Design of vehicle power management system for automotive electronics

The power on and off of the entire vehicle has become a seamless function, accompanying us every time we use the vehicle.

Before I became an engineer related to automotive electronics, my impression of the power supply of the whole vehicle came from the scenes in old movies: a car thief pried open the door of a car, pulled out two wires from under the steering wheel, and after rubbing them against each other and producing sparks, the car actually started successfully.

This left a deep impression on me as a child, so much so that for a while I always wanted to find a car on the side of the road to practice.

It was not until I became a "proud" automotive electronics engineer and was responsible for designing the vehicle's power management system that I realized that fairy tales were all lies. Now if a car is parked on the side of the road without the car key, you can't even turn on the radio, let alone drive away.

Although powering on and off the vehicle may seem like a simple and boring function, it actually requires many factors to be considered during the system design process, such as the vehicle's anti-theft certification, the robustness of the function, and emergency power-off.

To facilitate the explanation of the vehicle's power-on and power-off process, the author drew a simple state machine of a pure electric vehicle to assist understanding. In the actual design of the vehicle's power-on and power-off system, the state machine will be dozens of times more complicated than what the author drew.

01 Powering the vehicle

After entering the vehicle, step on the brake or press the one-button start switch inside the car (different OEMs have slightly different power-on strategies, which simply make it convenient for users while saving costs). The relevant controller in the car will first detect whether there is a legitimate key in the car. If so, the vehicle can be powered on normally. If not, it will continue to remain in the OFF position.

In the previous two articles "PEPS (Part 1), the attack launched by BLE" and "PEPS (Part 2), the revolution blown by UWB", the positioning principle of the key is introduced in detail. Those who are interested can go and have a look.

When the user steps on the brake or presses the one-button start switch, if there is a legitimate key in the car, we will immediately see the instrument or large screen start to light up, which means that the vehicle has been powered on successfully. This is an almost imperceptible process, and the whole process generally takes about 200ms, mainly used to confirm whether the key exists and is legitimate.

There is a slight difference between the power-on status of fuel vehicles and electric vehicles. Generally speaking, the power-on of a fuel vehicle only involves the energization of the KL15 relay. If it is an electric vehicle, the power-on process actually includes two steps. One is what we often call the energization of the KL15 relay, and the other is the energization of the high-voltage relay.

It should be noted here that the high-voltage relay is not actually directly energized. Before the high-voltage relay is energized, the capacitance in the circuit is zero, that is, there is no energy in the capacitor. If the high-voltage relay is directly closed, the moment of closing is equivalent to a direct short circuit. The current at this time is very large. If such a large current flows in the circuit without restriction, it will cause a huge impact on the entire power circuit and damage related components.

Therefore, the power system of the electric vehicle must be equipped with a pre-charging circuit, and the pre-charging relay must be opened before the high-voltage relay is energized, so that the high-voltage relay is only energized when the capacitor voltage in the circuit is consistent with the voltage of the power battery, thereby reducing the impact current during power-on and protecting the motor controller, battery, main relay, etc.

02 Power certification

After the vehicle is powered on, it does not mean that the power system is ready. The power system here usually refers to the engine of a fuel vehicle or the motor of an electric vehicle. Before the power system is ready, anti-theft certification is required, which is the common IMMO (Immobilizer, engine anti-theft locking system) certification.

AES128 or 256 algorithms are commonly used for anti-theft authentication. There are many introductions to this algorithm on the Internet. You can do in-depth research when you have time. I will not introduce it too much here. It is difficult for ordinary people to crack this set of passwords. Anti-theft authentication usually selects the controller that controls power on and off and the engine ECU or VCU for authentication.

When the vehicle is manufactured in the factory, there will be a special key generation device. After the key is generated, it will be written into the controller that needs to be authenticated, so that the vehicle can be authenticated by IMMO smoothly.

For electric vehicles, when the vehicle is powered on, the anti-theft authentication of the vehicle will be carried out at the same time. Once the authentication is passed, the user can drive the vehicle by simply switching to the forward gear.

For fuel vehicles, when we step on the brakes and press the one-button start switch, the engine will be dragged up first. Usually, IMMO certification will be carried out during this process. If the certification is successful, the engine can start normally.

During the transmission of the authentication message, the content of a certain bit of the message may be erroneous due to external interference, causing the IMMO authentication to fail. Therefore, under normal circumstances, when the IMMO authentication fails, at least three attempts are made to avoid the failure of anti-theft authentication due to a single failure.

03 Disconnect the vehicle

When the user stops the car and shifts the gear to P, press the one-button start switch, and the engine will be turned off. The purpose of this is to prevent the vehicle from accidentally touching the one-button start switch while driving, causing the engine to turn off.

The regulations also stipulate that there needs to be a switch for users to power off in an emergency. If an emergency occurs during driving, causing the vehicle's brakes to fail and the gears to fail to switch, the vehicle cannot stop. At this time, the vehicle can be powered off through the emergency switch. This switch will also be slightly different according to the strategy of each OEM. Generally, the more common ones are long-press one-button start switches or P gear switches.

I once saw a state machine for the power on and off of a vehicle, which contained dozens or even hundreds of jump paths. When the vehicle was powered on and off abnormally, I often had to look at the diagram one by one, and it would take a long time to find a problem, not to mention testing the entire function. I wonder how good a person must be to draw such a complex state machine. However, one day I suddenly realized that this was not the result of one or two days, but the accumulation of many years. Every time a problem occurred, I would patch it up on the original basis. This would really take another three years of patching. It would be better to give up the original things and redesign a simple power on and off process.

In the process of using our cars, usually no one will pay attention to the function of powering on and off the whole vehicle, because it is too inconspicuous. No one will praise the car for the smooth powering on and off of the whole vehicle, but the occasional failure to power on and off normally will cause great complaints. Therefore, the automotive electronic engineers responsible for the design of this part need to have a meticulous mind and a mentality of silently contributing, because if this function is done well, it is what it should be, and if it is not done well, it is your fault.