Asynchronous boost converter solution to create a new automotive start-stop system

　　Nowadays, car users are becoming more and more sensitive to fuel consumption, and they expect to save fuel costs, while objectively helping to reduce the impact on the environment. In order to keep up with this trend, car manufacturers have adopted various ways to reduce consumption, one of which is to apply the automatic "Start/Stop" function in new models, which helps reduce fuel consumption while also increasing sales points.

　　The so-called automatic start-stop function means that when the car stops because of traffic jams or waiting at red lights, these innovative systems automatically shut down the engine (turn off the engine); and when the driver's foot moves from the brake pedal to the accelerator pedal, the engine is automatically restarted (ignition). This helps reduce unnecessary fuel consumption and emissions during urban driving and stop-and-go traffic rush hours.

　　The impact of automatic start-stop system on automobile power system

　　But such an innovative system also brings some unique challenges to automotive electronic design. Because when the engine restarts, the battery voltage may drop sharply to 6.0 V or even lower. In the traditional automotive power architecture, the typical electronic module contains a reverse polarity diode to protect the electronic circuit in the event that the car is jump started and the jump cable is reversed. The protection circuit itself generates a voltage drop, which makes the downstream circuit voltage only 5.5 V or lower. Since many modules still require 5 V power supply, the low voltage difference does not have enough margin for the buck power supply to work properly. Therefore, the traditional automotive power architecture is not suitable for automatic start-stop systems.

　　Figure 1: Traditional automotive power architecture and its problems.

　　Common power supply solutions for automatic start-stop systems

　　There are three common solutions for selecting the appropriate power architecture for the automatic start-stop system. One solution is to use a low-dropout (LDO) linear regulator or a low-dropout switching power supply such as ON Semiconductor's NCV8852. This is an automotive-grade non-synchronous buck controller that can accept a wide input voltage range of 3.1 to 36 Vdc and withstand a 44 V load dump. The advantage of this solution is that the system voltage can work properly when the input voltage is as low as 5.5 V. However, its disadvantage is that if the input voltage drops lower, the system will not work properly.

　　Another solution is to use a buck-boost power supply as the primary power supply, such as a single-ended primary inductor converter (SEPIC) circuit built with ON Semiconductor's NCV8871 or NCV898031. The outstanding advantage of this solution is that the system can work normally even if the input voltage is as low as 4 V. However, its disadvantage is that the design needs to be changed and the system is relatively complex.

　　Figure 2: Common power supply scheme for automatic start-stop system.

　　The third solution is to use a pre-boost power supply before the primary high-voltage buck power supply. For example, the NCV8871 from ON Semiconductor can be used to build a pre-boost circuit, which will boost the voltage when the input voltage is lower than the set voltage. The advantage of this solution is that the system can work normally when the input voltage is lower than 4 V, and there is no need to change the power supply design. However, its disadvantage is that when the battery voltage is normal, it will generate additional power consumption, which is not conducive to reducing energy consumption as much as possible.

　　ON Semiconductor's Improved Front-end Boost Power Solution for Start-Stop Systems - NCV8876

　　As mentioned above, these three common automatic start-stop system power supply solutions each have their own advantages and disadvantages. In response to this situation, ON Semiconductor has launched the non-synchronous boost controller NCV8876 for automotive automatic start-stop systems, which is mainly used to provide sufficient operating voltage for subsequent circuits when the car automatically starts and stops. It is an improved pre-boost power supply solution that aims to give full play to the advantages of the pre-boost power supply solution while trying to avoid its disadvantages.

　　Figure 3: Typical application circuit of ON Semiconductor's improved front-end boost power solution NCV8876. 　　NCV8876 drives an external N-channel MOSFET, uses peak current mode control with internal slope compensation, and integrates an internal regulator to provide charge for the gate driver. NCV8876 operates with an input voltage of 2 V to 45 V and can operate under cold start and 45 V load dump conditions. The typical quiescent current of NCV8876 in sleep mode is only 11 μA, which meets the low quiescent current requirements of automotive applications. It provides ±2% output voltage accuracy over a wide temperature range. NCV8876 uses a SOIC8 micro package with an operating temperature range of -40°C to 150°C, which can meet the stringent requirements of automotive applications.

　　As shown in Figure 3, NCV8876 has a status monitoring function, which can provide working status information to the microcontroller. When the working state is low, NCV8876 works; when it is high, 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. NCV8876 integrates multiple protection functions, such as cycle-by-cycle current limit protection, intermittent mode overcurrent protection and overheating shutdown. Other features include: peak current detection, minimum COMP voltage clamping to improve the response speed during switching, etc. In general, the NCV8876 application circuit is simple and low cost, which is very suitable for automotive start-stop system applications.

　　NCV8876 Working Principle

　　The principle of the NCV8876 improved front-end boost power supply solution is: when the battery voltage is normal, NCV8876 enters sleep mode and consumes only very low quiescent current (typical value < 11 μA); when the battery voltage drops to the set voltage, NCV8876 automatically wakes up and starts boosting.

　　Specifically, when the car battery supply voltage drops below 7.3 V (factory preset), the NCV8876 is automatically enabled, and when the battery voltage drops below 6.8 V, the NCV8876 enables boost operation. Therefore, the NCV8876 can ensure that the subsequent circuit has enough margin to properly perform the step-down operation for use by the downstream system.

　　Figure 4: Detailed explanation of the working principle of ON Semiconductor NCV8876 non-synchronous boost controller.

　　ON Semiconductor's NCV8876-based demonstration board test showed that at an input voltage as low as 2.6 V, the output voltage was 6.8 V and the output current was 3.6 A, enabling the subsequent buck converter to work properly and power the downstream system.

　　Figure 5: NCV8876 demo board and measured waveforms.

　　Summarize

　　The car automatic start-stop system is a feature that many car manufacturers use as a unique selling point to help reduce fuel consumption and exhaust emissions. However, this innovative feature also brings unique engineering design challenges. This article analyzes the impact of the car automatic start-stop system on the traditional power architecture and the problems it faces, explores the common power solutions for the automatic start-stop system and their respective advantages and disadvantages, and introduces the functional features and working principles of ON Semiconductor's improved front-end boost power solution NCV8876 to help designers apply this non-synchronous boost controller to develop simple, low-cost power solutions for innovative car automatic start-stop systems.