Shocking protection with reinforced digital isolators

Article By : Bonnie Baker

When working in a dangerous, high voltage area, you should think about whether the systems around you meet international safety standards for two-barrier or reinforced isolation.

If you’re a person that’s working in a dangerous, high voltage area, you might take a pause and think about whether you have the right electrical protection. If the designers of the systems around you have selected devices that meet the international safety standards for two-barrier or reinforced isolation, you stand an excellent chance of being safe. Both types of barriers protect system electronics and, most importantly, the people using the equipment.

Figure 1 A handshake can go well or maybe not so well if the system lacks proper galvanic isolation protection.

High-voltage systems that are around other equipment or humans require multiple levels of galvanic isolation. Galvanic isolation is the prevention of unwanted AC currents that flow between functioning electric systems, sections, or humans, while still allowing signal and power transfers to occur. As they say, “Current, not voltage, kills.” Even a touch to a non-galvanic isolated circuit can harm equipment, circuits, and humans.

As you tackle digital isolation challenges, you have access to three IC options. These examples of galvanic isolation are the optocoupler and isolated digital coupler. Specifically, this includes optocouplers, which have an LED and a photodetector; magnetic couplers, which have stacked IC inductors with polyimide separators; and capacitive couplers, which have IC capacitors with a silicon dioxide (SiO2) dielectric between the capacitor plates. This blog goes into the details behind capacitive isolation technology.

Functional isolation

Two circumstances warrant isolation. The first is called functional isolation. Functional isolation ensures that the circuit functions correctly. A perfect example of this is breaking ground loops with an isolation device. When breaking loops, a single channel or multiple channels straddle the interface of the circuit grounds (Figure 2).

Figure 2 A single isolation channel produces effective functional isolation.

In Figure 2, this isolation barrier consists of a bottom plate, a SiO2 dielectric, and a top plate. The thickness of the SiO2 dielectric supports a maximum repetitive peak voltage (VIORM) of 630VP, a maximum working voltage (VIOWM) of 445VRMS, a maximum transient isolation voltage (VIOTM) of 6000VP, and maximum isolation withstand voltage (VISO) of 3000VRMS. The maximum surge isolation voltage (VIOSM) per IEC61000-4-5 equals 10kV. This four-channel capacitive isolation device can also work in a noise reduction strategy. In a noise-reduction circuit, there are several isolation channels across the barrier, such as an ADC’s digital SPI communication lines. This type of isolation separates the low-noise sensor circuits from the noisy remainder of the circuit.

Double isolation

Safety regulations require double or reinforced isolation that has a second isolation barrier for redundancy. The additional barrier provides shock protection even if the first barrier fails. Robust double isolation protects analog front-ends or external industrial backplane/communication ports designed for increased automation in harsh environments. Double isolation requires two equivalent isolation devices in a series (Figure 3).

Figure 3 A series arrangement of two single isolation barrier devices creates a double isolated barrier.

In Figure 3, two independent devices have equivalent isolation barriers. The thickness of both SiO2 dielectrics support the isolation specifications at VIORM equaling 630VRMS, VIOWM equaling 445VP, VIOTM equaling 6000VP, VISO equaling 3000VRMS, and VIOSM equaling 10kV.

The maximum surge isolation voltage (VIOSM) per IEC61000-4-5 equals 10kV, which quantifies the ability of the devices to withstand very high voltage impulses.

Reinforced isolation

Reinforced isolation makes systems more compact by using one device that has the required electrical strength, reliability, and shock protection of two levels of functional isolation. The high-voltage specifications of VIORM, VIOWM, VIOTM, VISO, and VIOSM still apply assuring that the device is capable of handling high-voltage stresses. The digital isolator, as with the double digital isolator, appears in real applications systems and end equipment that have standard isolation requirements (IEC 60664-1, IEC 61010-1, or IEC61800-5-1). A capacitive reinforced digital isolator contains two isolated capacitor barriers in series, surrounded by the active circuits (Figure 4).

Figure 4 Reinforced barrier with two separate isolation barriers inside a single device

Digital isolators that provide reinforced isolation, and specially reinforced digital isolators with actual dual isolation barriers internal to the device, provide the most robust protection. Factory and power grid equipment that operate at high working voltages for long periods of time in the field need this level of robustness to reduce maintenance costs and downtime.

Universal robotics and systems are a growing trend in the industry. Sometimes the same module may be used in places that are accessible to human operators as well as deep within a system, away from human contact. It is essential for reinforced digital isolators to use low power, consume little current, and produce little heat. Heat stresses materials and degenerates the isolation barriers. Low heat generation caused by power conservation is key to reliable digital isolation.

Galvanic isolations in industrial applications are commonly required, not only to provide a complete isolation barrier but also to send data across the barrier. When human operators work with dangerously high voltages and electronic signal chains, it is necessary to use at least two barriers of galvanic isolation, but preferably a reinforced digital isolator. Either way, each of these barriers can withstand the same test criteria as two basic isolation barriers together. Using two isolation barriers on the same PCB takes up a lot of space, which in turn increases the costs of the PCB. Many customers prefer to use only one isolation barrier on their PCB by using a reinforced digital isolator.

Bonnie Baker has been working with analog and digital designs and systems for more than 30 years and is writing this blog on behalf of Maxim Integrated.

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