A low-energy power supply design for automobile start-stop

To curb fuel consumption, many automakers are implementing "start-stop" features in next-generation cars, and a large number of these cars are already on the road. These systems shut down the engine when the car is stopped and automatically restart the engine when the foot moves from the brake pedal to the accelerator pedal - or releases the clutch pedal in a manual transmission to re-engage power. This feature is very helpful in reducing fuel consumption during city driving and stop-and-go rush hour.

However, this system also brings some unique engineering challenges to automotive electronics, because the battery voltage may drop to 6.0V or lower when the engine is restarted. In addition, typical electronic modules contain a reverse polarity diode to protect the automotive circuits when the car is jump-started and the jumpers are accidentally connected in reverse. This diode will reduce the battery voltage by another 0.7V, so the voltage available for downstream circuits is only 5.3V or even lower. Since many modules still require a 5V power supply, there is basically no margin, making it difficult to ensure that the circuit works properly.

Boost power supply

One method is to boost the power supply, that is, input a lower voltage and output a higher voltage. Currently, many suppliers use some type of boost power supply in the front end of the electronic module, so that the electronic module can work normally even under the reduced voltage conditions caused by the start-stop system.

As with most engineering problems, there are many ways to solve the problem. If the battery voltage at the input is only down to 6V, the first and simplest solution is to find a low dropout linear regulator that requires less than 0.3V of voltage difference to operate properly. This approach works well for modules with lower current requirements, but it is not suitable for modules with higher current requirements.

Another approach is to replace the standard PN junction diode used for front-end battery reverse polarity protection with a Schottky diode or a P-channel MOSFET. The forward voltage drop of a Schottky diode is about half that of a standard rectifier, so a few tenths of a volt of voltage margin can be added. Switching to a Schottky diode is fairly simple because it can be mounted directly on the same PCB pads as a standard diode, without requiring layout changes.

Figure 1: Using a P-FET to implement reverse polarity protection for batteries.

However, using a P-channel MOSFET requires modifying the PCB and adding some additional circuitry.

Figure 1 shows that three components are needed: a P-FET, a Zener diode, and a resistor. The P-FET needs to be sized to handle the voltage applied to the module input and the required load current. Also, consider the system's thermal requirements, since the FET power dissipation is equal to the square of the current multiplied by the FET's on-resistance. The Zener diode protects the MOSFET gate oxide from damage due to overvoltage conditions. Most P-FETs can handle 15V to 20V from the gate to source connection, so the Zener diode must be sized to clamp before this voltage point. The resistor pulls the gate to ground, turning on the P-FET, but it must also be sized to the right value. The resistor value cannot be too small, which will allow too much current to flow through the Zener diode, causing power dissipation issues. However, the resistor value cannot be too large, which will not reliably turn on the P-FET. The guiding principle of this approach is to reduce the voltage drop on the drain to source connection. Voltage drops below 5V

For a given application, one or a combination of the above solutions may be the solution. But what happens if the input voltage does drop below 5V? Some manufacturers have found that the voltage can drop to 4.5V during "cold crank" conditions. If this happens, a switching power supply is needed to increase the input voltage. The three most common switching power supplies are boost power supplies, buck/boost power supplies, and SEPIC power supplies.

The boost supply uses an inductor, an N-FET, a diode, and a capacitor. This is the simplest design, but it has some disadvantages. If the output is shorted, there is no protection because there is a direct path from the input to the output. In addition, when the input voltage rises above the output voltage setting, there is no way to prevent the output voltage from rising at the same time because the input voltage can pass directly to the output through the inductor and diode.

For example, most modules in a car must pass a load dump test. During this test, a voltage spike is generated and applied to the Vin terminal. In the case of a boost power supply, this voltage spike will be directly propagated to the output terminal. So if a 40V spike is propagated, all circuits connected to the Vout terminal must be able to withstand this voltage.

Another possibility is a non-inverting buck/boost design. This solution uses only one inductor and one capacitor, but requires two switches and two diodes. However, this solution allows the designer to prevent the output voltage from rising further if the input voltage is higher than the output voltage. By cutting off the first switch (FET1), this solution also protects against the effects of an output short circuit. The disadvantage of this design is efficiency, because there are losses in the two diodes and two switches.

Figure 2: Various boost power supplies.

The layout of the single-ended primary inductor converter (SEPIC) design is very similar to the direct boost converter, except that it adds an inductor to ground and a DC blocking capacitor. The benefit is that there is no longer a short circuit problem at the output because there is now a DC blocking capacitor in series with the output. The output is also no longer affected by the input voltage and can be lower or higher than the input voltage. It should be noted that, as with all switching topologies, a reverse polarity battery protection mechanism is still required because the reverse current will still flow back from ground to the input voltage through the body diode of the FET.

In summary, there are many issues to consider when using a start-stop alternator system. This article only briefly discusses the power supply for electronic modules, and there are actually other factors to consider. For example, when these voltages drop sharply, the interior and exterior lighting will dim. Although the interior lights flickering is annoying, it is not critical, while the brake lights and headlights directly affect safety, so corresponding power solutions are also needed to keep them working properly. Fortunately, there are corresponding solutions on the market today to solve these problems.