Technical information｜How electric explosion drivers and contactor drivers help improve the safety and efficiency of hybrid vehicle/electric vehicle battery disconnection systems

For hybrid vehicles (HEVs) and electric vehicles (EVs), the power distribution system in the battery management system (BMS) supplies the core functions of the vehicle and also provides a mechanism to safely disconnect high voltage or high current events. As the demand for higher voltages, currents, efficiency and reliability continues to grow, two core components of the power distribution system - the high-voltage relay and the disconnect fuse - face increasing design challenges. Figure 1 shows an overview of the high-voltage relay and disconnecting fuse.

In the event of an emergency, the non-resettable battery disconnect fuse will activate, disconnecting the battery from the rest of the vehicle. A high voltage relay (also called a contactor ) connects and disconnects the power lines throughout the HEV or EV during normal operation. In this article, we discuss emerging technologies in contactors and fuse-opening drivers that are helping to make BMS smarter, safer and more efficient.

In the event of a collision, power to downstream systems needs to be cut off to avoid further complications or damage. Two commonly used solutions today are blowout fuses and thermal fuses. Blow-type fuses are tripped by thermal conditions in the event of an overcurrent event, and these fuses are preset at the factory. Thermal fuses require an electronic driver to send a signal to disconnect, primarily driven by discrete circuits or traditional airbag electric blast drivers. As HEV and EV systems become more powerful, thermal fuses can provide greater reliability and enable faster deployment. However, the common solutions to achieve fast response and actuation of these thermal fuses can easily become very complex. The requirements of the International Organization for Standardization (ISO) ISO26262 also need to be met, adding to the complexity of these designs.

To achieve faster response, Texas Instruments' DRV3901-Q1 thermal fuse driver uses a direct 2-pin hardware interface that bypasses the serial peripheral interface (SPI). The DRV3901-Q1 driver can be paired with voltage, current, and resistance (UIR) sensors to further speed deployment. UIR sensors such as the BQ79631-Q1 device can communicate directly with the DRV3901-Q1 driver through hardware pins, eliminating the need for an MCU.

One of the more important functions of a thermal fuse is its ability to disconnect the battery from the rest of the system in the event of a crash or other serious failure. System designers must ensure that the thermal fuse function can be activated reliably. The DRV3901-Q1's built-in diagnostics monitor drive status, thermal fuse health, and backup power availability. In order to monitor the availability of backup power, the energy storage capacitor needs to be measured. This capacitor serves as backup power to the thermal fuse system if the main power provided by the battery is no longer available. By regularly checking the discharge voltage of this capacitor, the DRV3901-Q1 driver and MCU are able to detect its failure and alert the vehicle, rather than having to wait until it is needed to find out that it is unusable.

It's important that the fuse works when needed, but it's equally important that the fuse doesn't blow by mistake. The DRV3901-Q1 driver features integrated safety diagnostics to prevent accidental blowing of the thermal fuse. This is achieved by combining different features, including separate high-side and low-side drivers, redundant pins for direct hardware triggering, and cyclic redundancy check (CRC) protection on the serial peripheral interface.

A common connection in the power line of a HEV or EV is to connect the high voltage battery system to the main contactor of the traction inverter. There may also be other power rails, such as AC/DC charging contactors to connect the charging station to the battery, and auxiliary contactors to connect other electrical loads such as interior lights or heaters.

A contactor is a low-voltage solenoid valve used to control a mechanical relay switch capable of delivering high current at high voltage. To control systems with increasing power, contactors in HEV and EV vehicles continue to evolve. The low voltage solenoid valve element of the contactor is usually driven by a control circuit called an economizer. Energy saver circuits are becoming increasingly important and complex to meet higher requirements for efficiency, reliability and safety, and also help improve power supply efficiency under high power conditions. These circuits help reduce the current draw required to keep the contactor closed. This economizer can be integrated directly into the contactor or added externally. Contactors requiring external economizers can easily become very complex when trying to achieve system-level safety goals.

Fully integrated high-power contactor drivers, such as the DRV3946-Q 1 driver, can replace complex economizer designs. The DRV3946-Q1 driver enables efficient conduction of contactors and safe shutdown. ForFigure 2 shows this feature in action. Higher current can be supplied during startup to establish the initial connection. After the connection is established, the current can be reduced to a lower level during the "hold" phase. The ability to program the integrated peak and hold phases enables more robust and efficient switching of contactors.

Closing the contactor is also critical, being able to quickly open the contactor helps prevent the contacts from welding and provides the first line of defense for the rest of the vehicle system in the event of a failure. Common solutions utilize fast discharge capabilities for peak and hold current control, but result in circuit complexity. The DRV3946-Q1 driver combines these two functions on a single chip, helping to reduce system complexity and improve efficiency and security.

Improving the efficiency and reliability of contactors can help increase drivable range and enhance the safety of daily driving of HEVs and EVs. Integrating a thermal fuse driver into a single-chip solution helps make smarter, faster decisions about when to disconnect the battery. The DRV3901-Q1 thermal fuse driver and DRV3946-Q1 contactor driver provide system designers with multiple options to design smarter, safer vehicles.