time" switch, A1 is closed to charge the inductance (see below). In this cycle the current flows from the input through switch A1, the coil, and switch B1 into the output capacitor.

In the second phase of the cycle (OFF time) switch A1 is opened, and switch A2 is closed (below). The magnetically charged coil forces a current from ground through switch A2, the coil, and switch B1 into the output capacitor ( this condition is also called "free wheeling").

In a non-synchronous topology, switch A2 is replaced with a diode as passive free wheeling element. This reduces the number of required drivers and FETs, but decreases the efficiency of the converter. The switching duty cycle in this operation is depending on the input-to-output voltage ratio,

Boost-mode operation
For boost mode operation, the input voltage always must be below the output voltage. The device operates in basic boost topology. The converter's buck switches (A1 and A2) are not switching during this mode. Switch A1 is always closed to allow current flow from the input into the inductor. Switch A2 must be open to avoid shorting the input to ground.

During the "ON time" switch B2 is closed to charge the inductor (see below). In this cycle the current flows from the input through switch A1, the coil, and switch B2 into ground.

In the second phase of the cycle (OFF time) switch B2 is opened and switch B1 is closed (see below). The magnetically charged coil forces a current from the input through switch A1, the coil, and switch B1 into the output capacitor.

For a non-synchronous topology, switch B1 is replaced with a diode as passive free wheeling element. The consequences are the same as described in the buck-mode operation section above. The switching duty cycle in this operation also depends on the input-to-output voltage ratio,

Transition operation
When the input and output voltages are nearly the same, neither basic buck mode nor basic boost mode alone can maintain a stable closed loop controlled output voltage. One possibility is to switch over from one mode to another at a certain input voltage level (the threshold voltage has a hysteresis for stability). Another way would be to operate both, buck and boost, in alternating switching clock cycles to ensure stable output voltage as well as good transient response.

Conclusion
A variety of products are available that can help customers deal with the challenges of the wide 12V board-net voltage range including cold-cranking starting conditions, load dump, or battery run-down. With a fully integrated 5V, 1A buck-boost converter (such as the TPIC74100), a stable output voltage can be maintained without the need for costly, bulky transformer-type inductances. Thus, full operation under a wide range of battery voltage conditions is ensured.

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