Getting to know your photocoupler - key phrases & characteristics that you need to be aware of

As photocouplers are based on optoelectronic technology, they offer a highly robust, non-contact method by which to deal with electrical isolation that has numerous advantages over the conventional approach, where electro-mechanical components (such as relays) are employed. Among their key plus points are wear-free operation, relatively low bill-of-materials costs, minimal board space utilisation, immunity to the effects of electro-magnetic interference, strong reliability and extensive working lifespans.

The basic construction of a photocoupler is as follows. An optical emitter (such as a near IR LED) is positioned in such a way that it can pass a light signals across a small air gap to an optical receiver (normally a photoresistor, photostransistor or simple photodiode) placed opposite it. This allows communication to be undertaken while still upholding the necessary galvanic isolation between the circuit’s high and low voltage elements. The system is thus able to function with fully effectiveness, but is simultaneously protected from the potential harm that transients and current loops pose (which could otherwise destroy important components).

The emitter material generally used in photocouplers is gallium-arsenide (GaAs), which has been a staple optoelectronics process technology for many decades. However, some manufacturers are now making use of gallium-aluminium-arsenide (GaAlAs), as emitters based on this process will have markedly lower power budgets and also permit longer working lives. In some cases dual-channel operation may be required - with a pair of emitters and photodetectors then being co-packaged together. Occasional quad devices will be specified, with four emitters accompanied by four photo detectors.

In order to save valuable board real estate, modern photocouplers are being offered in ever smaller packages. Among the new options now available on the market are the compact SO8, mini-flat small outline package (MFSOP) and low profile SO6L formats.

Depending on the nature of its construction and various other parameters, a photocoupler will be rated to cope with voltage levels reaching up to a certain figure. This will basically be the maximum voltage that can be applied to the device with it still being able to maintain electrical isolation. It can be as much as several thousand Volts.

With circuits now needing to support higher degrees of performance, the bandwidths that the constituent photocouplers must achieve has been elevated considerably. Often tens of Mbps will be required. They may need to work in accordance with established serial interface standards (I²C, RS232, CAN, etc.).

The current transfer ratio of a photocoupler is simply the current that flows through its output divided by the current found at its input. From this it is possible to derive the relationship between the output current and the input current from which it stems. The ratios quoted in photocoupler datasheets can cover a wide range of possibilities, according to the type of device being specified and where it is destined to be employed.

The following white paper details how the new generation of photocouplers now entering the market are satisfying the plethora of demands being set by the industry and enabling the projected growth discussed earlier to be realised in its entirety.