要約

This application note summarizes the international safety standards and certifications that apply to digital isolators. Our example shows how to use a data sheet and the standards' specifications to determine which digital isolator is optimal for an application.

Introduction

Digital isolators provide the signal isolation and level shifting required for the correct operation of many circuits. They also insulate the user from electric shock. These isolators must undergo extensive testing and certification to ensure user safety because basic human safety considerations are pertinent. This application note summarizes the international safety standards and certifications for digital isolators. An exercise using the MAX2244x family shows how a system designer must use a data sheet and the standard's specification tables to determine the optimal digital isolator for an application.

Why Isolation?

Isolation prevents conduction between the two parts of a system (often called domains), while still allowing signal and power transfer between them. The term 'field side' refers to the higher voltage domains such as 24V I/Os and 'logic side' refers to the lower voltage domains such as 3.3V and below (common in microcontrollers and such ICs) in many industrial applications.

Isolation is required in electrical systems for several reasons, which can be simplified into two:

• High Voltage Systems: Isolation is implemented to prevent current surges from damaging equipment, protect humans from mains voltage, etc., when safety is a concern
• Analog Subsystems: Isolation is implemented to break ground loops where different ground potentials are involved, especially in a precision measurement system. It can also help to isolate digital noise from an analog system.

Insulation Grades

Table 1 lists the various insulation grades and describes the insulations. There are four types of insulation systems, indicating different levels of protection in the system.

• Basic insulation, i.e., if the system has only a single layer of basic protection.
• The second layer is the supplementary insulation if another layer of protection is added onto the basic insulation if the basis insulation is broken.
• The basic and supplementary insulations together are called the double insulation.
• Reinforced insulation, i.e., if a single-layer insulation can provide the equivalent protection as double insulation.

Analog Devices' Digital Isolator Technology

Analog Devices' proprietary process and edge-based capacitive isolation technology allow isolators to achieve fast data transmission rates with the lowest power consumption compared to direct competitors who use either inductive technology or on-off keying (OOK) modulation architectures. Figure 1 shows the simplified block diagram for both a basic and reinforced isolator, while Figure 2 shows a bonding diagram and SEM picture of how an isolator is constructed using multiple die within a single package. Two die are used for a 4-channel isolator, interconnected with eight bond wires due to the differential signals. The thickness of the silicon dioxide determines the basic vs. reinforced isolation rating.

Basics of International Safety Standards

Equipment Standards

Equipment safety standards apply to end equipment assembled from multiple components. These standards cover a variety of safety concerns including insulation, pressurized gasses, and strength of electrical cords. Typically, these standards regulate different aspects of a specific application or end equipment type. For example, the International Electrotechnical Commission (IEC) 61010-1 defines the safety requirements for electrical equipment for measurement, control, and laboratory use. It is concerned with the transient voltages that equipment experience, and the characteristics of the insulation used. This standard does not specifically address isolators. Neither the word isolator nor isolation is found anywhere in this IEC standard. It is yet relevant to isolators because it specifies the transient voltage and minimum creepage and clearance distances for the insulation. The insulation of a marketed isolator must, therefore, be compatible with these IEC requirements.

Support Standards

Support standards provide definitions, methods, or requirements applicable to numerous other standards. Support standards help with harmonization. An example is the IEC 60664-1 (insulation coordination for equipment within low-voltage systems). This standard also does not specifically mention the words isolation or isolator.

Component Certification Standards

Component certification standards apply to specific components used in a variety of applications. These standards define the required tests, sample size, and preconditioning. They may also define terms and information required for the data sheet. These standards, with a few exceptions, do not set performance levels. Instead, the performance requirements are according to the manufacturer's specifications. An example is the IEC 60747-17. It is a new standard for semiconductor devices using magnetic and capacitive couplers for basic and reinforced isolation.

Component Level Certifications

Let us quickly look at the specifications that directly impact the manufacture of digital isolators (Figure 3).

UL1577

The UL1577 specification is for optical isolators. However, it is also used to qualify capacitive and inductive isolators. The rating is based strictly on the voltage breakdown. It does not include any requirements on clearance or creepage. A device to be certified under this standard must withstand the isolation voltage, VISO (specified by each manufacturer), usually of 2.5kVRMS or 5kVRMS, for one minute. Also, the specification allows a production test of 120% of the isolation voltage for one second. Devices passing these requirements (plus the 150% of the VISO overload and thermal aging tests) are certified with a single protection rating. A double protection rating requires survivability to a 20kV discharge test (applied 50 times) and a one-second production test of the rated isolation RMS voltage or 2.5kVRMS, whichever is greater.

The New VDE 0884-11

The VDE 0884-11 superseded the VDE 0884-10 in 2020. The two primary changes from 0884-10 are the additions of the reinforced isolation rating and lifetime estimation (based on worst-case lifetime of the isolation barrier). All the sections were revised, considering the complete content of the previous release:

• Updated the section on the definition of the magnetic and capacitive couplers.
• Introduced basic insulation in addition to reinforced insulation.
• Defined the lifetime of basic and reinforced insulation.
• Introduced an extrapolation factor of the voltage to determine the lifetime.
• Introduced the cumulative failure rate for the rated lifetime of basic and reinforced insulation.

Analog Devices, like all other isolator manufacturers, sends its devices to UL and VDE for certification. UL is the US regulatory agency and VDE is based in Germany. All isolator data sheets have a table to show the safety regulatory approvals, ensuring that the devices are tested and certified by the international agencies for their isolation performances. Find further details of Analog Devices' certification document details in the data sheet (Table 2 is an example) and on the Analog Devices website.

Digital Isolation Safety Certifications

The Analog Devices digital isolators are tested and approved by recognized regulatory agencies like UL and VDE to meet or exceed product safety and quality requirements. Here is an overview of the safety certifications achieved by the Analog Devices digital isolators. Click on the product links in the table to view the certificates and product overviews.

Safety Limiting Values

A digital isolator must withstand its safety limiting specs during a fault event without compromising the isolation performance. This must cover the maximum current on any device pin (I_S), maximum power dissipation (P_S), and maximum junction temperature (T_S = 150°C). Operating beyond these specifications can cause the isolation barrier to no longer be functional or shorten its lifetime. The isolation barrier must remain intact to prevent downstream system failures or electrical hazards, which requires engineers to adopt system-level protection schemes to limit the isolator from exceeding the safety limits in the system design. There is a safety limits section in the isolator data sheet, which defines the maximum current or power dissipation limits that do not damage the isolation barrier or degrade its performance. During a fault event, such as the V_DD supply going beyond its ABS MAX (Absolute Maximum) rating, or injecting huge current into the pins from a short circuit, potentially damaging the device, system-level protection (such as protection in the power module, current limiting resistors) must be adopted to protect the isolator from exceeding its safety limits, thereby protecting the isolation barrier from being damaged.

Data Sheet Specs: Insulation Characteristics

All Analog Devices isolator data sheets have a table to specify insulation specifications, like the one shown in Table 3 from the MAX22444 – MAX22446 data sheet.

Table 3. MAX22444–MAX22446 Data Sheet Insulation Characteristics.

This specifies the isolation rating, working voltage, creepage and clearance of the package and package material CTI. All the specifications here show how robust the isolation barrier is and how much voltage the barrier can withstand.

The working voltage is defined with two values, maximum repetitive peak voltage (V_IORM) and maximum working voltage (V_IOWM), which is the maximum voltage that can be applied across the barrier on a continuous, day-to-day basis, throughout the lifetime of the isolator.

The isolation voltage is also defined with two values, maximum transient isolation voltage (V_IOTM), which is the value the barrier can withstand for one second, and the maximum withstand isolation voltage (V_ISO), which is the voltage that can be applied across the barrier for 60 seconds. This is the voltage always included in the title of the data sheet.

Creepage is the shortest distance between two conductive materials measured along the surface of the insulation, i.e., two pins.

Clearance is the shortest distance between two conductive materials measured through air. Clearance need not be the line of sight (Figure 4).

IEC standards define the minimum creepage and clearance in the system design based on V_ISO, V_IOWM, pollution degree, overvoltage category of the device, material rating, etc. Adequate creepage distance protects against tracking failure over the lifetime.

Tracking is the process that produces a partially conducting path of localized deterioration on the surface of an insulating material because of the electrical discharges on or close to an insulation surface. Tracking damage to the insulating material normally occurs because of one or more of the following reasons: humidity in the atmosphere, presence of contamination, corrosive chemicals, and altitude at which the equipment is operated. The degree of tracking depends on the Comparative Tracking Index (CTI) of the isolator package and pollution degree in the environment.

Clearance is like creepage such that the distance, pollution degree, temperature, and relative humidity influence the tendency for a breakdown. Breakdown along a clearance path is a fast phenomenon, where damage can be caused by a very short duration impulse (arc). Therefore, it is the maximum peak voltage, including transients, that should determine the required clearance spacing. Adequate clearance distance protects against the ionization of the air gap (arc) and subsequent flashover. Arc generation is related to the isolation voltage (peak voltage, short duration impulse, transient, etc.) and pollution degree.

Comparative Tracking Index (CTI) quantifies how good the insulating package material is (against electrical tracking) to prevent a conductive path forming on the package in the presence of pollutants over the product lifetime. A leakage path can form through carbonized tracks when a high enough voltage is applied across the insulating material. A higher CTI material allows smaller creepage and smaller package, when the working voltage and pollution degree are held constant. Table 4 shows the material classification according to the CTI index and Table 5 shows the pollution degree classification.

Common-Mode Transient Immunity (CMTI)

CMTI is specified in the electrical characteristic table although it is not an isolation or safety specification. CMTI quantifies the ability to tolerate fast changes in the potential difference between two grounds, common mode, without causing errors. It is specified in kV/µs and measured by applying a transient between the ground at each side of the isolation barrier. High CMTI is required in motor control or gate driver applications as two isolation grounds always have different potentials, and the ground potentials are always changing, such as for photovoltaic (PV) inverters or motor driving applications.

Figure 5 shows the MAX22701E isolated gate driver is switching signals and turning the gates (FETs) on and off constantly. The gate driver ground potential always changes compared to the microcontroller ground. This change can be very fast due to the switching signal frequency. The MAX22700–MAX22702 family of products has a CMTI of 300kV/µs (typical).

Conclusion

Signal isolation is a necessity in today's circuits not only for functionality but also to provide the required protection from electric shock. The designer is aided (or confused) today by the available international and regional standards and certifications. The UL1577, IEC60747-5, IEC 60747-17, and VDE0884-11 standards are the key component-level certifications required for digital isolators. Also, certifications under the IEC60950-1, IEC61010-1, and IEC60601-1 standards are required depending on the end application.