With the rapid popularization of electric vehicles (EVs), their safety has been receiving increasing attention from both users and manufacturers. As EVs become more commonplace on our roads, understanding and ensuring the safety of their high-voltage systems has become paramount to building consumer trust and meeting regulatory requirements.

Among the many safety features in modern electric vehicles, the high-voltage active discharge function stands out as a critical mechanism for preventing high-voltage personal hazards. This sophisticated system ensures that residual electrical energy is safely dissipated after the vehicle is powered down, protecting both technicians and first responders from potential electric shock risks.

We will comprehensively discuss this important safety feature, explaining several methods of high-voltage active discharge in EVs, along with their working principles, applicable scenarios, necessity, implementation methods, and the crucial role they play in improving the overall safety performance of electric vehicles.

High-voltage active discharge refers to the process in which the electrical energy stored in high-voltage capacitors is rapidly (typically within 1-2 seconds) released to a safe level (reducing the high voltage to below 60V) through a specialized discharge circuit and control strategy after the high-voltage system of the EV is powered off.

Its core purpose is to prevent the risk of electric shock and secondary hazards caused by residual voltage in the high-voltage system, ensuring the safety of personnel during vehicle maintenance, repair operations, and emergency rescue situations. This function is particularly critical when the vehicle is shut down, when the high-voltage battery is disconnected in an emergency, or in any situation requiring safe operation such as maintenance and repair.

High voltage systems in EVs, such as batteries and motor controllers, generate and store high voltages during operation. These high voltages can remain in the system when the vehicle is shut down or in an emergency situation, posing a potential threat to personnel who may come into contact with the system components.

Without an effective discharge mechanism, the residual voltage may cause electric shock accidents, potentially leading to serious consequences including injury or even fire. Therefore, the high-voltage active discharge function becomes an indispensable part of EV safety design. These risks can be effectively mitigated by actively discharging the system voltage to a safe level quickly after the vehicle is powered down.

Considering personal safety, there are clear requirements for high voltage safety in electric vehicles at the regulatory level. ISO 6469-Part4 specifically addresses electrical safety requirements for electric road vehicles, including provisions for active discharge systems. These international standards ensure that manufacturers implement adequate safety measures to protect users and service personnel from high-voltage hazards.

The working principle of the high-voltage active discharge function is primarily based on power electronics technology and sophisticated control strategies. When the high-voltage system of the vehicle needs to be discharged (triggered by commands from the Vehicle Control Unit or internal controller logic), the control system issues commands to rapidly release the high-voltage electrical energy through specific discharge circuits.

This process typically utilizes motor windings, built-in discharge resistors within motor controllers, or the linear region characteristics of power semiconductors as loads to convert electrical energy into heat energy, which is then safely dissipated. The entire process is carefully managed by the vehicle's control systems to ensure safe and efficient energy dissipation.

For EV power systems, there are three primary types of discharge mechanisms: motor active discharge, redundant active discharge, and passive discharge. Each approach has distinct characteristics and applications, working together to form a comprehensive safety system.

How does it work? Active discharge is a function that uses controlled current to actively reduce voltage through the motor. In EVs, this typically means using the motor controller (MCU) to manage the discharge process. When the system detects the need for discharge, the MCU sends signals to the motor, making the motor windings act as a load through specific control strategies, thereby consuming the energy in the high-voltage system.

How to achieve discharge? The active discharge path is implemented through the QM torque control function, which controls the d/Q current, discharges the DC-Link energy to the motor windings, and realizes energy consumption and voltage reduction through heat dissipation.

Application scenarios: Active discharge is commonly used to ensure that voltage in the high-voltage system is reduced quickly and safely during normal vehicle shutdown or maintenance procedures. It can also be employed to rapidly decrease high voltage following vehicle failures, such as after collisions, to reduce electrocution risks.

Advantages

Fast discharge speed, reducing voltage to safe levels quickly

Intelligent discharge strategies through software control

High flexibility and suitability for various situations

Disadvantages

Potential torque jitter during rapid discharge

Dependent on motor and controller functionality

How does it work? Redundant active discharge is similarly an active function but doesn't depend on the motor. Instead, it uses controlled current to reduce voltage through dedicated power electronics components, such as discharge resistors or specialized discharge circuits. These components provide backup discharge capability in case the primary motor discharge path fails.

How to achieve discharge? Redundant active discharge paths utilize specialized discharge resistors or discharge cells, controlling the discharge process through power electronic switches like MOSFETs. This provides additional discharge capacity when needed, ensuring system safety even if the primary discharge path fails. The design includes discharge resistance networks, switch control circuits, and necessary protection circuitry.

Application scenarios: Redundant active discharge primarily enhances system reliability, especially when motor discharge paths may be inoperable due to failures, such as in collision events. It also ensures voltage reduction to safe levels after extended vehicle parking.

Advantages

Provides additional safety layer and system redundancy

Independent of motor state

Wider applicability across various scenarios

Enhanced maintainability through independent monitoring

Disadvantages

Requires additional circuitry, increasing cost

Potential resistor aging and damage with repeated use

Introduces additional failure points

How does it work? Passive discharge is a discharge mechanism that doesn't require active control. It typically relies on passive system components like resistors or inductors that naturally consume or disperse electrical energy, gradually reducing voltage. While slower than active methods, passive discharge provides a constant, maintenance-free safety mechanism.

Application scenarios: Passive discharge ensures gradual voltage reduction within the high-voltage system when the vehicle is switched off and no other discharge mechanisms are active. It serves as a final safety barrier when active discharge paths fail.

Advantages

No additional control or monitoring required

Always active, providing continuous discharge

Cost-effective and simple implementation

High reliability with minimal failure points

Disadvantages

Slower discharge rate compared to active methods

Less efficient for rapid voltage reduction needs

Limited application in emergency scenarios

In the EV high-voltage electrical system, motor active discharge, redundant active discharge, and passive discharge together form a comprehensive discharge system. These complementary approaches ensure that high-voltage systems can be safely de-energized under various conditions, protecting both users and service personnel from electrical hazards.

There are multiple implementation methods for redundant active discharge, including resistance discharge, thyristor discharge, power switch discharge (module discharge), and other approaches. Each discharge path has its unique role, application scenarios, and advantages, working synergistically to ensure system safety and reliability. As EV technology continues to evolve, discharge systems are becoming increasingly sophisticated, incorporating smarter control algorithms and more robust hardware designs to enhance safety performance further.

The development of these sophisticated discharge systems represents a critical aspect of EV safety engineering, demonstrating the industry's commitment to protecting users while enabling the transition to sustainable transportation. As electric vehicles become more advanced, we can expect continued innovation in high-voltage safety systems, making EVs even safer for everyone who interacts with them.