Isolating low-power power supplies

Isolating low-power power supplies

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By Peter Clarke

There are two main reasons to use galvanic isolation, especially in analog applications. The first is the phenomenon known as a ground loop, where one circuit’s ground reference or voltage potential is different from that of a second circuit. The second reason for is the need for safety and/or the need to remove a high common mode signal. I discussed the signal aspect in a series of blogs on Analog Isolation Techniques Analog Isolation Techniques: Part 1, Analog Isolation Techniques: Part 2 and Analog Isolation Techniques, Part 3 but I passed over the options for isolated power supplies.

There are many switch mode power supplies (SMPS) that provide an isolated output (or outputs). Creating a regulated output in these systems requires feedback across the isolation barrier to the controller chip. Industrial signal isolation has an emphasis towards lower power and miniaturization. Because it is predominantly analog it is often preferable to pass a switch mode derived power signal through a linear power supply, so that a regulated output is not necessary. Let’s not forget that there are also digital systems that can also be isolated like RS485 and RS232. In addition, most SMPS controller chips are aimed at higher power output(s) and so I am going to concentrate on some lower power products.

Instead of designing your own it is always possible to use an isolated DC-DC converter. There are numerous options depending on your input voltage and output voltage requirements. The vast majority seem to me to be overpowered (you might only need a 150mWatt output) and relatively high profile (that is height above the PCB and not in the public eye). In terms of profile and low power you may find some exceptions from Analog Devices with their “isoPower” range and Texas Instruments’ range.

It is always possible to create an unregulated isolated supply with an oscillator like the 555 and a transformer. A search on the Internet will certainly turn up quite a few examples, but if you are a digital fellow like me, I am certain that the design would be sub-optimal. One of the most complete circuits I found was in the EDN Design Idea “Derive an efficient dual-rail power supply from USB” by RO Ocaya from June 2013.

I don’t know whether Maxim’s MAX1480 was made because Maxim had the MAX253 designed, or they designed the MAX253 for the MAX1480, but either way both made an impression on me and I was designing with both in no time. Maxim has always understood that digital engineers have a mental blockage about transformers and they made sure that some magnetics suppliers designed suitable parts and then made absolutely sure to list them in the data sheet. It allowed this lowly digital engineer to design low profile circuits along with sufficient isolation. Maxim has increased their offering and has even produced a spread spectrum driver, the MAX13253 for low EMI systems.

Not to be outdone, Linear Technology has jumped on the bandwagon with a part along with transformer support. The LT8300 in contrast to most others can handle up to 100VDC (yes, that’s right) on the input. It also has a sophisticated technique of regulating the isolated output from the input side. Another offering using the LT8331 can be found here.

Probably as a result of their work with their DC-DC converters mentioned above, TI also has the SN6501 available.

Some commercially available DC-DC converters have different and especially higher input voltage ratings and so may prove to have an advantage over most of the roll-your-own devices. Did I miss anything? Please comment below.

Aubrey Kagan was born and raised in what was then Rhodesia (now Zimbabwe) and completed his electrical engineering degree at the Technion Institute of Technology in Israel before obtaining an MBA at the University of the Witwatersrand in South Africa. Kagan lives in Canada and is engineering manager at Emphatec, a Toronto-based design house of industrial control interfaces, signal conditioners, and switch-mode power supplies.

This article first appeared on EE Times sister site Planet Analog.

Related links and articles:

Niche sensors and devices

Digital output isolation amplifiers target power converters

AC current sensor; differential output, galvanic isolation


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