Optocouplers and Optoisolators

Industrial terminology often blurs the distinction between similar concepts. Today, we find the terms optocoupler and optoisolator used interchangeably to refer to the same function.

The distinguishing feature between the two terms is the magnitude of the isolation voltage. Optocouplers are used to transfer analog or digital information from one potential to another while maintaining electrical isolation of less than 5000V. Optoisolators are designed to transfer analog or digital data between systems while maintaining isolation of power systems with isolation voltages of 5000 to 50 000V or more.

Working Principles

Both optocouplers and optoisolators allow signals and data to be transferred from one system to another within an electronic device without a direct electrical connection. This is done optically by placing a transmitter and detector separated by a light-transmitting medium in a single package, which then transmits a light beam to an optical receiver. This allows the circuit to maintain complete electrical isolation while information is transferred from one potential to another. In all optocouplers and optoisolators, the input signal is converted into a light pulse emitted by an LED. This light pulse is transmitted to a silicon-based light sensor.

                                                                   Figure 1. The structure of the optocoupler varies depending on the application.

Depending on the type of input signal to be transmitted within the device, the light sensor can be analog or digital. If an application requires an analog signal, such as 4 to 20 mA, the light sensor can be a photodiode or a phototransistor. Both types of devices provide analog output signals and can be used in many analog applications.

In applications where the amount of signal is important to the operation of the system, an analog response is necessary. The amount of current the device outputs relative to the LED input light intensity is called the current transfer ratio (CTR), which is equal to the output current divided by the input current. Depending on the gain of the system, the CTR value can vary from 10% to 5000% or even higher. Generally, the smaller the CTR, the faster the rise and fall times.

Analog devices usually require additional circuitry before they can be used by other parts of the system. Digital output devices are most suitable when the application requires a pulse or bit output.

These devices are very easy to use and can be directly used with other system components by providing a reliable signal. As long as the light shining on the light sensor is strong enough, the output signal will remain high or low, and the output level depends on the logic structure.

One thing the designer should consider is the transmission rate of digital data while minimizing transmission delay. Most optoisolators/optocouplers are used for data rates below 10Mb/s, and the transmission delay may be in the range of 2 to 20μm.

Optocouplers

Optocouplers are specifically designed to be small packages such as dual in-line packages (DIP) or surface mount devices (SMD), so that they take up minimal space while transmitting data using light. Designers should pay special attention to the isolation voltage potential because it may be defined as an AC, RMS, or DC value.

Optoisolators

Optoisolators are available in many package types, such as rectangular, cylindrical, and some special shapes. These package types are designed to provide higher isolation voltages than what can be achieved with DIP and SMD packages.

When using optoisolators, designers may need to take environmental conditions into account. For example, if the system requires DC 50,000V isolation, the humidity percentage of the environment will be an important consideration.

If the humidity of the air is too high, arcing may occur on the surface of the optoisolator or PCB, forming a conductive path and shorting around the device. Distance, temperature, atmospheric pressure, type and level of air pollution, and humidity all determine the breakdown voltage across the optoisolator surface.

Device Structures

Optocouplers and optoisolators are available with either a single LED or back-to-back LEDs (anode and cathode connected together). Optical sensor structures can vary between a single light sensor and a complete integrated receiver.