High-Voltage Interlock for
New Energy Vehicles
High-Voltage Interlock Introduction
Only when the interlock circuit forms a complete closed loop can the high-voltage components of the vehicle be considered normal, and only then can high-voltage electricity be connected. HVIL signal source voltages generally come in three forms: 5V/12V/PWM duty cycle.
Purpose of High-Voltage Interlocking
The battery packs of new energy vehicles typically operate at voltages as high as 300V-800V, with very large currents. Therefore, a safety protection device is needed to improve the safety of the vehicle and maintenance personnel.
The design of high-voltage interlocks primarily aims to solve the following problems:
1. Preventing maintenance personnel from plugging and unplugging while the circuit is energized: High-voltage connectors contain numerous metal contacts. If they are directly unplugged under load (current), a strong electric arc will be generated at the moment the contacts separate. This arc reaches extremely high temperatures, severely burning the connector's metal contacts, leading to increased contact resistance, overheating, and even fire. Simultaneously, the electric arc itself is also a significant source of electromagnetic interference.
2. Preventing Accidental Disconnection of High-Voltage Connectors: Vibrations may occur during vehicle operation, or maintenance or collisions may cause a high-voltage connector to become loose or intermittent. If the system remains unaware and continues to supply power, the intermittent connection may overheat due to excessive contact resistance, posing a fire risk.
3. Providing Safety Warnings: Before vehicle startup, the BMS (Battery Management System) or VCU (Vehicle Controller Unit) performs an HVIL self-check. If the circuit is not open, the system will report a fault and prevent the high-voltage system from being powered on, thus protecting personnel and vehicle safety.
Working Principle of High-Voltage Interlock
The essence of high-voltage interlock is a low-voltage, low-current signal circuit.
1. Basic Components
(1) HVIL Signal Source: Typically, a low-voltage (e.g., 5V or 12V) monitoring signal is provided by the Battery Management System (BMS) or Vehicle Controller Unit (VCU).
(2) HVIL Circuit: A thin, low-voltage signal wire, like a "string of candied hawthorns," is connected in series through all the high-voltage components that need to be monitored and their connectors.
(3) HVIL Receiver: The signal will eventually return to the monitoring port of the BMS or VCU.
2. Working Process (Taking the most common "series circuit" as an example)
(1) Normal State (All connectors are plugged in)
① The BMS sends a 5V HVIL monitoring signal.
② This signal flows sequentially through the interlock pins inside the connectors of all high-voltage components, such as the motor controller, air conditioning compressor, PTC, and DC/DC converter.
③ Since all connectors are plugged in, the entire circuit is conductive.
④ The signal eventually returns smoothly to the receiving end of the BMS.
⑤ The BMS detects the complete return signal, determines that "the high-voltage circuit connection is normal," and allows subsequent operations such as pre-charging and power-on.
(2) Abnormal State (Any connector is unplugged or loose)
① Suppose the technician unplugged the air conditioning compressor connector during maintenance.
② The moment this connector is disconnected, its internal HVIL interlock pins will disconnect first (the design typically uses a "break-before-make" pin configuration).
③ The entire HVIL signal loop is severed at this point.
④ The BMS receiver immediately detects no returned signal (voltage becomes 0).
⑤ The BMS can identify the fault within milliseconds and immediately implement the following safety measures:
1) Alarm: The high-voltage fault light illuminates on the instrument panel to alert the driver.
2) Power Cut-off: All relevant controllers are immediately ordered to stop operating, and the main positive and negative relays are disconnected, cutting off the power supply to the entire high-voltage system.
3) Discharge: Components such as the motor controller are instructed to actively discharge the bus capacitors, quickly reducing the voltage to a safe level.
