Under the hood: Contactors enable electric vehicle safety

What is a contactor?

They are electromechanical switches that, while similar to relays, perform heavy-duty applications that require higher current carrying capabilities. Contactors provide low voltage control that engages and separates copper plates to connect or disconnect leads for high voltage current paths.

These devices consist of a solenoid (a cylindrical coil of wire) and a plunger rod made of a material that resists permanent magnetization, such as steel. Current passing through the coil creates a magnetic field that attracts the plunger. Attached to the plunger is a moving contact on a thick copper plate that meshes with a fixed contact of the same material, providing a very low resistance path, typically less than one milliohm, to the main circuit. This construction allows the switch to provide electrical isolation between the control circuit and the main circuit being switched.

Contactor in operation

Today’s electric vehicles typically use either normally open (NO) or normally closed (NC) electromagnetic contactors. These devices find themselves in the path of anything that draws more than a few amps from the battery – motor drives, OBCs and even DC-DC converters that power 12 V and 48 V systems.

The key contactors that protect the battery pack are placed next to the battery pack or embedded in it, depending on the OEM's choice. The battery and inverter are isolated by these contactors to ensure safety when the vehicle is turned off. The main positive contactor is located between the positive terminal of the battery and the inverter, and the main negative contactor is located between the negative terminal of the battery and the inverter (Figure 2). The electrical isolation of the two power rails is for safety reasons, to provide redundancy, and to minimize the accidental design of alternative current paths.

The main contactor disconnects the high voltage positive and negative leads in the event of a crash or other fault, such as high speed braking causing high back EMF from the motor to the battery. The contactor also allows a mechanic to safely disconnect the battery pack for replacement.

The pre-charge contactor shown in Figure 2 has a series resistor to limit the current, and both are placed in parallel with the main contactor. This setup is necessary because the traction inverter has a bank of filter capacitors or DC link capacitors that absorb high inrush currents when the main contactor closes. Inrush currents can damage the battery and other components in its path. Therefore, the pre-charge current path is located during initial power-up.

A battery EV (BEV) also has a pair of DC fast-charging contactors that bypass the AC OBC, creating a direct connection between the battery and the off-board fast charger. Likewise, the OBC has contactors, as does the path from the DC-DC converter to low-voltage auxiliary systems such as heaters, air compressors, pumps, and steering actuators.

Failures in Fault Tolerance

While contactors are primarily used to improve the safety and fault tolerance of BEV systems, they are not without their own operational flaws and reliability issues, as discussed below.

Bounce: Whenever a contactor opens or closes, the spring action in the device causes the contacts to bounce several times before coming to rest in the new position. This damages the contacts in the long run and causes an immediate voltage spike in the circuit (Figure 3), which can damage other components. Physical damage roughens the contact surface and exacerbates sparking. Voltage spikes can be damped by capacitive loads but can cause problems with inductive loads.

Inductive load dump: When contactors open to disconnect high current carrying circuits, inductive loads in the path, such as traction inverters and motors, can induce high back electromotive force (EMF), or voltage overshoot, in the circuit. The inductive effect can temporarily result in a much higher voltage than originally applied, which can damage the contactor and other components. Note that contact dumping can also exacerbate this problem.

Holding Current: Electromechanical switches typically require a continuous current to flow through the solenoid to keep the switch closed (or open in the case of NC contactors). Although the energy required is low, it is an important consideration for applications where efficiency is paramount.

Stored Energy: Since the contactor itself is operated by an inductor (solenoid), the problem of stored energy also arises in the low-voltage control circuit. This inductive energy must be dissipated through a freewheeling diode (Figure 4) or a high-side MOSFET.

Latching: When a contactor is "latched" or closed as part of its normal operation, it requires magnetic force in the opposite direction to open the high voltage terminals carrying the high current. Switching very high currents can sometimes cause contact welding, which can keep the contactor latched. If a fault occurs, this can cause catastrophic failure, including fire.

Sparking: This occurs when a high-voltage circuit in an electric vehicle is closed or opened. The current bridges the gap between the contacts which causes a spark. Contactors are usually enclosed in a rugged housing to prevent the spark from affecting the surrounding environment. However, the spark can cause high enough power dissipation in the module to damage the copper interface and cause higher resistance. Electromagnetic interference (EMI) on surrounding components can also affect its function.

OEMs may have to choose between two mitigation options – a contactor filled with an inert gas that helps minimize or extinguish the spark, or a contactor with an attached electromagnet that “stretches” the spark from the contacts into the chute where it is “blown out.”

The Future of Solid-State Contactors

To address the above shortcomings, the industry, including the Wolfspeed design team, is investigating the use of solid-state switches (such as high-performance silicon carbide MOSFETs) to replace electromechanical contactors. MOSFET-based contactors will provide low-voltage control of the high-current path through a low-voltage isolated gate driver input.

A solid-state contactor like this would have significant advantages:

No moving parts subject to mechanical wear, resulting in longer service life

Avoid contact bounce, sparking and contact oxidation to improve reliability

Exponentially faster switching speeds compared to electromechanical switches — measured in nanoseconds — providing greater system safety and fault tolerance

Lighter and takes up less space

However, certain challenges must be overcome before such contactors become mainstream. The first of these is related to resistance. Since electromechanical contactors offer <1 mΩ, multiple solid-state switches may need to be connected in parallel to achieve the same performance. The surge current capability of MOSFETs is also much lower than that of mechanical switches. Inductive load dump issues present challenges for electromechanical switches with very high voltages across the air gap, potentially causing MOSFET breakdown.