HVIL in Electric Vehicles

As a new trend, new energy electric vehicles have gradually come into our lives.

According to the sales data, more and more people are using electric cars to replace traditional cars.

There are many interesting technology applications on new energy electric vehicles, HVIL is one of them.

This article will introduce HVIL from the following aspects.

1. What is HVIL?

HVIL (High Voltage Interlock) is a safety feature that uses a low-voltage loop to monitor the integrity of a high-voltage circuit.

The HVIL system is designed to protect people who may come into contact with high-voltage components of electric vehicles.

The main electrical components used in the electric vehicle high-voltage interlock loop are the HVIL connector and the Manual Service Disconnect (MSD connector).

HVIL Connector & MSD Connector
HVIL Connector & MSD Connector

2. Why do electric vehicles need HVIL?

From a system security point of view, every possible risk needs to be monitored by the appropriate technology to reduce the probability of its occurrence.

One of the risks of electric vehicles is a sudden power failure. A power failure can cause the car to lose power.

HVIL acts as a circuit breaker that sends an alarm to the driver if a high-voltage connection becomes loose, disconnected or damaged while the electric vehicle is in operation.

Another risk of electric vehicles is human error.

During the operation of a high voltage system, it is very dangerous to manually disconnect the high voltage connection points.

At the moment of disconnection, the entire circuit voltage is applied to both ends of the disconnection point.

This transient voltage can break through the air and cause damage to surrounding people and equipment.

3. HVIL’s Principle

There are two aspects of high voltage interlock design to consider.

  • How the low voltage circuit can fully detect the connection status of each connection point in the entire high voltage circuit.
  • How to make the low-voltage circuit complete information transfer before the high-voltage circuit is disconnected.

Let’s start with these two aspects.

3.1 High-voltage interlock circuit design principle

All high voltage connector docking terminals are equipped with low voltage signal terminals.

Contact Structure of HVIL Connector
Contact Structure of HVIL Connector

In a HVIL system, the low-voltage signal circuit and the high-voltage circuit are independent of each other.

For example, on high voltage, Electrical A and Electrical B together form a complete circuit.

  • We can set up a separate low-voltage signal circuit for each of Electric A and Electric B.
  • We can also connect the low-voltage signal circuits of Electric A and Electric B in series.

As shown below.

The high voltage circuit uses the power pack as the power source.

There is a low-voltage power supply in the low-voltage circuit, allowing the signal to pass along the low-voltage circuit.

Once the low voltage signal is interrupted, it means that one of the high voltage connectors is loose or disconnected.

High Voltage Interlocking System in Electric Vehicles

Components of the high-voltage interlock circuit

The implementation of high voltage interlocking technology requires the following devices to work together:

  • HVIL connectors and high/low voltage conductors
  • Closed low voltage signal circuit
  • High voltage interlock monitoring circuits and monitors (The monitoring module can be on the BMS or the VCU, or both have monitoring functions separately)
  • High-voltage relay directly controlled by the high-voltage interlock monitoring signal
  • High-voltage relay controlled by the VCU according to the high-voltage interlock monitoring results
Schematic diagram of high voltage interlock circuit

There are two types of high-voltage interlock monitors:

  • One is to monitor whether the high-voltage circuit connection is complete.
  • The other is to monitor whether the high voltage electrical enclosure is in place.

The two types of monitors are used in different HVIL systems and should not be mixed.

Role of HVIL

1. Preventing Accidents During Vehicle Power-Up. Before the vehicle is powered on, the HVIL ensures that the system remains inactive if any circuit is detected as incomplete. This precautionary measure avoids accidents resulting from false connections and potential issues.

2. Mitigating Human Misoperation Risks. During high-voltage system operation, the HVIL acts as a safeguard against safety accidents caused by human error. It ensures that high-voltage connections cannot be manually disconnected without the incorporation of a high-voltage interlock design.

3. Detecting Open High Voltage Circuits. The HVIL plays a crucial role in detecting open high voltage circuits, which could lead to high voltage disconnection, loss of vehicle power, and compromise passenger safety. It provides timely alarm information to the vehicle controller prior to high-voltage power failure, allowing the vehicle system to implement necessary countermeasures.

3.2 Working principle of HVIL connector

HVIL connectors consist of a housing, high-voltage conductive parts, low-voltage signal conductive parts and monitors together.

On the HVIL connectors, a pair of high-voltage terminals and a pair of low-voltage terminals are fixed respectively.

How to make the low voltage circuit get the disconnection information first and the connection information later than the high voltage circuit?

We need to ensure that the low-voltage circuit terminals are docked later than the high-voltage circuit terminals.

This can be achieved by adjusting the length of the high-voltage terminals and low-voltage terminals.

The monitor monitors the connection status of the low voltage signal circuit and sends it to the controller.

In this way, the controller has mastered the status of the connector before the high voltage circuit is actually turned on and off.

Basic implementation of HVIL connectors
Basic implementation of HVIL connectors

4. Example of HVIL

Case from Lifan Sun’s paperElectric Vehicle High Voltage Interlock Design


In the figure below, the thick solid line indicates 12V low-voltage power line circuit, and the dotted line is the HVIL monitoring circuit.

The HVIL monitoring circuit for high voltage appliances (including DC/DC, compressor, PTC) is controlled via the VCU.

The control pins of the VCU are directly connected to the low-voltage control terminals of the relays.

The low-voltage control outputs of the three high-voltage relays inside the battery box are directly overridden via test point 1.

Test point 1 monitors the operation of the emergency power-off circuit.

The HVIL control circuit of the motor and MCU is connected directly to the control coil of low-voltage relay 2, with the other end directly overridden.

HVIL monitoring loop scheme 1
HVIL monitoring loop scheme 1

The specific working principle is as follows.

  • When there is an emergency power failure, detection point 1 sends the information to BMS, which directly disconnects the three high-voltage relays. The BMS directly disconnects the three high-voltage relays.
  • When the motor and MCU are well connected, the low-voltage relay 2 coil is switched on and the BMS works normally.
  • When the motor and MCU are not connected properly, the BMS is disconnected by controlling the coil of low-voltage relay 2.
  • Detection points 3, 4 and 5 detect the connection of DC/DC, compressor and PTC respectively and send them to VCU.

In this scheme, the BMS is linked to the motor and motor controller through relay 2.

However, VCU needs to close and split the main circuit relay through the BMS.

Scheme 2

The thick solid line in the figure indicates the 12 V power line and the dotted line indicates the HVIL monitoring loop.

Compared to scheme 1, the connections of motor, MCU connector, DC/DC, compressor, and PTC connector are detected by VCU.

Example2 of HVIL system
HVIL monitoring loop scheme 2

The specific working principle is as follows.

  • When there is an emergency power failure, detection point 1 sends the information to BMS, which directly disconnects the three high-voltage relays. The BMS directly disconnects the three high-voltage relays.
  • The connection of motor and MCU is detected by detection point 2 and sent to VCU.
  • The connection of DC/DC, compressor and PTC in series is detected by test point 3 and sent to VCU.
  • If any connector is not connected properly, the VCU detects the result and controls the BMS, so that the BMS controls the high voltage relay to disconnect the power.

It is easy to see that in a high voltage interlock circuit, to know exactly which electrical’s connector has failed, a separate HVIL circuit needs to be designed for this electrical.

If not exactly necessary, it is preferable to install correlated electrics in the same HVIL loop to simplify control.

4 thoughts on “HVIL in Electric Vehicles

    • kinlin says:

      Thank you for your inquiry. Our sales team will be in touch with you shortly to discuss your requirements and provide further assistance.

    • kinlin says:

      Yes, High Voltage Interlock Loop (HVIL) cables often use shielding. The HVIL system is designed to ensure safety by detecting the connection and disconnection of high-voltage circuits in electric vehicles, and it is crucial that this system is reliable and free from interference.

      The shielding in a cable is designed to reduce the electromagnetic interference (EMI) that can be picked up by the cable. EMI can come from many sources, including the high-voltage circuits that the HVIL system is monitoring. If this interference were to cause a false signal in the HVIL system, it could lead to unsafe conditions. Therefore, to ensure the reliability of the HVIL system, the cables used often include a shield layer.

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