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Photorelays support mechanical relays in EV BMS applications

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Mechanical relays are often a good choice for EV applications as they are able to deal with the high voltages that are found in many use cases. However, one significant concern is if the contacts weld themselves together, risking a permanently ‘on’ condition that could clearly be extremely dangerous.

Although lower capacity, semiconductor-based relays (such as photorelays) are solid state and, therefore, cannot stick – significantly enhancing safety. They are also more flexible, allowing use in more applications.

Photorelays are simple devices, the primary side contains a light emitter (typically an infrared LED), which illuminates when a voltage is applied, causing a current to flow. On the secondary side is a light receiver (often a photodiode array) that detects the light from the emitter and switches the output MOSFETs.

Although photorelays have limited load-switching capacity compared their electro-mechanical counterparts, they have many valuable features that designers should be familiar with. As they are solid-state, there can be no physical wear thereby extending the lifetime and ensuring reliable operation. Ultimately, this also reduces cost as photorelays do not need to be replaced as part of a preventative maintenance schedule, while mechanical relays often require replacement. Photorelays are generally more compact than mechanical relays which is a significant benefit in space-constrained EV applications, and this reduces weight – another useful benefit.

In EV’s, the battery voltage is moving from 400V to 800V to improve efficiency, reduce size and speed up recharging. This means that photorelays must be able to withstand these voltages to be valuable in modern EV applications, requiring photorelay designers to enhance designs.

With these higher voltages present, safety mechanisms must be provided to prevent the voltages from harming vehicle occupants. Photorelays with enhanced voltage performance are suitable for detecting ground faults, which is a valuable protection as it safeguards against the risk of electric shocks through ground faults . It also detects short circuits that could deplete the battery, or even result in a fire.

Monitoring and managing the battery is another important function and this is usually done at the cell level via the battery management system (BMS) by monitoring the voltage. Mechanical relays are not suitable, due to their open/close contact limitations and relatively slow switching speeds. Additionally, if voltage monitoring is to be accurate, then the absolute minimum amount of current should be drawn, meaning that a photorelay is the best choice.

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