Automotive Start/Stop System Power Solution

    To limit fuel consumption, some car manufacturers are using Start/Stop functions in their new generation models. These innovative new systems shut down the engine when the car comes to a stop and automatically restart it when the driver moves his foot from the brake pedal to the accelerator. This helps reduce fuel consumption in urban driving and stop-and-go traffic.

　　But such a system presents some unique engineering challenges for automotive electronics, because when the engine is restarted, the battery voltage may drop to 6.0 V or even lower. In addition, typical electronic modules contain reverse polarity diodes to protect the electronic circuits in the event that the car is jump started and the jump cables are reversed. The diodes cause the battery voltage to drop another 0.7 V, leaving the downstream circuits with only 5.3 V or less. Since many modules still require a 5 V supply, there is not enough headroom for the power supply to work properly.

　　One solution is to use a boost power supply. A boost power supply accepts a lower input voltage and produces a higher voltage at the output. Vendors are currently using some type of boost power supply on the front end of the electronic module to enable it to operate properly under the voltage drop conditions caused by the start/stop system. The following article will review the different options that designers can use for these start/stop systems, including low dropout (LDO) regulators, reverse battery protection schemes, and various boost options.

　　Like most engineering problems, there are many ways to solve them. If the battery voltage drops to only 6 V at the input, then the first and simplest solution is to look for a very low dropout linear regulator that requires only <0.3 V headroom. This solution works for modules with lower current requirements, but for modules that require higher current, the designer needs more options.

　　Another option is to replace the standard PN junction diode used for reverse battery protection on the front end 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 it adds a few tenths of a volt of voltage margin. Switching to a Schottky diode is simple and straightforward enough because it usually fits right into the same PCB pads as a standard diode, without changing the wiring. But a P-channel MOSFET (P-FET for short) requires changes to the PCB and some additional circuitry.

　　

　　Figure 1: Reverse battery protection using a P-channel MOSFET

　　Figure 1 shows the three components required, including the P-FET, Zener diode, and resistor. The P-FET needs to be sized appropriately to handle the voltage applied to the module input and the required load current. In addition, it is important to consider the system thermal requirements, as the power dissipated by the FET is equal to the square of the current multiplied by the on-resistance of the FET. The Zener diode protects the gate oxide of the MOSFET from operation caused by overvoltage conditions. Most P-FETs can handle 15 to 20 V from the gate-to-source connection, so the Zener diode must be set to clamp before this point. The resistor pulls the gate down to ground to turn on the P-FET, but the resistor must also be sized appropriately. The impedance of the resistor cannot be too low, because if the impedance is too low, too much current will flow through the Zener diode, causing power dissipation problems in the Zener diode. However, if the impedance of the resistor is too high, the P-FET may not turn on as reliably as desired in this case, and the idea is to reduce the voltage across the drain to the source.

　　It is likely that one of these options, or some combination of them, will be suitable for a given application. But what happens if the input voltage actually drops below 5 V? Some manufacturers are looking at whether the input voltage will drop to 4.5 V during cold cranking conditions. The three most common switching regulators are the step-up voltage supply, the buck/boost supply, and the single-ended primary inductor converter (SEPIC) supply.

　　

　　Figure 2: Different boost power supply topologies

　　The boost power supply uses an inductor, an N-channel MOSFET (N-FET), a diode, and a capacitor. It is the simplest design, but it also has some disadvantages. If the output is shorted, there is no way to protect it because there is a direct path between the input and the output. In addition, when the input voltage rises above the output voltage set point, there is no way to prevent the output voltage from rising as well, because the input voltage will pass right through the inductor and diode to the output.

　　For example, most modules in a car must pass a load dump test. This test creates a voltage spike and is applied to the input voltage. In a boost power supply, this voltage spike propagates to the output. Therefore, if a 40 V spike propagates along the line, any circuit connected to the output voltage must be able to handle this high voltage.

　　Another possible switching regulator option is a non-inverting buck/boost design. This design uses only 1 inductor and 1 capacitor, but requires 2 switches and 2 diodes. However, this scheme does allow the designer to prevent the output voltage from rising when the input voltage rises above the output voltage. It also provides output short-circuit protection by opening the first switch (FET1). The drawback of this design is its energy efficiency, because the losses of 2 diodes and 2 switches need to be considered.

　　The SEPIC design is very similar to a direct boost converter in terms of wiring, except that this design adds an inductor and a DC blocking capacitor. The downside of this design is that it adds another inductor and a capacitor, but the upside is that there are no longer issues related to output short circuits because the DC blocking capacitor is now connected in series with the output. In this way, the output is no longer affected by the input voltage, so it can be lower or higher than the input voltage.

　　It should be noted that despite all the switching topologies listed above, a reverse battery protection scheme is still required because reverse current may flow from ground to the input voltage via the body diode on the back of the FET.

　　In summary, there are many issues to consider when designing a start/stop alternator system. This article only discusses the power supply issues of the electronic modules, but there are other issues that need to be addressed. For example, when the voltage drops, both interior and exterior lighting will dim. The interior lighting flickering problem is also annoying but not critical, while brake lights and headlights affect safety, so the power supply needs to keep these interior/exterior automotive lights bright and working continuously. Fortunately, there are solutions on the market today to solve these problems.