Beware of Misleading Isolation Voltage Ratings in Optocoupler, Digital Isolators, Transformers and DC/DC converters

Introduction

When a circuit requires galvanic isolation, there are millions of off-the-shelf components available for purchase, including optocouplers, digital isolators, transformers, and DC/DC converters. However, to make their ratings as impressive as possible, vendors often advertise the peak or surge isolation voltage ratings. These ratings cannot be used for the purpose of protecting human safety, and sometimes the components are not even guaranteed to be functional.

The safe isolation voltage in an actual system can be two orders of magnitude lower than the peak isolation voltage rating of a component. For example, an isolator with a “12800 volts” rating is only safe to use in a 300-volt AC circuit in practice.

A misunderstanding of these ratings by an inexperienced designer can result in violations of laws and regulations, equipment damage, even injury and death.

Device Ratings vs System Ratings

When an isolator enters production, the vendor performs a dielectric withstand voltage test using a test voltage that is several times above the working ratings. For a transformer, it’s colloquially called a “Hi-Pot” (high potential) test. For optocouplers and digital isolators, standards such as IEC 60065, UL 1577, and DIN V VDE V 0884-10 are commonly used in the industry. 3000 V is common for transformers, and for optocouplers and digital isolator, the voltage can be truly enormous, up to 12800 V! I’ll call it the surge rating of the isolator.

Checking the robustness of the insulator and spotting possible manufacturing defects are the only purposes of these tests. If we consider the fact that the duration of these tests is several microseconds (IEC 60065 uses the well-known 1.2/50 μs waveform), 1 second (common in transformer Hi-Pot production tests), or 60 seconds (UL 1577), it’s obivious that these tests, at best, shows the immunity of isolators to a transient overvoltage, it does not determine their long-term reliability and safety. How long can it survive when the high voltage is constantly applied across the isolator? It’s anyone’s guess.

A more practical rating is the working voltage rating of the isolator. This is the rating given by the vendor after performing a reliability and barrier lifetime analysis. For a digital isolator, a typical spec is “1000 V, 100-year lifetime”.

Unfortunately, these voltages are only appropriate in a controlled laboratory test, and are still unrealistic for practical applications due to a variety of non-idealities in the real world.

To solve these safety challenges, it’s inadequate and dangerous to use only a device-level rating, instead, system-level safety standards are enforced by regulations worldwide. The most widely used international standard in the computer industry was IEC 60950-1, “safety standard for information technology equipment, including electrical business equipment and associated equipment.” Recently, this standard has been deprecated and replaced by IEC 62368-1, but the rules are the same for our purposes. Since the language of IEC 60950-1 is simpler than IEC 62368-1, I’ll refer to IEC 60950-1 here.

Creepage distance sets the pracitical upper limit

IEC 60950-1 and IEC 62368-1 have many hundreds pages. But the requirement we care the most is the creepage distance, that is, the separation distance between two conductors along the surface of an insulator.

In the real world, dust, dirt, and other contaminants on an insulator (pollution) causes dielectric breakdown at the surface and progressive degrades the isolation barrier through a process called tracking.

Thus, the voltage rating of an insulator must be made with a safety margin according to the Material Group using its Comparative Tracking Index (CTI). The amount of dust in the working environment is also considered by the Pollution Degree (PD) rating. For most applications, we use Material Group IIIa or IIIb (the worst groups), and Pollution Degree 2 (some dust, but no conductive dust).

In addition, if human lives are at stake, the possibility of a failure must be further reduced by overdesigning the insulation with a even large safety margin. This is called Reinforced Insulation, and requires increasing the ceepage distance by 100%.

The following table shows the required creepage distance for Material Group III, Pollution Degree 2.

Using these data, we can start doing some case studies.

Case Study

Texas Instruments ISO782xLL - 8000-VPK Reinforced Isolated Dual-LVDS Buffer

The first chip that caught our attention is ISO782xLL from Texas Instruments.

• Signaling Rate: Up to 100 Mbps
• Isolation Barrier Life: > 40 Years
• Isolation Surge Withstand Voltage 12800 Vpk
• Safety-Related Certifications:
  • 8000-VPK Reinforced Isolation per DIN V VDE V 0884-10 (VDE V 0884-10):2006-12
  • 5700-VRMS Isolation for 1 minute per UL 1577
  • CSA Component Acceptance Notice 5A, IEC 60950–1 and IEC 60601–1 End Equipment Standards

Just look at the heading of the datasheet. Wow, 12800 V isolation!

Insulation Types

This standard defines five types of different insulations:

• Functional Insulation: It does not provide any safety protection, both sides of a functional insulation is treated as equally dangerous.

• Basic Insulation: It provides basic protection from electrical shocks, but it’s a risky single-point-of-failure, and thus cannot be relied upon by itself to protect human safety. For further protection, one should either shield the dangerous voltage with Protective Earthing, or insulate it further via Supplementary Insulation.

• Supplementary Insulation: It’s an additional stage of insulation used in combination with Basic Insulation, its requirement is mostly similar to identical to Basic Insulation.

• Double Insulation: A combination of Basic Insulation and Supplementary Insulation, suitable for protecting humans from electrical shocks.

• Reinforced Insulation: One stage of insulation, but overdesigned with a large safety margin to make any failure very unlikely. Equivalent to Double Insulation and suitable for protecting humans from electrical shocks.

The

Creepage and Clearance

The most challenging requirements in the standard is creepage, or the distance