Working Principle of High Voltage Interlock (HVIL) and PWM Hardware Implementation in New Energy Vehicles

Full English translation of HVIL professional introduction document

High-Voltage Interlock (HVIL) Principle & PWM Hardware Solution

1. Definition and Functions of HVIL

HVIL (High-Voltage Interlock Loop) is the core passive safety protection mechanism for the high-voltage system of new energy vehicles. It adopts a low-voltage signal monitoring architecture to supervise high-voltage system status. A continuous closed-loop low-voltage monitoring wiring harness connects the HVIL detection points of all high-voltage connectors, high-voltage cabinet cover travel switches, MSD, on-board charger, motor controller, PDU, battery pack and other vehicle components, to real-time monitor the physical connection integrity of the high-voltage circuit.

In case of abnormal conditions such as loose connectors, opened cabinet covers, pulled-out MSD or broken wiring harness, the HVIL loop will be disconnected instantly. The VCU (Vehicle Control Unit) or BMS (Battery Management System) will immediately block high-voltage power-on, cut off high-voltage relays and report fault codes. This prevents personnel electric shock risks, arc ablation caused by hot plugging/unplugging of high-voltage connectors, and breakdown damage of electronic control units.

2. Basic Architecture of Traditional HVIL
   1. Signal Source: VCU/BMS outputs DC 5V/12V or PWM pulse signals
   2. Series Monitoring Loop: HVIL pins and travel switches of all high-voltage components are connected in series to form a single closed loop
   3. Terminal Matching Circuit: Terminal resistors (50Ω ~ 1kΩ) are arranged at the loop end for current limiting and sampling matching
   4. Signal Sampling Loop: Signals flow back to the VCU/BMS sampling port, and the controller judges loop continuity and signal characteristics
   5. Arc Suppression Timing Principle of Long & Short Pins

High-voltage connectors adopt a long high-voltage pin + short HVIL pin design:

• Mating stage: High-voltage conductive pins contact first, followed by HVIL pins. High-voltage power-on is allowed only after the loop self-check passes
• Unplugging stage: Short HVIL pins separate first to cut off HVIL signals, while high-voltage pins separate later. The interlock signal is disconnected prior to high-voltage separation, completely eliminating electric arcs generated by hot plugging/unplugging.

4. Defects of Traditional DC HVIL

Conventional constant 5V/12V DC HVIL can only judge loop continuity, and cannot distinguish wiring harness ground short circuit, power supply short circuit, poor contact, resistance drift and electromagnetic interference crosstalk. This leads to high probability of false or missed fault alarms. Therefore, the PWM-based HVIL hardware solution is widely adopted in high-end vehicles.

1. Basic Working Principle of PWM HVIL Solution
2. Operating Logic of PWM HVIL

VCU/BMS actively outputs PWM square wave signals with fixed frequency and fixed duty cycle into the series HVIL loop. After loop transmission and voltage division by the terminal resistor, the controller collects the returned PWM waveform:

• Normal waveform frequency, duty cycle and amplitude → HVIL loop intact, high-voltage power-on permitted
• Waveform disappearance, voltage drop, distorted duty cycle or no feedback signal → Loop open-circuit/short-circuit/poor contact, high-voltage power supply locked immediately and fault reported

2. Advantages of PWM Solution Compared with DC Solution
3. Identifies multiple faults: open circuit, ground short circuit, 12V power short circuit, poor contact and abnormal resistance
4. Strong anti-electromagnetic interference performance, adaptable to full-vehicle high-voltage EMI environment
5. Supports accurate fault location, avoiding misjudgment from simple continuity judgment of DC schemes
6. Enables continuous online real-time diagnosis to monitor high-voltage connection status during vehicle driving

III. Overall Hardware Architecture of PWM HVIL

1. Hardware Composition
2. PWM Signal Generation Unit: MCU internal timer of VCU/BMS generates fixed PWM waves
3. Drive Output Circuit: Triode/MOSFET plus current-limiting resistors to improve PWM driving capacity
4. Series HVIL Loop: Battery pack, MSD, PDU, OBC, motor controller, AC/DC charging ports, PTC, compressor and all high-voltage cabinet cover travel switches connected in series
5. Terminal Matching & Sampling Unit: Precision terminal resistors, voltage divider sampling and filter circuits at the loop end
6. Signal Receiving & Shaping Circuit: Filtering, clamping and Schmitt shaping, then signals are sent to MCU sampling port
7. Hardware Topology

MCU PWM Output → Current-Limiting Drive Circuit → Series HVIL Interlock Loop → Terminal Resistor Voltage Divider → RC Filter → Clamp Protection → Waveform Shaping → MCU AD/IO Sampling Port

1. Key Hardware Circuit Design of PWM HVIL
2. PWM Signal Output Drive Circuit

NPN triode switching drive is adopted:

• 3V PWM signal output by MCU drives the triode through base current-limiting resistor
• Collector pulled up to 12V vehicle power supply to output stable 12V PWM square wave
• Emitter series current-limiting resistor limits loop current, preventing controller port burnout under short-circuit conditions

Function: Improve PWM load capacity and avoid waveform attenuation during long-distance harness transmission.

2. Series Topology of HVIL Loop

HVIL switches of all high-voltage components are connected end-to-end in series to form a single loop without branches or parallel connections. Disconnection of any single switch will block PWM signal return transmission.

Key series sequence: Battery pack HVIL → MSD → PDU → Motor Controller → OBC/DCDC → PTC/Compressor → AC/DC Charging Ports → High-Voltage Cabinet Cover Travel Switch → Terminal Resistor

3. Terminal Resistor & Voltage Divider Sampling Circuit
4. Precision chip resistors selected for terminal resistors, typical specifications: 100Ω, 220Ω, 510Ω
5. Resistor voltage divider network attenuates loop PWM voltage to MCU sampling range (0～3.3V)
6. Parallel capacitors form RC low-pass filter to suppress vehicle high-frequency EMI interference and smooth waveforms
7. Input Protection & Shaping Circuit
8. Zener clamping diodes prevent MCU port damage caused by high-voltage crosstalk and electrostatic discharge
9. RC filter circuits eliminate high-frequency interference coupled from wiring harnesses
10. Schmitt trigger reshapes distorted, glitchy PWM waveforms into standard square waves, ensuring MCU accurately identifies frequency and duty cycle
11. MCU Sampling & Hardware Diagnosis Logic Coordination

MCU relies on timer input capture function for real-time detection:

• Whether PWM waveform frequency is within the set range
• Whether high/low level duty cycle meets standard values
• Whether peak and valley voltage are normal

Once parameters exceed tolerance range, HVIL fault protection will be triggered immediately.

1. Hardware Fault Judgment Logic of PWM HVIL
2. Loop Open Circuit: No PWM feedback waveform; MCU detects constant low/high level and reports HVIL open-circuit fault
3. Ground Short Circuit: PWM waveform pulled down to nearly 0V and disappears
4. Power Supply Short Circuit: Waveform pulled up to 12V without pulse variation
5. Poor Contact / Increased Loop Resistance: PWM amplitude attenuates, waveform distorts and duty cycle shifts
6. Failed Terminal Resistor: Abnormal voltage division ratio, sampled voltage deviates from calibrated value and triggers fault
7. Recommended Key Hardware Design Parameters
8. PWM Frequency: 1kHz～5kHz recommended, balancing anti-interference performance and sampling accuracy
9. Standard Reference Duty Cycle: 50%, convenient for fault comparison and identification
10. Operating Voltage: 12V vehicle system as PWM amplitude reference
11. Terminal Resistor: Precision 220Ω / 510Ω resistors preferred
12. Output Loop Current-Limiting Resistor: 100～300Ω in series to restrict loop current
13. Filter Parameters: RC filter resistor 1kΩ, 104/105 ceramic capacitor

VII. Summary

1. HVIL realizes real-time monitoring of high-voltage system connection integrity via low-voltage series loop, and avoids electric arcs through long/short pin timing design, which is a mandatory safety mechanism for new energy vehicle high-voltage systems.
2. Traditional DC HVIL can only judge continuity with weak fault recognition capability; PWM hardware solution realizes multi-dimensional diagnosis of open circuit, short circuit, poor contact and abnormal resistance.
3. PWM hardware consists of PWM drive circuit, series interlock loop, terminal voltage divider sampling, filter protection and shaping circuit. Combined with MCU input capture, it achieves high-precision and high-immunity HVIL monitoring, which has become the mainstream hardware implementation scheme for current new energy vehicles.