There may come a time in your career as a PCB designer when you must adhere to regulatory agencies. Whether it be medical, automotive, military, or anything of the sorts, your design will certainly be scrutinized and held to very high standards.Oftentimes, when these regulations are in effect, power isolation (or lack thereof) is a very popular topic.
Linear power isolation is essentially what it sounds like; isolating the power supply from the rest of the applied circuit. This is common amongst these agencies for obvious reasons, especially in the medical field. With a non-isolated power supply powering your medical Printed Circuit Board, there is a greater risk for dangerous shocks to surge through the supply and into your device potentially harming the user (and maybe even the patient!) Yikes!
With a better understanding of the isolated power supply vs non-isolated power supplies, where their potential downfalls are, and how they best operate we can attempt to do better with making devices that will work without fail and, certainly, without harm. So let’s all do ourselves a favor, and consider these regulations well before you set out to operate on another human.
The Isolated Power Supply
Again, as it sounds, an isolated power supply is a power supply that is electrically isolated from the rest of the circuit it is powering. The ‘barrier’ to most of these isolated power supplies comes from the use of a transformer.
Transformers in their essence are great big magnets. These magnets take a typical alternating current and will aim to either step the output voltage up or down depending on your downstream requirements. The benefit of isolation is that the magnet literally acts as a magnetic wall of protection in that a large surge that may attempt to pass through the transformer will stop short. This keeps anything downstream of the transformer protected, or in other words ‘isolated.’
As previously stated, these power supplies are often required for regulated industries. However, the fact that a large magnet is most often used to achieve this, you may as well say goodbye to the space requirements you thought you had. But I (and likely many others) would prefer a large device that is isolated and safe, rather than a risk of getting dangerously shocked upon use!
Another caveat seen when using these isolated supplies comes in the form of efficiency. These transformers are simply just not as efficient as their non-isolated counterparts. This inefficiency comes from the physics of the transformer itself. Using the iron core as a ‘lever’ if you will, some energy is lost as heat dissipates through the metal and into the air (be sure to wave goodbye as it leaves your device).
Isolated power supplies often employ the use of a transformer such as this
The Non-Isolated Power Supply
Now that we know what isolates a supply from your board, it becomes rather obvious that taking the transformer out of the design chain, suddenly makes it a non-isolated power supply, who would have thought? These supplies often utilize chips to regulate their linear power.
Designing a board without power isolation is a common practice especially outside of the regulatory agencies, however I still urge you to consider the end-user along your design journey, as you might just save yourself a court case or two when your supply hits the fan and gives your favorite customer a shock they won’t soon forget.
The upsides of designing these non-isolated power designs are plentiful. First, you’ll enjoy a nice bump up in space allowance (relative to an isolated design) due to the fact that you won’t need to put a big fat magnet in your enclosure. [Full Disclaimer: Altium is an equal opportunity transformer company. We accept magnets of all shapes and sizes - come one, come all magnets!]
Non-Isolated supplies always hold the risk of electrical shock through the design.
All jokes aside, you additionally will benefit from an increase in efficiency in your power supply since you, again, will not require the use of a big, fat, and inefficient magnet. These magnets aren’t seeming like a crowd favorite these days. Many of these non-isolated units are well within the reaches of the 95th percentile of efficiencies.
It is worth noting that it is common practice to place these non-isolated supplies downstream of an isolated supply (sometimes physically separate from each other). This is a bit more complex but has the advantage of giving you the proper protection that checks off your safety requirements. An example of this may look like a standalone medical power supply (isolated) that feeds a handful of devices (non-isolated) downstream.
So Which One Is Right For You?
After the facts, benefits, and applications are presented, do you know which one is right for your application? A few concrete factors will likely decide this for you, but obviously, you have plenty of options to achieve either side of the coin.
Keeping in mind that isolating your supply will forgo much of the space otherwise needed, and will be slightly less efficient, but will allow for protection, and the fact that the non-isolated supply will give you space and efficiency, but at the (potentially dangerous) cost of forgoing protection will leave you with enough info to make an educated decision about which to choose.
Programs like Altium Designer can greatly assist in the design and routing of either of these designs. Great features like their PDNA can give you a great analysis of the power frequency distribution within any portion of your design.
If you would like to discuss which power supply design is correct for you, talk to an Altium expert today.
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