More safety for electric vehicle batteries

Fully integrated drivers for HV contactors More safety for electric vehicle batteries

2024-01-11 From Nicolò Cascone* | Translated by AI 5 min Reading Time

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Switching high voltages in electric vehicles according to safety requirements requires powerful switching elements. High-voltage contactors fulfill this task – and integrated driver solutions ensure correct and efficient control.

Image 1: Use of contactors (yellow symbol) in electric vehicles.(Image: STMicroelectronics) — Image 1: Use of contactors (yellow symbol) in electric vehicles.
(Image: STMicroelectronics)

Nicolò Cascone is a Technical Marketing Engineer at STMicroelectronics in Catania.

At the historical Paris Agreement of 2015, the ambitious goal was set to achieve CO₂ neutrality by 2050 to limit the long-term rise in global average temperature to 1.5 °C. To achieve this, it is necessary to focus the energy sector on renewables rather than fossil fuels, and part of this transition will also affect the automotive sector, where the aim is to reduce the production of vehicles with internal combustion engines.

Image 2: Forecast of global vehicle production.
(Image:STMicroelectronics)

According to a study by IHS Markit, electric vehicles (xEVs) will play a dominant role from 2026 and make up the majority of produced vehicles (Figure 2). Here, "electric vehicles" include purely battery-electric powered vehicles (BEVs), hybrid electric vehicles (HEV), and plug-in hybrid vehicles (PHEVs). Since energy in xEVs is primarily stored in batteries, efficiency, safety, and reliability are essential parameters.

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Image 3: Development of the failure rate of the VIPower technology.(Image: STMicroelectronics)

Image 4: Structure of a high-voltage contactor (with closed contact).(Image: STMicroelectronics)

Gallery with 6 images

The VIPower technology developed by STMicroelectronics specifically for the automotive sector, with its broad portfolio, provides very good preconditions. The manufacturer has enhanced this technology with improved performance data, new features, and increasing miniaturization, thus positioning it as a pioneer for the advancing electrification and digitalization of motor vehicles. As intelligent power switches, the VIPower devices can be used as high- and low-side drivers for electrical loads and electric motors, where their very low losses and high current measurement accuracy come into play.

Electronic components of all kinds play a crucial role in electric vehicles, and improvements here also involve raising the level of quality and reliability. Starting from a current failure rate of 0.08 ppm (parts per million), as shown in Figure 3, the most important goal for the near future is to make VIPower a zero-defect technology.

High voltages in EVs as a potential safety risk

Malfunctions in the automobile's electronics or in the charging infrastructure can potentially summon significant safety issues for both the vehicle occupants and other individuals. Therefore, the power electronics in every electric vehicle must be connected to the high-voltage battery in a safe manner. For this purpose, high-voltage contactors are used, which are electromechanical switches designed for high currents and high voltages - in contrast to relays, which are intended for relatively low load currents.

Two main contactors and a precharge contactor are installed in every electric vehicle - in a vehicle with hybrid drive as well as in a purely electrically driven vehicle. The main contactors close or disconnect the connection between the battery pack and the power electronics and can quickly interrupt this connection in the event of an accident or another event. The precharge contactor charges the initially empty DC link circuit before closing the main contactor, thus avoiding high inrush currents that could be harmful to the battery and the consumers.

The largest number of high-voltage contactors is used in purely battery-electric vehicles (BEVs), as they are required not only in the AC charging system and the DC fast charging system but also in auxiliary functions, as can be seen in Image 1. Overall, one can find eight contactors in a BEV, five in a PHEV (Plug-in Hybrid Electric Vehicle), and three in a HEV (Hybrid Electric Vehicle).

Reliable drivers for high-voltage contactors

As can be seen in Figure 4, a contactor consists of two iron cores. The rigid core is wrapped with the actuation coil made of insulated copper wire, and the movable core is responsible for opening or closing the contacts. The movable part is held in its default position by spring force as long as the actuation coil is not energized. For a contactor to close properly, the actuation coil requires the typical current profile shown in Figure 5, which is divided into three phases:

Image 5: Typical current profile in the actuating coil of a high-voltage contactor.
(Image:STMicroelectronics)

Pickup phase: Here the current should be sufficiently high and last long enough to ensure a safe closing of the contact (usually within milliseconds).
Holding phase: In this phase, the current is lower than in the previous phase to reduce power consumption and efficiently maintain the contactor in the closed state. This phase can last indefinitely (up to several hours).
Brief dropout phase: Here, the current drops quickly, in about 10 ms, to achieve a swift opening in the interest of the contactor's reliability and lifespan.

Image 6 shows the typical topology of a circuit for controlling a high-voltage contactor. It is a discrete circuit consisting of high-side and low-side drivers based on VIPower technology, which, in addition to control, also takes over protection and diagnostic functions.

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Image 6: Typical topology of a circuit for controlling a high-voltage contactor.
(Image:STMicroelectronics)

While the high-side driver monitors the current profile for correct control of the high-voltage contactor, the control of the low-side driver is achieved through pulse width modulation to modulate the profile of the load current. Both drivers are necessary to protect the high-voltage contactor from short circuits to ground and to the battery, and to achieve an adequate level of safety.

Fully integrated driver solution

The VNH7100BAS component made using VIPower technology is a full-bridge chip for driving DC motors and contains a monolithic, double high-side switch as well as two low-side switches in an SO-16N type package on galvanically isolated leadframes. Furthermore, the VNH7100BAS is equipped with a variety of diagnostic functions (current measurement, off-state diagnosis, and temperature monitoring) as well as protection functions (power and current limitation, overheat protection, and cross-current protection), allowing for solutions for safety-sensitive applications with a high degree of reliability and miniaturization.

Image 7: VNH7100BAS as a fully integrated driver for high-voltage contactors.
(Image:STMicroelectronics)

Beyond its application with DC motors, the VNH7100BAS can be used as an innovative solution for driving high-voltage contactors (see Figure 7). In a single component, the VNH7100BAS encompasses all the functions necessary for the correct actuation, monitoring, and protection of high-voltage contactors – including current recirculation in PWM operation and rapid demagnetization.

The three phases of the control current for a high-voltage contactor shown in Image 5 can be implemented here as follows:

Pickup phase: Clockwise control (HSA and LSB turned on)
Holding phase: Activation of the PWM signal at LS
Brief dropout phase: Control of the actuating coil with reverse polarity (only HSB turned on)

The VNH7100BAS thus reduces the component effort and the PCB area of the final application and actively contributes to improving safety and reliability with its monitoring and protection functions. (cg)