Analog Isolation in the 21st Century
Editor’s note: Here is an article regarding a new approach to analog isolation. Each application example makes use of a high-current PVI as an isolated power source, a micropower digital isolator, and a micropower data converter. All of the examples are “building block” type of circuits put together by Arlie Stonestreet whom I met at an Analog Aficionados dinner a few years ago. He designed these circuits out of need for a new system design on which he was working and thought that EDN readers might be interested in the various solutions here. We have here a great example of how designers think and use their resources to solve problems. (Note the hand-drawn schematics in Figures 4 through 8—that’s the way we roll as designers.)
Analog isolation is becoming increasingly important in a multitude of applications including industrial inverter designs, medical applications, metering, isolated sensor applications, and battery management to name just a few. While dedicated isolation amplifiers have been around for some time, often these amplifiers are application specific. Many are targeted for use in isolated current sensing applications and consequently have a limited input range, undesirably low input impedance, and poor gain accuracy.
An additional key challenge has also been in providing isolated power to these devices. While a typical miniature 1W DC/DC converter may sport a 1kV or higher isolation voltage, this is usually only a 1 second rating and the fine print often reveals that the true working voltage is but a paltry 42.4V peak – rendering such devices wholly unsuitable for use in numerous isolation applications. However, even with a DC/DC converter having a suitably high continuous working isolation voltage, the coupling capacitance between the isolated windings must also be kept low if one is to preserve high common mode transient immunity.
Today, a new class of high-current photovoltaic isolators, low-power high speed isolators, and micropower data converters are available. This is the trifecta needed to enable an endless number of isolated designs that no longer require the compromises mentioned above.
High Current PVI Milliwatt Power Source
The 8A100-1 and 8A100-2 family of devices from Dionics-USA are dual high-current PVI milliwatt power sources. Since these are dual devices, the independent outputs may be wired in parallel for increased output current, in series for higher output voltage, or as a split supply if needed. For example, in the application shown in Figure 1 below the outputs are paralleled and a micropower shunt reference provides a stable 3.00V regulated output.
For a 20mA input LED bias the shunt regulated output can source 200uA for the 8A100-1 and nearly 300uA for the 8A100-2 all while maintaining a stiff output voltage of 3.00V. Considerably higher output power in excess of 3mW can be achieved on a duty-cycle limited basis. Additionally, the true galvanic isolation provides high continuous working isolation voltage and vanishingly low coupling capacitance.
Another benefit of the PVI approach is no minimum loading requirement; the clean power produced on the output eliminates the need for EMI output filtering, and the LED input produces no inherent reflected ripple current. The 8-pin surface mount gull-wing or through-hole package is also smaller and less costly than any other isolated power source that could be envisioned in such an application. The relatively high output power of this device, in comparison with other PVI devices, is achieved using an optimized detector structure and customized for voltage levels used by most modern data converters and logic devices.
To achieve even higher output current and efficiency, the 8A100-2 uses a dual LED emitter in each channel thus yielding a 1.5X increase in output current at the expense of the increased forward voltage drop on the input. As we will soon see this flexible mW isolated power source hits the “sweet spot” for many potential isolation applications.
Figure 1: Dionics 8A100 High Current PVI Output Current vs. Input LED Drive