Photorelays are the best choice for EV BMS applications

EVs contain high voltages which tends to make mechanical relays the technology of choice. The primary downside is the concern that the physical contacts could ‘stick’, causing a dangerous fault, such as a short circuit.

Although less capable in terms of voltage / current, semiconductor-based relays (such as photorelays) do not have this issue as they have no moving parts, thereby enhancing safety. Their flexibility is an added bonus, permitting use in a wider variety of applications.

Photorelays are not that complex, consisting of a light emitter (typically an infrared LED) on the primary side which illuminates when a voltage is applied. This is paired with a light receiver (often a photodiode array) on the secondary side that switches the output MOSFETs when light is present.

The limited current handling capacity of photorelays often leads to them being overlooked in favour of mechanical relays. However, there are many useful attributes. As solid-state devices, there is no wear mechanism, so lifetime and reliability are enhanced. This removes the need for routine maintenance / replacement, so costs are improved. Additionally, the small size of photorelays (and their reduced weight) makes them easier to integrate into space constrained applications – such as EVs.

The automotive industry is increasing battery voltages to 800V, and beyond. This results in better efficiency, smaller size and more rapid charging but means that photorelays must be able to withstand voltages at this level.

Higher voltages mandate enhanced safety for vehicle occupants. One key use for photorelays is detecting ground faults, safeguarding occupants against the risk of electric shocks. A significant short circuit might cause a fire and would certainly drain the battery, so protection here is also essential.

The EV is completely reliant upon the battery. For this reason, batteries are usually monitored at the cell voltage level via the battery management system (BMS). This is not an application where mechanical relays can be used, due to their open/close contact limitations and relatively slow switching speeds.

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