What are the performance standards for solid state isolators?

Solid state isolators (SSIs) use coreless transformers (CTs) to provide electrical isolation between circuits while supporting power and signal connections. Isolation supports safety, noise immunity, reliability, and other performance benefits.

This article briefly compares SSIs with alternative technologies used in solid state relays (SSRs) and reviews international standards for operating lifetime and isolation performance.

Some of the common isolation technologies other than SSIs and CTs used in SSRs include (Figure 1):

Hermetically sealed reed switches use an external magnetic field to activate the switch and control the switching action.
Magnetic isolation utilizes a transformer with a magnetic core to provide electrical isolation and connections between the input and output. Another alternative (not shown in Figure 1) is capacitive isolation that replaces the transformer with a series of capacitors for isolation and signal transmission.
Optical, or photo-coupled, isolation combines an infrared LED on the input side with a photo transistor on the output to provide isolation and connectivity.

Figure 1. Basic structures of reed switches (left), magnetic isolators (center), and opto couplers (right). (Image: HUIMU)

CT isolation and lifetime

CT isolation is a galvanic isolation technology like conventional magnetic isolation, but it does not use magnetic core materials. CT is based on semiconductor fabrication processes and integrates spiral coils for the transformer directly into a CMOS structure with the isolation provided by silicon dioxide.

The CT enables control signals to be transmitted across the isolation barrier and switch power MOSFETs or IGBTs on the output. The level of isolation is a key performance parameter of CT technologies. CT isolation can meet the strict requirements of reinforced isolation, which provides a higher voltage withstand and lower failure rates than basic isolation.

For especially safety-sensitive applications, special tests are required for reinforced isolation to determine operating lifetimes. IEC 60747-17 assumes an application lifetime of 20 years. It requires 100% testing for transient overvoltage (V_IOTM).

Another key test parameter is the maximum rated repetitive peak isolation voltage (V_IORM) in accordance with IEC 60747-5-5. V_IORM represents the maximum repetitive peak voltage that the insulation barrier of a device can withstand without degradation or breakdown over its lifetime.

Magnetic and capacitive isolation technologies can meet various levels of lifetime testing required by IEC 60747. There is no lifetime model for optocouplers.

Testing requirements for magnetic and capacitive isolators are shown as the green area in Figure 2. Some manufacturers include additional testing margins like testing at 9.6 kV (peak) for devices with a V_IOTM specification of 8 kV (peak). IEC 60747-17 also imposes a safety factor of 1.5 on lifetime, while assuming an intrinsic failure rate of 1 ppm during this time. This means that a CT gate driver is expected to operate for up to 30 years without an isolation failure.

What else is in IEC 60747?

Figure 2. Lifetime testing requirements of IEC 60747 (green) compared with the performance of CT-based SSIs. (Image: Infineon)

IEC 60747 is a series of standards that define test methods and requirements. It focuses on magnetic and capacitive couplers but is also used by SSI makers and users. It establishes isolation levels for both basic used in general purpose applications and reinforced insulation that’s suitable when higher voltage or safety requirements are involved. Key requirements cover steady state and surge voltages (Table 1).

Surge tests apply high-voltage surges to the isolator to simulate transient voltages, and the device must withstand these surges without insulation breakdown.
Isolation resistance tests measure the resistance between different circuit elements to ensure proper isolation.

Table 1. Overview of IEC 60747 requirements for magnetic and capacitive isolators and optocouplers. (Table: Texas Instruments)

Where does UL 1577 fit in?

UL 1577 primarily focuses on the safety and performance of optical isolators. While it doesn’t address SSIs, many SSIs, especially those designed for applications requiring high-voltage isolation, are often certified using UL 1577. It focuses on preventing voltage breakdown and ensuring safety in equipment where a failure could lead to fire, shock, or injury.

UL 1577 includes various tests to evaluate the isolator’s dielectric strength. UL 1577 is particularly relevant for SSIs used in solid-state relays (SSRs). Users of high-voltage SSRs often specify compliance with UL 1577 to ensure user safety.

Summary

There are no SSI-specific performance standards. Instead, SSI makers and users have adopted existing standards, such as the IEC 60747 series and UL 1577, and tailored them to the needs of CT-based SSIs. SSIs can provide the same levels of isolation and lifetime as alternative isolation technologies.