Optically isolated relays inherently have a long lifespan, thanks to their lack of moving parts and the robustness of their solid-state electronics. You can, however, make them last even longer by accounting for LED power losses.
Keep in mind that LED power doesn’t remain constant over time. Instead, all LEDs experience a power loss in proportion to the time that current is applied to them. With optically isolated relays, including PhotoMOS, this loss of LED power affects the device’s operating characteristics and lifecycle.
As LED power falls, the relay’s operating currents will rise accordingly. On a typical PhotoMOS relay, for example, LED power might drop by roughly 3 percent after a 5-mA input current has been applied for 100,000 hours. As a result, the relay’s operating (IFon) and turn-off (IFoff) currents would rise from their initial value by 3 percent.
This change in the electrical characteristics of the PhotoMOS has lifecycle implications. As LED sensitivity degrades with continued usage, more current is needed to generate the same amount of light. This light is used to charge the gates of internal MOSFETs and ultimately turn the relay on.
The turn-on time of optically isolated relays slows as LED power falls. Going back to our example of a 3 percent degradation of LED power after 100,000 hours at 5 mA, the turn-on time would likewise slow down by 3 percent. Put differently, a PhotoMOS with a turn-on time of 0.03 ms out of the box will have a turn-on time of 0.0309 ms after 100,000 hours of use at 5 mA. This slowdown occurs because light intensity diminishes, which reduces the voltage and current output of the photo diode array in the IC. So it takes time to bias the MOSFET gates.
At elevated ambient temperatures, more LED current is needed to generate the same amount of lamination. This lamination will then be converted to produce the necessary electrical voltage and current to charge the gates of MOSFETs and maintain ON state. Careful design is required to set up the series limiting resistance of the input LED to ensure proper operation of the relay across the relay’s operating range.
In many applications, the electrical change related to optically isolated relays may not make a practical difference. Adding 3 percent to an already fast on-time, for instance, won’t matter in every application.
Yet even incremental changes in performance or lifecycle can be significant in cutting-edge applications. Examples include high-speed test and measurement systems, where the datasheet alone won’t tell you whether you’ve picked the right relay for the job. You’ll have to evaluate the relay based on the electrical characteristics that will emerge after an operating time horizon that corresponds to your application.
Aiman Kiwan is an engineering manager at Panasonic Electric Works of America.