Optocouplers and other power isolation technologies are widely used for signal isolation and high voltage level shifting in a wide range of products. These devices can also be used to provide insulation for safety reasons. In light of these electrical concerns, it is critical to comprehend the optocoupler or alternative isolator’s safety characteristics.
The Foundation of Electrical Safety
Electrical shock, caused by the passage of an electrical current through the human body, can cause physiological effects ranging from involuntary movement injuries to death from ventricular fibrillation.
Due to the variations in health, moisture levels, and body impedance, the voltage threshold of risk is somewhat erratic, but DC voltages up to 42V and AC voltages up to 60V are generally considered safe. Any electrical application that exposes people to voltages higher than this is considered dangerous, and adequate electrical insulation is required.
The Safety Factor Concept
When it comes to human safety, designers are forced to consider so-called safety factors. The goal of safety factors is to account for user conditions that are not fully deterministic, with the goal of ensuring a very low chance of failure.
Safety factors are commonly used in a variety of engineering disciplines. In civil engineering, for example, a common safety factor frequently used for scaling support members in building construction is typically 2. When the quality of the material is unknown, a higher factor can be used.
Continuous Working Voltage
It is expected that the optocoupler or isolator will be subjected to continuous voltage stress during normal operation. This voltage is commonly known as the working voltage. Since this stress voltage is continuous, the risk to people is much higher if the insulation fails. As a result, the working voltage rating is typically derated by a factor of two from the power isolation optocoupler or isolator’s designed continuous voltage stress capabilities.
Transient Voltage Capabilities
The optocoupler or isolator must not only be capable of withstanding the continuous working stress voltage, but it must also be capable of withstanding or surviving high transient voltages. Transient voltages are classified as either high energy or low energy transients.
High energy transients have a high potential for danger. Although low energy transients are not directly hazardous to health, they do pose a significant risk to the health of the power isolation material, which could result in a safety hazard.
Low Energy Voltage Transients
This category includes ESD (Electrostatic Discharge), which is a particularly common voltage transient event. Due to the fact that ESD events can easily exceed 15 kV, they can and frequently do exceed the creepage and clearance distance requirements of most optocouplers or alternative isolators. As a result, there is flashover across the power isolation optocoupler or isolator. Fortunately, this flashover does not pose a significant direct safety risk.
The occurrence of flashover is a self-limiting event in terms of insulation stress, minimizing the maximum voltage stress to the insulation. Regardless, transient voltage loading can be extremely high up to the point of flashover. Because of environmental conditions such as altitude and humidity, the flashover inception voltage varies greatly.
Even low-level ESD events have the potential to cause significant insulation damage, either as latent damage or as immediate damage. If the continuous working voltage falls within the hazard limits, this can create a dangerous situation.
This failure scenario is well addressed in the case of optocouplers by scaling the insulation thickness. Thick insulation material is used in particular, ensuring that the internal insulation’s breakdown voltage is sufficiently higher than the external flashover voltage. However, in alternative technologies where the fundamental operation is dependent on the use of extremely thin insulation layers, such protection is much more difficult to achieve.
These devices are especially vulnerable to ESD breakdown. Alternative isolator technologies are classified into two types based on their construction: Type 1 devices use spin-on polyimide coatings for primary insulation, and Type 2 devices use silicone dioxide (SIO2) insulation for primary insulation. SIO2 insulation is particularly vulnerable to ESD damage in type 2 devices. In fact, most integrated circuit designers go to great lengths to provide ESD protection structures for SIO2 structures on exposed interconnects.
High Energy Voltage Transients
Power distribution systems are frequently subjected to high-energy surge events. Such power surges can be caused by the operation of heavy machinery connected to the same distribution network, or, in rare cases, by lightning strikes. Because such surges can be directly life threatening, it is critical that the isolator dimensions are properly scaled to ensure protection against such events.
The installation category or overvoltage class addresses this issue within the end equipment standards. The relevant equipment standard determines the maximum surge voltage transient that the insulation should be capable of withstanding for each application usage.
Electrical Safety Optocoupler/Isolator Construction Requirements
When discussing constructional requirements for safety, the two main areas to consider are internal construction and external mechanical dimensions.
Before deciding on the construction requirements, it is necessary to determine whether the insulation will be basic or reinforced:
- Basic insulation is used to provide functional insulation properties on its own and may not be used to provide protection against electrical shock risks.
- Reinforced insulation is used when electrical shock protection is required. The terms reinforced and double insulation are frequently used interchangeably.
Double insulation literally means the ability to support twice the rated electrical voltage. The physical method of accomplishing this can also be literal, i.e., providing two separate insulation layers, each capable of holding off the required voltage. In some cases, the requirement for reinforced insulation can be met with a single layer of solid insulation.
The appropriate single solid layer of insulation for safe insulation varies slightly depending on regulatory standards. As an example, according to the end equipment standard IEC60950, a single thick (>0.4 mm) homogenous material is suitable for providing double or reinforced insulation.
In terms of the definition of solid insulation, it is important to consider not only the material itself, but also the material processing. For example, thick polyimide insulation can be considered solid insulation, but solvent-based polyimide (enamel) layers may not be.
For decades, optocouplers have been widely used in electrical safety-related applications. Regardless, it can be argued that in some cases, equipment and component-related safety standards do not fully address the requirement of absolute proof of safe use. The main areas of concern are high voltage lifetime and high voltage transient damage.
Fortunately, this is only a theoretical risk, as this is already addressed by the optocoupler’s inherent design and can be demonstrated experimentally and phenomenologically. Unfortunately, in the case of alternative isolator technology, this theoretical risk becomes a very real safety hazard. In many cases, the use of alternative isolator technologies for reinforced insulation on a construction basis is effectively prohibited by equipment standard definitions.