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Capacitive, optical, and inductive isolation technologies are design principles that have been around since the beginning of modern electronics. While devices such as capacitors and inductors serve a variety of functions within an electronic device, they, along with optoelectronics, also can be used to isolate circuits from each other.
As new applications arise and others evolve, such as factory automation, motor drives, grid infrastructure, energy, and electric vehicles (EVs), the ability to isolate circuits becomes critical due to the high-voltage components of these applications. Isolating these high-voltage circuits from the low-voltage circuits, which are oftentimes digital (e.g., voltage- and current-sensing communication and signal processing) becomes paramount. It’s also critical to ensure that users are completely isolated from those high voltages.
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An elegant method for accomplishing this is through galvanic isolation (GI). Galvanic isolation is an ideal method for isolating partitions. While GI isn’t new, the technology has evolved thanks to numerous advances. Each GI technology has its particular advantages and disadvantages, and the best solution depends on the application.
On that front, TI offers an array of leading-edge solutions such as the TIDA-010232 AFE reference design for insulation monitoring in high-voltage EV charging and solar energy. Products range from the TPSI2140 and TPS13050/52 solid-state relays to signal-isolating products such as gate drivers, amplifiers, comparators, interfaces, analog-to-digital converters (ADCs), and modules.
Drilling Down
The most commonly used circuit is transformer-based. The venerable is well-understood and has stood the test of time, until recently. As the digital transformation gains traction, this inductive technology starts to bump up against some limitations in the digital domain at higher frequencies. Typically, these are susceptible to stray magnetic fields and increased power consumption, but there are other issues.
A major advantage of magnetic isolation is its main component is a mechanical device—a transformer. That makes it cheap and relatively bullet-proof, and it’s well-entrenched. It offers other advantages over capacitive, too. Still, it has the disadvantages mentioned above.
Capacitive GI
Capacitor-based GI is a new paradigm for high-voltage use cases. Unlike inductive technology, capacitors can’t pass dc. Thus, they become the ideal solution for many emerging GI use cases. Capacitive solutions are insensitive to stray electromagnetic (EM) fields, can run at very high speeds, and have lower power consumption. Also crucial is that they’re much more compact, which is generally a standard requirement for circuits today.
The capacitive trend has become possible thanks to solutions from companies such as TI. Its silicon-dioxide (SiO2) process offers insulating properties with a dielectric strength of ~500 VRMS/µm, and it can be used in high-voltage applications.
This newly developed manufacturing process uses silicon dioxide, the basic on-chip insulation, for the dielectric. As a result, the isolation circuit can be integrated on-chip with the other circuitry to prevent the pass-through of high-voltage, high-frequency signaling. Furthermore, the integrated solution provides shock protection in a single package. The process and technical details are well described in their brief.
Optical GI
Optical isolation is the third methodology for GI. Optical isolators are capable of working with voltages of up to 5 kV, even more in special cases.
Optical isolators are good for many cases but come with some significant disadvantages. First, they’re low-speed devices. Such isolators also are nonlinear for most analog measurements. Response time is slow and they’re not generally recommended for high-speed applications. their specification characteristics tend to drift over time. And they’re inexpensive.
However, since many high-voltage applications have circuits that aren’t necessarily high speed, they’re still applicable to high-voltage GI use cases.
Typical Circuits
Todays advanced high-voltage isolation circuits are IC-based. Such circuits integrate the best properties of optical, capacitive, and inductive technologies. They offer outstanding dc and low-frequency ac blocking while passing the desired digital, analog, and power signal to propagate through. The figure shows a simplified diagram of each circuit type.