How to choose safe isolation technology in engineering design?
As energy demand expands, current and voltage values will also increase. In many applications, higher voltages are becoming more common, even in standalone applications. Applications with higher voltages are considered hazardous, including electric vehicles (EVs), energy infrastructure applications such as direct current fast charging (DCFC), solar, energy storage and uninterruptible power supplies (UPS).
Galvanic isolation is necessary when interacting with high-voltage equipment and is defined by safety standards to ensure safe operation. There are several ways to create isolation, and the choice depends on the application, technical requirements, or applicable standards. This technical article will explore the different methods and discuss how to choose the appropriate method for your application.
What is quarantine?
Often called "electrical isolation", meaning no metallic contact, the purpose is to isolate functional parts or electrical electronic circuits to prevent current from flowing between them.
Figure 1. Principle of electrical isolation
One of the primary reasons for isolation is safety; this is especially important when personnel may interact with powered equipment. Providing appropriate isolation ensures that no user, even in the event of a catastrophic failure, is exposed to hazardous voltages.
Isolation is typically defined by the safety standards applicable to the application. The definition may refer only to breakdown voltage, or it may include requirements for physical separation ("creepage distances and clearances").
In addition to protecting the user, isolation separates the low-voltage (LV) and high-voltage (HV) portions of the circuit and plays a role in filtering common-mode noise. Since there is no metallic conductive path, ground current cannot circulate, eliminating the noise associated with ground loops in isolated circuits.
In addition to protecting personnel, isolation protects high-value circuit components and subsystems from damage caused by catastrophic failures in other areas of the circuit.
While isolation keeps different circuit parts separate, signals need to cross the isolation boundary, so something like an optocoupler or digital isolator is used that allows the signal to be transmitted while maintaining the isolation level.
The most common method is to use an optocoupler that integrates an LED and a phototransistor. This technology is over 50 years old and continues to dominate the isolation device market with approximately 85% share.
Digital isolators can be magnetic or capacitive, and although these technologies are relatively new, approximately one in seven isolation devices sold is a digital isolator. They are particularly valuable in specific applications, especially automotive, and growth in these areas will allow digital isolators to capture a larger market share in the future.
Comparison of various isolation technologies
Each technology has different performance and price, and is suitable for different application areas.
“Traditional” optocoupler devices use epoxy or silicone gel to provide a relatively robust isolation distance (DTI) of >400μm, in compliance with EN 60950 requirements. However, over time, the operation and performance of these devices degrade relatively and the temperature becomes unstable. They also have a relatively short life expectancy of about 10 years, and few meet AEC standards.
Figure 2. Overview of isolation technology
Optocouplers are generally not suitable for multi-channel designs because they require a lot of space. However, on the positive side, optocouplers have no EMI/EMC issues because they use light to transmit, and they do not require a modulated signal, thus reducing circuit design complexity while saving space and cost.
The performance of the two on-chip digital technologies (magnetic and capacitive) is similar. The obvious difference is that magnetic devices use a magnetic field to transmit signals through a 20μm polyimide insulator, while capacitive devices use an electric field to pass through the SiO2 isolation barrier.
Both digital technologies deliver faster propagation and performance over time and temperature, with a life expectancy of about 20 years, twice as long as optocouplers. They also offer higher common-mode transient immunity (CMTI), around 100kV/μs.
The downside is that the DTI of digital devices is only about 20μm, which cannot meet the requirements of EN60950. They also require signal modulation since they cannot pass DC, and EMC/EMI design considerations must be taken into account when using these devices.
ON Semi's Digi-Max™ technology offers a unique type of digital isolator called off-chip capacitive. It uses a 500μm ceramic substrate as a capacitor, separating it from the primary and secondary chips, hence the name "off-chip". Metal layers are deposited on both sides of the ceramic substrate to build the capacitor, providing an isolation distance of 0.5 mm. Typically, two sets of capacitors are used in differential mode communications to help eliminate common mode noise.
Figure 3. Off-chip digital isolators build capacitors on separate ceramic substrates, offering several advantages
Off-chip isolators work similarly to on-chip solutions. On-off keying (OOK) is used so that when the input signal is low, the signal applied to the capacitor is not modulated and therefore the output remains low. When the input is high, the modulating signal passes through the capacitor, causing the output to rise.
Figure 4. Digital isolators use on-off keying (OOK) to transmit signals across isolation boundaries.
Digi-Max technology multiplexes data across capacitors so multiple channels of parallel data can be transmitted as a serial data stream. This eliminates the need for multiple capacitors, thereby reducing the solution size.
By their very nature, isolation devices may experience electrical overstress (EOS) during use. However, the key consideration is whether they are still safe. To illustrate this point, we exposed two devices, an ON Semiconductor Digi-Max isolator and a similar competitive device, to a breakdown-limiting voltage between VDD and ground.
As expected, both devices failed. However, they are all tested for insulation before being unsealed. Competing devices fail at approximately 2.76kV, while ON Semiconductor's Digi-Max digital isolator maintained its datasheet-specified 5kV performance for 60 seconds. This clearly shows that with Digi-Max, even after a severe EOS event, the isolation barrier remains intact and protection remains even if a functional failure occurs.
Why use ceramic materials?
Ceramic materials are used off-chip to create capacitors that act as isolation barriers and signal transmission media, using on-off keying (OOK) technology to transmit signals from one end to the other.
The off-chip isolation structure uses ceramic materials to protect the insulating barrier in the event that EOS/ESD damages one of the ICs.
There are several advantages to using ceramic materials for isolation capacitors. Ceramic materials are good electrical and thermal insulators with low electrical and thermal conductivity. Flame propagation from the primary side to the secondary side is almost impossible because ceramic materials have a high melting point and are heat resistant. They are durable and have considerable durability, hardness and strength. Ceramics are chemically inert and therefore do not react with other chemicals. Simply put, there is no material that can compete with SiO2 material when it comes to making isolation capacitors.
Which technology is used?
Optocouplers are often the ideal solution if only a single or dual channel solution is required. Optocouplers are also ideal if EMC requirements need to be easily met, or if cost is a key consideration.
However, if the application requires a long lifetime (>10 years) or stable performance over time and temperature, digital isolators are the ideal technology. They are also better suited for high channel count applications and those requiring bidirectional communication. Since few opto-isolators are AEC compliant, almost all automotive isolators are digital isolators.
Digi-Max technology offers the same performance benefits and features as other digital isolators without the risk of safety failure due to an EOS event. In addition, due to the unique capacitor structure, it can provide the same level of reliability as optocoupler devices.
Figure 5. Technical summary
Isolation is a key aspect in many designs, and as with many technologies, designers have several options. Although optocouplers have been around for a long time, they have some limitations, especially in automotive applications. Today's digital isolators overcome many of these issues and are AEC compliant.
With a unique off-chip structure, ON Semiconductor's Digi-Max digital isolators provide designers with another key advantage by maintaining isolation levels even after a catastrophic EOS event.
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