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Optocouplers for safe, reliable electrical systems

Posted: 15 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:optocouplers? AC coupling electrostatic discharge?

Electrical systems, no matter what their purpose, share three main requirements: reliability, safety and long operating life. To ensure safe operation, users must be insulated from any dangerous high voltages the equipment employs. To ensure reliable and long-life operation, control electronics must also be protected from hazards such as electromagnetic interference and voltage spikes. Although there are multiple technologiescapacitive, magnetic, RF, and opticalthat can provide electrical isolation, optocouplers deliver safety and protection unmatched by any other isolation technology.

Designers must consider many factors when selecting an isolation technology. The primary factor is the safety of equipment and personnel. Industrial equipment typically operates using signals of several hundred to several thousand volts. Yet the threshold of human safety can be as low as 42 V DC or 60 V AC. Electronic equipment can be even more sensitive since integrated circuits can typically be damaged by even a few tens of volts applied across the wrong pins.

To prevent humans from harm and electronic systems from electrical damage, both the people and systems must work in the "safe" extra-low-voltage (SELV) realm even though other parts of the electrical system use high voltages. Keeping these two voltage realms separated while also passing information between them is the job of the isolation device. These isolation devices must be able to operate with a continuous stress of hundreds of volts across their isolation barrier.

A second factor to consider is the isolation device's insulation rating. There are three levels of insulation rating: functional, basic, and reinforced (or double). Functional insulation is that needed for the device to operate and implies nothing about safety. Basic insulation provides protection for users from electrical shock, as long as the insulating barrier remains intact. Reinforced, or double, insulation provides failsafe operation in that should one level of insulation fail a second level will continue to protect the user. All signal lines going from the high voltage realm to electronic circuits driving interfaces that a user might touch, such as switches and displays, require isolation with a reinforced insulation rating. One of the prime considerations in achieving a reinforced insulation rating is the distance through insulation (DTI) that a high-voltage signal must traverse in order to reach a human.

Consider more than safety
While not directly related to human safety, an important factor for the safety of electronic equipment as well as for reliable operation of the equipment is electromagnetic compatibility (EMC). Parameters such as common-mode noise immunity and EMI susceptibility are important in assuring that an isolation device will transmit control signals without error. Radiated emissions are an important measure of whether or not an isolation device will affect nearby devices and generate errors in other signal lines.

Designers should also be aware of the wear out mechanisms that over time can lead to failure in isolation devices. High-voltage transients such as electrostatic discharge (ESD) and voltage surges represent one type of failure mechanism. ESD most often arises from static buildup on human operators or tooling while voltage surges arise as the result of changing loads on system power as well as "kickbacks" from switching inductive loads. These voltage transients may not themselves result in immediate device failure, but can cause damage that can later lead to failure.

Continuous high-voltage stress across the isolation barrier can also lead to failures, particularly when there are voids in the insulation material. Partial discharges within those voids can wear away the insulating material, eventually leading to failure. To ensure that this failure does not occur during the working lifetime of equipment, designers must consider the high-voltage life rating of their isolation device.

Isolation technologies
There are several different types of isolation technologies for system designers to evaluate. One of the simplest uses a capacitor to prevent DC voltages on either side of the isolation barrier from equalising. Also known as AC coupling, capacitive isolation only passes changes in logic signal levels, not the logic levels. Capacitive coupling depends upon changes in the electrostatic field between plates to carry information.

Magnetic isolation uses the equivalent of a transformer in the signal path, magnetically coupling across an insulation barrier from an input coil to an output coil. Such magnetic coupling can only pass high-frequency AC signals, not DC levels. A method for encoding logic levels as AC signals must be included in a magnetic isolation device.

RF isolation uses "on C off" encoding to convert logic signals into radio pulses that magnetically or capacitively couple from a transmitter to a receiver. This approach solves the problem of preserving DC logic levels. It suffers, however, from the additional complexity of needing active RF components.

Optocouplers use light to carry information through an isolation barrier. Input signals modulate the output intensity of a light-emitting diode. A photodiode responds to the optical signal by switching an output transistor on and off. Unlike the magnetic or electrostatic fields used in other isolation techniques, optical coupling does not require extremely close proximity to be effective.

Distance through insulation
This freedom from the need for proximity gives optocouplers a tremendous advantage over other isolation techniques when evaluating the critical parameter C DTI. As shown in figure 1, the DTI of optocouplers can be one or more orders of magnitude greater than that of other isolation techniques. The typical magnetic isolation device, for example, is built on monolithic CMOS IC material with a thin layer of spin-on polyimide as insulation. Its DTI can be as low as 17?m. Similarly, capacitors for both capacitive and RF isolation use layers of silicon dioxide (SiO2) as thin as 8?m. In contrast, optocouplers can typically have insulation thicknesses of 80?m to 1000?m, even when housed in smaller SO5 and SO16 surface-mount packages.

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