Isolating low-power power supplies

November 24, 2016 //By Aubrey Kagan
Isolating low-power power supplies
Aubrey Kagan, engineering manager at Emphatec in Toronto, considers options for isolating low-power power supplies.

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

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