Extending The Life Of A PhotoMOS Relay

May 01, 2012
By Anonymous

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.

LED Characteristics

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.    

                                    

Figure 1

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).

Conclusion

A limitation on PhotoMOS life comes from LED degradation. PhotoMOS can normally operate >20 years with 50% duty cycle provided that the LED input current is 5mA. Using a lower input current will significantly reduce the lifetime of the device. Turn on time also gets slower due to decreased LED power, which in turn reduces the photo diode array current necessary to charge the gate of output MOSFETs.
 
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