What’s Under the Hood: Contactors Implement EV Safety

Placed beside the more keenly discussed parts of the electric vehicle (EV) powertrain are electromechanical switches, called contactors. Despite their unglamorous existence under the hood, they are part of both functional and safety essentials that keep the EV running smoothly. Given that EV registrations grew by 41% in 2020¹ and each vehicle has several contactors, this is indeed an important component segment.

This chapter of Wolfspeed’s What’s Under the Hood series focuses on the switches that play various roles in an EV, from helping manage cell balancing and routing out on-board chargers (OBCs) in favor of fast charging to protecting batteries from overcurrent conditions.

What are contactors?

They are electromechanical switches that, while similar to relays, perform heavier duty applications that require higher current carrying capacity. Contactors offer low-voltage control that engages and disengages a copper plate to connect or disconnect the leads of a high-voltage current path.

The devices comprise a solenoid (a cylindrical coil of wire) and a plunger rod of material that resists being permanently magnetized, such as steel. Passing current through the coil creates a magnetic field that attracts the plunger (Figure 1). Attached to the plunger are moving contacts on a thick copper plate that engage with fixed contacts of the same material, offering a very low resistance path, usually less than a milliohm, to the main circuit. The construction allows the switch to offer galvanic isolation between the control circuit and the main circuit being switched.

Contactors in action

Today’s EVs generally all use electromagnetic contactors that are either normally open (NO) or normally closed (NC). These devices find themselves in the path of anything that draws more than a few amperes from the battery — the motor drive, the OBC, even the DC-DC converter that supplies 12 V and 48 V systems.

The key contactors protecting the battery pack are placed beside it or embedded in it, depending on the OEM’s choice. The battery and the inverter are isolated by these contactors for safety when the vehicle is switched off. The main positive contactor is placed between the battery’s positive terminal and the inverter, and the main negative contactor is between the battery’s negative terminal and the inverter (Figure 2). Electrical isolation of both rails is done for safety — to provide redundancy as well as to minimize accidental design-ins of alternative current paths.

The main contactors disconnect the high-voltage positive and negative leads in the event of a crash or another fault, such as when braking at high-speed causes a high back EMF from the motor to the battery. The contactors also allow mechanics to safely disengage the battery pack for replacement.

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

Battery EVs (BEVs) also have a pair of DC fast-charge contactors that bypass the AC OBC to establish a direct connection between the battery and the off-board fast charger. Similarly, the OBC has contactors as does the path from the DC-DC converter to low-voltage auxiliary systems, such as the heater, the air compressor, the pumps, and the steering drive.

The faults in fault tolerance

Although contactors are used primarily to increase the BEV system’s safety and fault tolerance, they are themselves not without operational drawbacks and reliability issues as described below.

Bounce: Whenever a contactor is opened or closed, spring action in the device causes the contacts to bounce several times before coming to a rest in the new position. This damages the contacts in the long run and causes immediate voltage spikes (Figure 3) in the circuit that can damage other components. The physical damage roughens the contact surfaces and worsens sparking. The voltage spikes can be dampened by capacitive loads but cause problems with inductive loads.

Inductive load dump: When contactors are opened to break a high-current-carrying circuit, inductive loads in the path, such as the traction inverter and motor, cause a high back electromotive force (EMF) or voltage overshoot in the circuit. The inductive action momentarily causes a much higher voltage than was originally applied, which can cause damage to both the contactors and other components. Note that contact bounce can also exacerbate this problem.

Hold current: The electromechanical switches generally need a continuous current flowing through the solenoid to hold the switch closed (or open in case of NC contactors). Although the energy needed for this is low, for an application that seeks efficiency above all else it is an important consideration.

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

Latch-up: When a contactor is “latched” or closed as part of its normal operation, it requires a magnetic force in the opposite direction to open the high-voltage terminals carrying high currents. Switching very high currents can sometimes lead to contact welds so that the contactor remains latched up. In the event of a fault, this can lead to catastrophic failure, including fire.

Sparking: This happens when a high-voltage circuit in the EV is closed or broken. Current bridges the gap between contacts leading to sparking. Contactors are typically packaged in tough housings to prevent the sparks from affecting the surrounding environments. However, sparking can cause a high enough power dissipation in the module to destroy copper interfaces and cause higher resistances. The electromagnetic interference (EMI) on the surrounding components can also affect their functions.

OEMs may have to choose from between two mitigating options — contactors filled with an inert gas that helps minimize or extinguish the spark or those with an additional electromagnet that “stretches” the spark away from the contacts and into a chute where it is “blown out.”

A future of solid-state contactors

To address the disadvantages described above, the industry, including Wolfspeed’s design team, is investigating the use of solid-state switches, such as high-performance Silicon Carbide MOSFETs, to replace electromechanical contactors. The MOSFET-based contactor would offer a low-voltage control of the high-current path via a low-voltage isolated gate driver input.

A solid-state contactor, like this, would offer significant advantages:

• There are no moving parts subject to mechanical wear and tear, thereby increasing lifespan
• Avoidance of contact bounce, sparking, and contact oxidation increases reliability
• Exponentially faster switching — in nanoseconds compared to milliseconds of electromechanical counterparts — offers greater system safety and fault tolerance
• Lighter and potentially smaller footprint

Before such contactors can go mainstream, however, certain challenges must be overcome. The first of these is related to resistance. Since electromechanical contactors offer <1 mΩ, several solid-state switches may be required in parallel to achieve the same performance. The MOSFET surge-current capabilities are also much lower than those of mechanical switches. The inductive load-dump issue, which challenges electromechanical switches with very high voltages across the air gap, may cause the MOSFETs to breakdown.

When Wolfspeed and EV makers find a solution to these problems, yet another EV functional block will open up for Silicon Carbide technology to address.

Learn more from Wolfspeed’s application notes and tutorials in the Power Products Knowledge Center.

References

1. International Energy Agency, Global EV Outlook 2021, April 2021, Source: https://www.iea.org/reports/global-ev-outlook-2021/trends-and-developments-in-electric-vehicle-markets