Opto-Isolator Element


An Opto-Isolator element is missing from the library. A primitive element may be constructed using a LED model and a few other elements as shown in Figure 1.


(Figure 1) Simple OptoIsolator model

The input current is supplied through the A (anode) to C connections of the circuit. The ammeter measures the LED current and it is the input to the H1 element. The output terminals are the b (base), e (emitter) and c (collector) pins of the output device. The base may or may not be available as an input for a specific device; however it need not be used if it is not available in a specific device and/or it is unused in the final topology. One might wish to make adjustments to this model to account for the diode reverse currents being applied to this model if the LED D1 device is reversed biased. Otherwise it is not required.

In this model, D1 is modeled to represent the input diode v-i characteristics. Its current is transformed to a voltage by element H1 to a shaping network. In this case it is a simple R-C network. Inasmuch as the light coupling cannot be perfect, the overall gain from the LED current to the current applied to the output transistor base must be less than unity. One may provide element H1 with a gain less than unity to account for this, however it could be done in G1 (with the H1 gain then being unity) or in combination.

The voltage across the shaping network is transformed into a current, and applied to the output transistor base. The output transistor should be modeled to represent the actual output device.

Riso represents a leakage or isolation resistance between input and output. In many cases the Opto-Isolator element will receive a path from the external connections where the device a or c pins, as well as the c, b and e connections and the Riso will, being large, be of no effect. Where the output transistor is isolated, Riso will, along with whatever other resistance leakage resistances are used for other elements, may or may not be the one of the dominant resistive coupling elements.

It is expected that this model will be particularized to represent a specific Opto-Isolator device, and that device will be added to a subcircuit as a specific part model. The symbol for this device is of course arbitrary, as is the device name, dependent on the whims and creativity of the person creating this model.