A T-box power architecture design with eCall function

0 Preface

With the development of automobile networking, the installation rate of T-box , as a connected terminal device,  in automobiles continues to increase. Combined with mobile communication networks, T-box  can provide functions such as remote control, anti-theft, data routing, emergency calls (commonly known as eCall ), etc.; since eCall  plays a very important role in the timely rescue of people after a traffic accident, new cars in Russia, the European Union and other regions are required to install devices with eCall  functions. At present, China's eCall related standards are also being formulated.

1 Power requirements for vehicle-mounted equipment

Since automotive electronic equipment has very high requirements for reliability, life, electromagnetic compatibility, power consumption, etc., these must be considered when designing the power supply of automotive equipment. For example, power consumption. Since the equipment is powered by the battery after the car is turned off, the static current requirements of the equipment are very high. Some manufacturers require the static current of a single device to be at the μA level, which requires the equipment to have better countermeasures in power architecture design and power management strategy; secondly, the power supply environment of automotive equipment is very harsh, such as the battery voltage drop when the car is ignited, the load dumping of large current equipment on board, etc., so there are many test requirements for the equipment power supply, such as ISO7637, overvoltage, reverse voltage, power supply conduction radiation, power supply conduction immunity, etc.

2 Architecture Design

This article mainly involves a T-box power supply architecture design with eCall function  . The main difference between it and the general vehicle-mounted electronic equipment power supply is that the T-box with eCall function needs to be equipped with a backup battery and a dedicated battery management circuit. Since the typical operating voltage of the currently used network module processor is 3.8 V, when powered by a backup battery , in most cases, the voltage of the fully charged battery will cross 3.8 V from high to low during the discharge process of powering the T-box. This requires the power supply of the T-box to meet many test requirements of the vehicle-mounted equipment power supply under the condition of adding a battery management circuit. Figure 1 is the power supply architecture diagram of the T-box.

Figure 1 T-box power architecture

3 Functions and design considerations of each power module

1) The battery voltage detection module is used to detect the voltage of the car battery. When the car battery voltage is abnormal, such as the vehicle equipment dumping load or the car being left unignited for a long time, resulting in the battery being unable to charge, the microcontroller needs to shut off the power supply of most modules according to the overvoltage and undervoltage signals fed back by the voltage detection module to protect the internal circuit of the T-box or reduce the loss of the car battery.

2) The first-stage buck module is directly connected to the vehicle battery (about 12.6 V when the battery is in good condition and not ignited) and steps down the voltage, for example, to 5 V. Generally, this output voltage and the second and third-stage power outputs are constant power, which is used to power the device in standby mode. As long as the vehicle battery is not disconnected, its output is always present, mainly to power the microcontroller and bus transceiver circuit. Due to the special position of this power supply in the entire power architecture, it will affect the performance, reliability and stability of the entire power supply, and play an important role in whether many power-related tests can be passed.

3) The bus transceiver is used for signal transmission and reception between the microcontroller and the car CAN bus. The microcontroller controls the working status of each power module according to the car ignition and shutdown messages on the bus, so that the T-box can work in different modes, such as full function, standby, and deep sleep.

4) Each power module provides power with corresponding voltage and load capacity to each functional module. The following should be considered when designing these power modules:

● Set a suitable working voltage error range;

● Select appropriate load capacity according to the requirements of the functional module. The load capacity needs to have a certain margin to meet the peak current requirements and avoid cost waste caused by over-design;

● Control strategies for multiple power supplies of associated modules (e.g. modules related to the eCall function) to achieve low power consumption requirements and make the control strategy not too complicated;

● Meet the power-on timing/power-off timing requirements of some modules;

● Meet the power supply requirements of special circuits, for example, noise-sensitive GNSS circuits require low-noise power supplies;

● It is necessary to design a suitable filtering and decoupling circuit to prevent the noise from being transmitted to the subsequent stage as well as to prevent it from being transmitted to the previous stage, which would eventually lead to the test indicators related to the whole machine port and power supply exceeding the standard.

5) The main application scenario of the built-in backup battery of T -box is that when the car is in a collision or other accidents, the car battery is damaged or the battery connection line is disconnected and cannot power the T-box. The backup battery can power the T-box to allow it to continue to work for a certain period of time to complete the upload of vehicle emergency information, emergency calls with the backend, etc. eCall can be automatically activated by manual buttons or collision airbag signals, and the backup battery management circuit needs to have

The following functions and design considerations:

● It must have a switching circuit to switch power between the vehicle battery and the backup battery after activating eCall. The power switching must not cause the machine to restart, and it must also avoid frequent switching to the backup battery due to changes in the vehicle battery voltage when the car is ignited;

● Backup battery charge and discharge control: Due to the harsh working environment of vehicle-mounted products and the requirements for high reliability and long life, it is necessary to design appropriate charging logic according to the characteristics of the backup battery (such as charging condition judgment, charging current setting, charging time, whether to charge when the car is turned off, etc.) to ensure the safety, ready availability and life of the battery. As mentioned earlier, the voltage of the backup battery will cross 3.8 V during the discharge process, so the discharge circuit can consider the buck + boost solution, and set the appropriate discharge termination voltage to avoid damage to the battery due to over-discharge;

● Minimize the number of systems powered by backup batteries and take into account the power conversion efficiency to extend their working time;

● Battery life detection is required, usually detecting the internal resistance of the battery. When the detection value exceeds the preset value, the information needs to be sent to the backend and then pushed to

Users are reminded to replace batteries in time just in case;

● Design considerations for special scenarios, such as Tier 1 manufacturers completing T-box

The transportation mode from production to the car factory before installation on the car requires appropriate design of the battery peripheral circuit to minimize the impact on the battery power during this period.

6) The external power supply must have open circuit, short circuit and other diagnostic functions, such as the power supply to the GNSS antenna and the power supply to the eCall external button. In this way, when an open or short circuit fault occurs, the information can be sent to the background and pushed to the user, reminding the user to go to the 4S store for processing in time.

4 Applications

In the actual application of T-box, in order to realize functions such as remote control or remote monitoring after the car is turned off, the power supply of some modules of T-box cannot be turned off in standby mode, so that it can be woken up at any time; when the car is not ignited for a long time, such as 7 days, in order to reduce the loss of the vehicle battery, T-box needs to enter deep sleep mode from standby mode. The circuit corresponding to this power architecture can realize the above functions and meet the quiescent current requirements of mA level in standby mode and μA level in deep sleep mode, and can also meet other strict requirements of vehicle electronic product testing standards, such as electrical performance, EMC, reliability, etc.