EV switching – photorelays deliver convenience and reliability

Many automotive applications require high levels of energy to be switched, requiring the high voltage / high current capability of mechanical relays. However, while these devices can handle the energy involved, there is potential for the contacts to stick and ultimately weld together. If this happened, it would mean that power could not be removed from the load, potentially resulting in catastrophic failure or even fire.

Detecting the existence of this contact welding condition is a key requirement for EV designers. As this is a low-energy task, it is an ideal application for a high voltage photorelay that exhibits no risk of sticking as it is a solid-state device, with no moving parts.

The ability to detect contact welding of mechanical relays and ground faults as well as monitoring battery voltages are important requirements of many battery management systems (BMS). There is a need to switch high voltages here, but currents are generally low so photorelays are ideally suited to this type of application.

BMS are very important in EVs – as the traction inverters use huge levels of energy, active monitoring of individual cell voltages and the entire voltage is very necessary. As this must be carried out during use and charging, mechanical relays with limitations on contact opening/closing and slow switching speeds are often considered unsuitable. However, measurement is often performed using a high value shunt resistor meaning that a photorelay is very suitable for use as an ON/OFF switch in the monitoring circuitry.

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 currents due to the battery voltage cannot flow into the chassis.

One particular risk is that vehicle occupants could be subjected to an electric shock as the result of a ground fault or short circuit. To eliminate this risk, some form of ground fault detection circuit is normally necessary in vehicles equipped with high-voltage batteries.

Given the potentially serious consequences of such a fault, the presence or absence of ground faults is scanned frequently, including when driving, stopping or charging. For this application, a photorelay is normally used as there is no contact wear or risk of welding. If a mechanical relay is used and the ground fault detection switch welds, remaining in the ON state, ground fault detection cannot be performed.

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