The Tech Blog

A few folks from the last post wanted to know more about the design considerations of optocouplers. Let’s briefly go over them. The important consideration is the forward LED current, If. Its current-limited by a resistor Rf, which is straightforward to calculate based on the maximum current which can pass through the LED from the datasheet and the LED forward voltage. Standard KCL will give you the current. The question here is should you be driving the LED at its max current? Problem is that LED driven at its maximum current loses its brightness over time. For any LED application where it needs to last a long time, a common rule of thumb I personally follow is to limit the current through the LED to half the maximum current possible. I don’t push the LED to its limits and derate itself. Yes, this means that I leave a larger margin which can effectively increase the cost but it can help the circuit perform normally over a long time.

The second is ambient temperature. Like the current through the LED, choose optocouplers based on the operating temperature. It does have a severe impact again on the lifetime failure of the device. Obviously, lower is better for longevity. In good datasheets, you will find a curve which shows current and temp deratings.

The next consideration is to figure out what sort of isolation voltage you need from the input to the output. It heavily depends on the voltage applications you use this device. The important parameter in this is also the creepage distance between the input and output side to prevent the arcing over the isolator for higher voltage application.

The current Transfer Ratio(CTR, refer prev post for more) is another parameter in design. Its ratio of output to input current. For analog applications, try not to push the transistor to saturation by increasing the If current. If it saturates, the input current has relatively no role in determining the output side current(It will only be decided by the max current limits on the output). On datasheets, you will see CTR vs If curves, which increase till a point and then drops down. Usually(not always) you are looking for a high CTR to transfer max current at the output for a small change in input.

I have only glanced through a few points for more read App Notes ANO006(Wruth, Agilent’s OptoCoupler Design Guide and Toshiba’s Photocoupler App Note.

Optocouplers are one of the key building blocks of designing an isolation system. They provide electrical isolation between 2 circuits, of which one of them(Not necessarily always) is a high-voltage section. One of the main advantages of optoisolators is their ability to provide galvanic isolation, which means that there is no direct electrical connection between the input and output sides. This can prevent the transfer of electrical noise, voltage spikes, and other disturbances from one circuit to another, helping to improve the overall performance and reliability of the system.

The first versions of the optocouplers were mentioned in the patent in 1963. It consists of an input side LED and a phototransistor on the output side(Check images). When a current passes on the LED side, it glows and that light is used by the base of the phototransistor to detect that there was a change and current flows on the output side. Here the communication is effectively done via Light hence you get electrical isolation from 2 sides. The amount of isolation you get depends on the construction of the ICs and internal spacings. Usually, parts with isolation above 5KV between input and output sides are termed optoisolators. If used for isolation, always think of these as 2 separate circuits and don’t connect their grounds together. It’s a common rookie mistake to avoid.

Two major considerations while designing circuits with optocouplers are CTRs and Speed. The current Transfer Ratio is the ratio of the output side current to the input side current. Optocouplers are built to last for a long time, the major failure point is the degradation of LEDs(the light it emits) with time. The current through the LEDs needs to be increased to produce the same result. For digital communication circuits, you have to select units with a higher rate of transfer as it depends heavily on switching times. Always check the datasheets for speed before finalizing the part for your use case. Usually photodiodes are used instead of phototransistors for high-speed.

There are a few more design considerations which I can cover in future posts if there is interest.