Photorelays support mechanical relays in EV BMS applications

Mechanical relays have long been the sole choice for high-energy automotive applications where hundreds of volts and / or amperes must be switched. While mechanical relays provide the load capacity for this type of application, there is potential for the contacts to stick and ultimately weld together. Should this occur, then a dangerous situation arises where power cannot be removed from the load, leading to the potential for injury or fire.

Therefore, EV designers must check for contact welding of any mechanical relay deployed in their designs. Doing so does not require significant current handling, making it an ideal application for a high voltage photorelay that, as a solid-state device, exhibits no risk of sticking.

Photorelays are highly valuable in the battery management system (BMS) within EV applications where they are used in mechanical relay welding detection, battery voltage monitoring and ground fault detection. While current handling is low, these photorelays are often required to switch high voltages.

EV and hybrid vehicle batteries provide very high levels of electrical energy for drivetrain / traction. As a result, it is important to know the individual battery cell voltages and the total voltage of the entire battery, meaning that they should be actively monitored.

This voltage monitoring is required during use and charging, therefore mechanical relays with limitations on contact opening/closing and slow switching speeds are generally considered unsuitable. However, as it is common to measure using a high ohmic shunt resistor, a photorelay is ideal as an ON/OFF switch in voltage monitoring circuits.

Electrification of all types of vehicles, including EVs, is benefitting the environment, operating efficiency, functionality, and comfort. However, the presence of high voltages throughout the vehicle mandates higher levels of equipment safety, especially ensuring that the battery voltage cannot flow into the chassis.

In particular, there is a significant risk of electric shock to vehicle occupants in the event of a ground fault or short circuit. For this reason, a ground fault detection circuit is required for vehicles equipped with high-voltage batteries.

The presence or absence of this ground fault is checked frequently, including when driving, stopping, charging, and at other times. However, if the ground fault detection switch welds and remains in the ON state, normal detection cannot be performed. For this reason, it is common to use a high withstand voltage photorelay that is free from contact wear and welding risk.

Toshiba has published a white paper on high voltage photorelays in automotive applications – to download your free copy, please click here: