Today, automotive users are paying more and more attention to fuel consumption and expect to save on fuel costs, which can also help reduce environmental impact. In order to match this trend, automakers have adopted various ways to reduce fuel consumption. One way is to apply automatic “Start/Stop†function in new models to help reduce fuel consumption.
The so-called automatic start-stop function means that when the car stops due to traffic jam or red light, these innovative systems automatically shut down the engine (extinguish); when the driver's foot moves from the brake pedal to the accelerator pedal, the engine is automatically restarted. (ignition). This will help reduce unnecessary fuel consumption and reduce emissions during busy periods of urban driving and stop-and-go traffic.
The effect of automatic start-stop system on automotive power system and common power supply scheme
But such innovative systems also present some unique challenges for automotive electronics design. Because when the engine is restarted, the battery voltage can drop to 6.0 V or even lower. In a typical automotive power architecture, a typical electronic module includes a reverse polarity diode that protects the electronic circuit in the event of a jump start and a jumper cable reversal. The protection circuit itself creates a voltage drop that causes the downstream circuit voltage to be only 5.5 V or less. Since many modules still require a 5 V supply, an excessively low dropout voltage leaves the buck power supply with insufficient headroom to function properly. Therefore, the traditional automotive power architecture is not suitable for automatic start-stop systems.
Figure 1: Traditional automotive power architecture and its problems.
There are three common scenarios for selecting the appropriate power architecture for an automatic start-stop system (see Figure 2). One solution is to use a low dropout (LDO) linear regulator or a low dropout switching power supply. Another solution is to use a buck-boost power supply as the primary power source. The third option is to use a pre-boost power supply before the primary high-voltage step-down power supply.
Figure 2: Common power scheme for an automatic start-stop system (Scheme 1 is a low-dropout power supply, not just an LDO).
ON Semiconductor's improved pre-boost power solution for start-stop systems - NCV8876
The non-synchronous boost controller NCV8876 used by ON Semiconductor in automotive automatic start-stop systems is mainly used to provide sufficient operating voltage for subsequent circuits when the vehicle is automatically started and stopped. It is an improved pre-boost power solution.
The NCV8876 drives an external N-channel MOSFET that uses internal ramp-compensated peak current mode control and integrates an internal voltage regulator to charge the gate driver. Operating from an input voltage of 2 V to 45 V, the NCV8876 is capable of operation with cold start and 45 V load dump. The quiescent current of the NCV8876 in sleep mode is typically only 11 μA, which is suitable for low quiescent current requirements in automotive applications. It provides ±2% output voltage accuracy over a wide temperature range. The NCV8876 is available in a SOIC8 miniature package and operates over the -40°C to 150°C temperature range to meet the stringent requirements of automotive applications.
Figure 3: Typical application circuit for ON Semiconductor's improved pre-boost power solution, the NCV8876.
As shown in Figure 3, the NCV8876 has a status (STATUS) monitoring function that provides operational status information to the microcontroller. When the working state is low, the NCV887 works; when it is high, the NCV8876 sleeps. This device can set the frequency through the external resistor RDSC. It can also internally set multiple parameters such as current limit value and maximum duty cycle. The NCV8876 integrates a variety of protection features such as cycle-by-cycle current limit protection, discontinuous mode overcurrent protection, and thermal shutdown. Other features include peak current detection and minimum COMP voltage clamping for improved response speed during switching. In general, the NCV8876 has a simple application circuit and low cost, which is very suitable for automotive start-stop system applications.
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