Inside of the light that is being backfed builds up heat. Here's what happens to the electronics. An SCR is simply a p-n-p-n structure with a gate terminal . We can break the structure down as back-to-back transistors, one p-n-p, the other n-p-n. With that simplification, we can see that temperature analysis of the bipolar transistor extends logically to the SCR structure.
It is a well known empirical fact that leakage current approximately doubles with every 10? C increase in temperature.1 In a bipolar transistor, this increase in leakage is accentuated by the "transistor action" of the device. This can be explained by using an n-p-n transistor as an example. As we increase the temperature, more and more electrons are able to jump the barrier from the emitter to the base. This further biases the base region with respect to the emitter and collector, causing an increase in collector current. In fact, a transistor can be turned on simply by applying high temperature - sufficient leakage current can be generated to trigger the transistor action.
This discussion extends to the SCR, which is nothing more than two bipolar transistors driving each other. Any effect felt by the bipolar transistor is only magnified when discussing the SCR. The effect is not additive, it is multiplicative.
There is another temperature-related phenomenon we must point out: as we increase temperature, diode voltage decreases at an approximate rate of 2 mV/?C.2 Therefore, a transistor in the on state will have a tendency to not only stay on at high temperature, but to conduct even more fully; i.e., the barrier between p- and n-type regions is reduced even more.
The result of these two phenomena is that the bipolar transistor has a negative temperature coefficient; the higher the temperature, the higher the collector current at a given base drive.
Let's now extend the discussion to the SCR, specifically in Solid State devices. The S742 uses two SCRs in the output; thus, it can only be utilized in AC applications. This is because the only way to prevent these particular SCRs from conducting once they are turned on is to reverse the voltage across their terminals. This is dictated by the output of the application. In a DC application, once the SCR is turned on, there is no way of turning it off. Under DC, the SCR never experiences the reverse voltage condition across its terminals necessary to prevent conduction.
An SCR in the off state will tend to turn on and stay on (latch) at high temperatures. Of course, in the S742 one SCR will always be non-conducting because of the reverse voltage on the output. But the other will tend to turn on even without an input signal because of the above considerations.
In SSO's MOSFET-output devices, the driver consists of 14 series diodes. These diodes generate sufficient voltage to drive the gate of the output MOSFET, allowing conduction.
These results support theoretical expectations; namely, SCR-output devices can be expected to fail short , while MOSFET-output devices can be expected to fail open.
