Electric vehicles (EVs) commonly use mechanical contactors — high-voltage switches that control the flow of electricity — for charging. However, unlike most other vehicle and infrastructure parts, contactors have not advanced in efficiency.
Most OEMs still rely on mechanical contactors, but there is a better way to implement charging in EVs. For example, solid-state contactors react faster, last longer, and are safer and smaller, allowing for the inclusion of advanced diagnostics and safety controls. One reason solid-state contactors have yet to become the go-to option for EV charging is because of a critical drawback: higher conduction losses.
So, what the industry would most benefit from is a bidirectional power semiconductor designed to reduce those conduction losses.
Figure 1. Contactors, or hight-power switches, are a critical component in modern high-power applications such as electric vehicles and EV chargers. Their function is to convert energy to/from alternating current (ac), to/from direct current (dc), setting the proper voltage to meet the end use requirement.
How mechanical contactors work
Contactors are large switches controlled by electromagnets, which are managed by software to enhance safety. Alongside micro inverters, contactors link the battery pack to chargers, enabling the battery to be charged.
For instance, the Tesla Model S battery pack has one contactor for the positive terminal and another for the negative. This design prioritizes safety by allowing software to externally disconnect the battery if either the positive or negative side shorts.
The contactors widely used in today’s EVs are electromechanical devices designed to handle higher current capacities than relays, making them ideal for heavy-duty applications. They operate by engaging and disengaging a copper plate to control low voltages, effectively connecting or disconnecting the leads in a high-voltage circuit.
Each contactor consists of a cylindrical coil of wire, known as a solenoid, and a steel plunger rod or a similar material that resists permanent magnetization. When current flows through the coil, it generates a magnetic field that attracts the plunger, which has moving contacts on a copper plate. When this plate engages, touching the fixed contacts made of the same material, it creates a low-resistance path to the main circuit.
Mechanical versus solid-state contactors
A key benefit of mechanical contactors is their extremely low conduction losses, which has made them an effective choice for EVs. However, advancements in technology are pushing the case for replacing mechanical contactors with solid-state alternatives.
Mechanical contactors are slower to react than solid-state contactors, which not only react more quickly but do so with precise current direction. Mechanical contactors are also extremely heavy and bulky, particularly for high-voltage batteries with more than 800 volts. Solid-state contactors weigh significantly less, depending on the type of switch technology used. Lightweight components offer a significant advantage to EV manufacturers as they seek to optimize vehicle performance, efficiency, and range.
Additionally, mechanical contactors are less reliable than solid-state ones due to issues like arcing and the wear and tear of moving parts, as evidenced by the audible clicking sound when they engage or disengage. This clicking is one reason mechanical contactors are more prone to wear and tear and tend to break down much faster than solid-state contactors.
Overall, solid-state contactors have a longer lifespan and can handle more cycles when used with a power semiconductor switch. Lastly, mechanical contactors lack the intelligent diagnostic and monitoring capabilities that can be implemented with solid-state contactors, which improves the safety of the system.
Bidirectional power semiconductors
Before solid-state contactors can become standard in EVs and chargers, some challenges need to be resolved, particularly with resistance. Mechanical contactors offer less than 1 mΩ of resistance, meaning multiple solid-state switches are needed in parallel to match their performance.
Figure 2. Bipolar Junction Transistor (B-TRAN) is a novel four-quadrant, semiconductor power switch with ultra-low forward voltage and low switching losses that can be used in bidirectional switching applications, such as for EV charging.
However, a simple solution exists: bidirectional power semiconductor switches that can replace multiple solid-state switches. Bidirectional switching results in lower conduction losses, leading to the higher efficiency EV manufacturers aim for. These switches also enable smaller, more cost-effective solid-state contactors compared to widely used MOSFET switches.
These switches can be produced using mature silicon wafer processing equipment. Newer generations made with advanced materials promise even greater performance improvements.
Becoming standard
So, when when will bidirectional power semiconductors become standard? It may take some time yet before these switches become the EV standard. Since efficiency is critical, it’s likely adoption will occur faster than expected once the technology enters the testing phase at a significant number of automakers.
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Filed Under: Charging, FAQs