Extending The Life Of A PhotoMOS Relay
LED Current And PhotoMOS Lifespan
Panasonic PhotoMOS Relays are well known for their extremely long lifespan. However, there is a caveat - engineers must take into consideration the LED current when incorporating PhotoMOS in their designs. Let's take a look at how LED current defines a PhotoMOS Relay lifespan.
The LED life curve below shows LED power getting lower in proportion to the number of hours of use. It is not the number of operations, but the number of hours the LED current is applied.
The change in LED power causes electrical changes on PhotoMOS Relay. IFon & IFoff both get higher than initial value after continuous operation.
Impact On IF Characteristics
A typical PhotoMOS part, AQY221N2S: Initial IFon=0.9mA (typical), initial IFoff = 0.85mA(typical). After applying 5mA current for 100,000 hours, LED power is 3% lower. Panasonic recommends 5mA as input current IF. As a result, IFon increases by 3% (0.9mA x 1.03 = 0.927mA). IFoff also increases by 3% (0.85mA x 1.03 = 0.8755mA). Hence, a 5mA input current is good for 100,000 hours of usage. In normal applications, duty cycle (on/off ratio) is around 50%. So, the duration of the applied current in hours is half of the specified value. In summary, for an Input current (IF) = 5mA, with 50% duty cycle the PhotoMOS Relay would normally operate around 200,000 hours (22.3 years).
PhotoMOS vs. Opto Coupler
Opto couplers (Photo transistor output) are different. The LED light of Opto coupler makes the base current for photo transistor directly. Opto coupler also has CTR (current transfer ratio). Therefore, lower LED power results in a lower output current (load current). While the Photo diode array of PhotoMOS makes the charge current and voltage for MOS-FET gate. The voltage is decided by number of photo diodes (each diode generates around 0.5V). Current is decided by LED current.
Turn On Time Of PhotoMOS
Ton gets slower than the initial value. This is because photo diode array current decreases due to the decreased LED power.
For Example: AQY221N2S : initial Ton = 0.03mS. After applying 5mA current for 100,000 hours, LED power is 3% lower. In addition, the photo diode current is 3% lower. Thus, Ton will be 3% slower (0.03mS x 1.03 = 0.0309mS).